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BME

Unit 2

Application of Thermodynamics

Question 1) Explain the difference between 2 stroke and four stroke engine?

Answer 1)

Difference Between Two Stroke and Four Strokes
Two Strokes	Four Stroke
It has one revolution of the crankshaft during one power stroke	It has two revolutions of the crankshaft during one power stroke
ii. It generates high torque	It generates less torque
iii. Its uses port for fuel’s outlet and inlet	It uses valves for outlet and inlet of a fuel
iv. Its engines result in lesser thermal efficiency	Its engines result in higher thermal efficiency
v. It has a larger ratio in terms of power to weight	It has a lesser ratio in terms of power to weight
vi. It generates more smoke and shows less efficiency	It generates less smoke and shows more efficiency
vii. Requires more lubricating oil as some oil burns with the fuel	Requires less lubricating oil
viii. Due to poor lubrication, more wear and tear occurs	Less wear and tear occurs
ix. Engines are cheaper and are simple for manufacturing	Engines are expensive due to lubrication and valves and are tough to manufacture
x. Engines are basically lighter and are noisy	Engines are basically heavier because its flywheel is heavy and are less noisy

Question 2) Explain the difference between compressed ignition engine or Diesel engine and spark ignition engine or Petrol engine?

Answer 2)

S.no		Parameter		SI Engine	CI Engine
1.		Definition		It is an engine in which the spark is used to burn the fuel.	It is an engine in which heat of compressed air is used to burn the fuel.
2.		Fuel used		Petrol is used as fuel.	Diesel is used as fuel.
3.		Operating cycle		It operates on Otto cycle.	It operates on Diesel cycle.
4.		Compression ratio		Low compression ratio.(6 to 10)	High compression ratio.(15 to 25)
5.		Thermal efficiency		Low thermal efficiency because of low compression ratio .Up to 26 %.	High thermal efficiency because of high compression ratio up to 40%.
6.		Method of ignition		Spark plug is used to produce spark for the ignition.	Heat of compressed air is used for the ignition.
7.		Engine Speed		High speed engines.	Low speed engines.
8.		Pressure generated		Low pressure is generated after combustion. About 10 bar	High pressure is generated after combustion. High pressure is generated about 35 bar.
9.		Constant parameter during cycle		Constant volume cycle.	Constant pressure cycle.
10.		Intake		Air + fuel.	Only air.
		Weight of engine		Si engine has less weight.	CI engine are heavier.
12.		Noise production		It produces less noise.	It produces more noise.
13.		Production of hydrocarbon		Less Hydrocarbon is produced.	More hydrocarbons is produced.
14.		Starting		The starting of SI engine is easy.	The starting of CI engine is difficult.
15.		Maintenance cost		Low	High
16.		Vibration problem		Less	Very High
17.		Cost of engine		Less cost	High cost
18.	Volume to power ratio		Less			High
19.		Fuel supply		carburettor	Injector
20.		application		It is used in light commercial vehicles like motorcycle, cars.	It is used in heavy duty vehicles likes bus, trucks, ships etc.

Question 3) Explain valve timing diagram of two stroke S.I engine?

ANSWER 3) In the valve timing diagram, as shown we see that the expansion of the charge (after ignition) starts as the piston moves from TDC towards BDC.

Valve timing diagram for 2 stroke petrol engine

First of all, the exhaust port opens a fraction of the crank revolution, the transfer port also opens and the fresh fuel-air mixture enters into the engine cylinder. This is done as the fresh incoming charge helps in pushing out the burnt gases.

Now the piston reaches BDC and then starts moving upwards. As the crank moves a little beyond BDC, first the transfer port closes and then the exhaust port also closes. This is done to suck fresh charge through the transfer port and to exhaust the burnt gases through the exhaust port simultaneously.

Now the charge is compressed with both ports closed and then ignited with the help of a spark plug before the end of the compression stroke. This is done as the charge requires some time to ignite. By the time the piston reaches TDC, the burnt gases (under high pressure and temperature) push the piston downwards with full force, and the expansion of the burnt gases takes place.

It may be noted that the exhaust and transfer ports open and close at equal angles on either side of the BDC position.

Question 4) Explain valve timing diagram of four stroke C.I engine ?

ANSWER 4) In the valve timing diagram as shown we see that the inlet valve opens before the piston reaches TDC; or in other words while the piston is still moving up before the beginning of the suction stroke.

valve timing diagram for 4 stroke CI Engine

Valve timing diagram for 4 stroke CI Engine

Now the piston reaches the TDC and the suction stroke starts. The piston reaches the BDC and then starts moving up. The inlet valve closes, when the crank has moved a little beyond the BDC.

