basics of internal combution engine

INTERNAL COMBUTION ENGINE

Made by:

Assistant Professor : NAPHIS AHAMAD

MECHANICAL ENGINEERING

6/10/2017 Naphis Ahamad (ME) JIT 1


UNIT I

INTRODUCTION What is IC Engine?

An internal combustion engine is a thermal system (power plant) that

converts heat obtained from chemical energy sources (gasoline, natural gas)

into mechanical work.

Where are IC Engines Used?

IC engines are used as the propulsion systems for land transport vehicles

such as automobiles (cars, etc.), marine vehicles (boats, etc.) and small

airplanes. IC engines are also used in portable electrical generators and as

prime mover in grass cutting machine, etc.6/10/2017 Naphis Ahamad (ME) JIT 3

INTRODUCTION


Basic Components of IC Engines

• Cylinder, piston, inlet valve and exhaust

valve.

• Piston moves from the top dead center

(TDC) to the bottom dead center (BDC).

• Clearance volume, Vc is a spacing

between the top of the piston and the

valve’s heads when the piston is at the end

of the delivery stroke.

• Swept volume or displacement volume,

Vs is the volume between TDC and BDC.

Classifications:

Internal Combustion Engines (IC Engines)

External Combustion Engines (EC Engines)

Internal Combustion Engines (IC Engines):

IC Engines are those in which combustion of fuels take places inside engine

cylinder (Example: Petrol, Diesel, GAS)

External Combustion Engines (EC Engines):

EC Engines are those in which combustion of fuels take places outside engine

cylinder (Example: Steam Engine, Steam turbine)6/10/2017 Naphis Ahamad (ME) JIT 5

Classification of IC Engines:

a. According to the cycle of operation

Spark ignition engines(Petrol or Constant volume or Otto cycle)

Compression ignition engines(Diesel or Constant Pressure)

b. According to the type of fuel used

Petrol, Diesel, Gas Engine.

c. According to the method of fuel

SI & CI

d. According to the Process of combustion

Otto cycle, Diesel cycle, Dual combustion cycle


About Petrol Engine:

A petrol engine (also known as a gasoline engine in North America) is an

internal combustion engine with spark-ignition, designed to run on petrol (gasoline).


Working Cycles Four Stroke Petrol Engine:

It is also known as Otto cycle or constant volume cycle.

Cycle of operation is completed in 4-strokes of the piston or 2 revolutions of

the crankshaft.

Each stroke consists of 180°, of crankshaft rotation and hence a cycle

consists of 720°of crankshaft rotation.

2-stroke: 1 power stroke per 1 crankshaft rev



Principle of Operation:

The series of operations of an ideal four-stroke SI engine are as follows

Suction Stroke or charging stroke

Compression Stroke

Expansion or Power Stroke or working stroke

Exhaust Stroke


Principle of Operation


Principle of Operation:

Stroke Valve Position

Suction StrokeSuction Valve open

Exhaust Valve closed

Compression Stroke Both Valves closed

Expansion or Power Stroke Both Valves closed

Exhaust StrokeExhaust Valve open

Suction Valve closed


Working Principles of Four Stroke Diesel Engine:

Increased pressure of combustion gases acts on piston -> converted to rotary motion

Can be 2 or 4 stroke engines



Engine stroke:

A stroke is a single traverse of the cylinder by the piston (from TDC to BDC)

1 revolution of crankshaft = 2 strokes of piston


Exhaust

Suction Compression Ign.-Combn. Expansion Exhaust


Working Principles Of Four Stroke Diesel Engine:

Intake stroke

Intake valve open, exhaust valve shut

Piston travels from TDC to BDC

Air drawn in

Compression stroke

Intake and exhaust valves shut

Piston travels from BDC to TDC

Temperature and pressure of air increase


Working Principles Of Four Stroke Diesel Engine:

Power stroke


Fuel injected into cylinder and ignites

Piston forced from TDC to BDC

Exhaust stroke

Intake valve shut, exhaust valve open

Piston moves from BDC to TDC

Combustion gases expelled


Working Principles Of Two Stroke Petrol

Engine



Two Stroke Cycle Petrol Engine Construction :

•A piston reciprocates inside the cylinder

•It is connected to the crankshaft by means of connecting rod and crank

•There are no valves in two stroke engines, instead of valves ports are cut

on the cylinder walls.

