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Page 1 of 15 Internal Combustion Engines (IC Engines) The internal combustion engine (IC Engine) is a heat engine that converts heat energy (chemical energy of a fuel) into mechanical energy (usually made available on a rotating output shaft). Applications of IC Engines: Mainly used as ‘prime movers’, e.g. for be the propulsion of a vehicle i.e., car, bus, truck, locomotive, marine vessel, or airplane. Other applications include stationary saws, lawn mowers, bull-dozers, cranes, electric generators, etc. Classifications of IC Engines: IC engines can be classified according to: 1. Number of cylinders – 1, 2, 3, 4, 5, 6 to 16 cylinder engines. 2. Arrangement of cylinders – Inline, V-type, Flat type, etc. 3. Arrangement of valves and valve trains – In-block camshaft, OHC, DOHC, etc. 4. Type of cooling – Air-cooled, Water-cooled, etc. 5. Number of strokes per cycle – 2-stroke, 4-stroke engines. 6. Type of fuel burned – Petrol, diesel, CNG, etc. 7. Method of ignition – Spark Ignition (SI), Compression Ignition (CI). 8. Firing order – 1-3-4-2, 1-2-4-3, etc. 9. Primary mechanical motion – Reciprocating, rotary. Fig. 1: Major Components of a reciprocating single cylinder Petrol Engine.

Internal Combustion Engines (IC Engines)

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  • Page 1 of 15

    Internal Combustion Engines (IC Engines)

    The internal combustion engine (IC Engine) is a heat engine that converts heat energy (chemical energy of a fuel) into mechanical energy (usually made available on a rotating output shaft).

    Applications of IC Engines:

    Mainly used as prime movers, e.g. for be the propulsion of a vehicle i.e., car, bus, truck, locomotive, marine vessel, or airplane. Other applications include stationary saws, lawn mowers, bull-dozers, cranes, electric generators, etc.

    Classifications of IC Engines:

    IC engines can be classified according to:

    1. Number of cylinders 1, 2, 3, 4, 5, 6 to 16 cylinder engines. 2. Arrangement of cylinders Inline, V-type, Flat type, etc. 3. Arrangement of valves and valve trains In-block camshaft, OHC, DOHC, etc. 4. Type of cooling Air-cooled, Water-cooled, etc. 5. Number of strokes per cycle 2-stroke, 4-stroke engines. 6. Type of fuel burned Petrol, diesel, CNG, etc. 7. Method of ignition Spark Ignition (SI), Compression Ignition (CI). 8. Firing order 1-3-4-2, 1-2-4-3, etc. 9. Primary mechanical motion Reciprocating, rotary.

    Fig. 1: Major Components of a reciprocating single cylinder Petrol Engine.

  • Page 2 of 15

    Fig. 2: Main Geometric parameters of a reciprocating IC Engine. Four-stroke Petrol Engine.:

    A 4-stroke petrol engine operates on air standard Otto cycle. It completes the Otto cycle in 4 strokes (4 TDC to BDC movements of the piston), namely, (1) Suction Stroke, (2) Compression Stroke, (3) Power Stroke, (4) Exhaust Stroke. Otto cycle shown below consists of four processes: 1 2 : Isentropic Compression Process 2 3 : Constant Volume Combustion 3 4 : Isentropic Expansion Process 4 1 : Constant Volume Blowdown

    Fig. 3: Four-strokes of an IC (Petrol) Engine.

    Problem-1: A four cylinder car engine has bore x stroke = 79 mm x 77 mm.

    What is the capacity of the engine in cc? Solution: Capacity in cc = N.(/4).B2. S Here, N= number of cylinders B= bore diameter in cm S= stroke length in cm Therefore, Engine Capacity in cc = 4 x (/4) x (7.9)2 x (7.7) = 1509 1500 cc Ans.

