Unit 3 Internal Combustion Engines

ACHARYA INSTITUTE OF TECHNOLOGY DEPARTMENT AUTOMOBILE ENGINEERING

SUB: ELEMENTS OF MECHANICAL ENGINEERING II SEMESTER ACADEMIC YEAR 2012-13 Sub Code : 10EME24Staff: SRINIVAS Y V

INTERNAL COMBUSTION ENGINES

Heat engine is a thermal prime mover, which derives the heat energy from the combustion of fuels and converts this energy in to mechanical work.Heat engines are classified into

1. External combustion engines(E.C) ex: Steam engine ,steam turbine

2. Internal combustion engines(I.C) ex: Diesel and petrol enginesIf the combustion fuel takes place outside the engine cylinder then the engine is called the External combustion engine. If the combustion fuel takes place inside the engine cylinder then the engine is called the Internal combustion engine.

Classification of IC engines

I C engines are classified as given below1. According to nature of thermodynamic cycle (Cycle of combustion)

i) Otto cycle engine – combustion takes place at constant volume ii) Diesel cycle engine-combustion takes place at constant Pressure iii) Dual combustion cycle engine - combustion takes place at constant volume and then at constant pressure3. According to the fuel used

i) Petrol engine ii) Diesel engine iii) gas engine iv) Bio fuel engine

4. According to the cycle of operation

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i)Two stroke cycle engine – working cycle is completed in 2 different strokes ii) Four stroke cycle engine - working cycle is completed in 4 different strokes

4. According to Method of ignitioni) Spark ignition engine (S I) ii) Compression ignition engine (C I)

5. According to the number of cylindersi) Single cylinder engine ii) Multi cylinder engine

6. According to the arrangement of cylinders i) Horizontal engine ii) Vertical engine iii) In –line engine iv) Radial engine v) opposed type engine VI) Vee engine

7. According to the Method of cooling asi) Air cooled engine ii) Water cooled engine

8. According to their uses i) Stationary engine ii) Automobile engine iii) Locomotive engine iv) Marine engine

Parts of IC Engine

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Cylinder: Cylinder is the heart of the engine, in which fuel is burnt and the power is developed. Inside diameter is called “Bore”. The function of cylinder is to contain the working fluid under pressure and to guide the piston while reciprocating inside the cylinder. To prevent the wearing of the cylinder block, a sleeve will be fitted tightly in the cylinder.Cylinder Head: Top end of the cylinder is closed by a removable cylinder head. It consists of 2 valves inlet and exhaust valve.Piston: Piston is a cylindrical shaped component that fits perfectly inside the cylinder. Function of the piston is to compress the charge during compression stroke and receive the force produced by the combustion of fuel and to transmit the energy to the crankshaft.The function of the oil rings is to extract the lubricating oil walls and to send it back to the oil sump through holes.Piston rings: These maintain a tight joint between the piston and the cylinder. They also help in conducting the heat from the piston to the cylinder. Prevent high pressure gases escaping in to the crankcase.

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Connecting Rod: forms the link between the piston and the crankshaft. It converts the rectilinear motion of the piston into rotary motion of the crankshaft.Crank and crankshaft: crank is a lever with one of its end connected to the end of the connecting rod by a pin joint and the other end is rigidly connected to crankshaft.Valves: controls the flow of intake and the exhaust gases to and from the engine cylinder. They are also called as poppet valves. the valves are actuated by means of springs, cams and lever.Flywheel: Heavy mass of rotating wheel mounted on the crankshaft and used as energy storing device .It stores the energy during the power stroke and supplies the same during other strokes.Crankcase: Encloses the crankshaft and serves as a sump for the lubricating oil.Cam: Element designed to control the movement of both inlet and exhaust valves.

I C Engine Terminolgy:

Bore: Inner diameter of the engine cylinder is called Bore.Stroke: Linear distance traveled by the piston when it moves from one end of the cylinder to the other end. It is equal to twice the radius of the crank.Cover end or Top dead Centre (TDC): Extreme position of the piston nearer to the cylinder head is called TDC.Crank end or Bottom dead centre (BDC): Extreme position of the piston near to the crank is called BDC.

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Clearance Volume (Vc): Volume of the cylinder above the top of the piston when the piston is at the TDC is called clearance volume. It is denoted by Vc.

Swept Volume or Piston displacement or stroke volume (Vs): Volume swept by the piston as it moves from BDC to TDC is called swept volume.Compression Ratio: It is the ratio of the total cylinder volume to the clearance volume.Total cylinder volume = Stroke volume + Clearance volume

Rc = (Vs + Vc) / VcPiston Speed: the average speed of the piston is called piston speed.

