Se prod thermo_chapter_4_i.c.engines

Analysis of Internal Combustion Engines

S. Y. B. Tech. Prod Engg.

Analysis of

Internal Combustion Engines

ME0223 SEM-IV Applied Thermodynamics & Heat Engines

Applied Thermodynamics & Heat Engines

S.Y. B. Tech.

ME0223 SEM - IV

Production Engineering


S. Y. B. Tech. Prod Engg.ME0223 SEM-IV Applied Thermodynamics & Heat Engines

Outline

• Otto, Diesel and Dual Combustion Cycles, Air Standard Efficiency and

Mean Effective Pressure.

• Constructional Details of I.C. Engines.

• Four and Two – Stroke Cycles, S.I. and C.I. Engines.

• Ignition System of S.I. Engines.

• Valve Timing Diagram.

• Calculation of I.P., F.P. and B.P. Determination of Indicated and Brake

Thermal Efficiency and Specific Fuel Consumption , Heat Balance

Sheet.



Heat EnginesAny type of engine or machine which derives Heat Energy from the combustion of

the fuel or any other source and converts this energy into Mechanical Work is

known as a Heat Engine.

Classification :

1. External Combustion Engine (E. C. Engine) :

Combustion of fuel takes place outside the cylinder.

e.g. Steam Turbine, Gas Turbine Steam Engine, etc.



2. Internal Combustion Engine (I.C. Engine) :

Combustion of fuel occurs inside the cylinder.

Heat Engines

e.g. Automobiles, Marine, etc.



Heat EnginesAdvantages of External Combustion Engines over Internal Combustion Engines :

1. Starting Torque is generally high.

2. Due to external combustion, cheaper fuels can be used (even solid fuels !).

3. Due to external combustion, flexibility in arrangement is possible .

4. Self – Starting units.

Internal Combustion Engines require additional unit for starting the engine !

Advantages of Internal Combustion Engines over External Combustion Engines :

1. Overall efficiency is high.

2. Greater mechanical simplicity.

3. Weight – to – Power ratio is low.

4. Easy Starting in cold conditions.

5. Compact and require less space.



Classification of I. C. EnginesA. Cycle of Operation :

B. Cycle of Combustion :

2. Four – Stroke Engine1. Two – Stroke Engine.

1. Otto Cycle (Combustion at Constant Volume).

2. Diesel Cycle (Combustion at Constant Pressure).

3. Dual Cycle (Combustion partly at Constant Volume + Constant Pressure).



Classification of I. C. Engines

C. Arrangement of Cylinder :

1. Horizontal Engine. 2. Vertical Engine

3. V – type Engine 4. Radial Engine



Classification of I. C. EnginesD. Uses :

1. Automobile Engine. 2. Marine Engine

3. Stationary Engine 4. Portable Engine




E. Fuel used :

1. Oil Engine. 2. Petrol Engine

3. Gas Engine 4. Kerosene Engine

F. Speed of Engine :

1. High Speed 2. Low Speed

G. Method of Cooling :

1. Air – Cooled Engine. 2. Water – Cooled Engine



Classification of I. C. EnginesG. Method of Ignition :

2. Compression – Ignition (C.I.) Engine1. Spark – Ignition (S.I.) Engine.




I. No. of cylinders :

1. Single Cylinder Engine. 2. Multi - Cylinder Engine



Application of I. C. Engines

APPLICATIONS

Road vehicles. Aircrafts.Locomotives.

Construction EquipmentsPumping Sets

Generators for Hospitals, Cinema Hall, and Public Places.



Air – Standard Cycles

OPERATING Cycle of an I. C. Engine ≡ Sequence of separate Processes.

1. Intake

2. Compression

3. Combustion

4. Expansion

5. Exhaust

I.C. Engine DOES NOT operate on a Thermodynamic Cycle, as it is an Open System.

i.e. Working Fluid enters the System at 1 set of conditions (State 1) and leaves at

another (State 2).

