auto electronics projects

The Maplin ser ies

This book is part of an exc i t ing s e r i e s deve loped by

Bu t t e rwor th -He inemann and Maplin E l e c t r o n i c s P i c .

Books in the se r ies are prac t ica l guides which offer e lec-

t ronic cons t ruc to r s and s tudents c lear in t roduct ions to

key top ics . Each book is written and compiled by a lead-

ing e l ec t ron ics author.

Other books published in the Maplin se r ies include:

Computer Interfacing

Logic Design

Music Projects

Starting Electronics

Audio IC Projects

Video and TV Projects

Test Gear & Measurement

Integrated Circuit Projects

Home Security Projects

The Maplin Approach

to Professional Audio

Graham Dixey 0 7506 2123 0

Mike Wharton 0 7506 2122 2

R A Penfold 0 7506 2119 2

Keith Brindley 0 7506 2053 6

Maplin 0 7506 2121 4

Maplin 0 7506 2297 0

Danny Stewart 0 7506 2601 1

Maplin 0 7506 2578 3

Maplin 0 7506 2603 8

T.A.Wilkinson 0 7506 2120 6

Auto

Electronics

Projects

U N E W N E S

Newnes

An imprint of Butterworth-Heinemann Ltd Linacre House, Jordan Hill, Oxford 0X2 8DP

- ^ J j A member of the Reed Elsevier group

OXFORD LONDON BOSTON MUNICH NEW DELHI SINGAPORE SYDNEY TOKYO TORONTO WELLINGTON

1995 Maplin Electronics Pic.

All rights reserved. No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1P 9HE. Applica-tions for the copyright holder's written permission to reproduce any part of this publication should be addressed to the publishers.

The publisher, copyright holder and author have taken all reasonable care to prevent injury, loss or damage of any kind being caused by any matter published in this book. Save insofar as prohibited by English law, liability of every kind including negligence is disclaimed as regards any person in respect thereof.

British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 0 7506 2296 2

Library of Congress Cataloguing in Publication Data A catalogue record for this book is available from the Library of Congress

Edited by Co-publications, Loughborough

^ Typeset and produced by Sylvester North, Sunderland

all part of The Sylvester Press ^

Printed in Great Britain by Clays L t d , St Ives pic

Preface

This book is a col lect ion of art ic les and projec t s previously

published in Electronics The Maplin Magazine.

Each project is se lected for publication because of its special

features , b e c a u s e it is unusual, b e c a u s e it is e lectronical ly

c lever, or simply because we think readers will be interested

in it. Some of the devices used are fairly specific in function

in other words , the circuit is designed and built for one pur-

pose alone. Others , on the other hand, are not specific at all,

and can be used in a number of applicat ions.

This is just one of the Maplin series of books published by

Newnes books covering all a s p e c t s of computing and e lectron-

ics. Others in the series are available from all good bookshops.

Maplin Elec tronics Pic supplies a wide range of e lec tronics

c o m p o n e n t s and o ther p r o d u c t s to private individuals and

trade cus tomers . Telephone: ( 0 1 7 0 2 ) 552911 or write to Maplin

E lec tron ics , PO Box 3, Rayleigh, Essex SS6 8LR, for further

details of product cata logue and locat ions of regional s tores .

V

1 Car electrical systems

The modern motor veh ic le is a precision-buil t highly-

t u n e d m a c h i n e . High s p e e d p e r f o r m a n c e , low fuel

consumpt ion and quiet smooth-running engine all rely

on efficient ignition, ba t te ry charging and general e lec-

t r ical sys tems throughout the car .

The e lec t r ica l sys tem is very complex . One only has to

look behind a dashboard to s ee the hundreds of wires of

all s izes and co lours , in te rconnec t ing the ins t ruments ,

high vol tage and high current c i rcu i t s . Also, the e lect r i -

cal sys tem is very prone to breakdown, whether this is a

blown lamp bulb, a faulty dynamo or badly adjusted con-

tac t breaker points .

1

Auto electronics projects

No two models of cars have identical e lec t r ica l c i rcui t s .

The e lec t r ica l c i rcui ts are, however, similar and fall into

ca tegor i e s such as convent iona l ignition or e lec t r i ca l

ignition, dynamo or alternator, positive or negative earth.

This chap te r desc r ibes the bas ic sys tems: it is left to the

individual car owner to interpret the descr ip t ions and

diagrams to suit their par t icular vehic le .

One word of warning. Car e l e c t r i c c i rcu i t s can cause

damage to e i ther the car or to the user if tampered with.

For ins tance a shor t c i rcui t a c r o s s the ba t te ry can gen-

era te hundreds of amperes and a lot of heat, even a fire:

the ignition circui t genera tes very high vol tages indeed:

tampering with the instrument c i rcui t s , can cause mis-

leading readings and a poss ib l e safe ty hazard to the

dr iver . Befo re embark ing on any c h a n g e s to the ca r

e l ec t r i c s , make every effort to understand how the cir-

cuit works. In this way fault finding should be greatly

simplified.

The ignition circuit

The purpose of the ignition circui t (Figure 1.1) is to sup-

ply the high vol tage required to opera te the spark plugs

in the co r r ec t s equence and so ignite the air /petrol mix-

ture in each cylinder. The explos ions generated push the

pis tons and so turn the engine, causing motion. The cir-

cui t c o m p r i s e s the ca r ba t t e ry , an ignit ion co i l , the

dis t r ibutor and four (or s ix) spark plugs. The principle

of operat ion is descr ibed later .

2

Car electrical systems

Figure 1.1 The ignit ion c i rcu i t

Battery charging

All e lec t r ica l sys t ems draw their power from the 12 volt

ba t te ry (Figure 1.2). If the ba t te ry was not cont inual ly

charged it would b e c o m e exhaus ted very quickly, par-

t icular ly if the lights, wipers and s ta r te r motor were in

cons tan t use. The turning of the engine charges the bat-

tery by connec t ing it to a dynamo, via the fan bel t . A

3


pulley network at the front of the engine cons tant ly turns

the dynamo which genera tes enough power to charge up

the bat tery . A control box cont ro l s the charging rate and

informs the driver via the ignition light if the ba t te ry is

not charging. Some cars use an a l te rnator in preference

to a dynamo. T h e s e are more efficient but genera te a.c.

ra ther than d.c. and so require rect i f icat ion of the a.c.

output. Ba t te ry charging is desc r ibed later .

Figure 1.2 The battery charging c i rcu i t

4


Lighting

The lighting c i rcui t s are the s imples t of all these , com-

prising a simple connec t ion of the 12 volt lamp to the

ba t te ry via the instrument panel swi tches (Figure 1.3).

T h e s e c i rcu i t s are comple te ly independent of the igni-

tion and charging c i rcu i t s , the one connec t ion to each

lamp being taken via a single wire and respec t ive switch

to the bat tery; the o ther connec t ion uses the car chas -

s is . The lighting c i rcu i t s are desc r ibed in more detail

later .

Figure 1.3 The l ighting c i rcu i t

5


Figure 1 .4 The indicator and accessories c i rcu i t

6

Indicators and accessories

Contained within this circui t is the s ta r te r motor which

draws hundreds of amperes from the ba t te ry to turn the

engine until it fires (Figure 1.4). Heavy duty cab le and a

heavy duty solenoid car ry out this operat ion, which is

prone to t rouble for various reasons . Also there is the

fuel pump which is a small solenoid opera ted device to


pump petrol from the tank to the ca rbure t to r , the indi-

ca tor light c i rcui t ry with hazard warning lights, the radio

and c a s s e t t e player c i rcu i t s , the hea te r and wiper mo-

tors , horns , instrument gauges, and heated rear sc reen .

T h e s e c i rcui t s are relat ively s imple and are desc r ibed

toge ther with fault-finding t echn iques later .

Wiring diagram

Car wiring diagrams are often very difficult to read and

interpret . The reason for this is that , in a modern car

with a large number of ins t ruments , l ights, a c c e s s o r i e s

and motors , all are to be in te rconnec ted on one compre-

hensive diagram. Fuses and switches must also be shown,

toge ther with the co lours of the wires and cab le s ; many

manufacturers use an international colour code for easier

identification of the r e spec t ive c i rcui t c ab l e s .

Some of the more popular symbols used in car wiring

diagrams are i l lustrated in Figure 1.5. The cab les are of-

t en c o d e d and c o l o u r e d for i d e n t i f i c a t i o n and a

s h o r t h a n d m e t h o d of s implifying t h e d iagram often

groups all in one bundle (cal led a cable-form) as a single

line. To t r ace the s tar t and finish of one cab le involves

almost m ic ro scop i c analysis of all connec t ions , search-

ing for the required code and colour .

E l e c t r o n i c dev ices such as e l e c t r o n i c ignition or the

dashboard m i c r o p r o c e s s o r are shown as simple b locks .

Fault finding within t h e s e dev ices must be left to the

specia l i s t dealer .

7

Auto electronics projects ~L

Z~.*J

|||

|||

1|1|

_#

%

H

IX

Ban

ery

Bat

tery

B

atte

ry

_.

~ ^

Dis

trib

utor

MM

-j-j

v

, A1?

. .

_ W

ires

cro

ssed

wit

hout

join

ing

Wir

es jo

ined

JL

Bul

b, d

oubl

e lo

op or

is

hea

dlam

p

Cap

acito

r D

iode

I

II

I C

-JI

/\

/^Zm

\ .