•There are three ports, namely inlet, exhaust and transfer ports.

•The closing and opening of the ports are obtained by the movement of

piston. The crown of piston is made in to a shape to perform this.

•A spark plug is also provided.


Two stroke cycle Petrol Engines - Working

• The piston moves up from Bottom Dead Centre (BDC) to

Top Dead Centre (TDC)

• Both transfer and exhaust ports are covered by the piston.

• Air fuel mixture which is transferred already into the engine

cylinder is compressed by moving piston.

• The pressure and temperature increases

• at the end of compression.



First Stroke : (b) Ignition and Inductance:

• Piston almost reaches the top dead centre

•The air fuel mixture inside the cylinder is ignited by means of an

electric spark produced by a spark plug

•At the same time, the inlet port is uncovered by the plane.

•Fresh air fuel mixture enters the crankcase through the inlet port



(c)Expansion and Crankcase compression

•The burning gases expand in the cylinder

•The burning gases force the piston to move down. Thus useful work is

obtained.

•When the piston moves down, the air fuel mixture in the crankcase is

partially compressed.

This compression is known as Crank case compression.



(d) Exhaust and transfer:

•At the end of expansion, exhaust port is uncovered.

•Burnt gases escape to the atmosphere.

•Transfer port is also opened. The partially compressed air fuel mixture enters

the cylinder through the transfer port.

•The crown of the piston is made of a deflected shape. So the fresh charge

entering the cylinder is deflected upwards in the cylinder.

•Thus the escape of fresh charge along with the exhaust

gases is reduced

Compression


Piston travels from BDC to TDC

Temperature and pressure of air increase

Power stroke


Fuel injected into cylinder and ignites

Piston forced from TDC to BDC

Working Principles Of Two Stroke Diesel Engine


Application Of Four stroke Cycle Engine

Used in heavy vehicles

Buses,

Lorries,

Trucks etc.,

Application Of Two stroke Cycle Engine

Used in light vehicles

Bikes,

Scooters,

Mopeds

Ship propulsion


Application Of Four stroke Petrol Engine

Buses ,Trucks

Mobile electric generating sets.

Small pumping sets with side cars

Application Of Four stroke Diesel Engine

30kw-Tractors

40to 100kw – jeeps, buses and trucks

200 to 400kw-Earthmoving m/c

100 to 35000kw-Marine application



I.C ENGINE TERMINOLGY


standard terms used in I.C Engines are

1. Bore: Inside diameter of the cylinder is termed as Bore and it is designated by the

letter d and is usually expressed in millimeter (mm)

2. Top Dead Center (TDC): The extreme position reached by the piston at the top of

the cylinder in the vertical engine is called Top Dead center. It is also called the Inner

dead centre (IDC).

3. Bottom Dead Center (BDC): The extreme position reached by the piston at the

Bottom of the cylinder in the vertical engine is called Bottom Dead center. It is also

called the Outer dead centre (ODC).


5. Compression ratio (r): It is the ratio of Maximum cylinder volume to the

Clearance volume.

6. Cylinder volume (v): It is the sum of swept volume and the Clearance volume.

V = Vs + Vc

7. Displacement (or)Swept volume (Vs): It is the volume of space generated by

the movement of piston from one dead center to another dead center. It is expressed

in terms of cubic centimeter (cc) and given by

VS = A * L = * d2 * L / 4

8. Clearance Volume( Vc): It is the space in the cylinder, when the piston is at Top

Dead Center It is designated as VC and expressed in cubic centimeter (cc).

Comparison of Petrol and Diesel Engines

Petrol Engines

A petrol engine draws a mixtureof petrol and air during suctionstroke.

The carburetor is employed tomix air and petrol in therequired proportion and tosupply it to the engine duringsuction stroke.

Pressure at the end ofcompression is about 10 bar.

The charge (i.e. petrol and airmixture) is ignited with the helpof spark plug.

Diesel Engines

A diesel engine draws only airduring suction stroke.

The injector or atomizer isemployed to inject the fuel atthe end of combustion stroke.

Pressure at the end ofcompression is about 35 bar.

The fuel is injected in the formof fine spray. The temperatureof the compressed air issufficiently high to ignite thefuel.