Piston speed = 2 L NN = speed of the engine in rpm

L= Stroke length in m

Four stroke petrol engine

Four stroke petrol engine or spark ignition engine all the events of the cycle i.e. suction, compression. Expansion and exhaust take place in two revolutions of the crankshaft i.e.720 degree of the crank rotation. Thus each stroke is of 180 degree crankshaft rotation. Therefore the cycle of operation for an ideal four stroke engine consist of the following stroke:

1. Suction or induction or intake stroke.

2-compression stroke.

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3-expansion or working or power stroke.

4-exhaust stroke.

Suction stroke:

The piston move downward by the crankshaft i.e. from TDC position to BDC position. The crankshaft is initially revolved either by the power generated by the electric starting motor or by the momentum of the flywheel. The inlet valve opens and the exhaust valve remains closed. The movement of piston increases the space, thereby reducing the pressure in the cylinder. The higher pressure of the outside atmosphere forces the charge i.e. air fuel & petrol) through the inlet valve into the cylinder. The operation completes till the piston reaches the BDC position where the inlet valve closes. During this period the crankshaft rotated through 180 degree i.e. half revolution. The suction stroke is represented by a line 5- 1 in the PV diagram.

Compression stroke:

The piston move from BDC position To TDC position and both the valves (inlet and exhaust) remain closed. The upward movement of the piston compresses the charge during suction stroke. Due to compression (adiabatic compression) the pressure and temperature of the air fuel mixture increases continuously and depend upon the compression ratio. This is shown by the curve 1-2 on PV diagram .When the piston is just reaching the end of compression stroke the spark occurs and the charge is ignited at constant volume. This is shown by the line 2-3 on PV diagram.

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Power stroke:

The increased high pressure of combustion exerts a greater amount of force on the piston and pushes it downward i.e. from TDC to BDC position with both the valves closed. This is the only stroke during which power is produced and thus it is named ad power stroke or working stroke. Also gases expand and do work on the piston. So the stroke is also called expansion stroke.The expansion of gases is adiabatic in nature and this adiabatic expansion is shown by the curve 3-4 o PV diagram .When the piston reaches BDC position the exhaust valve open and the pressure falls to slightly above the atmosphere pressure at constant volume and is shown by the line 4-1 on PVdiagram .

Exhaust stroke:

During the exhaust stroke the exhaust valves open and the inlet valve remain closed. Pressure of the burnt gases being slightly above the atmosphere pressure and the piston moves from BDC to TDC position. The upward movement of the piston pushes out the exhaust gases through the exhaust valve into the atmosphere. This is shown by the line 1-5 on PV diagram. when the piston reaches TDC the exhaust valve closes .Thus the cycle is completed.

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P-V Diagram of Petrol engine

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Four stroke Diesel engine Four stroke Diesel engine is similar to that of petrol engine except fuel injector is placed instead of spark plug. A fuel pump supplies the fuel oil to the injector at higher pressure.

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Suction stroke:

The piston move downward by the crankshaft i.e. from TDC position to BDC position. The inlet valve opens and the exhaust valve remains closed. The movement of piston increases the space, thereby reducing the pressure in the cylinder. The higher pressure of the outside atmosphere forces the air through the inlet valve into the cylinder. The operation completes till the piston reaches the BDC position where the inlet valve closes.. The suction stroke is represented by a line 0 - 1 in the PV diagram.

Compression stroke:

The piston move from BDC position To TDC position and both the valves (inlet and exhaust) remain closed. The upward movement of the piston compresses the air. Due to compression (adiabatic compression) the pressure and temperature of the air increases continuously and depend upon the compression ratio. This is shown by the curve 1-2 on PV diagram .When the piston is just reaching the end of compression stroke the fuel (Diesel) is injected in to the cylinder in the form of fine sprays by a fuel injector. The fuel ignites as it comes in contact with compressed air .combustion of fuel takes place at constant pressure and this is shown by the line 2-3 on PV diagram.