Igni

tion

coil

^if

W^

Wlo

om

1

Gen

erat

or

Star

ter

mot

or

-

-

_/^\

_ O

pb

on^

wir

,

^

V

V

V

V

Inst

rum

ents

E

lect

rici

an

Am

met

er

Tra

nais

tor

Rad

io

Con

tact

poi

nts

Rea

ieto

ra

Con

trol

box

L

*1

| S

S *

\

\ Si

x sy

mb

ols

used

tor s

wit

ches

B

lock

of s

nap

con

nec

tors

Igni

tion

coil

Blo

ck o

f co

nnec

tors

mot

or

T

Hea

ted

rear

win

dow

S

par

kp

lug

Six w

ays

to s

how

fuse

s Fo

ur w

ays

to s

how

eer

th c

onn

ecto

ns

Aer

ial

lerm

inai

blo

ck

Fig

ure

1.

5 C

om

mo

n sy

mb

ols

u

sed

in

ca

r w

irin

g

dia

gra

ms


The engine

The most common small to medium car engine is the 4-

c y l i n d e r p e t r o l i n t e r n a l c o m b u s t i o n e n g i n e . M o r e

powerful engines have six cyl inders , some have eight;

motor cyc le s and mopeds have one or two. The arrange-

ment of cylinders varies, some being overhead cam shaft,

some pushrod and rocker , and o the r s with cy l inders

aligned in the shape of a V.

This brief descr ipt ion of the 4-cylinder engine, highlights

the impor tance of a c c u r a t e timing so as to maximise

power and performance. Figure 1.6 shows the arrange-

men t of c y l i n d e r s and t h e four s t r o k e s , i l l u s t r a t e d

separa te ly in Figure 1.7:

induction compression

Crank position {degrees)

Cylinder no. 1

Cylinder no. 2

Cylinder no. 3

Cylinder no. 4

0 - 1 8 0 1 8 0 - 3 6 0 3 6 0 - 5 4 0 5 4 0 - 7 2 0

Power Exhaust Induction Compression

Exhaust Induction Compression Power

Compression Power Exhaust Induction

Induction Compression Power Exhaust

Figure 1.6 4-cy l inder and 6-cyl inder engines

9


Figure 1.7 The four stages of combustion

10


induction the petrol /a i r mixture is sucked into

the cylinder,

compress ion the piston compresses the mixture,

power the spark plug ignites the mixture caus-

ing an explosion which pushes the piston down,

exhaust the piston pushes the burnt gases out

of the cylinder.

The four cyl inders opera te in se r ies so that, at any one

t ime, one is being powered. The crank shaft posi t ions

the pis tons in the c o r r e c t s equence , two comple te revo-

lutions (720) comprising the comple te four-stroke cyc le .

The e lec t r ica l c i rcu i t s have the j ob of supplying each

spark plug with a high voltage pulse to power the piston

in the co r r ec t s equence , and at the t ime when the piston

is at the top of its s t roke ( top dead c e n t r e ) . The distribu-

tor ensures that the pulses travel in s equence to the four

spark plugs and, at the same time, t ime the pulse to top

dead cen t re .

Basic ignition

The main componen t s of the ignition circui t are the igni-

t i on c o i l a c y l i n d r i c a l t r a n s f o r m e r wi th two

connec t ions SW and CB and a high tension cab le going

to the dis t r ibutor ( s e e Figure 1.8) and the dis t r ibutor

a mechanica l device coupled to the engine via skew

gears . This ac t s as a four-way switch to route the high

tension to the spark plugs, and as a means of generat ing

the high tension vol tage.

11

Auto electronics projects liiilllllillfj^

Figure 1.8 Basic high voltage generating c i rcu i t

Figure 1.8 shows the bas i c high voltage generat ing cir-

cuit . The operat ion is as follows, assuming the con tac t

breaker points are initially c losed ( see Figure 1.10):

the piston in one cyl inder ( say number 1) r i ses to

top dead cen t re , compress ing the petrol /a i r mixture,

the ro tor arm in the dis t r ibutor cap points to the

a p p r o p r i a t e high t e n s i o n c o n n e c t i o n to s p a r k plug

number 1 and,

the con tac t breaker points open,

the magnetic field in the primary of the ignition coil

(Figure 1.9) quickly c o l l a p s e s . The turns ra t io of the

t ransformer of about 10,000 to 1 transforms this col lapse

into a vol tage of about 20 ,000 volts a c ro s s the second-

ary,

12


'To distributor

^ J \ High tension

^ ^ ^ ^ Secondary

Figure 1.9 The ignit ion coi l

I Sparking plugs winding ^ J g n M o n wteh ^ | I

winding^ "Jl ^ . g ^ o ^ ^ ^ ^ a m ^ ^ ^ ^

Figure 1.10 Sparking plugs f i r ing c i rcu i t

13


the high tension pulse ignites the petrol /a i r mix-

ture in cyl inder 1 causing the engine to ro ta te ,

the dis tr ibutor shaft ro ta tes to again c lo se the con-

t ac t b reake r poin ts . The c a p a c i t o r a c r o s s the points

suppresses the high voltage pulse genera ted by this c lo-

sure,

the distr ibutor shaft turns the rotor arm to the next

cyl inder and the procedure repea t s .

The timing of the opening of the points is cr i t ica l . The

dis t r ibutor shaft cam opens the gap as in Figure 1.12,

the posit ioning of the con tac t breaker points a ssembly

is cr i t ica l toge ther with the gap width. The points , after

a period of wear, tend to co r rode and pitting occur s ; a

deposi t which builds up and reduces the effective gap.

The gap is usually about 25 thousands of an inch wide,

opens and c l o s e s s o m e ten mill ion t imes every 1000

miles. One o ther adjustment to opt imise the timing is

the dwell angle. This is the number of degrees that the

points remain c losed; refer to the maker ' s manual for

the recommended value.

Ignition timing is carr ied out in the following sequence :

c h o o s e cyl inder number 1 consul t the manual,

loca te the timing marks on the fan belt pulley ( see

Figure 1.13),

turn the engine crank shaft until the marks align at

top dead cen t re ( t . d . c ) . The engine can be turned by

placing the car on level ground, take out all the spark

plugs, p lace in top gear, r e lease the brakes and move

the car to and fro,

14


ensure that the dis t r ibutor ro tor arm points to the

high tension lead to cyl inder number 1. If not, turn the

engine through a further 360 ,

connec t a 12 V lamp between the con tac t breaker

spring ( see point X in Figure 1.12) and a good earth point,

ro ta te the engine by about 20, then inch it slowly

backwards until the lamp just l ights,

if the t .d.c . reading is i nco r rec t , align the t .d.c .

mark, then loosen the dis t r ibutor clamping nut (point Y

in Figure 1.11) and turn the ent i re dis t r ibutor ant ic lock-

wise until the light just goes out. Then turn c lockwise

until it jus t l ights. Clamp the nut,

c h e c k the t .d.c. set t ing once again,

rep lace the plugs, put on the brakes and take out

of gear! A faster method uses a s t r o b o s c o p e with the

engine running, a Xenon tube flashing as the points open

and c lose .

Electronic timing

The sys tem so far desc r ibed somet imes fails because of

pitting of the points and wear and tear of the moving

parts of the dis t r ibutor . Two types of e l ec t ron ic sys tem

are found:

t rans i s to r i sed ignition or capac i to r d ischarge igni-

tion see Figure 1.14 and,

con t ac t l e s s (opt ica l or magne t ic ) ignition.

15


Figure 1.11 The distr ibutor

16


Figure 1 .12 Contact breaker assembly

Figure 1.13 Timing marks on fan belt pulley

Trans i s to r ignition uses a power d .c . -d .c . conver te r , a

two t rans i s to r push-pull osc i l la tor , to genera te 400 V or

so , to feed to the ignition coil and produce a higher volt-

age and heal th ier spark. At the same time, the con tac t

b reakers no longer switch the full 12 volt ba t te ry cur-

rent: they merely switch a 12 volt low current signal to

the d .c . -d .c . c o n n e c t o r . T h e points the re fore last far

longer and the sys tem is virtually maintenance-free.

17


Contac t less ignition uses a moving magnet or infra-red

ray to rep lace the cumber some con tac t b reakers , a tran-

sis tor ised d.c.-d.c. conver ter circuit being used as before

to deliver the high tension pulses to the plugs. Both sys-

tems can be installed into an exist ing c i rcui t in a very

small t ime, a number of modern ca rs having such sys-

tems built in when new.

12 V O-

Figure 1.14 Transistorised and capacitor-discharge ignit ion

circui ts

18


The battery

A car ba t te ry is a real powerhouse and should always be

maintained in prime condi t ion. It is compr ised of a se-

r ies of s ix lead-acid 2 volt ce l l s (Figure 1.15) which,

together , cons t i tu te 12 vol ts at capac i t i e s varying from

about 30 to 100 ampere-hours . A 70 ampere-hour ba t te ry

delivers a cons tan t 70 amps for one hour, or one amp for

70 hours , or on a very cold day, 400 amps for a few s e c -

onds to s tar t the engine.