Accurate Analysis of I. C. Engine processes is very complicated. Advantageous to analyse the performance of an Ideal Closed Cycle that closely

approximates the real cycle.

i.e. Air – Standard Cycle.


S. Y. B. Tech. Prod Engg.

Assumptions

ME0223 SEM-IV Applied Thermodynamics & Heat Engines

1. The working medium is assumed to be a Perfect Gas and follows the relation PV = mRT

2. There is no change in the mass of the working medium.

3. All the processes that contribute the cycle are reversible.

4. Heat is assumed to be supplied from a constant high temperature source; and not

from chemical reactions during the cycle.

5. Some heat is assumed to be rejected to a constant low temperature sink during the cycle.

6. It is assumed that there are no heat losses from the system to the surrounding.

7. Working medium has constant specific heat throughout the cycle.

8. Physical constants viz. Cp, Cv, γ and M of working medium are same as those of air at

standard atmospheric conditions.

Cp = 1.005 kJ / kg.K Cv = 0.717 kJ / kg.K

γ = 1.4 M = 29 kg / kmole



Otto Cycle

Basis of Spark – Ignition Engines.

0 -1 : Suction

1 -2 : Isentropic Compression

2 -3 : Constant Vol. Heat Addition

3 -4 : Isentropic Expansion

1 -0 : Exhaust

0 1

Pre

ssu

re, P

Volume, V

Isentropic

2

Qs

3

4

QR

Qs

1

2

Tem

per

atu

re, T

Entropy, s

3Isochoric

4QR

4 -1 : Constant Vol. Heat Rejection



Otto Cycle – Thermal Efficiency

)(

)(1

)(

)()(

)(

)()(

23

14

23

1423

23

1423

TT

TT

TT

TTTT

TTCm

TTCmTTCm

Q

QQ

otto

otto

V

VVotto

S

RSotto

0 1

Pre

ssu

re, P

Volume, V

Isentropic

2

Qs

3

4

QR

1

3

4

4

3

1

2

1

1

2

V

V

T

TAND

V

V

T

T

3

4

2

1

V

VAND

V

VRationCompressior




0 1

Pre

ssu

re, P

Volume, V

Isentropic

2

Qs

3

4

QR

4

3

1

2

3

4

2

1

T

T

T

T

V

V

V

Vr

23

14

2

1

3

4

TT

TT

T

T

T

T

1

2

1

2

1

11

1

VV

T

Totto

1

11

rotto




0 1

Pre

ssu

re, P

Volume, V

Isentropic

2

Qs

3

4

QR

1

11

rotto

Thermal Efficiency is a function of :

1.Compression Ratio (r) and

2.Ratio of Specific Heat (γ)

),( rfth

Thermal Efficiency is a Independent of :

1.Pressure Ratio (P2 / P1) and

2.Heat Supplied (Qs)



Otto Cycle – Work Output

0 1

Pre

ssu

re, P

Volume, V

Isentropic

2

Qs

3

4

QR

1111224433

VPVPVPVPW

)....(1

4

2

3

4

3

1

2 sayrP

P

P

PANDr

P

P

P

PP

3421 VrVANDVrV

11

11

11

11

11

22

11

44

11

3311

11224433

r

rr

r

rrVPW

VP

VP

VP

VP

VP

VPVPW

VPVPVPVPW

PP

111

111

rr

VPW P



Otto Cycle – Mean Effective Pressure

0 1

Pre

ssu

re, P

Volume, V

Isentropic

2

Qs

3

4

QR

)1(

111

1

2

111

rV

rrVPP

P

m

VolumeSwept

OutputWorkPm

)1(221 rVVVVolumeSwept

)1(1

11 11

r

rrrPP Pm

Work Output α Pr. Ratio, (rp)

&, MEP α Internal Work Output

Pr. Ratio ↑ ≡ MEP ↑



Diesel Cycle

In S. I. Engines, max. compression ratio (r) is limited by self – ignition of the fuel.

