Ignition Timing Accuracy : Causes, Impact, and Solutions Presented by: Fred Husher

12/9-10/2014 1

Assuming the engine is capable of perfect performance:

Ignition system performance is limited by the cumulative errors:

mechanical elements, sensors, and signal processing

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The Ignition Timing Shifts, what does it do to Engine Performance? If retarded:

Easy start Acceleration is slowed because of HP drop Engine heat increases because combustion temp has

increased Higher exhaust temps

If advanced: Hard start Acceleration is aided because max HP possible Engine runs cooler Knock & pre-ignition can occur If too advanced there wll be excessive stress on engine

parts

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As with a Crime Mystery: Who Are the Possible Perpetrators?

Engine mechanical tolerances to driving the trigger sensor Trigger sensor behavior to its stimulus Trigger detector edge detection accuracy & repeatability Signal processing delay through ECU/ICU Managing the spark gap between the rotor and cap on the distributor and the spark plug

The focus of this discussion will be on: trigger sensor, sensor signal processing and their mechanical supporting elements

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The mechanical causes are often the easier to identify: Distributor

Shaft end play shifts timing • CW: advance on acceleration • CCW: retard on acceleration

Rotational wobble causes timing bobble between cylinders

Backlash between cam & distributor gears Mismatch of advance springs & weights to cam, if

used Cam Shaft

End play imposes timing shift to the distributor Crankshaft

Torque can move crank timing disk and change the trigger timing

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The Electrical Causes

Poor head ground return: RF vs. DC Skin vs. bulk current conduction

Insufficient current supply capacity to the ignition module MDI pulse to < 8A CDI can pulse to >200A

Overdriven ignition coil

Distributor rotor contact shape, metal alloy, and surface finish

Often the most insidious as they are not easily measured with the common mechanics tools

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Trigger Sensor Behavior to its Stimulus & Environment

Mechanical motion variations Temperature: magnetic or optical Induced EMI to signal lines Ground loop currents such as spark return Load dump sensitivity from solenoid release Battery voltage sag and failure to/from alternator, battery, & ignition module

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The input signal type has considerable impact on how it can be processed Variable Reluctor coil Digital (open drain/collector) Hall Effect Opto-interrupter sensors

Digital or analog variable reluctor mimics RS-232 output – Crane Pro-Race distributor Capacitor coupled outputs – Crane Race Billet

distributor Differential RC outputs – Crane Crank Sensor

The exotics: magnetostrictive, piezoelectric, and Wiegand

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Variable Reluctor Coil

Developed voltage by the VR is Voltage developed = N(ΔΦ/Δt),

Attributes: Unable to function at slow RPMs Noise immune with differential detector Noise sensitive with single ended detector Rise/falling trigger edge choice is by simply

exchanging the coil’s leads, MAG+ & MAG- Fragile due to the coil’s wire winding breaking from vibration

From US6278496

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Digital Attributes Dominated by open drain/collector output Types Hall Effect Opto-interrupter sensors

Output low is limited to Vsat ~ 0.2V

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Opto-interrupter Triggers

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Pro-race supports individual cylinder adjustment for advance/retard, developed initially for race use Etch lines on either side of the window provide 1° markers that the user can file or mill to adjust the trigger timing advance/retard for each cylinder Points replacement for 4/6/8/12-cylinder distributors Accuracy of chemical etched plate & opto-interrupter is <0.01 °

Crane Race Billet Crane Pro-Race Crane Points Replacement

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Digital or Analog Variable Reluctor Mimics

RS-232 output – Crane Pro-Race distributor Capacitor coupled outputs – Crane Race Billet distributor Differential RC outputs – Crane Crank Sensor

Variable Reluctor Mimic: RS-232 Output

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Single ended output +/- 7 to 10V output swing Very high noise immunity to VR trigger detector

Signal never at 0V Low output impedance

GNDMAG+

U105MAX3232

V

I O

G

+5V +12V

+IGN

CON3

123

Simplified circuit

Variable Reluctor Mimic: Capacitor Coupled Output

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Single ended output +/- 6 to 12V differentiated digital signal Idles at ground

Output is susceptible to transmission of ground impulse noise

2.2K

2.2K

MAG++IGN

GND

0.1CON3

123

Simplified circuit

Variable Reluctor Mimic Differential RC Coupled Output

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Differential output signaling MAG + & MAG- can be swapped just as with the VR sensor to change

trigger edge Either output can be shorted to ground Output signals can be floated to +2.5V to support use with single supply differential input detectors Very high noise immunity rejection into VR trigger detector

+5V

R242.4K

R252.4K

R264.7K

U3ISL3294EFHZ-T

V2

G5

A6

B4

DI1

DE3

R271K

R284.7K

U6HALL SENSOR

V+1

G2

O3

+C41UF/16V

+

C31UF/16V

SIMPLIFIED DIFFERENTIALOUTPUT HALL SENSOR

MAG-

MAG+

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Hall Sensor Attributes

Uni-polar Hall sensor Operational to zero RPM Rugged and stable throughout -40 to