Comparison of Petrol and Diesel Engines(contd..)

The maintenance cost is less.

The thermal efficiency is about 26%.

Overheating trouble is more due to low thermal efficiency.

These are high speed engines.

The petrol engines are generally employed in light duty vehicle such as scooters, motorcycles and cars. These are also used in aeroplanes.

The maintenance cost ismore.

The thermal efficiency isabout 40%.

Overheating trouble is lessdue to high thermal efficiency.

These are relatively lowspeed engines.

The diesel engines aregenerally employed in heavyduty vehicles like buses,trucks, and earth movingmachines.


VALVE TIMING DIAGRAM A Valve timing diagram is a graphical representation of the exact moments, in the

sequence of operations, at which the two valves(i.e. inlet & exhaust valves) open

and close as well as firing of fuel. It is, generally, expressed in terms of angular

positions of the crankshaft.

THEORETICAL VALVE TIMING DIAGRAM FOR

FOUR STROKE CYCLE DIESEL ENGINE


The theoretical valve timing diagram for a four stroke cycle engine is shown in this

fig. In this diagram, the inlet valve opens at A and the suction takes place from A to

B. The crankshaft revolves through 180 degree and the piston moves from T.D.C. to

B.D.C. At B, the inlet valve closes and the compression takes place from B to C.

The crankshaft revolves through 180 degree and the piston moves from B.D.C. to

T.D.C. At C, the fuel is fired and the expansion takes place from C to D. the

crankshaft revolves through 180 deg. And the piston again moves from T.D.C. to

B.D.C. At D, the exhaust valve opens and the exhaust takes place from D to E. The

crankshaft again revolves through 180 deg. And the piston moves back to T.D.C.


ACTUAL VALVE TIMING DIAGRAM FOR FOUR

STROKE CYCLE DIESEL ENGINE


As we seen in the theoretical valve timing diagram, the valves open and close at the

dead centre position of the piston. But, in actual practice they do not open and close

instantaneously at dead centres. The valve operates some degrees before or after the

dead centres. The ignition also occurs a little before the top dead centre. The valve

timing diagram shown is the actual valve timing diagram, where we see that the inlet

valve opens before the piston reaches T.D.C. or in other words while the piston is

moving up before the beginning of the suction stroke. Now the piston reaches the T.D.C.

and the suction stroke starts. The piston reaches the B.D.C. and then starts moving up.

The inlet valve closes, when the crank has moved a little beyond the B.D.C. This is done

as the incoming air continues to flow into the cylinder although the piston is moving

upwards from B.D.C. Now the air is compressed with both valves closed. Fuel valve


opens a little before the piston reaches the T.D.C. Now the fuel is injected in the form

of very fine spray, into the engine cylinder, which gets ignited due to high

temperature of the compressed air. The fuel valve closes after the piston has come

down a little from the TDC. This is done as the required quantity of the fuel is

injected into the engine cylinder. the burnt gases (under high pressure and

temperature) push the piston downwards, and the expansion or working stroke takes

place. Now the exhaust valves opens before the piston again reaches B.D.C. and

the burnt gases start leaving the engine cylinder. Now the piston reaches B.D.C. and

then starts moving up thus performing the exhaust stroke. The inlet valve opens

before the piston reaches T.D.C. to start suction stroke. This is done as the fresh air

helps in pushing out the burnt gases. Now the piston again reaches T.D.C. and the

suction starts. The exhaust valve closes when the crank has moved a little beyond6/10/2017 Naphis Ahamad (ME) JIT 34

THEORETICAL VALVE TIMING DIAGRAM FOR

TWO STROKE CYCLE DIESEL ENGINE

done as the burnt gases continue to leave the engine cylinder although the

piston is moving downwards.


The theoretical valve timing diagram for a two stroke cycle engine is shown in fig. In

this diagram, the fuel is fired at A and the expansion of gases takes place from A to B.

the crankshaft revolves through approximately 120 deg. and the piston moves from

T.D.C. towards B.D.C. At B, both the valves open and suction as well as exhaust take

place from B to C. At B, both the valves open and suction as well as exhaust takes

place from B to C. The crankshaft revolves through approximately 120 deg. and the

piston moves first to BDC and then little upwards. At C, both the valves close and

compression takes place from C to A. The crankshaft revolves through approximately

120 deg. and the piston moves to B.D.C.


