I C ENGINES CLASS NOTES
Need for IC engines in automobiles and power
generation is increasing day by day. Therefore
design an efficient and economical engines.
Judge the performance of the engine from the
various parameters listed below:
Indicated Horse Power, IHP
Brake Horse Power
Frictional Horse Power
Relative fuel Air Ratio
Brake Specific Fuel Consumption
Mean Effective Pressure
Indicated thermal efficiency
1. INDICATED HORSE POWER
An IC engine produces power by the combustion of fuel.
Power is rate of doing work. Power is the product of torque
and angular velocity. Measure the torque by a
Dynamometer and RPM by a Tachometer. Engine produced
total power is Indicated horse power (IHP). The NET output
useful power available from the engine is the brake horse
IHP =n pm L A N” / 60
n is the number of cylinders
pm = mean effective pressure on the piston, N/m2
L= length of stroke, m
A= area of the piston, m2
But in 2 Revolutions, one cycle is completed
N” = N/2
IHP is indicated horse power in Watts
(ii) TORQUE, T
T = Tangential force x radius
(iii) BRAKE HORSE POWER
It is useful power available from engine shaft.
BHP = 2 πN T/60
T is torque in N.m
N is RPM
BHP is brake horse power in Watts
(iv) MECHANICAL EFFICIENCY, ηmech
ηmech = BHP / IHP
(v) FRICTIONAL POWER, FP
Power lost in friction.
FP = IHP – BHP
(vi) FUEL AIR RATIO
Fuel-Air Ratio = mass of fuel/mass of air
Fuel air ratio is of three types:
Rich fuel air ratio—Fuel is more and air is less than required for complete combustion
Normal fuel air ratio (Stoichiometric fuel air ratio)
Fuel and air ratio required for complete combustion
3. Lean fuel air ratio—fuel is less and air is much more than required for complete combustion
(vii) RELATIVE FUEL AIR RATIO
Used relative fuel air ratio in the analysis.
Relative fuel air ratio = Actual fuel air ratio/ Stoichiometric fuel air ratio
PERFORMANCE PARAMETERS-I C ENGINES-2
ηth = BHP in heat units / heat produced from the fuel combustion in heat units
Mechanical efficiency = BHP/IHP
Fuel Consumption efficiency = It is combustion efficiency
= % of the fuel completely burnt
(viii) BRAKE SPECIFIC FUEL CONSUMPTION, BSFC
Firstly BSFC = rate of mass of fuel burnt / BHP produced =kg/s/ BHP) =kg/kWh
Secondly BSFC is fuel the engine consumes at a certain RPM to produce one brake horse power.
Thirdly BSFC is different at different speeds for the same power produced.
Therefore there will be a range of BSFC for an engine.
Lower BSFC is good as it consumes less fuel per unit time for unit power produced.
Really it is fuel consumption efficiency.
BSFC is less for a bigger piston displacement.
At high speeds, BSFC increases due to increased friction.
At lower speeds also, BSFC increases. It is due to increased time for heat losses from the gas to the cylinder and piston wall.
Hence there has to be an optimum speed for lower value of BSFC for a certain engine.
Increase of compression ratio decreases the fuel consumption and hence decreases BSFC and is desirable.
In actual practice, its value varies between 0.4 and 0.6.
BSFC compares the performance of different engines.
(ix) ENGINE EXHAUST
Engine exhaust is wastage of heat to the atmosphere. It contains gases like carbon monoxide, carbon dioxide, Sulphur dioxide, hydrocarbons, lead, nitrogen oxides and particulate matter. It causes air pollution as well Global Warming. Smoke in exhaust is an indication of incomplete combustion of fuel. It adds more of carbon monoxide and drastically decreases the thermal efficiency.
(x) MEAN EFFECTIVE PRESSURE
pmep = IHP x 60 / L A N
It is mean pressure acting on the piston during the entire power stroke. Calculate it from IHP or BHP.
(xi) VOLUMETRIC EFFICIENCY, ηcv
There has to be clearance between the piston and cylinder head otherwise piston will strike the cylinder head. This will decrease the life of the cylinder. This clearance volume will contain high pressure and high temperature exhaust gases. These gases on expansion during the suction stroke will occupy significant volume of the cylinder. As a result of this, volume sucked will decrease. This efficiency varies between 70 to 80 %.
ηcv = actual volume sucked / stroke volume = 1 + C–C (PR) 1/n