+125°C Not usable with VR detectors without

signal conversion

The developed signal is independent of any rate of change in the magnetic field acting upon the Hall Effect bridge

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Static Hall Sensor Characterization Fixture

Motion adjustments are Rotation of trigger disk Translation to flux concentrator / magnet to Hall

Sensor Translation of Hall assembly to the trigger disk

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Crank Trigger Disks

STEEL TOOTH – BIASED HALL MAGNET TOOTH – UNBIASED HALL

Crank Trigger Disks

Magnetic Circuit Iron core flux concentrator for embedded magnet trigger disks

• Embedded magnets are matched for flux density Back biased sensor with integral magnet for steel tooth trigger disks

Timing accuracy depends on all teeth being of Equal geometry & spacing Width, slope, and gap distance Tooth shape to optimize the magnetic field state change

For embedded magnet disks The magnets are tilted relative to axis of rotation by 45-50° to

accelerate the field collapse in the variable reluctor sensor core

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Trigger Disk Eccentricity

Changes in gap distance = changes in flux density Variable Reluctor Sensor Gap change = amplitude modulation of the developed coil voltage.

Fixed threshold detectors are mostly unaffected Adaptive threshold detectors using (amplitude * rate-of-change)

are sensitive to gap modulation, but the jitter will be below 0.05°

Hall Effect Sensor Gap change = change in when flux threshold levels are passed

Fixed threshold & adaptive threshold Hall sensors react differently

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Trigger Disk Eccentricity

Hall Sensor with Fixed Threshold Sees a change in gap as a change in flux density >> timing shift

Hall switch sensors incorporate their own threshold detectors • Therefore, a change in gap will translate to a rotational shift • This will be a linear relationship less than a 0.02°total impact.

Tooth shape will play a role in how sensitive this will be.

Hall Sensor with Adaptive Threshold Uses each peak to set the next threshold level. Even with a 20% change in

gap distance the change in phase delay is insignificant.

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Adaptive Threshold Hall Sensor

From Melexis data sheet

Prediction threshold level correction on tooth-by-tooth basis

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Helical Gear End Play (Cam and Distributor)

If the end play is restricted to the distributor alone, the system appears as a transverse helical rack. The relationship of end play to timing error is:

Δ angle per 0.001” of end play = (0.001 * 360)/πPD, where PD = Pitch Diameter

Example: Chevy SB, 14 Pitch, 13 Tooth, PD=1.107194” distributor gear the timing shift is:

0.103549° per 0.001” of distributor shaft axial movement 0.067288° per 0.001” of the mating cam end play

Performance engines • 0.005” of cam + 0.010” distributor end play = 1.37°

Street engines • 0.010” of cam + 0.025” of distributor = 3.26°

Crane Pro Race & Race Billet distributors with near zero end play means error is all cam so, 0.005” cam = 0.336°

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VR Replacement Hall based Crank Sensor

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Flux concentrator used for embedded magnet trigger disk Biasing magnet used for steel tooth trigger disk Built-in LED to indicate trigger tooth present Operational from 6 to 24VDC with reverse polarity protection Load dump protected to +-200V

Does not suffer from accumulated overload damage Available in 4-configurations: steel/magnet trigger disk, digital/analog output

Trigger Detector Edge Detection Accuracy & Repeatability

Every trigger detector consists of: Input signal conditioning Protection from excessive input signals

Translation between the sensor signal and the digital needs within the ignition/fuel injection control modules Analog comparator which introduces delay in

recognizing a state change: <1usec

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Digital – Points Detector

+3.3V

S1POINTS

ISO1OPTO IINTERRUPTER390

+IGN

DIGITAL INPUT

OUT#

OUT#

D8 D9100

4.7K220K

15K

47K

-

+

LM29033

21

84

+15V+5V

+IGN

TRIG

Protected against shorts to ground, +Vbatt, or load dump signals Sensor Vsat up to +4V is acceptable

Very high ground noise rejection Insignificant timing delay over all RPM

Typical points/digital input detector

Digital trigger

Points

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Variable Reluctor Detector Circuits

Fixed threshold Single ended Differential

Adaptive threshold

Single ended Differential

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Single-ended VR detector

Negligible timing shift over all RPM Sensitive to ground impulse noise & impressed EMI

-

+3

21

84

R8301

R99.82K

R10301K

R114.7K

R1224.3K

R1510K

+0.226V

THRESHOLDS:+0.200V / -0.200V

+5V

TRIGGER OUTL1VR COIL

R3893.1K

True zero-crossing discrete fixed threshold VR detector

Simplified circuit

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Adaptive Threshold VR Detector

Threshold level changes with signal amplitude and RPM Unable to cope with constant amplitude input without serious timing retard error vs. RPM Sensitive to ground impulse noise and impressed EMI