The negative plates are cons t ruc t ed from spongy lead

plates and the posi t ive plates from lead dioxide. Dilute

sulphuric acid with a specif ic gravity of about 1.2 s ta r t s

the chemis t ry into act ion, current from the ba t te ry turn-

ing the plates into lead sulphate . A ba t te ry charger , by

Figure 1.15 The battery

19


way of the dynamo or a l ternator , r everses this p roces s

by restor ing the ba t te ry plates to their original compo-

sit ion.

Modern ba t te r ies are self maintaining and the e lec t ro-

lyte (ac id ) levels remain cons tan t . Older ba t te r ies are

prone to deter iorat ion and last only 3 or 4 years . The

performance of a ba t te ry falls at low tempera tures , giv-

ing problems on a cold morning and sulphation of the

terminals which causes leakage currents to chass i s ; this

is avoided by smearing petroleum jel ly onto the termi-

nals. A more common cause of bat tery trouble, other than

an old and tired ba t te ry itself, is damp and dirty wiring,

part icular ly around the s ta r te r motor which drains most

of the ba t te ry power.

Ba t te ry charging is carr ied out in one of two ways:

the dynamo a d.c. generator , like a motor in re-

verse , which delivers current to the ba t te ry as long as

the engine is running fast,

the a l ternator an a.c. genera tor which, although

requiring an a .c . /d .c . rect if ier circui t , has greater effi-

c i ency and charges the ba t te ry even when idling.

Figure 1.16 shows a cut away picture of the dynamo and

the circui t which con t ro l s the charging of the bat tery,

cal led the cut-out or cont ro l box. This unit s enses the

dynamo output vol tage and, if low, cuts the dynamo out

of c i rculat ion. As the voltage r i ses the cut-out connec t s

the dynamo to charge the ba t te ry and if it r i ses beyond

a prese t value, the regulator winding reduces the effec-

tive dynamo output by adjusting the current in the field

winding, excess ive current going direct ly to the car e lec-

tr ical c i rcu i t s .

2 0


Figure 1.16 Dynamo and control box

The a l te rnator is shown in Figure 1.17 toge ther with its

cont ro l c i rcui t ry and rect if ier d iodes . The th ree s ta tor

windings are connec t ed internally to the diodes and a

d.c. output is obta ined. A t rans i s to r i sed cont ro l c i rcui t

maintains a cons tan t ba t te ry charging current by adjust-

ing the current in the ro tor winding.

21


Stator windings in which current is generated

Diodes convert alternating current to d.c.

Rotar turns inside stator assembly

Figure 1.17 Alternator and control c i rcu i t ry

Both sys tems have a built-in ignition warning light with

one side connec t ed to the ba t te ry +12 V terminal , the

o ther to the dynamo or a l ternator output. If the genera-

tor is not working, when the engine is swi tched off for

22


ins tance , or when the fan-belt is slipping or broken, the

12 V bulb has 12 vol ts a c r o s s it and it l ights. Normally

the lamp has 12 vol ts on e i ther side and it goes out.

Lighting

Little needs to be said about the normal lighting c i rcui t s

excep t to say that the headlamp bulbs can consume sev-

eral amperes each and so cab le of the c o r r e c t size must

be used to prevent heating (or melt ing) of the wiring.

Many bulbs , as in Figure 1.18, have two fi laments for

compac tness . Quartz halogen bulbs, with a gas surround-

ing the tungsten fi laments, give off greater br ightness .

Figure 1.18 Dual fi lament bulbs

23


As the headlamps between them consume several am-

peres , the headlamp (or f lasher) switch has to be heavy

duty and high current wires must be sent to the dash-

board. Consequent ly a relay is often posi t ioned near the

headlamps, as in Figure 1.19, this being act ivated via a

(preferred) low current switch and wiring. Operating the

switch act ivates the relay which connec t s the headlamps

direct ly to the ba t te ry terminal .

One final lighting device in common use is the spring

s tee l flasher unit ( s ee Figure 1.20) which turns the indi-

ca to r lamps on and off.

Figure 1.19 Headlamp relay

2 4


While cold, the c o n t a c t s are held toge ther by the dia-

phragm. When current passes through the con t ac t s , by

indicating to turn left or right, the res i s t ance metal heats

up, expands and pushes the con t ac t s apart . They then

cool again, c l o s e and the s e q u e n c e repea t s 60 to 120

t imes a minute. Emergency light units are similar excep t

that heavy duty con t ac t s are used.

Current from A I Current to indicator switch | indicator lamps

Indicator lamps on' Indicator lamps off

Figure 1.20 Flasher unit

Starter motor and other accessories

In a similar way to the headl ights being opera ted via a

remote control relay, a s ta r te r solenoid is used as in Fig-ure 1.21 to switch the 400 amps to the s ta r te r motor .

This wiring is the th ickes t to be seen under the bonnet

and every s tep is taken to minimise any heat generated

25


despi te the c o s t s of the thick copper wire. The s ta r te r

motor engages with the engine via the flywheel to s tar t

the engine, as seen in Figure 1.22. If the ignition circui t

is working well, a few turns of the engine should cause

the engine to fire and cont inue under its own s team. The

s ta r te r motor is then d i sconnec ted from the engine.

Figure 1.21 Starter solenoid

Two methods are used, a pre-engaged motor whose pin-

ion is always linked to the flywheel, a solenoid operat ing

a plunger to engage the s tar ter motor with its pinion (like

a small c lu t ch ) , and the inert ia type whose pinion sl ides

along the shaft to engage with the flywheel as soon as

the s ta r te r motor opera tes . T h e s e are shown in Figure

1.23. Figures 1.24 to 1.28 il lustrate a number of other e lec-

tr ical a c c e s s o r i e s which are essent ia l , and some legally

required, in the modern motor car .

26


Figure 1.22 Flywheel

Petrol pumps ope ra t e e i ther via a mechan ica l rocke r

assembly coupled to the engine forming a small mechani-

cal pump (Figure 1.24), or an e lec t r ica l diaphragm pump,

ra ther like a vibrator , which pumps the petrol from the

tank to the engine, as in Figure 1.25. The petrol gauge

opera tes using a small float coupled to a var iable resis t -

ance unit. As the petrol level r i ses or falls, the current

to the gauge r ises or falls accordingly . This unit, similar

to a WC bal l-cock, is sea led for fire r easons , see Figure

1.26.

27


28

c*

_

>


29

Fig

ure

1.

23

Co

nti

nu

ed


Figure 1.24 Mechanical fuel pump

Horns come in all shapes and s izes . Figure 1.27 shows a

simple type, working like a v ibra tor whose diaphragm

output is mechanica l ly amplified to warn pedes t r ians to

get out of the way.

Ammeters can be fitted in any car : a s imple means of

installat ion necess i ta t ing a minor change to the wiring

30


Figure 1.25 Electr ic fuel pump

31


Figure 1 . 2 6 Fuel gauge and float

as shown in Figure 1.28. By this means the ammeter does

not record the s ta r te r motor current , but all o ther cur-

rents taken by the car c i rcui t ry .

Figure 1 .27 Horn diaphragm

32


Figure 1.28 Ammeter wiring

Finally, a look into the compute r i sed dashboard now

found in a number of high performance ca r s . Transduc-

ers cons tan t ly read r.p.m., p ressures , t empera tures and

so on; t hese are moni tored and the computer checks and

warns the driver of impending t rouble ( see Figure 1.29).

The day of the J a m e s Bond superca r or the Night Rider 's

Kit looms nearer everyday.

33


Figure 1.29 Computerised dashboard

34

2 Electronic ignition

The e l ec t ro -mechan ica l ignition sys tem that has been

used to fire the fuel/air mixture in an internal combus-

tion engine for severa l decades , and which is familiar to

home mechan ic s everywhere , has prac t ica l ly been re-

placed by e lec t ron ic methods in recen t t imes . Some of

the reasons for this are not quite as obvious as you might

suppose , but cer ta inly, as with everything e lse , a mod-

ern e l e c t r o n i c a l t e r n a t i v e is s u p e r i o r to i t s

e lec t ro-mechanica l ances to r . To be fair though, the lat-

ter has had a lot going for it, it originally rep laced a

method so a rcha ic as to be unbel ievable .

Automotive ignition a brief history

Earl iest motor ca r s , or in fact anything using the new-

fangled gas engine (many of which were a lso used for

35


powering agricultural machinery) , of slightly over a cen-

tury ago had to make do with a device compr is ing a

thin-walled copper tube with c losed ends, supported in

the middle with a porcelain insulator or some-such simi-

lar item. The insulator sc rewed into the cyl inder head,

like a modern plug indeed the word plug p robably

or iginates from this t ime.

To s tar t the engine, the outs ide end of the tube is heated

with the flame of a spirit burner until glowing. Then at-

t empts can be made to get the engine going, using a

start ing-handle. When the fuel/air mixture arr ives at the

o ther end of the tube, on the inside, in the right quanti-

t ies (a bit of a juggling a c t ) , it should (hopefully!) burn.

Once the engine is warmed up and running, the spirit

burner can be put out and thereaf ter the tempera ture of

the tube will be maintained by the heat of internal com-

bust ion, in the same way tha t the engine of a model

aeroplane keeps its glow-plug hot.