  • Page 3 of 15

    For a petrol engine working on air standard Otto cycle,

    Compression ratio, 1 4

    2 3

    d c

    c

    v v v vrv v v+

    = = =

    For an isentropic process, x y ,

    11

    kkk

    yx x

    y y x

    vT pT p v

    = =

    Otto cycle Efficiency = Work Output / Heat Supplied = (Heat Supplied Heat Rejected) / Heat Supplied,

    or, in out

    in in

    Q QWoutQ Q

    = =

    or, 3 2 4 1 4 1

    3 2 3 2

    ( ) ( ) 1( )

    a v a v

    a v

    m c T T m c T T T Tm c T T T T

    = =

    So, 4 1

    3 2

    1 T TT T

    = (1)

    Now for isentropic compression process,1 2 :

    1 11 2

    2 1

    1k kT v

    T v r

    = =

    or, 1

    2 1kT T r = (2)

    Fig. 4: Air standard Otto cycle for petrol engine.

  • Page 4 of 15

    Again, for isentropic expansion process, 3 4 : ( )1

    13 4

    4 3

    kkT v r

    T v

    = =

    or, 3

    4 1k

    TTr

    = (3)

    Putting (2) and (3) into (1), we get,

    311

    13 1

    1k

    k

    T Tr

    T T r

    =

    or,

    13 1

    1

    13 1

    1

    k

    k

    k

    T T rr

    T T r

    =

    Therefore, 111Otto kr

    = where, p

    v

    ck

    c= and k = 1.4 for air.

    So we see, the efficiency of the petrol engine depends only on the compression ratio r.

    For a diesel engine working on air standard Diesel cycle,

    Compression ratio, 1

    2

    d c

    c

    v v vrv v+

    = = and Fuel cut-off ratio, 3

    2

    cut c

    c

    v v vv v

    += =

    Diesel cycle Efficiency = Work Output / Heat Supplied = (Heat Supplied Heat Rejected) / Heat Supplied,

    or, in out

    in in

    Q QWoutQ Q

    = =

    Diesel cycle shown above consists of four processes: 1 2 : Isentropic Compression Process 2 3 : Constant Pressure Combustion Process 3 4 : Isentropic Expansion Process 4 1 : Constant Volume Blowdown

    Fig. 5: Air standard Diesel Cycle for Diesel Engine

    Diesel Engine working on diesel cycle

  • Page 5 of 15

    or, 3 2 4 1 4 1 4 1

    3 2 3 2 3 2

    ( ) ( ) 11 1( )

    a p a v v

    a p p

    m c T T m c T T c T T T Tm c T T c T T k T T

    = = =

    So, 4 1

    3 2

    11 T Tk T T

    = (1)

    Now for isentropic compression process,1 2 :

    1 11 2

    2 1

    1k kT v

    T v r

    = =

    or, 1

    2 1kT T r = (2)

    For constant pressure combustion process, 2 3 : 2 2

    3 3

    1T vT v

    = =

    or, 1

    3 2 1kT T T r = = (3)

    Again, for isentropic expansion process, 3 4 :

    1 13 4

    4 3

    k kT v rT v

    = =

    34 4 1

    3 2 2 2 3 2

    1since, ( / )

    vv v v rv v v v v v

    = = =

    or, 1 1 1

    3 14 11 1

    .k k k kk k

    T T rT Tr r

    = = = (4)

    Putting (2) and (3) into (1), we get, 1 11 1

    1 1

    11k

    k k

    T Tk T r T r

    =

    or, 1

    11

    ( 1)11( 1)

    k

    k

    Tk T r

    =

    Therefore, 11 11

    ( 1)

    k

    Diesel kr k

    =

    where, p

    v

    ck

    c= and k = 1.4 for air.

    So we see, the efficiency of the diesel engine depends not only on the compression ratio but also on the fuel cut-off ratio.

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    Comments: The fuel cut-off ratio of the diesel engine is usually greater than 1 and k is 1.4 for air so the expression in square bracket is always greater than 1. So for the same compression ratio, petrol engine is more efficient than the diesel engine. But usually the compression ratio of diesel engine is much higher than that of petrol engine (from 8 to 12). Therefore, the diesel engine is more efficient due to it higher compression ratio (from 15 to 23). Problem-2: Calculate the air standard cycle efficiencies of an Otto cycle engine and a Diesel cycle with a compression ratio of 10:1. Take = 1.5 for the diesel engine.