This can be released if air and fuel are compressed separately and brought together

at the time of combustion.

i.e. Fuel can be injected into the cylinder with compressed air at high temperature.

i.e. Fuel ignites on its own and no special device for ignition is required.

This is known as Compression Ignition (C. I.) Engine.

Ideal Cycle corresponding to this process is known as Diesel Cycle.

Main Difference :

Otto Cycle ≡ Heat Addition at Constant Volume.

Diesel Cycle ≡ Heat Addition at Constant Pressure.



Diesel Cycle

Basis of Compression – Ignition Engines.

0 -1 : Suction


2 -3 : Constant Pr. Heat Addition


1 -0 : Exhaust

Qs

1

2

Tem

per

atu

re, T

Entropy, s

3

Isobaric

4QR


0 1

Pre

ssu

re, P

Volume, V

Isentropic2

Qs 3

4

QR

Isochoric



Diesel Cycle – Thermal Efficiency

)(

)(11

)(

)()(

)(

)()(

23

14

23

1423

23

1423

TT

TT

TTC

TTCTTC

TTCm

TTCmTTCm

Q

QQ

Diesel

P

VPDiesel

P

VPDiesel

S

RSDiesel

CrV

V

T

TIsobaric

T

V

T

V

2

3

2

3

3

3

2

2 ......

0 1

Pre

ssu

re, P

Volume, V

Isentropic2

Qs 3

4

QR

2

1

V

VrRationCompressio

2

3

V

VrRatiooffCut C




1

1

2

1

1

2

rV

V

T

T

11

1

3

1

4

2

2

334

1

4

3

3

4

CC rTr

rT

V

V

V

VTT

V

V

T

T

11

111

1

.

111

.

111

rrr

r

rrrT

rT

C

C

C

Cotto

1

111

1C

Cotto r

r

r

0 1

Pre

ssu

re, P

Volume, V

Isentropic2

Qs 3

4

QR

11

23 TrrTrT CC

AND




Efficiency of Diesel Cycle is different than that of

the Otto Cycle by the bracketed factor.

0 1

Pre

ssu

re, P

Volume, V

Isentropic2

Qs 3

4

QR

1

111

1C

Cotto r

r

r

This factor is always more than unity. (> 1)

Otto Cycle is more efficient than Diesel Cycle, for

given Compression Ratio

In practice, however, operating Compression

Ratio for Diesel Engines (16 – 24) are much

higher than that for Otto Engines (6 – 10).

Efficiency of Diesel Engine is higher than that

of Otto Engine



Diesel Cycle – Work Output

1

1.11

1

.11

111

11)(

11

22

124322

2122442322

11224433232

rrrrrVPW

rPPrPrPrPVW

VrPVPVrPVrPrVPW

VPVPVPVPVVPW

CCC

CC

CC

1

1.1.

11

11

CC rrrrVPW

0 1

Pre

ssu

re, P

Volume, V

Isentropic2

Qs 3

4

QR



Diesel Cycle – Mean Effective Pressure

11

11..

1

1

11

V

rrrVPP CC

m

VolumeSwept

OutputWorkPm

11

1.1..1 r

rrrrPP CC

m

0 1

Pre

ssu

re, P

Volume, V

Isentropic2

Qs 3

4

QR



Dual Cycle

Combustion process is neither Constant Volume nor Constant Pressure Process.

Real engine requires :

1. Finite time for chemical reaction during combustion process.

Combustion can not take place at Constant Volume.

2. Rapid uncontrolled combustion at the end.

Combustion can not take place at Constant Pressure.

Hence, a blend / mixture of both the processes are proposed as a compromise.



