INTERNAL COMBUSTION ENGINE

Part #3

Engine Efficiency and Performance:

Power of Engine:

✓ Power is defined as the rate of doing work and is equal to the product of force and linear velocity or the product of torque and angular velocity.

✓ Thus, the measurement of power involves the measurement of force (or torque) as well as speed.

The force or torque is measured with the help of a dynamometer and the speed by a tachometer 

                          
✓ The power developed by an engine and measured at the output shaft is called the brake power(bp).

✓ The total power developed by combustion of fuel in the combustion chamber is, however, more than the (bp) and is called indicated power (ip).

✓ Of the power developed by the engine, i.e. ip, some power is consumed in overcoming the friction between moving parts, called friction power (fp).

Mean Effective Pressure:

✓ Mean effective pressure is defined as a hypothetical/average pressure which is assumed to be acting on the piston throughout the power stroke.

✓ Mean effective pressure is also written in terms of power as :

where, L = Length of the stroke,

A = Area of the piston,

N = Rotational speed of the engine (It is N/2 for four stroke engine )

 k = Number of cylinders.

 ✓ If the mean effective pressure is based on bp it is called the brake mean effective pressure (bmep) and if based on ip it is called indicated mean effective pressure (imep). Similarly, the friction mean effective pressure (fmep) can be defined as

                  

Specific Fuel Consumption:

✓ Specific fuel consumption is defined as the amount of fuel consumed for each unit of brake power developed per hour.

where, mf = Mass of fuel supplied, kg/sec

Thermal Efficiency:

✓ Thermal efficiency of an engine is defined as the ratio of the output to that of the chemical energy input in the form of fuel supply. It may be based on brake or indicated output. It is the true indication of the efficiency with which the chemical energy of fuel (input) is converted into mechanical work.

where, Cv = Calorific value of fuel, kJ/kg, and

mf = Mass of fuel supplied, kg/sec.

Mechanical Efficiency:

✓ The difference between the ip and bp is the indication of the power lost in the mechanical

components of the engine (due to friction) and forms the basis of mechanical efficiency; which is

defined as follows

Volumetric Efficiency:

✓ Volumetric efficiency of an engine is an indication of the measure of the degree to which the engine fills its swept volume. It can be defined as the ratio of the actual volume inhaled during suction stroke measured at intake conditions to the swept volume of the piston.

SOME QUESTION FROM THESE TOPICS:

1.A two stroke IC engine has a stroke length of 120 mm and cylinder diameter 80 mm. Its mean effective pressure is 4 × 105 N/m2 and the crankshaft speed is 1500 r.p.m. What is its indicated power (in kW)?
(A) 1.92
(B) 24.12
(C) 60.3
(D) 6.03

3.The chemical correct stoichiometric ratio for petrol is
(A) 18 : 1
(B) 11 : 1
(C) 15 : 1
(D) 14.7 : 1

2.The total power developed by combustion of fuel in the combustion chamber is called
a) brake power
b) indicated power
c) mean effective pressure
d) none of the mentioned

Fuels:

AIR - FUEL RATIO:

✓ Fuel-air ratio (F/A) is the ratio of the mass of fuel to the mass of air in the fuel-air mixture. Air fuel ratio (A/F) is reciprocal of fuel-air ratio. Fuel-air ratio of the mixture affects the combustion phenomenon in that it determines the flame propagation velocity, the heat release in the combustion chamber, the maximum temperature and the completeness of combustion.

✓ Rich air – fuel mixture mean, the amount of air less than stoichiometric air-fuel ratio.

✓ Lean air – fuel mixture mean, the amount of air greater than stoichiometric air-fuel ratio.

✓ stoichiometric air-fuel ratio is 14.7 : 1 (14.7 is air, and 1 is fuel) for Petrol engine

Air fuel ratio for different engine condition:

At starting condition(cold/idle start):

✓ At the starting condition, all engine components such as cylinder head, cylinder block, intake manifold, are on the cold temperature so Rich air – fuel mixture needed to get the best perform to starting engine.

 At warning up condition:

✓ In this case also, The rich air – mixture, still needed.

At accelerate condition:

✓ Rich air – fuel mixture needed.

At cruising and constant speed condition:

✓ Near stoichiometric or lean air-fuel ratio needed.

At heavy load condition:

✓ A rich mixture in the order of 11:1 is required for idle heavy load.

✓ The best fuel economy is obtained with a 15:1 to 16:1 ratio.

✓ while maximum power output is achieved with a 12.5:1 to 13.5:1 ratio.