Not surprisingly, while the gas engine was still only a few years young, engineers thought hard about improv-

ing this less than ideal s i tuat ion. It was only a quest ion

of t ime before the e lec t r ica l ly powered hot wire type of ignition, a glow-plug then, was pressed into se rv ice for the petrol engine. The t rouble with glow-plugs however,

is that the wire burns away quite quickly and a s tock of

spares must be carr ied around at all t imes .

Then, just prior to the turn of the century, a method was

devised which, though it s eems obvious now, must have

taken a good deal of working out at the t ime. It was reli-

able in opera t ion like nothing e l se previously, it was

sophis t ica ted , it was state-of-the-art. It was spark igni-

tion.

3 6

Electronic ignition

37

The advantages included much eas ie r s tar t ing simply

energise the sys tem and crank the handle. Also, b e c a u s e

the plug was no more than a spark gap at the business

end, and the e l ec t rodes were far more robus t than thin

wire or copper tube, it had a working life h i ther to un-

seen.

From the engine des igners ' point of view it ra ised two

important poss ib i l i t ies :

the moment of ignition of the fuel/air mixture could

be p r e c i s e l y c o n t r o l l e d . P rev ious ly , the c o m b u s t i o n

chamber had to be designed to prevent the charge ignit-

ing prematurely during compress ion , a shape which did

nothing for efficiency (or performance, if you l ike) ,

engines with multiple cyl inders could be ca te red

for jus t as easi ly as s ingles . Prior to this engines were

most ly a single cyl inder type the ignition parapherna-

lia for jus t one was usually quite enough to cope with.

There are basical ly two types of e lectro-mechanical spark

ignition sys tems: the magneto, and what ' s cal led coil ig-

ni t ion. T h e only d i f ference is tha t the magne to a l so

genera tes its own e lec t r i c power to opera te . With coil

ignition the power supply is external . In the beginning,

there was only the magneto. In the 1920s , the Americans

p ioneered coil ignition, which used power h i ther to gen-

erated exclus ively for ancillaries lights and so forth. The power supply compr ised a d.c. genera tor in the form

of a dynamo, with a back-up for the per iods when the dynamo couldn ' t provide the n e c e s s a r y current an

accumulator (a ba t t e ry ) . In Europe there was great re-s i s t ance to coil ignition, espec ia l ly among the Bri t ish,

who thought it too gimmicky. Customers wouldn't buy a


car if it had coil ignition manufacturers had to revert

to the magneto in order to be able to maintain sa les .

Would you bel ieve that such a r e spec ted manufacturer

as Rolls Royce couldn ' t shift their la tes t spor t s tourer

until they had put a magneto back into every car? Such

was the r e s i s t ance to change. Perhaps there is a modern

parallel here , about cus tomers (and m e c h a n i c s ) being

frightened of the complexi ty of fuel in ject ion. . .

Spark ignition the principles

An e lec t r i c arc is an e lec t r i c current flowing through a

gas, which for the purposes of this d iscuss ion, is air. Air,

as with most insulators r es i s t s the flow of e lec t r i c cur-

rent . If forced, it ionises as e l e c t r o n s begin to move

between molecules . As with any o ther res is tor , this mo-

lecular friction genera tes heat from the amount of

energy required to cause air to succumb, quite a lot of

heat . The arc is a whi te /blue colour , and hot enough to

s tar t a fire.

It is worth descr ibing how the e lec t ro-mechanica l igni-

tion sys tem opera tes first, s ince there is no substant ia l

difference between it and any e lec t ron ic equivalent

they all have to do the same thing, make a spark. We

shall s tar t here and work backwards .

Air needs a little persuading in order to ca r ry an e l ec t r i c

current and produce an arc . At normal a tmospher ic pres-

sure it is not all that difficult, but still requires a high

voltage to break down the air be tween a pair of e lec-t rodes . The narrower the gap, the eas ie r it is . However,

38

Electronic ignition

whilst it is quite easy to bridge a gap of 0.02 inches (a

typical spark plug gap) in open air, it is much more diffi-cult inside the combus t ion chamber . This is b e c a u s e air

ionises more easi ly the thinner it is ( the typical demon-

st ra t ion is an e lec t r i c a rc in a glass vesse l with a vacuum

pump a t t ached ) , it cor respondingly b e c o m e s more re-

s is t ive the more dense it is, like inside the combus t ion

chamber of an engine. Universally, the fuel/air mixture

is compressed before ignition, the main reason being that

this r e leases more energy on combus t ion (but a lso be-

cause the piston, being a rec iproca t ing part linked to a

revolving part, can ' t help i tself) . The upshot of all this is

that it is more difficult to bridge the gap to produce a

spark in c o n s e q u e n c e , requiring a very high vol tage to

do so , which accoun t s for the 25 to 35 kV HT vol tage

range typical at the plug's live end. I labour on this point b e c a u s e it causes problems for the design of e l ec t ron ic

ignition amplifiers, as will be seen later .

Obviously it is impract ica l for this sor t of potent ial to

be produced and cont ro l led di rect ly from some engine

driven genera tor , so ins tead a step-up t ransformer is

used, which is where the coil c o m e s in. All the genera-

tion and t imed-switching is done at a more manageable

low voltage, and is conver ted by the coil to the neces -

sary high vol tage.

Actually the sys tem is c levere r than that . The s e q u e n c e

shown in Figure 2 .1 (a ) to 2 .1(d) reveals the sys tem to be

a form of flyback converter. Figure 2 .1 (a ) shows the com-ponents of a mechanica l sys tem at rest. With switch SI open, nothing is happening. When S I c l o s e s in Figure

2 . 1 ( b ) , current flows in the primary winding LI of T l ,

39


the ignition coil . T l has a laminated s tee l co re and a fi-

n i t e t ime is t aken for t h i s c o r e to r e a c h m a g n e t i c

saturat ion, by which time the primary current will a lso

be at a maximum. This maximum is set by choos ing a

d.c. impedance for LI by using res is t ive wire, or e l se it

will a t tempt to short-c i rcui t the supply after the co re

sa tura tes! For 12 V sys tems the impedance is chosen for

a maximum current of around 3.5 to 4 A, as a typical

value.

(c) GO

Figure 2.1 Sequence of act iv i ty in contact breaker ignit ion

system

40

Electronic ignition

In Figure 2 . 1 ( c ) , SI opens and unwanted effects take place

in its vicinity, but we'll ignore them for the moment . Suf-

fice to say that as the magnetic field col lapses , it a t tempts

to maintain the current flow in LI in the same direct ion,

and at the same t ime induces a current in L2. B e c a u s e L2

has many more turns than L I , its output vol tage is much

higher. In the cha rac t e r i s t i c manner of flyback conver t -

ers , the coil will a t tempt to output the same amount of

power that went into it. If a path on the primary side is

denied it, then the only r ecou r se is to find an outlet on

the secondary side.

The load is the plug air gap, which basical ly doesn ' t want to know at first, but the coil will keep pushing the volt-

age up until the gap is bridged. If the total power input

was 50 W and the output r e ached 30 kV then the gap

cu r r en t is ini t ia l ly 1.6 mA. However , o n c e the a r c is

s tar ted , the vol tage level required to maintain it can re-

duce substant ia l ly allowing a grea ter current flow and a

nice heal thy spark. This is indicated in Figure 2 .1 (d ) .

The snag is that a smal ler representa t ion of this act ivi ty

also appears a c r o s s the primary, L I . The effect is an ini-

tial pulse of up to severa l hundred vol ts . At the point of

breaking the circui t , the mechanica l switch SI has a very

narrow gap between its c o n t a c t s which might be meas-

ured in m i c r o n s . Such a gap is e a s y for a coup le of

hundred volts to bridge; the coil expends all its energy

in producing an arc between the switch c o n t a c t s , and

there is none left for the plug. If you want to prove the

effect for yourself t ry it with the coil of a relay, a pair of

tes t leads and a ba t tery .

So this is where the o ther c lever bit c o m e s in, the third

componen t in the set-up, C I . To this day it is still cal led

41


a condenser, a very old-fashioned name for a capac i to r . Its function is to momentar i ly take over from the switch.

As S I opens , current flow is diverted into CI , charging

it. T h e idea is tha t by the t ime the pr imary vo l t age

reaches a high level, the contac t gap is unattainably wide,

forcing the coil to go for the plug gap instead. This has

two main disadvantages:

it consumes some power which might o therwise

con t r ibu te to the spark, and,

it s lows down the ra te at which the HT level can

increase , the output of which takes on more of a milder

ramped pulse shape ra ther than a true pulse. The value

of CI is cr i t ica l : if too small , it will encourage switch arc-

ing; if too large, it will absorb too much power and defeat

the whole ob jec t . A value of 220 nF is usually about right.

Switch arcing and power loss still occur , but at accep t -

able levels .

A third anomaly is that , after the main pulse has oc -

curred , what you are left with is LI and CI , with the

supply as a common terminal , forming a tuned circui t

which rings or r e sona tes slightly. Figure 2.2 shows the vol tage waveforms a s soc ia t ed with this se r ies of events .

It was ment ioned that the ignition coil has an inbuilt d.c.

impedance to limit current flow while the con tac t break-

ers are c losed . During this t ime the coil is drawing its

maximum power of 45 to 50 watts , to no effect o ther than

that this manifests i tself as heat . Consequent ly an igni-

tion coil has been safeguarded against this , and hence is

a lmost universal ly cons t ruc ted as shown in Figure 2.3. It

is supported in the cen t re of an aluminium can, which is

filled with oil . An ignit ion coi l is , the re fo re , a liquid

cooled component .