Dual Cycle

0 -1 : Suction


2 -3 : Constant Vol. Heat Addition


1 -0 : Exhaust


0 1

Pre

ssu

re, P

Volume, V

Isentropic

2

Qs

34

QR

5

Qs Qs

1

2

Tem

per

atu

re, T

Entropy, s

3

Isobaric4

QR

Isochoric

Isochoric

5

Qs

2 -3 : Constant Pr. Heat Addition



Dual Cycle – Thermal Efficiency

)()(

)(

)()(

)()()(

3423

15

3423

153423

TTTT

TT

TTCmTTCm

TTCmTTCmTTCm

Q

QQ

Dual

PV

VPVDual

S

RSDual

112

1

2

1

1

2 .

rTTV

V

T

T

2

1

V

VrRationCompressio

2

3

V

VrRatiooffCut C

0 1

Pre

ssu

re, P

Volume, V

Isentropic

2

Qs

34

QR

5

Qs

2

3PrP

PrRatio P

11

2

323

2

3

2

3 ..

rrT

P

PTT

P

P

T

TP




1134

3

4

3

4 ....

rrrTrTTr

V

V

T

TPCCC

1.1

1.11

1CPP

CPDual rrr

rr

r

...

...

...

.

15

11

15

1

5

4115

1

5

445

PC

CPC

PC

rrTT

r

rrrrTT

V

VrrrTT

V

VTT

0 1

Pre

ssu

re, P

Volume, V

Isentropic

2

Qs

34

QR

5

Qs




0 1

Pre

ssu

re, P

Volume, V

Isentropic

2

Qs

34

QR

5

Qs

1.1

1.11

1CPP

CPDual rrr

rr

r

For ( rp ) > 1;

ηDual ↑ for given ( rc ) and ( γ )

Efficiency of Dual Cycle lies in

between that of Otto Cycle and

Diesel Cycle.

With ( rc ) = 1 ≡ Otto Cycle

With ( rp ) = 1 ≡ Diesel Cycle



Dual Cycle – Work Output

1

1.......

111

11)(

1111

11

11

22

11

55

11

44

11

33

11

4411

11225544343

rrrrrrrrrrVP

VP

VP

VP

VP

VP

VP

VP

VP

VP

VPVP

VPVPVPVPVVPW

CPPPPC

1

1.11.. 11

11

CPPCP rrrrrrrVPW

0 1

Pre

ssu

re, P

Volume, V

Isentropic

2

Qs

34

QR

5

Qs



Dual Cycle – Mean Effective Pressure

1

1.1.1....

1 11

1121

CPPCP

m

rrrrrrrVP

VVP

VolumeSwept

OutputWorkPm

11

1..1.1...1 r

rrrrrrrrPP CPPCP

m

0 1

Pre

ssu

re, P

Volume, V

2

Qs

34

QR

5

Qs



Four – Stroke / Compression Ignition (C.I.) Engine



Four – Stroke / Compression Ignition (C.I.) Engine



Four – Stroke Engine – Valve Timing Diagram



Two – Stroke / Spark Ignition (S.I.) Engine



Two – Stroke / Spark Ignition (S.I.) Engine



Two – Stroke Engine – Valve Timing Diagram



Comparison : Two – Stroke Vs. Four Stroke

Sr. No.

ParticularsFour – Stroke

CycleTwo – Stroke

Cycle

1. Cycle Completion 4 strokes / 2 revolutions

2 strokes / 1 revolution

2. Power Strokes 1 in 2 revolutions 1 per revolution

3. Volumetric Efficiency High Low

4. Thermal and Part – Load Efficiency

High Low

5.Power for same Engine

Size

Small; as 1 power stroke

for2 revolutions

Large;as 1 power stroke

per revolutions

6. Flywheel Heavier Lighter

7. Cooling / Lubrication Lesser Greater

8. Valve Mechanism Required Not Required

9. Initial Cost Higher Lower



Comparison : S.I. Vs. C.I. Engines

Sr. No.