ACTUAL VALVE TIMING DIAGRAM FOR TWO

STROKE CYCLE DIESEL ENGINE


As like as four stroke cycle engine, the actual valve timing diagram of two stroke

cycle engine is also different from it’s theoretical valve timing diagram. The actual

valve timing diagram of two stroke cycle engine is shown in fig. As we see that the

expansion of the charge (after ignition) starts as the piston moves T.D.C. towards

B.D.C. First of all, the exhaust port opens before the piston reaches B.D.C. and the

burnt gases start leaving the cylinder. After a small fraction of the crank revolution,

the transfer port also opens and the fresh air enters into the engine cylinder. This is

done as the fresh incoming air helps in pushing out the burnt gases. Now the piston

reaches B.D.C. and then starts moving upwards. As the crank moves a little beyond

B.D.C. first the transfer port closes and then the exhaust port also closes. This is

done to suck fresh air through the transfer port and to exhaust the burnt gases

through the6/10/2017 Naphis Ahamad (ME) JIT 38

exhaust port simultaneously. Now the charge is compressed with both the ports closed.

Fuel valve opens a little before the piston reaches the T.D.C. Now the fuel is injected in

the form of very fine spray into the engine cylinder, which gets ignited due to high

temperature of the compressed air. The fuel valve closes after the piston has come

down a little from the T.D.C. This is done as the required quantity of fuel is injected into

the engine cylinder. Now the burnt gases (under high pressure and temperature) push

the piston downwards with full force and expansion of the gases takes place. It may be

noted that in a two stroke cycle diesel engine, like two stroke petrol engine, the

exhaust and transfer ports open and close at equal angles on either side of the B.D.C.

position.


thnet

in

W

Q

th CarnotL

H

T

T, 1

Upon derivation the performance of the real cycle is often measured in

terms of its thermal efficiency

The Carnot cycle was introduced as the most efficient heat engine that

operate between two fixed temperatures TH and TL. The thermal

efficiency of Carnot cycle is given by

40

Carnot Cycle


The ideal gas equation is defined as

mRTPVorRTPv

where P = pressure in kPa

v = specific volume in m3/kg (or V = volume in m3)

R = ideal gas constant in kJ/kg.K

m = mass in kg

T = temperature in K

41 41


Air continuously circulates in a closed loop.

Always behaves as an ideal gas.

All the processes that make up the cycle are internally reversible.

The combustion process is replaced by a heat-addition process

from an external source.

42

Air-Standard Assumptions


A heat rejection process that restores the working fluid to its initial

state replaces the exhaust process.

The cold-air-standard assumptions apply when the working fluid is

air and has constant specific heat evaluated at room temperature

(25oC or 77oF).

No chemical reaction takes place in the engine.

43

Air-Standard Assumptions


Top dead center (TDC), bottom dead center (BDC), stroke, bore,

intake valve, exhaust valve, clearance volume, displacement

volume, compression ratio, and mean effective pressure

Terminology for Reciprocating Devices

44


The compression ratio r of an engine

is defined as

rV

V

V

V

BDC

TDC

max

min

The mean effective pressure (MEP)

is a fictitious pressure that, if it

operated on the piston during the

entire power stroke, would produce

the same amount of net work as that

produced during the actual cycle.MEP

W

V V

w

v v

net net

max min max min45


46

Otto Cycle

The Ideal Cycle for Spark-

Ignition Engines


The processes in the Otto cycle are as per following:

Process Description

1-2 Isentropic compression

2-3 Constant volume heat addition

3-4 Isentropic expansion

4-1 Constant volume heat rejection

1

2

3

4

Qout

Qin

PvgConstant

v1v2 v

P

s

T

Qout

Qin

1

2

3

4

(a) P-v diagram (b) T-s diagram 47


Related formula based on basic thermodynamics:

Process Description Related formula





1

2

1

1

2

2

1

n

nn

T

T

V

V

P

P

1

2

1

1

2

2

1

n

nn

T

T

V

V

P

P

3 2in vQ mC T T

4 1out vQ mC T T

48


Thermal efficiency of the Otto cycle:

thnet

in

net

in

in out

in

out

in

W

Q

Q

Q

Q Q

Q

Q

Q

1

Apply first law closed system to process 2-3, V = constant.