Example of TI LM1815 implementation

C130.01

R3620K

R373.16K

SIMPLIFIED LM1815 IN ADAPTIVE TRIGGER MODE

PEAKDETECTOR

TRIGGER OUT

-

+3

21

-

+5

67-

+3

21

C110.1

C120.001

R341M

ONE-SHOT

R35182K

+5V

RC

QTRIG

L2VR COIL

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Differential VR Trigger Detector

Inputs are floated to +2.5V True zero-crossing detection largely independent of signal amplitude CMRR ensures all impressed EMI is rejected Not affected by ground noise

Maxim MAX9924-9927 family are the only integrated differential input detectors

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MAX9924 With external support detector can be configured to support:

VR Digital/points All VR mimics

Both adaptive and fixed threshold modes are supported With suitable stimulus, measurement, and control the sensor can be identified causing the MAX9924 to be configured for optimal support and processing of the sensor’s signal

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How does trigger detection time affect timing?

In a perfect world the sensor’s trigger edge would be processed with no advance or retard error Reality, however includes: signal filtering, adaptive threshold predictive algorithms, and comparator delay

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Dynamic Characterization of Hall & VR Sensors

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Crank sensor to trigger wheel gap can be adjusted as in vehicle Spindle can be set from static to 10,000 RPM Sensor gap can be adjusted to centerline of trigger disk with X & Y adjustments

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

-80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80

RPM

TRIGGER DETECT in usec ADVANCE -- RETARD

RPM vs. ZERO-CROSSING to DETECTOR OUTPUT TIME

Crane HI6 - CRANK SENSOR

CRANE HI6 - VR SENSOR

LM1815 - CRANE CRANK SENSOR

LM1815 - VR SENSOR

MAX9924 - CRANE CRANK SENSOR

MAX9924 - VR SENSOR

COMPETITOR - CRANE CRANKSENSOR

COMPETITOR - VR SENSOR

Detector – Sensor Results For a 0.080” sensor – disk gap

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Detector – Sensor Results For a 0.080” sensor – disk gap

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

-0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5

RPM

TRIGGER DETECT in DEGREES ADVANCE -- RETARD

RPM vs. ZERO-CROSSING to TRIGGER DETECT OUTPUT PHASE ERROR

CRANE HI6 - CRANK SENSOR

CRANE HI6 -- VR SENSOR

LM1815 - CRANE CRANK SENSOR

LM1815 - VR SENSOR

MAX9924 - CRANE CRANK SENSOR

MAX9924 - VR SENSOR

COMPETITOR - CRANE CRANKSENSOR

COMPETITOR - VR SENSOR

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Managing Rotor to Distributor Gap Sparks always jump from a point to a surface, which may not be the shortest distance Electrons are emitted from the negative potential, the rotor, where the electrical field strength is greatest: a point The rotor blade geometry dictates how much of the ignition energy is lost in the cap via

Shape Edge roughness & sharp corners Metal alloy & surface chemistry

From http://tesladownunder.com/tesla_coil_sparks.htm

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Managing Rotor to Distributor Gap

Increased spark length = energy wasted as heat to maintain plasma

The stator contact should be curved to reduce the spark gap distance throughout the spark duration(s)

Cutting back the trailing edge of the rotor blade ensures the next is the shortest path

If the spark is unable to slide along the edge of the rotor blade then the spark duration will be cut short If the preferred launch point to stator is too far, timing will be retarded

Ignition Wiring Spark event in cylinder is really a RF transmitter within a metal container RF energy only flows on metal surfaces Braided straps provide the low RF impedance path for spark current between

Head(s) to block Block to ignition ground. The head bolts

do not conduct the spark current even though they measure as a DC short circuit.

CDI recharge demands are at low RF frequencies that demand both DC & RF wiring considerations

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Sensor Wiring Any connection of a sensor ground to the engine provides a pathway for ground loop currents to the measurement circuit EMI can be impressed upon any single-ended sensor signal EMI cannot be impressed upon a differential sensor signal as it will be common mode rejected on receipt Twisted pair or triad ensures that all leads get the same EMI exposure

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Sensor Wiring

If load 1 = load 2 then Voltmeter M1 is > Voltmeter M2 because of the addition of 2R more sheet resistance in the head Thus, spark currents will change the ground potential spatially depending upon the position and path to ground

BT112V BATTERY

LOAD-1

R R R R R

ENGINE HEAD

A-

+ M1VOLTMETER

A-

+ M2VOLTMETER

LOAD-2

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Closing

Ignition triggering accuracy can be well within 0.1 degree of accuracy over all RPM by choice of trigger sensor, measurement site, trigger signal detector circuit, and distributor Differential signal processing rejects EMI and ground loop currents since only the difference is considered Very valuable in noisy engine environments

Treatment of grounding from a RF basis will greatly reduce the radiation of EMI signals

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