    Solution: Otto cycle efficiency, otto = 1 rk 1

    1

    = 1 10

    114.1 = 1 10

    14.0 = 0.6019 = 60.19% Ans.

    Diesel cycle efficiency, diesel = 1

    )1(

    111

    k

    k

    kr

    = 1

    )15.1(4.1

    15.1

    101

    4.1

    14.1 = 0.5654 = 56.54% Ans.

    Problem-3: Calculate the air standard Diesel cycle efficiency of the engine with a compression ratio of 11:1; if the fuel supply is cut-off at 6% of the stroke (or swept or displacement volume). Solution:

    Given, 0.06cutstroke

    vv

    =

    Now from Fig.5, we can write,

    3 2

    4 2

    0.06v vv v

    =

    or, 3 2

    1 2

    0.06v vv v

    =

    or, 2 3 2

    2 1 2

    ( / 1) 0.06( / 1)

    v v vv v v

    =

    or, 1 0.061r

    =

    or, 1 0.06

    11 1

    =

    or, 1.6 = Diesel cycle efficiency,

    diesel = 1

    )1(

    111

    k

    k

    kr

    1.4

    1.4 1

    1.6 111 0.57531.4(1.6 1)11

    = =

    = 57.53 % Ans.

  • Page 7 of 15

    Actual Indicator Diagram of a 4-stroke Petrol Engine

    Valve Timing Diagram for a 4-stroke Petrol Engine

    Valve overlap: The duration (10o + 10o = 20o) when both the inlet and exhaust valves remain open is called valve overlap. Spark Advance: The ignition is initiated 20 30o before TDC. This is called spark advance. Differences between Petrol and Diesel engines

  • Page 8 of 15

    Two-Stroke Engines: Two strokes engines (both petrol and diesel engines) completes the cycle (both Otto and Diesel cycle) in just two strokes of the piston. In moving from BDC to TDC, both the suction and compression occur whereas, during TDC to BDC movement, both power and exhaust occur thus completing the total cycle. In Brief: (a) and (b): Up-stroke of the piston : Suction and Compression. (c) and (d) : Down stroke of the piston : Expansion and Exhaust.

    drilling machines, etc.

  • Page 9 of 15

    Fig. 6: Two-stroke IC (Petrol) Engines. Scavenging in 2-Stroke Cycle Engine: At the end of the expansion stroke, the exhaust port opens and allows the exhaust gas to exit. As the piston starts moving down, air-pressure in the crank case increases and air, through the scavenging port, enters the cylinder (combustion chamber) and pushes out the burnt gas clear from the cylinder. This is referred to as scavenging. Differences between 2-stroke and 4-stroke engines

    4-stroke Engine 2-stroke Engine 1. The intake, compression, combustion and exhaust occur in two upward and two downward strokes of the piston.

    1. All four events are accomplished in one downward stroke, and one upward stroke.

    2. Needs complicated valve train arrangement for intake and exhaust strokes.

    2. Intake and exhaust are both integrated into the compression and combustion movement of the piston, eliminating the need for valves.

    3. Outputs power once in every two revolutions of the crankshaft.

    3. The engine delivers power on every revolution.

    4. The engine is heavier for the same power rating, i.e., low power to weight ratio.

    4. Higher power-to-weight ratio because it is much lighter.

    5. More expensive than the 2-stroke engines.

    5. Less expensive because of its simpler design.

    6. It has limited orientation if oil is to be retained in the sump.

    6. It can be operated in any orientation because it lacks the oil sump

    7. More fuel efficient, less noisy, less polluting and longer lifespan.

    7. Less fuel-efficient because of the simpler design, resulting in poorer mileage than a four stroke engine.

    8. Less noisy. 8. Twice as much noisy. 9. Less polluting. 9. Very much polluting. 10. Usually lasts longer. 10. Does not last very long.