Power stroke:

The increased high pressure of combustion exerts a greater amount of force on the piston and pushes it downward i.e. from TDC to BDC position with both the valves closed. This is the only stroke during

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which power is produced and thus it is named ad power stroke or working stroke. Also gases expand and do work on the piston. So the stroke is also called expansion stroke. The expansion of gases is adiabatic in nature and this adiabatic expansion is shown by the curve 3-4 o PV diagram .When the piston reaches BDC position the exhaust valve open and the pressure falls to slightly above the atmosphere pressure at constant volume and is shown by the line 4-1 on PV diagram

Exhaust stroke:

During the exhaust stroke the exhaust valves open and the inlet valve remain closed. Pressure of the burnt gases being slightly above the atmosphere pressure and the piston moves from BDC to TDC position. The upward movement of the piston pushes out the exhaust gases through the exhaust valve into the atmosphere. This is shown by the line 1-0 on PV diagram. When the piston reaches TDC the exhaust valve closes .Thus the cycle is completed.

Two Stroke Petrol Engine

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Two stroke engine performs only 2 strokes to complete one cycle. In these engines there are no suction and exhaust strokes. Instead they are performed while the compression and the power stroke are in progress.In a 2 stroke engine, there are no valves, instead the cylinder is provided with ports .The ports act as valves and are open and closed by the moving piston. There are 3 ports viz, the inlet port for admitting charges into the crank case. Transfer port through which charge is transferred from the crankcase to the cylinder.Exhaust port through which gases are discharged out of the cylinder.Two stroke engine works on the principle of Otto cycle. The charge used in a 2 stroke petrol engine is a mixture of air and fuel and is supplied by a carburetor. The charge is ignited by a spark plug .The two different strokes performed are upward and downward stroke.Downward stroke: At the beginning of this stroke, the piston is at TDC .At this position air and petrol mixture is compressed to a high pressure and temperature. The inlet port will be opened and the fresh charge enters the crankcase .The compressed charge in the cylinder is ignited by means of an electric spark produced by the spark plug. As the combustion takes place, the burnt gases expand and exert a large force on the piston. Due to this piston moves from TDC to BDC.The power impulse is transmitted down through the piston to the crankshaft through the connecting rod. This causes the crankshaft to rotate at higher speeds. Thus work is obtained. The movement of piston from TDC to BDC uncovers the exhaust port and part of the burnt gases due to their own expansion escapes through the exhaust port.

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Further downward movement of the piston uncovers the transfer port. During the downward movement the charge in the crankcase is compressed by the piston. The compressed charge rushes into the cylinder through the transfer port .The charge enters the cylinder and expels the remaining exhaust gases through the exhaust port.

This process of removing / sweeping out the exhaust gas with the help of fresh charge is known as scavenging.

The top of the piston has a projection known as “Deflector” that deflects the fresh charge coming through the transfer port to move towards the top end of the cylinder. By this fresh charge will drive the entire burnt gases out of the cylinder.UPWARD STROKE: Piston moves from BDC to TDC.During this it covers the transfer port, there by stopping the flow of fresh charge in to the cylinder.Further movement of the piston covers the exhaust port and actual compression of the charge begins. Inlet port is opened and the upward movement creates suction in the crankcase. The fresh charge enters in to the crankcase through the inlet port. The compression of the charge in the cylinder continues till the piston reaches TDC.Thus one cycle is completed.

Comparison between PETROL and DIESEL enginePETROL DIESEL

Works on otto cycle/ constant volume cycle

Works on constant pressure or Diesel cycle.

Petrol/ gasoline is used as fuel Diesel is used as fuelMixture of air and petrol enters the cylinder during suction During

Only air enters the cylinder during suction stroke. At the end of compression stroke,

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suction stroke, a carburetor is used to supply air-petrol mixture in constantproportion.

the fuel is injected into the hot compressed air.

Fuel consumption is more Fuel consumption is less.Can be easily started in cold weather

Difficult to start in cold weather.

Due to light weight they run at high speed

Due to heavy weight they run at low/medium speed.

Ignition takes place by means of spark plug hence these are called (S.I) engines

The fuel is ignited as it comes in contact with the hot compressed air hence these are called compression ignition(C.I) engine.

Initial cost is less but running cost is high

Initial cost is high but running cost is low

Thermal efficiency is low due to lower compression ratio.

Thermal efficiency is high due to higher compression ratio.

Compression ratio(C.R) varies from 6:1 to 10:1

Compression ratio is high and varies from 16:1 to 22:1.

Light weight and occupies less space.

Heavy weight and occupies more space.

Quantitative method of governing employed.

Qualitative method of governing employed

Used in light duty vehicles such as cars, motorcycle etc.