42

Electronic ignition

Figure 2 . 2 Voltage waveform from Figure 2 . 1 at coi l primary

Brass HT socket

Terminal Terminal

Moulded insulator

Aluminium can

Oil filled cavity

Synthetic rubber support

Twisted pair of wires.

Primary and HT Common

is + terminal

Coil windings

Laminated core as a bundle of steel strips.

HT * connects to this, and is

passed via the coil spring at the top

to HT socket.

Figure 2 . 3 Internal construction of a typical ignit ion coi l

43


Advantages of electronic ignition

The first two problems are prac t ica l ly solved by e lec-

t ronic switching, the third by using the coil in a different

way. The re are o ther p rob lems that can be solved at a

s t roke , like mechanica l wear.

The heel of the moving half of a con tac t b reaker wears

on the dis t r ibutor cam. The con t ac t sur faces b e c o m e

damaged, developing a hole or pit in the posi t ive side

and a raised pip on the negat ive surface, as the inevita-ble arcing causes metal to migrate from one surface to

the other . The lumpy result causes irregular timing and

bad separat ion, but it may be poss ib le to rescue them with the skilful appl icat ion of a fine s tone .

Then there is the ( somet imes be t te r than dreadful) me-

chanica l auto-advance mechanism, with its centrifugal

bobweights , springs, cam con tours and vacuum ass is t

device. To be fair, in p rac t ice a mechanica l sys tem which

is both well designed and 100% fit is difficult to beat ,

even by an e lect ronic equivalent, but sooner or later wear

takes its toll , affecting engine efficiency, and so it needs

per iodic examinat ion and co r rec t ion or even replace-

ment.

But owners put off having the car serv iced until it des-

perate ly needs it b e c a u s e of exorbitant garage bil ls . In the meant ime the vehic le is wasting valuable fossil fuel

and polluting the a tmosphere in a way that it wouldn't if

properly tuned. Also of conce rn to car manufacturers ,

under pressure to reduce pollution and fuel consump-

t ion, is the D.I.Y, home m e c h a n i c t inker ing with his

engine. If he knows what he is doing then fine. If he

doesn ' t . . .

44

Electronic ignition

Consequen t ly fac tory se t and m a i n t e n a n c e free e l ec -

t ronic ignition, and ca rbure t to r s with secur i ty blanking

plugs seal ing off the vital b i t s , prevent unauthor i sed

hands fiddling with these and getting it wrong. And you

thought it was all done for your benefit . It a lso explains

the lack of really meaningful information in the modern

owner ' s handbook. Refer servicing to your dealer, or

warranty is void, and that sort of thing. Basical ly it means

s lapped wrist to the potent ial D.I.Y'er.

Electronic ignition how it works

The good news is that e l ec t ron ic ignition for the average

modern car has boiled down to a recognisab le s tandard

formula, with a long t rack record of rel iabil i ty. The bad

news is that if it does go wrong, you can ' t fix it yourself . Having a c i rcui t diagram is no help (which you won't be

able to get hold of anyway); both the s enso r and the

amplifier are sealed in resin and you can ' t get inside with-

out destroying them. And assuming you could get into

the amplifier you will most p robably find thick film re-

s i s to rs bonded straight onto a ce ramic base which they

share with o ther micro-mount componen t s and a very

spec ia l i sed cus tom chip, with which you will be able to

do nothing.

The h is tory of t rans i s tor i sed ignition goes back as far as

the 1960s . Unfortunately s e m i c o n d u c t o r s of the t ime,

being made of germanium instead of s i l icon, were some-

what fragile, requiring that spec ia l beefed-up ones be manufactured to cope . Consequent ly e l ec t ron ic ignition

was expens ive and usually only found a t t ached to simi-

larly unaffordable spor t s ca r s .

45


Timing sensors

In the 1970s , solid s ta te ignition with th ree vers ions of

timing sensor proliferated. The simplest was the so called

transistor assisted ignition, which still required a me-chanica l switch. The second type had an opto-e lec t r ic

timing sensor , which might use e i ther vis ible light or an

infra-red coupler . Here the beam is interrupted by a ro-

tating shut ter with blades like a fan. The third type uses

a magnet ic sensor .

Many of t hese were available as after-market bolt-on kits for both ca rs and mo to rcyc l e s . After some twenty years

only one type has c om e out on top as the s implest and

most re l iable the magnet ic sensor .

The senso r genera tes an e lec t r i c pulse which tr iggers

the amplifier, which in turn drives the coil primary. Fig-

ures 2 .4 (a ) and ( b ) show the now archetypal , s tandard

design in operat ion. Here a permanent magnet couples

to a ferromagnet ic e lement which is mounted on the dis-

tr ibutor shaft and rota tes with it. As this element ro ta tes ,

the s t rength of the field var ies , being largest when the

air gap is smal les t . The t ime varying magnet ic field in-

duces a current in the coil which is proport ional to the

rate of change of the magnet ic field, and which outputs

a vol tage waveform as i l lustrated in Figure 2 . 4 ( c ) . Each

t ime one of the teeth , or r idges, on the ro tor passes un-

der the co i l ' s axis , one of the sawtooth shaped pulses is

generated. The rotor has one tooth for each cylinder and

the voltage pulses cor respond to the spark t ime of the

relevant cylinder. Figure 2 .4(d) shows an advanced ex-

ample of this idea following exac t ly the same principle,

46

Electronic ignition

except that the rotor is a star shaped wheel and the s ta t ic magnet ic sys tem has a cor responding number of poles ,

in this c a s e six of each , for a s ix cyl inder engine.

Auto advance

One reason why this triggering method has come out on

top over rival designs is simply due to one staggering

implicat ion. B e c a u s e the sys tem is magnet ic ; it is, in ef-

fect, a very simple a.c. genera tor on a small sca le , and

its output is, therefore, proport ional to the driven speed.

What this means is that at slow rotor speeds the output

vol tage is low, while for higher speeds the output is a lso

higher by a proport ional amount. If the tr igger thresh-

old of the amplifier 's input is vol tage dependent , then

triggering can be made to o c c u r at the required point

anywhere on the leading s lope of the output waveform.

Figure 2.5 shows how, from different output levels as

produced by cor respond ing ro tor speeds , the t r igger

level is near the peak of the s lope if the output is low,

and near the beginning if it is high. At a s t roke, what we

have here is, by way of an added bonus , an automat ic

ignition advance mechanism, and this with just one mov-

ing part the rotor!

The need for ignition advance

While the fuel/air mixture in the combus t ion chamber

burns at a cons tan t rate , the engine as a whole however

47


48

Distributor Rotary shaft ferromagnetic

\ element

W il low reluctance \ J U / /

P e r m a n e nt

and r e s u l t s in m a g n et

s t r o n g m a g n e t i c I . j j

f i e l d f o r c o i l / /

P i c k u p co l l ^ 1 ^ / /

(^ ) ^ 1 N a r r o w Gap

V o l t a g e d u e t o m a g n e t i c M a x i m u m n a r r o w f i e l d c h a n g i n g a s g a p v o l t a g e

r o t o r m o v e s t o w a r d s e n s o r ^ /

V o l t a g e d u e t o m a g n e t i c M a x i m u m w i d e f i e l d c h a n g i n g a s g a p v o l t a g e

r o t o r m o v e s a w a y f r o m s e n s o r

(c)

Figure 2.4 Magnetic timing sensor

Electronic ignition

49

Wide air gap offers ^^^S I/ I I high reluctance / / and results in **^e> 7/

weak magnetic I I I field for coil ^ - / /

( b ) + I W ide G a p

Rotor a r m key

/^^^^^^^^^^^^^^^^^^^^^^^^^^\^^\ R e ' U C t 0 r I / o V ^ ^ ^ T - | (ts. Li^) / ^ ^ V ^ \ | | Coi l

a n t^ m a g n e t

1 | w f r - - ~ " ^ \ ^ ^ / / || jj-i s y s t e m u n d e r rT 1 d f r * ^ \ v ^ X / X / / J / Ii d u s t c o v e r

( j | ^ ^t a

* ' Po l es

" I L D is t r ibu to r

l b o d y

V J (D)

Figure 2.4 Continued


Figure 2.5 Auto-advance plot using waveform of Figure 2 .4(c )

is required to opera te over a range of crankshaft speeds .

For this reason the moment of ignition must occu r ear-

lier at higher r.p.m. Full combus t ion of the fuel gas must

occu r during the period where the piston has full lever-

age on the crankshaft , and at high revs the burn actual ly

needs to begin well in advance of this point; at lower

speeds , not so much, at idle, hardly at all. The magnet ic

re luc tance type of ignition timing senso r ach ieves this

auto advance act ion in a much more linear manner than

do compromised mechanica l or e lec t ron ic methods , and

barring the odd rare mishap such as a s c rew coming

loose , once se t it does not need readjustment for any-

one who has persona l ly endured the long drawn out

p roces s of ignition retiming, the subt le t ies of the opera-

tion do not need rei terat ion!