Thus, for constant specific heats

Q U

Q Q mC T T

net

net in v

,

, ( )

23 23

23 3 2

,23 ,23 23

3

,23 ,23 ,23

2

0 0

net net

net other b

Q W U

W W W PdV

49


Apply first law closed system to process 4-1, V = constant.

Thus, for constant specific heats,

Q U

Q Q mC T T

Q mC T T mC T T

net

net out v

out v v

,

, ( )

( ) ( )

41 41

41 1 4

1 4 4 1

The thermal efficiency becomes

th Ottoout

in

v

v

Q

Q

mC T T

mC T T

,

( )

( )

1

1 4 1

3 2

,41 ,41 41

1

,41 ,41 ,41

4

0 0

net net

net other b

Q W U

W W W PdV

50


th Otto

T T

T T

T T T

T T T

,

( )

( )

( / )

( / )

1

11

1

4 1

3 2

1 4 1

2 3 2

Recall processes 1-2 and 3-4 are isentropic, so

Since V3 = V2 and V4 = V1,

3 32 4

1 4 1 2

T TT Tor

T T T T

11

32 1 4

1 2 4 3

kk

TT V Vand

T V T V

51


The Otto cycle efficiency becomes

th Otto

T

T, 1 1

2

Since process 1-2 is isentropic,

where the compression ratio is

r = V1/V2 and

th Otto kr,

1

11

1

2 1

1 2

1 1

1 2

2 1

1

k

k k

T V

T V

T V

T V r

52


The processes in the Diesel cycle are as per following:

Process Description


2-3Constant pressure heat addition


4-1Constant volume heat rejection

53

Diesel Cycle


cv rratiooffCutv

vandrrationCompressio

v

v,,

2

3

2

1

54

Diesel Cycle


Related formula based on basic thermodynamics:

Process Description Related formula


2-3 Constant pressure heat addition



1

2

1

1

2

2

1

n

nn

T

T

V

V

P

P

1

2

1

1

2

2

1

n

nn

T

T

V

V

P

P

3 2in PQ mC T T

4 1out vQ mC T T

55


Thermal efficiency of the Diesel cycle

th Dieselnet

in

out

in

W

Q

Q

Q, 1

Apply the first law closed system to process 2-3, P = constant.


Q U P V V

Q Q mC T T mR T T

Q mC T T

net

net in v

in p

,

,

( )

( ) ( )

( )

23 23 2 3 2

23 3 2 3 2

3 2

,23 ,23 23

3

,23 ,23 ,23

2

2 3 2

0 0

net net

net other b

Q W U

W W W PdV

P V V

56


Apply the first law closed system to process 4-1, V = constant

Q U

Q Q mC T T

Q mC T T mC T T

net

net out v

out v v

,

, ( )

( ) ( )

41 41

41 1 4

1 4 4 1


The thermal efficiency becomes

th Dieselout

in

v

p

Q

Q

mC T T

mC T T

,

( )

( )

1

1 4 1

3 2

,41 ,41 41

1

,41 ,41 ,41

4

0 0

net net

net other b

Q W U

W W W PdV

57


PV

T

PV

TV V

T

T

P

P

4 4

4

1 1

1

4 1

4

1

4

1

where

Recall processes 1-2 and 3-4 are isentropic, so

PV PV PV PVk k k k

1 1 2 2 4 4 3 3 and

Since V4 = V1 and P3 = P2, we divide the second equation by

the first equation and obtain

Therefore,

34

4 2

k

k

c

VPr

T V

, 1

111

1

k

cth Diesel k

c

r

r k r

58


Dual cycle gives a better approximation to a real engine. The heat addition

process is partly done at a constant volume and partly at constant pressure.

From the P-v diagram, it looks like the heat addition process is a

combination of both Otto and Diesel cycles.

59

Dual Cycle


Process Description



3-4 Constant pressure heat addition



The same procedure as to Otto and Diesel cycles can be applied to Dual

cycle. Upon substitutions, the thermal efficiency of Dual cycle becomes

111

11

k

vcpp

k

cp

thrrckr

rr

60

Dual Cycle


basics of internal combution engine

Engineering