Used in heavy vehicles like trucks, buses and heavy machinery

Pollution is less. Exhaust pollution is morePower developed is lower due to lower C.R Lower developed is higher due to higher C.R

Comparison b/w two stroke and four stroke engineFour Stroke Two Stroke

Requires 4 different strokes of piston Requires only 2 strokes of piston or

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or two revolution of crankshaft to complete one cycle of operation.

one revolution of crankshaft to complete one cycle of operation.

There is one power stroke for every 2 revolution of the crankshaft.

There is one power stroke for every revolution of the crankshaft

Heavy flywheel is required. Light flywheel is requiredThe charge directly enters the engine

cylinder

Charge first enters the crankcase and then transferred to the engine cylinder

The exhaust gases are pushed out of the cylinder by the piston.

Incoming fresh charge pushes the exhaust gasses out of the cylinder

Consists of inlet and out let value Consists of ports.Requires less cooling and lubrication since combustion takes place in alternate revolutions of crankshaft

Requires greater cooling and combustion takes place in every revolutions.

Less fuel consumption because fresh charge never mixes with exhaust gas.

More fuel consumption because fresh charge gets mixed up.

Mechanical efficiency is low Mechanical efficiency is highHeavy and bulky. Lighter and compact.Used in Cars, Jeeps, Buses, Aero planes, trucks and power generators.

Used in light vehicles such as Motor cycle ,3 wheelers, lawn movers etc

Noise will be less. Noise will be more.

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Mean Effective Pressure: (Pm)

Mean or average pressure acting on the piston throughout the power stroke.Indicated mean effective pressure of an engine is obtained from the indicated diagram. The indicated diagram is a p-v diagram for one cycle at the load, drawn with the help of an indicator fitted on the engine.

(Spring value of the spring used in × (Net area of the the indicator in bar/m) indicator diagram (a) in m2)

Pm = ------------------------------------------------------------------------------------- Length of the indicator diagram (l) in m

Pm = S a / l

Indicated power:The total power developed inside the engine cylinder is called indicated power; It is denoted by IP and expressed in KW (kilo watts)

IP = n Pm L A N K (10/6) KW

For four stroke engineIP = n Pm L A × N (10/6) (1/2) kw

For two stroke engine

IP = n Pm L A × N (10/6) kwPm = Mean effective Pressure

L= Stroke length in mA = cross sectional area of the cylinder in mA = d2/4: d= diameter of cylinder in m2

N = Speed of the crankshaft in rpmN = number of cylinders

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Brake Power: The net power available at the crankshaft is called brake power.

BP = 2 N T /60 *1000 kWN= speed of the engine in RPMT = Torque in N-mTorque is measured by using either belt or rope brake dynamometerBelt Dynamometer

Torque (T) = Force * DistanceT = (T1 – T2) * R

T1 = tension in tight side of the belt in NT2 = tension in slack side of the belt in N

R= radius of the belt in mRope brake dynamometer

T= effective brake load * drum radiusT = (W – S) R N – m

W =suspended weight in NS = spring balance reading in N

R = radius of the pulley measured to the center of rope in mR = (D+ d)/ 2

D = diameter of pulley in m and d = diameter of the rope in m

Friction Power: The amount of power lost due to friction of the moving parts inside the engine cylinder is called friction power. Friction power is the difference between indicated power and brake power

FP = IP - BP kW19



Mechanical efficiency: It is defined as the ratio of brake power to the indicated power

mech = (BP/IP) *100Thermal efficiency: defined as the ratio of power output to the heat supplied by the combustion of fuel.

th = (power output/heat supplied ) *100heat supplied = (mf ) * CV

CV = calorific value of fuel in KJ/kgIndicated thermal efficiency = Ith = { IP/( mf )* CV )} *100Brake thermal efficiency =Bth = { BP/( mf )* CV )} *100

Specific fuel consumption: Specific fuel consumption is a term used to describe the fuel efficiency of an engine design. It measures the amount of fuel needed to provide a given power for a given period.Indicated SFC = (Fuel consumed in Kg/hr) / IP in Kw kg/k W hr

Brake SFC = (Fuel consumed in Kg/hr) / BP in Kw kg/k W hr

Problems: 1. The following observations were recorded during a test on a 4 – S engineBore = 2 cmStroke = 40 cmCrank speed = 250 rpmNet load on brake drum = 700 NDiameter of brake drum = 2 mIndicated M E P = 6 bar

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Fuel consumption = 0.0013 kg/sSp. Gravity of fuel = 0.78CV of fuel =43900 kJ/kgDetermine a) BP b) IP c)FP d)mechanical efficiency e)Indicated thermal efficiency f) Brake thermal efficiency