50

Electronic ignition

Furthermore , s ince this requi rement has already been

taken ca re of by the sensor , it makes the amplifier much

s impler . Otherwise e l ec t ron ic advance might take the

form of f requency sens i t ive switches se lec t ing from a range of t ime delays, the minimum number of which is

two in the crudes t example of such a sys tem. More than

this requires ra ther more logic gates , or a mic roproces -

sor . Instead the magnet ic re luc tor allows the use of a

compara t ive ly very few t rans i s to rs to produce an ampli-

fier.

The electronic ignition switch

Obviously the hear t of an e l ec t ron ic sys tem which simu-

lates the act ion of a mechan ica l switch to opera te the

coil primary in the traditional way is a t rans is tor , and you might suppose that any power t r ans i s to r ab le to

ca r ry the maximum on-time current of the primary will

suffice. But oh dear me no. Remember that the primary

potent ial is sufficient to produce an arc a c ro s s the me-

chanica l switch, and that the ignition coil as a whole,

primary included, must be allowed to genera te however

high a vol tage is n e c e s s a r y to bridge the plug gap? We

are therefore obliged to use a high vol tage power tran-

sistor , with a V rating of several hundred volts , and such ' ce '

devices are notor iously inefficient, which means to say

that the current gain (H f e) is very small, measured in tens

or less ra ther than hundreds.

The usual biasing method is to use a base bias res i s to r

which typical ly c o n n e c t s di rect ly between the t ransis-

to r ' s base and the supply rail, and this r es i s to r can be

51


formidably beefy to provide the n e c e s s a r y bias current

for the t rans is tor to do its job properly, with the attend-

ant power consumpt ion and heat dissipat ion problems.

I have actual ly seen one design where the base bias re-

s is tor is no more than 9.2 !

No, that wasn' t a printing error . It 's an illustration of how

ex t reme base biasing may have to be to ensure that the

switching t rans is tor ach ieves a sa tura ted on s ta te , es-

sential to get the maximum available vol tage ac ro s s the

primary of the coil and therefore the maximum primary

current . Suppose, in a worst c a s e example, that our tran-

s i s t o r has an Hfe of 3 at 1 A ( y e s , j u s t 3 al though

fortunately later devices are be t t e r than that now), but

then in order to conduct 4 A this value reduces to say

Electronic ignition

( swi tches off) as fast as poss ib le . This is n e c e s s a r y s ince

the coil needs to be swi tched off quickly in order to de-

velop its high tens ion output (a slowly swi tched ignition

coil fails to make a spark) .

High speed switching

Figure 2.6 shows the essen t ia l s of a typical ignition am-

plifier as used with a magnet ic r e luc tance type of timing

sensor . To summarise so far, TR5 is the inefficient, high

vol tage power t rans i s to r switch for the coil , and R9 is

the base bias res i s tor . In this c a s e the bias current origi-

nates from TR4, which is cont ro l led by a Schmit t tr igger

comprising TR2, TR3, and res i s tors R3 to R6. The Schmit t

tr igger is essent ia l to produce the fast edged switching

waveform from the s lower changing input, provided by

T R I .

T R I is the bas is of the input s tage which incorpora tes

the input level th reshold as indicated in Figure 2.5. This

cons i s t s of diode Dl and the base /emi t te r junct ion of TRI

itself, which toge ther will not begin to conduc t until the

applied level is >1.2 V. This signal is of cou r se the ramp

shaped output from the s enso r coil and you can see now

that while the amplitude of the ramp is var iable , the in-

put th reshold is cons tan t . Dl a lso b locks the negative

going part of the input waveform, which is superfluous,

while R l is a current l imiter to p ro tec t Dl and T R I in the

event that for example the input is acc iden ta l ly con-

nec ted to the supply while the power is on.

53


Fig

ure

2.

6 E

ss

en

tia

l ig

nit

ion

a

mp

lifi

er

for

a m

ag

ne

tic

relu

cto

r b

ase

d sy

ste

m

Electronic ignition

Protec t ion for the engine 's mechanica l bi ts can be pro-

vided by including CI , which ac t s as a rev limiter. While it is charged quickly v i a D l , this charge leaks away slowly

via the base emit ter of T R I due to this dev ice ' s current

gain offering a relat ively high impedance , and in conse -

quence the waveform at T R l ' s emi t ter takes on a more

triangular shape . As engine speed inc reases the mean

average d.c. vol tage drop a c r o s s R2 also inc reases until

a point is r eached where even the lowest level of the

waveform exceeds the low threshold of the Schmit t trig-

ger; the amplifier c e a s e s to opera te and no sparks are

generated.

CI a lso affords some RF filtering, but it might be surpris-

ing to learn that the input leads are rarely s c reened . The

senso r coil is of such low impedance that this is unnec-

essa ry and in any c a s e s ince both these wires are run

toge ther as a pair, any external ly induced current will

be equally present in both, cancel l ing each o ther out.

A real working amplifier

Figure 2.7 shows a c ircui t which is the culmination of s ix

months development including test ing in the field on-board a real motor vehic le which, for ear l ier vers ions ,

proved to be des t ruc t ive ( to the c i rcui t , not the vehi-

c l e ) . Such is the way of r e sea rch and development , and

t h e s e even t s made defini te ind ica t ions tha t the unit

should be:

e lec t r ica l ly robust ,

mechanica l ly robust ; and,

ut terly weatherproof .

55


56

^

-H2V

te

st

^j4

V

LLK

-7

S220n

F

22R

S

IOO

uF

y

w/

T20X

M

lOW

T

35V

1

1

u

L

J

3

Oth

er

inp

ut

ca

ptu

re/o

utp

ut

I ^

>

Co

mp

are

m

od

ule

s/p

ins

/"

Fig

ure

3

.8

Co

nti

nu

ed


rent speed . This number of engine degrees is the differ-

e n c e b e t w e e n t h e a n g l e m a t c h e d to t h e p u l s e

accumula tor value, and the exac t number of engine de-

grees at which the pulse must begin s e e Figure 3.9.

The o ther input parameters of Figure 3.7 are measured

using an analogue-to-digital (A-to-D) conver te r , which is

usually integrated on-chip as part of the microcont ro l le r .

As previously mentioned, the air inducted by the engine

can be used as a measure of the engine load. A value for

this is obta ined, via a vane device in the air intake that

opera tes a potent iometer , or a l ternat ively via a hot-wire

sensor . The lambda senso r is a fairly recen t addition to

engine management s y s t e m s prompted by increas ing

anti-pollution regulations that have led to the use of cata-

lytic conve r t e r s . The ca ta ly t ic conve r t e r is a de l ica te

ob jec t and very str ingent control of the engine emiss ions

must be obta ined if the ca ta ly t ic conver te r is to opera te

' M i s s i n g ' r e f e r e n c e

t o o t h

O u t p u t c o m p a r e t o g g l e s p i n at

d e s i r e d e n g i n e a n g l e

T o o t h . s i g n a l | [_

P u l s e a c c u m .

v a l u e I n p u t c a p t u r e i n t e r r u p t

r o u t i n e d e t e c t s m i s s i n g t o o t h & r e s e t s p u l s e a c c u m u l a t o r

Microcontrollers

efficiently. The lambda senso r is bas ica l ly a hot plati-

num/ce ramic dev ice that p roduces an output vol tage

which var ies , depending on the oxygen con ten t of the

gas it is surrounded by. By insert ing such a s enso r into

the exhaust manifold, it is poss ib le to determine the air/

fuel compos i t ion current ly being burned in an engine.

This effectively t ransforms the engine management sys-

tem, from an open-loop control sys tem into a closed-loop

one, where def ic iencies in the desired output ( c o r r e c t

air/fuel mixture) can be de tec ted and the input var iables

(ignition timing/fuel quant i ty) adjusted to compensa te .

This means that much c lose r control of the exhaust emis-

s i o n s can b e m a i n t a i n e d , he lp ing to m a x i m i s e t h e

e f f ic iency of the c a t a l y t i c c o n v e r t e r mounted down-

s t ream in the exhaust sys tem.

Having m e a s u r e d all t h e s e p a r a m e t e r s , t h e

microcont ro l l e r must de termine the cor responding out-

puts i.e. the timing of the spark ignition pulses , and

the t iming/duration of the pulses which fire the fuel in-

j e c t o r s . Th i s is a c h i e v e d by a c c e s s i n g the so -ca l l ed

engine maps t h a t a r e s t o r e d in t h e m e m o r y of t h e microcont ro l le r . T h e s e maps are, in fact, t ab les of data

that hold the ignition and fuelling cha rac t e r i s t i c s of a

par t icular engine type against a number of input vari-

ab les . B e c a u s e it is impract ica l to try and s to re all the

pos s ib l e c o m b i n a t i o n s of output t iming ve r sus input

cha rac t e r i s t i c s , a number of points are held in the map

table , and the C must then perform an ar i thmet ic cal-

culat ion to in terpolate between the two c lo se s t points

given, to the exac t input condi t ions obta ined from the

various s enso r s .

As there are a number of var iables to be taken into con-

siderat ion, t he se interpolat ion ca lcu la t ions are complex

95


and require a lot of p rocess ing power to be comple ted

quickly, in t ime to set up the output timings for the next

engine cyc le . This is the reason why 16 and now 32-bit

mic rocon t ro l l e r s are replacing older 8-bit sys tems for

engine management . They allow more complex calcula-

t ions to be comple ted quickly so that c lo se r cont ro l can

be maintained on a cycle-by-cycle bas i s .