Some question from these topics:

1.Lean air fuel mixture is required for
(A) Idling
(B) Cruising
(C) Acceleration
(D) Starting

Octane number:

✓ It is measure of the ability of a gasoline or petrol fuel to resist knocking when ignited in a  mixture with air in the cylinder of an internal-combustion engine.

✓ Octane number denotes the percentage (by volume) of iso-octane (a type of octane) in a combustible mixture (containing iso-octane and normal-heptane) .

✓ The higher the octane number, the more compression the fuel can withstand before detonating (igniting).

✓ High octane numbers were achieved by adding lead tetraethyl to the fuel (the 'leaded gas'), a pollutant that contributes to lead poisoning (see Lead).

✓ In India, the minimum octane rating for fuels as prescribed by Government through Bharat 3 emission norms is 91 Octane.

Cetane number:

✓ Cetane number of a diesel fuel is indicative of its ignition characteristics.

✓ Higher the cetane number better it is in its ignition properties (less knocking).

✓ Cetane number denotes the percentage (by volume) of Cetane in a combustible mixture (Diesel).

✓ Normal modern highway diesels run best with a fuel rated between cetane number 45 and 55.

Some question from these topics:

1.For complete burning of 1 kg of carbon, the air required will be about
(A) 2.67 kg
(B) 12.7 kg
(C) 11.6 kg
(D) 14.5 kg

2.Cetane number of a fuel is a measure of its
(A) Viscosity
(B) Volatility
(C) Ignition quality
(D) API specific gravity

3.Octane number of iso-octane is:
(A) 50
(B) 70
(C) 0
(D) 100

4.The commercially available petrol in India has an octane rating of
(A) 85-90
(B) 20-30
(C) 40-50
(D) 60-75

Supercharging in IC engine:

✓ Supercharging is the artificial raising of the pressure of the air above the atmospheric pressure to increase the density of the air to improve the volumetric efficiency of the engine.

✓ It is a method to increase power output of the engine without increasing the weight of the engine.

✓ As air density can be increase hence more air(by mass) can be available for a given suction volume.



Advantages of Supercharging:

✓ High power output

✓ Better atomization of fuel

✓ Better mixing of air & fuel

✓ Better scavenging of products

✓ Quicker acceleration of vehicle

✓ Reduced exhaust smoke

✓ More complete and smoother combustion

✓ Reduced specific fuel consumption

Limitations of Supercharging:

✓ Increased thermal stresses

✓ Increased cooling requirements of piston and valves

✓ Increased heat losses due to turbulence

✓ Increased gas loading

Lubrication of the IC Engines:

Main parts to be lubricated in case of I.C. engines are:

1. Cylinder walls

2. Crank shaft main bearings

3. Big end bearing of a connecting rod i.e. crank pin

4. Small end bearing of a connecting rod i.e. gudgeon pin

Types of lubrication systems used:

The wet sump lubrication system:

The wet sump lubrication system, the lubricating oil is drawn from the engine sump which contains the oil. The oil is placed in the sump and it is drawn by pump through the strainer. The sump contains the lubricating oil and supplies continuously to system.

The wet sump lubrication system:

In this system, the engine sump remains dry and the oil is placed in the tank near to the sump. The oil is drawn from the tank and supplied by pump to the lubrication system.

Mist lubrication system:

This system is used in the two stroke engine for lubrication. In this lubrication system, the oil in certain proportion of fuel is pre mixed in the fuel tank and lubricating oil is supplied along with the fuel to the crank case of the engine. The lubricating oil gets deposited on the surfaces of the main pin, crank pin and piston pin bearing and piston rings.

Engine Cooling System:

The cooling system has four primary functions :

➢ Remove excess heat from the engine.

➢ Maintain a constant engine operating temperature.

➢ Increase the temperature of a cold engine as quickly as possible. 

➢ Provide a means for heater operation (warming the passenger compartment).

➢ Approximately 25% of heat of combustion is lost to the jacket cooling water used for cooling

Some question from these topics:

1.Which of the following engine can be associated with heterogeneous combustion?
(A) Spark ignition
(B) Compression ignition
(C) Both spark ignition and compression ignition
(D) Neither spark ignition nor compression ignition

2.What approximate percentage of heat of combustion is lost to the jacket cooling water
(A) 5%
(B) 10%
(C) 15%
(D) 25%

3.Using lubricants on engine parts is an example of reducing
(A) Motion
(B) Force
(C) Acceleration
(D) Friction

4.How self-ignition temperature of petrol be compared to that of diesel?
(A) Higher
(B) Lower
(C) Same
(D) Insufficient data


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