Particulars S. I. Engine C. I. Engine

1. Thermodynamic Cycle

Otto Diesel

2. Fuel Used Gasoline Diesel

3. Air : Fuel Ratio 6 : 1 – 20 : 1 16 : 1 – 100 : 1

4. Compression Ratio Avg. 7 – 9 Avg. 15 – 18

5. Combustion Spark Ignition Compression Ignition

6. Fuel Supply Carburettor Fuel Injector

7. Operating Pressure 60 bar max. 120 bar max.

8. Operating Speed Up to 6000 RPM Up to 3500 RPM

9. Calorific Value 44 MJ/kg 42 MJ/kg

10. Running Cost High Low

11. Maintenance Cost Minor Major



Comparison : Gasoline Vs. Diesel Engines

Sr. No. Gasoline Engine Diesel Engine

1. Working : Otto Cycle Working : Diesel Cycle

2. Suction Stroke : Air / Fuel mixture is taken in

Suction Stroke : only Air is taken in

3. Spark Plug Fuel Injector

4. Spark Ignition generates Power Compression Ignition generates Power

5. Thermal Efficiency – 35 % Thermal Efficiency – 40 %

6. Compact Bulky

7. Running Cost – High Running Cost – Low

8. Light – Weight Heavy – Weight

9. Fuel : Costly Fuel : Cheaper

10. Gasoline : Volatile and Danger Diesel : Non-volatile and Safe.

11. Less Dependable More Dependable



Battery / Coil Ignition Systems1. The Connector is introduced in the

circuit.

2. Current flows from Battery to the

Circuit Breaker.

3. Condenser prevents the sparking.

4. Rotating cam of the Contact

Breaker successively connects

and disconnects the circuit.

5. This introduces the high magnetic

field, thereby generating high voltage.

( 8,000 – 12,000 V).

6. Spark Jumps in the gaps of the Spark

Plug. and the air / fuel mixture gets

ignited.



Magneto – Ignition Systems



Magneto – Ignition Systems

1. The Rotating Magnet offers positive

and negative magnetic field.

2. As the magnetic field changes from

positive to negative, current and

voltage is induced in the Primary

Windings.

4. This introduces the high magnetic

field, thereby generating high voltage.

5. Spark Jumps in the gaps of the Spark

Plug. and the air / fuel mixture gets

ignited.

3. Turning of magnet results in

breaking the circuit.



Sr. No.

Battery Ignition System Magneto – Ignition System

1. Current obtained from Battery Current generated from Magneto

2. Sparking is good even at low speeds Poor sparking at low speeds

3. Engine starting is easier Difficult starting

4. Engine can not be started, if battery is discharged

No such difficulty, as battery is not needed

5. More space requirement Less space requirement

6. Complicated wiring Simple wiring

7. Cheaper Costly

8. Spark intensity falls as engine speed rises

Spark intensity improvesas engine speed rises

9. Used in cars, buses and trucks Used in motorcycles, scooters and racing cars

Comparison : Battery Vs. Magneto Ignition



Performance of I.C. Engines

Engine Performance ≡ Indication of Degree of Success for the work assigned.

(i.e. Conversion of Chemical Energy to useful Mechanical Work)

Basic Performance Parameters :

1.Power & Mechanical Efficiency

3.Specific Output

5.Air : Fuel Ratio

7.Thermal Efficiency and Heat Balance

9.Specific Weight

2. Mean Effective Pressure & Torque

4. Volumetric Efficiency

6. Specific Fuel Consumption

8. Exhaust Emissions




A. Power and Mechanical Efficiency :

Indicated Power ≡ Total Power developed in the Combustion Chamber,

due to the combustion of fuel.

)(6010

)10(..

3

5

kWNkALpn

PI i

n = No. of Cylinders

Pmi = Indicated Mean Effective Pressure (bar)

L = Length of Stroke (m)

A = Area of Piston (m2)

k = ½ for 4 – Stroke Engine,

= 1 for 2 – Stroke Engine

N = Speed of Engine (RPM)




A. Power and Mechanical Efficiency :

Brake Power ≡ Power developed by an engine at the output shaft.