When the microcont ro l l e r has obta ined the desired out-

put t imings, it must actual ly genera te the pulses to fire

the spark plugs and in jec tors . This is done via the out-

put match facility of the t imer sys tem, where the CPU writes a value into a specia l regis ter . When the value of

the incrementing t imer-counter r eaches the same value

as that in the regis ter , the hardware of the t imer sys tem

automat ical ly changes the output pin s ta te to a desired

level. This mechanism allows very accu ra t e p lacement

of the various pulses required in the engine cyc le , as we

have seen from the descript ion of the Motorola M68HC11.

The method desc r ibed above, using the input capture

and output match t imer functions, is used in virtually all

of today 's production engine management sys tems . How-

ever, this sys tem is not perfect as the CPU still has to

respond to a large number of interrupts generated by

the t imer, thus slowing down its cont ro l ca lcu la t ions .

This interrupt overhead has se t the performance limits of today 's sys tems , and so a new approach will be re-

quired for the even more complex cont ro l a lgori thms

required for tomorrow's emiss ion regulat ions.

Motorola has been the first microcontrol ler manufacturer

to address this problem by introducing the innovative

MC68332 device . Not only does this device have a pow-

erful 32-bit 68000-based CPU, but is unlike any o ther

9 6

Microcontrollers

microcontro l le r in that it a lso has a second on-board CPU

d e d i c a t e d to con t ro l l i ng t imer func t ions . Th i s T ime

Process ing Unit, or TPU, is in effect a mic rocont ro l l e r

within a microcont ro l le r ! The TPU is used to handle al-

mos t all of the in te r rup t s a s s o c i a t e d with the t imer

channels , thus freeing the main CPU to spend more t ime

on complex cont ro l ca lcu la t ions . At sui table points in

the cont ro l cyc le , the main CPU obta ins new input read-

ings from the TPU and presen ts new data for the TPU to

ca lcu la te and schedule the output pulse t imings.

Vehicle alarms

The huge inc rease in car-related c r imes in the 1980 /90s

has been paral leled by an equally large inc rease in the

demand for car a larms. Originally based on simple logic

c i r c u i t s and t r i g g e r e d d i r e c t l y from i n t e r i o r l ight

swi tches , the complexi ty of a larms has grown to try and

match the skill of the potent ia l in t ruder . Figure 3.10

shows the s c h e m a t i c of a typical soph i s t i ca t ed MCU-

based alarm sys t em. Using a m i c r o c o n t r o l l e r in th is

application provides a great deal of sophis t icat ion within

a very low componen t count , allowing the alarm to be

small and thus easi ly concea led .

An MCU chosen for this j ob should have a low power mode s ince the alarm must be powered up for long peri-

ods of t ime without the engine running. It should also be

poss ib le to wake the device from this mode via several sou rces , so that a number of c i rcu i t s can trigger the de-

vice into sounding the alarm. A simple 8 or 16-bit on-chip

t imer is a lso des i rable to t ime the output audio/visual

warning p u l s e s , and to r e s e t t h e a la rm af ter it has

9 7

Auto electronics projects 00

IC2q,

IC3b

TR

6

Voltage

dro

p

Additio

nal

horn

/

sensor

I siren

driver

inte

rface

D1

TR

1,

R1

, C

1,

ZD

1.

S1

Mic

rocontr

oller,

IC

1

TR

7,

RL4

"2

; |

I TR

8, ,

RL2

, 3

Backup

battery

^_' 6

^

I

I ^

J

Centr

al

lockin

g

[=

" 2

//

Y

inte

rface

=

TR

5,

RL1

w

8_

Bit

Periphera

l/

\ |

" [V

eh

icle

im

mobiliz

ation|

CP

U

mem

orv

C1^

1

T

R1

Q

'

A

1 1

contr

ol

| m

E

lectric

w

indow

/sunro

of

oo

1

driver

inte

rface

Ignitio

n

security

J

V%

'C

om

puguard

* I

TR

1 ,

circuit

I

ig

contr

ol

pro

gra

m

^

, !

1 1

^

L

Siren

inte

rface

Fig

ure

3.

10

A

uC

-ba

se

d c

ar

ala

rm s

ys

tem

Microcontrollers

99

D25.

26.

R41,

C7

"

Panic

sw

itch

M

1

39

4

41

inte

rface

|

..

security

____

. 1

. sw

itches

_,

Oscill

ato

r IC

2b,

1,

2

I

I

RV

1,

IC3c

^

J

D42,

43

Shock

dete

cto

r '

| N

.C.

security

' '

3

TR

3,

4,

14,

IC3d

yj^

j U

ltra

sonic

sensor

| |

C9.

10

D27.-33,

TR

12,

13

inte

rface

Sensor

pro

gra

mm

ing/

1 arm

/dis

arm

m

odule

in

terf

ace

D20

, T

R15.

16

1

1

Ele

ctric

cooling

fan

sensor

Fig

ure

3.1

0

Co

nti

nu

ed


sounded for a se t t ime this is a legal requirement . The

t imer can also be used to arm the alarm after a defined

period, if it is not armed via a remote cont ro l .

A.B.S.

The increased performance of everyday ca r s , along with

their increasing numbers (and therefore greater densi ty

on the roads ) , has resul ted in a cont inual improvement

in braking performance. This t rend has included the pro-

gress ion from all-drum braking, drum/disc braking and

vent i la ted d isc /drums, through to the all-disc braking

sys tems found on today 's higher per formance ca r s . The

most recen t improvement has been the introduct ion of

ABS.

The Antilock Brake System does not i tself inc rease the

braking capac i ty of the vehic le , but improves safety by

maintaining optimum braking effort under all condi t ions .

It does this by preventing the vehic le wheels from lock-

ing, due to over -app l ica t ion of the b r akes , and thus

maintains s teerabi l i ty and reduces stopping d i s tances

when braking on difficult surfaces such as ice .

ABS allows shor te r s topping d i s tances than with locked

wheels , due to the friction or mu-slip cha rac t e r i s t i c of

the tyre-to-road interface; as a wheel brakes , it slips rela-

tive to the road surface producing a friction force . A

typical mu-slip curve is dep ic ted in Figure 3 .11 . This

shows that peak friction occu r s at about 10 to 20% slip,

and then falls to approximately 30% of this value at 100%

slip ( locked whee l ) .

100

Microcontrollers

mu 0.5H

.

10 20 30 40 100 % Slip

Fully locked

Figure 3.11 A typical mu-slip characterist ic for the tyre-to-road

interface

The aim of the ABS sys tem is to cont ro l the braking force

so as to s top the slip for any wheel exceeding this opti-

mum value by more than an a c c e p t a b l e window.

At t h e h e a r t of al l ABS s y s t e m s ( e x c e p t t h e a l l -

mechan ica l sys tem implemented by Lucas ) is an e lec -

t r o n i c c o n t r o l unit (ECU) b a s e d a round a powerful

microcont ro l le r . Figure 3.12 shows a b lock diagram of

such a sys tem. The solenoid valves that form part of the

hydraulic modulator allow cont ro l of the p ressure avail-

able to the individual wheel brake cylinders, independent

of the force supplied by the driver via the brake pedal.

T h e s e three-way valves can connec t the brake cyl inders

to:

the normal master cylinder circuit , so that the brak-

ing pressure will be di rect ly cont ro l led by the driver,

the return pump and accumula to r in the hydraulic

modulator , so that the p ressure in the brake cyl inders

will fall as the fluid returns to the mas te r cylinder,

101


102

r-----------

, W

he

el

4

Bro

ke

1 i

I1 c

yli

nd

er

~ I I I I

Clo

ck

m

on

ito

r

clo

ck

Figu

re 3

.12

Bloc

k di

agra

m o

f an

elec

tron

ic A

BS s

yste

m

Microcontrollers

nei ther of the above two c i rcui ts , thus isolating the

brake cyl inder so that the p ressure will be maintained

at the value immediately preceding the move to this po-

sit ion.

The cont ro l for t hese valves is supplied via drive cir-

cui ts from the output por ts of the microcont ro l le r .

T h e b a s i s for all e l e c t r o n i c ABS s y s t e m s is t h e

mic rocon t ro l l e r ' s abil i ty to determine the speeds of the

individual wheels (al though some front-wheel drive ve-

h i c l e s s h a r e a c o m m o n s p e e d s e n s o r for b o t h r ea r

whee l s ) . It does this via an inductive senso r and too thed

ring that produce an output waveform, the f requency of

which represen t s the speed of the wheel . This arrange-

ment is a lmos t ident ica l to the engine speed s e n s o r

d i scussed earl ier , excep t that s ince no angular posi t ion

information is required the re are no missing or ex t ra

teeth. It follows from this that the explanation previously

given on determining engine speed a lso applies to deter-

mining wheel speeds in an ABS sys tem.

In this ca se , there are around 50 to 100 tee th on the en-

coder ring, and this could result in a pulse f requency of

6000 Hz when the vehic le is travelling well in e x c e s s of

100 mph. As the re can be a speed senso r on each of the

4 wheels, a total of 24,000 pulse edges have to be resolved

every second . The solenoid valves in an ABS sys tem typi-

ca l l y have a r e s p o n s e t ime of 10 to 20 ms , and t h e

mic rocon t ro l l e r must be able to sample the inputs at

least twice that often, to reso lve lock-ups in 5 to 10 ms.

