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BKM 3313

APPLIED THERMODYNAMIC

CHAPTER 5

INTERNAL COMBUSTION ENGINES

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5.1 INTRODUCTION One of the most significant

inventions of the 20th century is the internal combustion (IC) engine

Definition An engine in which the

chemical energy of the fuel is released inside the engine and used directly for mechanical work

E : Exhaust cam shaft

I : Intake Cam shaft

S : Spark plug

V : Inlet and exhaust valve

W : Water Jacket for cooling flow

P : Piston

R : Connecting Rod

C : Crank shaft

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IC engines use reciprocating piston in a cylinder (block)

The piston operates between the “top dead center” (TDC) and the “bottom dead center” (BDC)

Valves are used to control the flow of gas into and out of engine

Stroke is the largest distance the piston travels

Bore is the diameter of the piston Other components are piston,

block, crankshaft, connecting rod etc.

tdc

bdc

bore

stroke

l

aθ

l = connecting rod

a = crank shaft

θ = crank angle

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5.2 TYPES & CLASSIFICATIONS OF IC ENGINES

IC engine can be classified according to:

applications Automobile, truck, locomotive, light aircraft, marine,

portable, power system etc

basic engine design Reciprocating engine, rotary engine

no of cylinders 1, 2, 3, 4, 5, 6, 8, 10, 12 etc.

arrangement of cylinder In-line, V-type, opposed, radial

working cycle 4-stroke, 2-stroke

fuel Gasoline, diesel, nitro methane, alcohol, natural gas,

hydrogen etc

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ENGINE DESIGN & CYLINDER ARRANGEMENT

V-type, 6 cylinder

(V6)

Inline, 4-cylinder

(Straight 4)

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Opposed, 4-cylinder

(Flat 4)

ENGINE DESIGN & CYLINDER ARRANGEMENT

Rotary egine

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4-STROKE ENGINE4-Stroke

1. Requires 4 stroke of piston to complete a cycle

1-2 Induction stroke

Inlet valve open. Exhaust valve is closed. BDC to TDC. Air + fuel is induced.

2-3 Compression stroke

Air + fuel is compressed to TDC. Spark occurred at S and combustion occurs mainly at constant volume. Large increase in pressure and temperature.

3-4 Working stroke

Hot gas expand pushing the piston down to BDC. Exhaust valve open at E to assist exhaustion. Inlet valve is still closed.

4-1 Exhaust stroke

The gas is force to exit the cylinder. Piston moved to TDC. Inlet valve is still closed.

2. 2 revolution of crank shaft per cycle

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START INTAKE COMPRESSION SPARK

POWEREXHAUST

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2-Stroke

1. Requires 2 stroke of piston to complete a cycle

First stroke : BDC – TDC (Both compression and induction stroke)

As piston ascends on the compression stroke, the next charge is drawn into crankcase C as the spring loaded valve, S open automatically. Ignition occur before TDC. Both transfer and exhaust port is uncovered.

Second stroke: TDC – BDC ( Both working and exhaust stroke)

At TDC working stroke begin. As the piston descend through about 80%, the exhaust port is uncovered and exhaust begin. The transfer port is uncovered later due to the shape of the piston and the position of the port. The descending piston push the air to enter the cylinder through the transfer port.

2. 1 revolution of crank shaft per cycle

3. Less efficient compared to 4 stroke

4. High power-to-weight ratio

5. Suitable for small applications

2-STROKE ENGINE

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5.3 THE AIR STANDARD CYCLES

Before we could discuss in depth about IC engines, let us look at several types air standard cycles.

We will discuss three standard cycles : Otto cycle Diesel cycle Dual combustion cycle

The air standards cycles are ideal cycles used as a yardstick for the actual cycles.

There are few assumptions applied to the cycles: Working fluid is air behaving as ideal gas All process in the cycle are internally reversible Combustion process is replaced by a heat addition process from an

external source Exhaust process is replaced by a heat rejection process No chemical reaction

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5.4 PERFORMANCE CRITERIA OF IC ENGINES

GEOMETRICAL PARAMETERS Referring to the diagram

Note: “Indicated” refers to the values obtained by analysis on

the cycle (i.e. Indicated Power, Indicated MEP) “Brake” refers to the values obtained through

experimental methods (i.e. Brake Power, BMEP) VD is multiplied by no of cylinder for multi-cylinder

engines

tdc

bdc

b

L

l

a

θ

VC

VD

C

CDv

D

V+VV

=

Lb=V

Ratio nCompressio4

nt VolumeDisplaceme

2

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INDICATED POWER Definition: The rate of indicated cycle work. How it’s being measured: Using indicator diagram obtained

from the engine.