    (a) Suction (b) Compression (c) Expansion (Power) (d) Exhaust

  • Page 10 of 15

    Engine Knocking: Engine knocking (also called detonation) is a sudden blow on the piston just like a hammering. Knocking occurs due to localized ignition inside the combustion chamber. This can be explained thus: at the end of the compression stroke the sparkplug gives

    electric spark to initiate ignition of the air fuel charge. Ignition takes place and very quickly advances like a heat wave to all corners of the combustion chamber. Consequently, localized ignition starts before the flame reaches it. Therefore, knocking is a post ignition phenomenon. Detonation or knocking is harmful for the engine and causes the engine-running shaky. Both high combustibility of fuel and the

    Fig. 7: Engine Knocking high compression ratio are responsible for knocking. To stop engine knocking generally a special fuel or a chemical (tetraethyl lead) is mixed with gasoline. Mixing of a small amount satisfactorily stops knocking. High Octane rating also prevents engine knocking. Octane Number The property that describes how well petrol will or will not self-ignite is called the octane number of petrol or just octane. The higher the octane number of petrol, the less likely it will self-ignite. Engines with low compression ratios can use petrol with lower octane numbers, but high-compression engines must use high-octane petrol to avoid self-ignition and knock. Common octane numbers (anti-knock index) for petrol used in cars range from 87 to 95, with higher values available for special high-performance and racing engines. A 93-octane petrol is more knock resistant than an 89-octane petrol. Reciprocating SI aircraft engines usually use low-lead fuels with octane numbers in the 85 to 100 range. Cetane Number In a compression ignition engine, self-ignition of the air-fuel mixture is a necessity. The correct fuel must be chosen which will self-ignite at the precise proper time in the engine cycle. It is therefore necessary to have knowledge and control of the ignition delay time of the fuel. The property that quantifies this is called the cetane number. The larger the cetane number, the shorter is the ID and the quicker the fuel will self-ignite in the combustion chamber environment. A low cetane number means the fuel will have a long ID. Normal cetane number range is about 40 to 60.

  • Page 11 of 15

    Engine Subsystem: The main engine subsystems are:

    1. Fuel System ---- carburetor, Electronic Fuel Injection (EFI), etc. 2. Lubrication System ---- splashed lubrication, pump-forced lubrication, etc. 3. Cooling System ---- air-cooling system, water-cooling system. 4. Ignition System ----Spark Plug Ignition (SI), Compresion Ignition (CI). 5. Starting System ---- Battery/Starting motor, manual cranking, compressed air

    motor, etc. Carburetor: Carburetor is a vital component of the fuel system of a conventional petrol engine. It composed of 4 main parts:(i) Air horn, (ii) Venturi, (iii) Fuel nozzle and (iv) Throttle valve Fig. 8: Engine Carburetor The above figure shows how petrol is practically atomized while passing through the carburetor venture. At the venture, the passage is the smallest, resulting in high velocity of the air and high kinetic energy as well. Since total energy must be unchanged, the rise in kinetic energy is balanced by the fall in pressure (vacuum) at the venture. Therefore, the air flowing through the venture creates a sort of vacuum (lower than atmospheric pressure) at the other end (downstream) of the narrow portion of the passage, which causes suction (air pushes the petrol) of the liquid petrol from the float bowl.

    Float bowl

    Petrol pumped from tank

  • Page 12 of 15

    Fuel Injection System A fuel injection system is used usually in diesel engines to inject diesel fuel at the end of compression stroke at a very high pressure. A fuel injection pump and its operation are shown below:

    Fig. 9: Fuel injection pump and its control unit The plunger is operated by a cam as shown. As the cam pushes, the plunger moves upward against a heavy spring and the fuel is delivered towards injection nozzle. The fuel then enters from the nozzle into the combustion chamber under high pressure. It will be seen that there is a spiral groove on the body of the plunger to control the amount of fuel injection. There is a hole (feed hole) on the body of the plunger connected to its central hole on top. The plunger can be rotated right way or left way (clock wise or anti clockwise) with the help of a rack and pinion mechanism fitted with the body of the plunger. Thus the actual amount of fuel per stroke as desired by the operator can be injected. The rack and pinion again is operated by the engine governor.