Put another way, the mic rocon t ro l l e r must be able to

de te rmine 4 independent wheel speeds from 6000 Hz

103


signals within a 5 ms window, and still have t ime to carry

out p rocess ing on this data to determine the new valve

s t a tes . T h e s e str ingent timing requi rements mean that

ABS sys tems are the domain of high performance 16-bit

mic rocon t ro l l e r s that can respond quickly to interrupts

from the t imer sys tem which is capturing the speed sen-

sor edges .

So far it has been s ta ted that the mic rocon t ro l l e r in an

ABS sys tem must prevent the wheel-slip value from ex-

ceeding the optimum, and we have d i scussed how the

C measures the wheel speeds (angular ve loc i ty ) . How-

ever, it may not be c lea r how these wheel speeds are

related to the slip values that the sys tem is at tempting

to cont ro l . The slip of any wheel can be defined as the

difference between the angular ve loc i ty of the slipping

and non-slipping wheels , divided by the angular ve loc-

ity of the non-sl ipping wheel . Th i s makes s e n s e and

sounds quite simple, but for one problem; how to find a

non-slipping wheel? The ABS algorithm s e a r c h e s for the

fastest spinning wheel and uses this as the re fe rence for

calculat ing the slip values of the o ther wheels . If the slip

value of a wheel is greater than the peak friction value

by a cer ta in margin ( i .e . the wheel is heading towards a

locked condi t ion) , then an ABS cont ro l cyc le is execu ted

on that wheel .

First the mic rocon t ro l l e r will i so la te the wheel brake

cyl inder from the brake c i rcui t to prevent further pres-

s u r e i n c r e a s e . It will t h e n r e c h e c k t h e s l ip and

acce le ra t ion values to determine if the wheel is still de-

celerat ing, and whether the slip value is still exceeding

the desired value. If so , then the valve posi t ion is moved

104

Microcontrollers

momentar i ly to the return posi t ion, reducing the brak-

ing effort on that wheel . This pulsed re lease of p ressure

is cont inued until the mic rocon t ro l l e r de tec t s that the

wheel acce le ra t ion is posi t ive, at which point it s tops

reducing the pressure , and r e c o n n e c t s the wheel cylin-

der to the b rake c i rcu i t to prevent o v e r s h o o t of the

acce le ra t ion . This ent i re cont ro l cyc le of holding/reduc-

ing/ increasing brake pressure is repea ted until the slip

value for the wheel has been brought back into the ac-

cep tab le window.

This is obviously a simplified explanat ion of how ABS

works and the algori thms are in fact very complex and

will vary from one ABS implementat ion to another . When

you remember that this algori thm must be execu ted on

all wheels in jus t a few mil l i seconds , it is not surprising

that ABS is among the most demanding mic rocon t ro l l e r

appl ica t ions .

An important point worth discussing about ABS is that it

is one of the most safety cr i t ica l p r o c e s s o r appl icat ions

in ex i s t ence . The c o n s e q u e n c e s of a faulty ABS sys tem

could be potent ial ly d isas t rous if the brakes were pre-

vented from operating, or were applied er roneously . For

this reason ABS manufacturers take great ca re in the

safety a spec t s of the sys tem design. It is not uncommon

for two identical mic rocon t ro l l e r s to be implemented,

running the same software in parallel and cont inual ly

checking each o ther via a communica t ion pro tocol for

any e r roneous operat ion.

Another solution to this problem is to have a s impler

( lower c o s t ) s lave ( that ac t s as a watch-dog for the main ABS m i c r o c o n t r o l l e r . Th i s s lave dev i ce is pro-

105


grammed to monitor the major act iv i t ies of the mas te r

and it has the abil i ty to shut down the ABS sys tem if

a fault is de tec ted , thus revert ing full braking cont ro l to

the driver.

A subjec t worth mentioning here is t ract ion control . Trac-

tion cont ro l is a fairly recen t development and can be

thought of as applying ABS in reverse . The idea of t rac-

tion cont ro l is to prevent wheel-slip due to e x c e s s power

on loose surfaces by applying a braking force to the slip-

ping w h e e l ( n o t e t h a t t r a c t i o n c o n t r o l is o n l y

implemented on the driven whee l s ) . This feature is a

natural progress ion for ABS, as all the n e c e s s a r y com-

ponents and measurements required for t rac t ion cont ro l

are inherent in the ABS sys tem excep t some means of

applying a braking force when the driver is not depress-

ing the b rake pedal . Th i s is usual ly a c h i e v e d via an

e lec t r i c pump arrangement .

With the cons ide rab le improvement in safety provided

by ABS, there can be little doubt that the next few years

will s ee this sys tem becoming more popular, poss ib ly

becoming a s tandard feature on all but the lowest co s t

ca r s .

The future

Hopefully this chap te r will have given the reader some

insight into the fascinating and challenging appl icat ions

for mic rocon t ro l l e r s in automotive appl ica t ions . It has ,

of course , been imposs ib le to cove r all of the applica-

t ions l isted earl ier in this chapter , or even to cover some

106

Microcontrollers

of t hose we have in great t echn ica l depth (engine man-

agement or ABS themse lves could each fill a text book) ,

but the se lec t ion chosen has shown just how varied in

complexi ty the automot ive mic rocon t ro l l e r applicat ion

can be .

As a finishing thought, it may be worth pondering what

t h e fu ture h o l d s for e l e c t r o n i c s , and p a r t i c u l a r l y

mic rocon t ro l l e r s , in ca r s .

Perhaps the next major advance, one which all the ma-

j o r v e h i c l e manufac tu re r s and s t anda rds b o d i e s a re

working on, is the multiplexed wiring system. As the e lec-

t r i ca l c o n t e n t of v e h i c l e s e s c a l a t e s even higher , the

weight and cos t of all the in te rconnec t ing cab le s is be-

coming a major concern , and the number of e lec t r ica l

c o n n e c t o r s poses a rel iabil i ty problem most veh ic le

breakdowns are due to e lec t r ica l faults. The concep t of

the mult iplexed wiring sys tem is to use a very high per-

f o r m a n c e s e r i a l c o m m u n i c a t i o n s n e t w o r k b e t w e e n

intelligent and semi-intelligent modules s ta t ioned at stra-

tegic points around the veh ic le . This means that only

power and the serial link need be dis t r ibuted about the

car all the loads have shor t connec t ions to the near-

est intell igent sub-module.

The poss ib i l i t ies of this sys tem are enormous; the en-

gine management sys tem could talk to the e l ec t ron i c gearbox cont ro l le r and to the ABS/ t rac t ion cont ro l sys-

tem. No longer would turning on your l ights s imply

connec t power direct ly to the bulb it would signal one

unit to send a command to another unit, instruct ing it to

turn on the bulb using a Smart Power device .

107


This scenar io is not fantasy, it is going to happen and

because the microcont ro l le r has a p lace at the hear t of

every one of t hese intelligent modules, it is safe to say

that its future in the automotive market is very secu re

indeed.

108

4 Car battery monitor

Any number of things from a faulty a l te rnator to left-on

headlights (or s idel ights , even) can result in a flat bat-

tery and the first you are likely to know about it is

when you turn the key one morning and the car won't

start! This car ba t te ry monitor is a useful little unit de-

signed to warn you in advance by displaying the bat tery ' s

s ta te of charge with a row of ten LEDs.

The moni tor consumes a miser ly 20 mA (it would take

2000 hours to d ischarge a 40 Ah ba t t e ry ) , so it can be

left permanent ly connec t ed to the ba t te ry if required.

Alternatively, it could be connec ted to the ancillaries side of the ignition switch.

109


The car ba t tery monitor will even reveal faults like a slip-

ping fan-belt: a problem which prevents the ba t te ry from

charging proper ly , but l eaves the da shboa rd ba t t e ry

warning light off. It will even show how the ba t t e ry is

handling the s t renuous work of s tar t ing the ca r (did you

know it takes some twenty minutes of driving to put back

what a five-second s tar t takes ou t? ) .

Circuit

The heart of the monitor circuit (Figure 4 .1) is the LM3914

bar-graph driver IC, used to drive a row of red, orange

and green LEDs which toge ther indicate a magnitude of

the ba t te ry charge vol tage in ten s teps , approximately

V2 V each s tep from 9 V to 14 V. The IC conta ins an input

buffer, a potential divider chain, compara to r s , and an

accura t e 1.2 V reference source . Logic is a lso included

which gives the c h o i c e of bar or dot-mode operat ion the la t ter is used in this appl icat ion. The compara to r

causes the LEDs to light at 0.12 V intervals of the input

voltage. TRI ac t s as an amplified diode, raising the lower

end of the divider chain and the negative terminal of the

re ference source (ICI pins 4 and 8 ) to 1.9 V. The upper

end of the chain at ICI pin 6 is connec t ed to a re ference

sou rce output vol tage of approximate ly 3.1 V from pin 7.

The potential divider formed by R l and RV1 a t tenuates

the supply voltage and produces the signal input to the

compara tor , such that a supply range of 9 - 1 4 V covers

the span of the divider chain and is indicated over the

whole of the ten segment LED display. The LED bright-

ness is held cons tan t by an internal cons tan t current

source .

110

Car battery monitor

Construction

Component pos i t ions and printed c i rcu i t board t rack

layout is shown in

auto electronics projects

Documents