This is done as the incoming air continues to flow into the cylinder although the piston is moving upwards from BDC. Now the air is compressed with both valves closed. The fuel valve opens a little before the piston reaches the TDC. Now the fuel is injected in the form of very fine spray, into the engine cylinder, which gets ignited due to the 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 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 valve opens before the piston reaches BDC and the burnt gases start leaving the engine cylinder.

Now the piston reaches BDC and then starts moving up thus performing the exhaust stroke. The inlet valve opens before the piston reaches TDC to start suction stroke. This is done as the fresh air helps in pushing out the burnt gases.

Now the piston again reaches TDC, and the suction starts. The exhaust valve closes when the crank has moved a little beyond the TDC. This is done as the burnt gases continue to leave the engine cylinder although the piston is moving downwards.

Question 5) Explain different kind of thermal efficiency in internal combustion engine?

Answers 5) There are different kind of efficiency are there

Indicated power (I.p): The power actually developed by engine cylinder or Indicated power is defined as the power developed by combustion of fuel in the combustion chamber (ip).

Indicated power (I.p): brake power + friction power

The indicated power (I.P.) of the engine can be calculated as follows:

I..P. = (pm x 105 x L. x A x N x n x k )/60

Pm = Inicated mean effective pressure in bar

L = Length of stroke in meter

A = Piston areas in m2

N = Speed in R.P.M.

n = Number of working stroke per minute.

n = 1 for two stroke engine

n= 1 / 2 for four stroke engine

k= number of cylinders

ii. Brake power (B.P): The power available at crankshaft is called brake power. It is measured by means of brake mechanism either Prony or rope brake.

For Prony brake = 2 Π N T / 60 Watt

For rope brake =(w-s) Π (D+d)N / 60

iii Mechanical efficiency : it is ration of brake power to indicated power

Efficiency = B.P / I.P

iv Indicated thermal efficiency : It is the ration of indicated work done to the energy supplied by the fuel .

η (ith)= I.P (KJ/s)/ energy in fuel per second (KJ/s)

= I.P / mf x cv

V Brake thermal efficiency: It is the ratio of power obtained at shaft to the energy supplied by fuel.

η (b.t.e)= B.P (KJ/s)/ energy in fuel per second (KJ/s)

= B.P / mf x cv

Vi Air standard efficiency: It is the ratio of work output to heat input

For petrol η = 1- 1/ ry-1

For diesel η = 1- 1/ ry-1 [ py-1 / y(p-1) ]

Where p = cut off ratio = v3 / v2 .

Question 6) In an air standard Otto cycle, the compression ratio is 7 and the compression begins at 35oC and 0.1 MPa. The maximum temperature of the cycle is 1100oC. Find

(a) The temperature and the pressure at various points in the cycle,

(b) The heat supplied per kg of air,

(d) The cycle efficiency and

(e) The MEP of the cycle

Answer 6) Given T1=35oC=308 K

P1=0.1 Mpa

T3=1100oC=1373 K

r=v1/v2=7

(a) the temperature and the pressure at various points in the cycle,

we know that P2/P1 = (V1 /V2)y = 71.4

Hence, P2=1524 k Pa

We also know that T2 / T1 = (V1/V2) y-1 = 71.4

We get T2=670.8 K

For process, 2-3, P2V2 / T2 = P3V3 / T3

P3 = T3 P2 /T2

P3 = 1373 X 1524 / 670.8

P3 = 3119.34 kPa

Process 3-4 is isentropic T3/T4 = (V4 /V3)y-1 = 71.4-1

T4 = 1372 / 2.178

T4 =630.39 K

(b) the heat supplied per kg of air,

Heat input = Qin= cv(T3-T2)

=0.718(1373–670.8)

= 504.18 kJ/kg

Heat rejected = Q out= cv(T4-T1)

=0.718(630.34–308)

=231.44 kJ/kg

The network output Wnet=Qin-Qout

The network output, Wnet=Qin-Qout

=272.74 kJ/kg

(d) the cycle efficiency

Thermal efficiency, ηth otto=Wnet/Qin

=0.54 =54 %

Otto cycle thermal efficiency, ηth,otto =1-1/rγ-1

= 1-1/70.4 = 0.54 or 54 %

v1=RT1/P1 =0.287 x 308 / 100=0.844 m3/kg

(e) the MEP of the cycle

MEP = Wnet/(v1–v2) = 272.74/v1 (1-1/r)

=272.74/0.844(1-1/7)

=360 kPa

Question 7) Explain classification of IC engine?