Net work done per cycle = area of power loop – area of pumping loop

Therefore indicated mean effective pressure, Pi POWER LOOP

PUMPING LOOP

P

V

constantdiagram of lengthdiagram of areanet

ip

Note: the constant will depends on the scales of the recorder. Normally the units of the constant are either bar/mm or kPa/mm

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INDICATED POWER For one cylinder engine,

where L = stroke

A = area of piston

For N rpm and n cylinder, indicated power can be written as:

LAW

VW

VVW

p net

s

netneti

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engine stroke- 2for

engine stroke- 4for 2NnLApip

NnLApip

i

i

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Based of the data given, determine the engine indicated power:

Net are of diagram = 210 mm2

Length of diagram = 28 mm

Constant = 0.8 bar/mm

Engine type = 4 stroke

No of cylinder = 4

Piston area = 100 cm2

Stoke = 12 cm

Engine speed = 3000 rpm

EXAMPLE 5.4

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BRAKE POWER

It is the measured output of the engine (actual power). The engine is connected to a brake dynamometer which can be

loaded in such a way that the torque exerted by the engine can be measured.

The torque is obtained by reading off a net load, W at a known radius, r from the axis rotation and hence the torque is given by

The brake power is given by

Nowadays torque can be measured directly and bp is obtained directly using above equation.

rW

Nbp 2Nbp 2

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ENGINEDYNAMOMETER

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19

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FRICTION POWER & MECHANICAL EFFICIENCY

The difference between the ip and bp is the friction power, fp. It is defined as the power required to overcome the frictional

resistance of the engine parts.

The mechanical efficiency is defined as

The value lies between 80 to 90%.

bpipfp

ipbp

m

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BRAKE MEAN EFFECTIVE PRESSURE

The brake power of an engine can be obtained accurately using dynamometer.

From equation and

we will get

Since m and ip are difficult to obtain, they may be combined and replace by a brake mean effective pressure pb.

ipbp m 2

or N

ALnpipALnNpip ii

2or

NALnpbpALnNpbp imim

2or

NALnpbpALnNpbp bb

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BRAKE THERMAL EFFICIENCY

Generally we define efficiency as

For IC engine, the actual output is brake power and the input is the chemical energy of the fuel supplied

The value of NKR is a standard value. For diesel, NKR = 45.5 MJ and for petrol, NKR = 44.2 MJ

inputoutput

valuecalorific low

fuel of rate flow mass

power brake

NKR

m

where

NKRm

f

fbt

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INDICATED THERMAL EFFICIENCY

Indicated thermal efficiency is defined as

If we divide bt with it

NKRmfit

power indicated

itmbt

mti

bt

it

bt

IPBP

NKRmfIPNKRmf

BP

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SPECIFIC FUEL CONSUMPTION

It is defined as mass flow rate of fuel per unit power output. It is a criterion of economical power production

).

(3600jamkW

kgbpm

sfc f

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Based of the data given, determine the brake power, brake mean effective pressure, brake thermal efficiency and sfc.

Type of engine = 4 cylinder, 4 stroke petrol engine

Bore = 55 mm

Stroke = 100 mm

Engine speed = 3500 rpm

Torque arm, r = 0.45 m

Net brake load = 160 N

Mass rate (fuel) = 0.0014 kg/s

NKR = 44.2 MJ

EXAMPLE 5.5

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5.5 OTHER TYPE OF ENGINE TESTING-MORSE TEST

Since it is very difficult to obtain indicated power, Morse Test is introduce to give alternative solution.

The test is suitable for multi cylinder engine. Testing procedures:1. Engine is set to run at certain speed, N and torque is measured

2. One cylinder is cut out by shorting the plug.

3. When the speed falls, load is reduced to restore the engine speed.

4. The torque is measured again.

5. The plug is placed back and another cylinder is cut out by shorting its plug.

6. Speed is again restored and torque is again measured.

7. The procedures is repeated until all cylinder is simultaneously cut out.

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MORSE TEST

If it is a 4 cylinder engine:

totalSSSS

SSSSSSSS

SSSS

FPIPIPIPIPBP

FPIPFPIPFPIPFPIPBP

BPBPBPBPBP

4321

44332211

4321

When cylinders are cut off

totalSSSoff

totalSSSoff

totalSSSoff

totalSSSoff

FPIPIPIPBP

FPIPIPIPBP

FPIPIPIPBP

FPIPIPIPBP

0

0

0

0

3214

4213

4312

4321

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MORSE TEST

Subtracting the equations, for cylinder 1

So for each cylinder,

Then for the engine

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432

43211

0

Soff

totalSSS

totalSSSSoff

IPBPBP

FPIPIPIP

FPIPIPIPIPBPBP

offSoffS

offSoffS

BPBPIPBPBPIP

BPBPIPBPBPIP

4433

2211

4321 SSSS IPIPIPIPIP

offoffoffoff WWWWWWWWNRIP 43212

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A Morse test is carried out to a 4 cylinder, 4 stroke petrol engine. Based on the data given, determine the mechanical efficiency of the engine:

W = 120N

W1-off = 86.8N

W2-off = 81.4N

W3-off = 88.6N

W4-off = 82.1N

EXAMPLE 5.5