  • Page 13 of 15

    Engine Performance

    Efficiency: Mechanical Efficiency = Brake Power (bhp)/ Engine Power (hp) Thermal Efficiency = Engine Power (hp) / Fuel Power (hp) Overall Efficiency = Mechancal x Thermal Brake Specific Fuel Consumption (bsfc): Fuel consumed per unit power generation per unit time. Unit kg/kW-hr. Bsfc = brake power/ fuel consumed per unit time An Engine is not 100% efficient (typical efficiency is about 30%). Because it has some losses. Heat balance is as follows: Heat loss to cooling water

    30%

    Heat loss to exhaust gases

    30%

    Heat loss to lubricating oil

    5%

    Heat loss due to friction of mechanical components

    5%

    Useful brake power available from engine 30% Total Heat Input to the engine 100%

    Mean effective pressure

    Generally, the mean effective pressure is the ratio of the net work done to the displacement volume of the piston. It is a valuable measure of an engine's capacity to do work that is independent of engine displacement.

    It is defined as,

    Here, pmep = mean effective pressure, Pa T = torque, N-m Vd = displacement volume, m3 nc = number of revolutions per cycle (for a 4-stroke engine nc = 2, for a 2-stroke engine nc = 1 )

  • Page 14 of 15

    This is useful for comparing engines of different displacements (a specific torque of sorts, i.e. torque per unit displacement). Mean effective pressure is also useful for initial design calculations; that is, given a torque, we can use standard mep values to estimate the required engine displacement.

    Brake Mean Effective Pressure (bmep) is, calculated by putting the measured dynamometer torque into the above equation.

    For spark-ignition engines : maximum values are in the range 8.5 to 10.5 bar (850 to 1050 kPa; 125 to 150 psi), at the engine speed where maximum torque is obtained. At rated power, bmep values are typically 10 to 15% lower.

    For four-stroke diesels: the maximum bmep is in the 7 to 9 bar range (700 to 900 kPa; 100 to 130 psi).

    Problem-4: A four-stroke engine producing 160 Nm from 2 litres of displacement. What will be it brake mean effective pressure (bmep) ?

    Solution: bmep is given by,

    So, bmep = (160 Nm) (4)/(0.002 m)

    = 1,005,000 N/m2 =1005 kPa (10.05 bar).

    Here, T = 160 N-m Vd = 2 liters = 2 x 10-3 m3 nc = 2 for a 4-stroke engine

    Problem-5: If the same engine (i.e., four-stroke, 2 liters) as above produces 76 kW at 5400 rpm (90 Hz), Find its bmep.

    We have,

    Power, P = T

    T=P/ =(76 x 103)/565.5=134.4 N.m

    Now,

    So, bmep = (134.4 Nm) (4)/(0.002 m)

    = 844460 N/m2 =844.5 kPa (8.34 bar).

    Here, P = 76 x 103 W =2N/60=2(5400)/60 =565.5 rad/s Where,

    Vd = 2 liters = 2 x 10-3 m3 nc = 2 for a 4-stroke engine

  • Page 15 of 15

    Problem-6: A 4-Cylinder, 2-stroke IC engine has the following particulars: engine speed = 3000 rpm, bore = 120 mm, crank radius = 60 mm, mechanical efficiency = 90% and the engine develops 75 bhp. Calculate the swept volume and mean effective pressure (MEP).

    Mechanical efficiency, )()(

    ihprEnginePowebhpBrakePower

    =

    Or, 0.9 = P75 , i.e., P = 83.33 hp

    Now, Engine Power, P = T

    Here, P = 83.33 hp = 83.33 x 746 W = 62166.67 W =2N/60=2(3000)/60 =314.16 rad/s

    T=P/ =(62166.67)/314.16=197.88 N.m

    We get, Mean Effective Pressure (MEP or Pmep) as follows:

    where,

    Vd = N.(/4).B2. S Here Stroke, S = 2 x crank radius = 2 x 0.06 m = 0.12 m Vd = 4.(/4).(0.12)2(0.12) = 5.43 x 10-3 m3 = 5.43 liter nc = 1 for a 2-stroke engine Therefore,

    MEP = (197.88 Nm) (2)/(0.00543 m)

    = 228971.77 N/m2 =228.97 kPa