)(1060

2..

3kW

X

TNPB

N = Speed of Engine (RPM)

T = Torque (N – m)

Frictional Power (F. P.) = I. P. – B.

P.

..

..

PI

PBmech




B. Mean Effective Pressure :

Mean Effective Pressure ≡ Hypothetical Pressure which is thought to be

acting on the Piston throughout Power Stroke.

Fmep = Imep – Bmep

Imep ≡ MEP based on I.P.

Bmep ≡ MEP based on B.P.

Fmep ≡ MEP based on F.P.

Power and Torque are dependent on Engine Size.

Thermodynamically incorrect way to judge the performance w.r.t. Power / Torque.

MEP is the correct way to compare the performance of various engines.




C. Specific Output :

Specific Output ≡ Brake Output per unit Piston Displacement.

LXA

PBOutputSp

...

)(.. RPMinSpeedNXBXConstOutputSp mep

D. Volumetric Efficiency :

Volumetric Efficiency ≡ Ratio of Actual Vol. (reduced to N.T.P.) of the Charge

drawn in during the suction stroke, to the Swept Vol. of

the Piston.

Avg. Vol. Efficiency = 70 – 80 %

Supercharged Engine ≈ 100 %




E. Fuel : Air Ratio :

Fuel : Air Ratio ≡ Ratio of Mass of Fuel to that of Air, in the

mixture.Rel. Fuel : Air Ratio ≡ Ratio of Actual Fuel : Air Ratio to that of

Schoichiometric Fuel : Air Ratio.

F. Sp. Fuel Consumption :

Sp. Fuel Consumption ≡ Mass of Fuel consumed per kW Power.

)./(..

.. hrkWkgPB

mcfs



Performance of I.C. EnginesG. Thermal Efficiency :

Thermal Efficiency ≡ Ratio of Indicated Work done, to the Energy Supplied by the fuel.

..

.., .).(

VCXm

PIEfficiencyThermalIndicated

f

PIth

)/(..

sec)/(

kgMJfuelofValueCalorificVC

kgusedfuelofmassm f

..

.., .).(

VCXm

PBEfficiencyThermalBrake

f

PBth




H. Heat Balance :

Heat Balance ≡ Indicator for Performance of the Engine.

Procedure :

1. Engine run at Const. Load condition.

2. Indicator Diagram obtained with help of the Indicator.

3. Quantity of Fuel used in given time and its Calorific Value are measured.

4. Inlet and Outlet Temperatures for Cooling Water are measured.

5. Inlet and Outlet Temperatures for Exhaust Gases are measured.



Performance of I.C. EnginesH. Heat Balance :

)(.. kJVCXm f

Heat Supplied by Fuel =

)(60.. kJXPIHeat equivalent of I.P. =

)(12 kJTTXCXm ww Heat taken away by Cooling Water =

mw = Mass of Cooling Water used (kg/min)

Cw = Sp. Heat of Water (kJ/kg.°C)

T1 = Initial Temp. of Cooling Water (°C)

T2 = Final Temp. of Cooling Water (°C) )(kJTTXCXm rePge Heat taken away by Exhaust Gases =

me = Mass of Exhaust Gases (kg/min)

CPg = Sp. Heat of Exhaust Gases @ Const. Pr. (kJ/kg.°C)

Te = Temp. of Exhaust Gases (°C)

Tr = Room Temperature (°C)




Sr. No.

InputAmount

(kJ)Per cent

(%)Output

Amount (kJ)

Per cent (%)

1.Heat Supplied

by FuelA 100

Heat equivalent to I.P.

B α

2.Heat taken by Cooling Water

C β

3.Heat taken by

Exhaust GasesD

γ

4.Heat

UnaccountedE = A – (B+C+D)

Eδ

Total A 100 Total A 100

H. Heat Balance :



Thank You !

Date post:	12-Sep-2014
Category:	Education
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