Answer 7) Classification of Internal Combustion (IC) Engine:

Internal combustion engines may be classified on the basis of:

Thermodynamic cycle used – Otto cycle, diesel cycle, and dual cycle engine

Fuel used – Petrol engine, diesel engine, gas engine, and kerosene oil engine

Number of strokes for completion of cycle – Four-stroke engine and two-stroke engine

Types of fuel ignition – Spark ignition (SI) and compression ignition (CI)

Number of cylinders – Single cylinder engine and multi-cylinder engine

Position of cylinders – Horizontal, vertical, V-engine, and radial engine

Method of cooling engines – Air-cooled engine and water-cooled engine

Question 8) What is carburettor?

Answer 8) Petrol engines are designed to take in exactly the right amount of air so the fuel burns properly, whether the engine is starting from cold or running hot at top speed. Getting the fuel-air mixture just right is the job of a clever mechanical gadget called a carburettor: a tube that allows air and fuel into the engine through valves, mixing them together in different amounts to suit a wide range of different driving conditions.

The carburettor is called the 'Heart' of the automobile, and it cannot be expected that the engine will act right, give the proper horse-power, or run smoothly if its 'heart' is not performing its functions properly.

Question 9) The following data was recorded during testing of a TWO STROKE gas engine:

Diameter of the piston d= 150 mm

Stroke length L= 180 mm

Clearance volume Vc = 0.89 litre

RPM of the engine N = 300

Indicated mean effective pressure pm= 6.1 bars

Gas consumption m. = 6.1 m3/h

Calorific value of the gas (fuel) CF = 17000 kJ/m3

Determine the followings:

a) Air Standard Efficiency

b) Indicated power (IHP)developed by the engine

c) Indicated thermal efficiency of the engine

Answer 9)

Swept volume Vs = πd2L/4 = π(0.150)2 x 180/4 = 0.00318 m3

Clearance volume Vc= 0.00089 m3

Total volume = Swept volume + clearance volume

= 0.00318 + 0.00089 = 0.00407 m3

Compression ratio γ = Total volume/Clearance volume

= 0.00407/0.00089 = 4.573

a) Air standard Efficiency

η = 1 –1/(r)γ—1 = 1—1/(4.573)1.4—1

= 0.456 = 45.6 %

b) Indicated power IHP = 100 pLAN/60

When p is in bars

= 100 x 6.1x 0.180 x [π (0.150)2 /4] x 300 / 60

= 9700 W

(c ) Indicated Thermal Efficiency

ηit = Indicated power in (kJ/s)/Heat supplied in (kJ/s)

= (9700/1000)/ (6.1 x 17000/3600)

= 0.3367 = 33.67 %

Question 10)

A FOUR STROKE petrol engine have given data below

Air fuel ratio 15.5: 1

Calorific value of fuel 16000 kJ/kg

Air Standard Efficiency: 53%

Mechanical Efficiency: 80 %

Indicated Thermal Efficiency: 37 %

Volumetric Efficiency: 80 %

Stroke/bore ratio: 1.25

Suction pressure: 1 bar

Suction Temperature: 270C

RPM: 2000

Brake Power: 72 kW

Calculate the followings:

a) Brake specific fuel consumption

b) Bore and stroke

Answer 10)

Find compression ratio from air standard efficiency

η = 1 –1/(r) γ—1

0.53 = 1 –1/(r) 1.4—1

r = 6.6

IHP = BP/Mechanical efficiency = 72/0.80

= 90 kW

ηit =IHP/(Specific Fuel Consumptions x Cal value)

0.37 = 90/sfc x 16000

sfc is specific fuel consumption

sfc = 0.0152 kg/s

a) Brake specific fuel consumption

Brake sfc = sfc IHP/BP

= 0.0152/72

=0.00021 kg/s /kW

Brake sfc = 0.7601 kg/kWh

b) Bore and stroke

Bore and stroke of the engine

Mass of air fuel mixture/kg of fuel = 15.5 +1 = 16.5

Mass of fuel supplied to the engine = 0.0152 x 16.5 = 0.2508

Volume of air fuel mixture = mRT/p

=0.2508 x 287 × 300/ (1×105)

V = 0.2159 m3/s

Swept volume = volume of mixture supplied/vol efficiency

Vs = 0.2159/0.80= 0.2699

Vs = (πd2L/4) n x (rpm/2) /60

Where n is the number of cylinders

= (πd2 x 1.25 d/4)n rpm/120

d= 0.152 m= 152 mm

L = 190 mm

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