Solution :Data: D= 25 * 10-2 m A = d2/4 = 0.049 m2

L = 40* 10-2 m = 0.4 mN = 250 rpmNet load on brake drum = (W – S) R = 700 NDiameter of brake drum =D = 2 m R= 1m Pm = 6 barFuel consumption = 0.0013 kg/sSp.gravity = 0.78C V =43900 kJ/kg

a) Brake power BP = 2 N T /60 *1000 kW

T = (W – S) R N – m = 700 *1= 700 N-m

BP = (2 * 250 * 700) /(60 *1000) kW BP= 18.32 kWb) Indicate power IP

Four stroke engine k =1/221



n =1 single cylinder IP = n Pm L A × N (10/6) (1/2) kW

IP = (1* 6 * 0.4 * 0.049 * 250 * 10)/ (6*2) kWIP = 24.5 kW

c) Friction power FPFP = IP - BP kW

FP=24.5 – 18.32 = 6.18 kWFP = 6.18 kW

d)Mechanical efficiency(mech) :

mech = (BP/IP) *100mech = (18.32/24.5) *100

mech = 74.7%e) Indicated thermal efficiency (Ith )

Indicated thermal efficiency = Ith = { IP/( mf )* CV )} *100 mf = sp.gravity * Fuel consumption = 0.78 * 0.0013 = 1.014*10-3

Ith = 24.5 / (1.014*10-3)* (43900) = 55%

f) Brake thermal efficiency (Bth ):

Brake thermal efficiency =Bth = { BP/( mf )* CV )} *100 Bth = 41.1%

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2. A single cylinder 4-S I c engine has a swept volume of 6 liters and runs at a rated speed of 300 rpm .At full load , the torque developed was measured with a belt dynamometer whose pulley diameter is 1m . The tension in the tight side and slack side of the belt is 700 N and 300N rep. kg of fuel was consumed in one hour .The indicated mean effective pressure is 6 bar and the CV of the fuel is 42000kJ/kg.Calculate the BP,IP,mechnical efficiency, indicated thermal efficiency, brake thermal efficiency .Solution:Data:

n=1k=1/2 for 4 –S engineN=300 rpm

Pm = 6 barSwept volume = L A =6*10-3 m3

Diameter of pulley = D =1mR =Radius =0 .5 mT1 = 700 NT2 = 300 N

T = (T1 – T2) * RT = (700 – 300) * 0.5 = 200 N – m

mf = 4kg/hr = 4/60 *60 = 1.11*10-3 kg/sec CV = 42000 kJ/kga) BP

BP = 2 N T /60 *1000 kWBP = 2 * * (300) * (200)/ (60*1000) kW

BP = 6.28 kW

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b) IPFour stroke engine k =1/2n =1 single cylinder

IP = n Pm L A × N (10/6) (1/2) kWIP = (1* 6 * 6*10-3 * 300 * 10)/ (6*2) kW

IP = 9 kWc) Mechanical efficiency(mech) :

mech = (BP/IP) *100mech = (6.28/9) *100mech = 69.7 %

d) Indicated thermal efficiency (Ith )

Indicated thermal efficiency = Ith = { IP/( mf )* CV )} *100 Ith = 9 / (1.1*10-3)* (42000) = 19.4 %

Ith = 19.4 %e) Brake thermal efficiency (Bth ):

Brake thermal efficiency =Bth = { BP/( mf )* CV )} *100Bth = { 6.28/(1.1*10-3)* (42000) } *100

Bth = 13.59%f) Brake specific fuel consumption

BSFC = Fuel consumed in kg/hr /BpBSFC= 4/6.28 = 0.63 kg/BP –hr

BSFC = 0.63 kg/BP –hrQuestion Bank

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1. Explain with a neat sketch naming all parts, the working of single cylinder four stroke petrol engine.

2. How are IC engines classified?3. with the help of a p-v diagram ,explain the working of 4-stroke

diesel engine.4. Differentiate between SI and CI engines.5. Differentiate between 4S and 2S engine6. Name the thermodynamic cycle for petrol and diesel

engines .Draw the p-v diagram of the same indicating various processes.

7. with the help of a p-v diagram ,explain the working of 2-stroke petrol engine

8. Explain with a neat sketch the various parts of IC engine9. Define IP,BP,FP10. Define Mean effective pressure and indicated thermal

efficiency.11. what are EC and IC engines give examples.

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Unit 3 Internal Combustion Engines

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