Thermodynamic Analysis of I.C. Engines
- Before we carry out the thermodynamic analysis of the internal combustion (I.C.) engine cycles, it is desirable to carry out the energy balance of the engine system from first law of thermodynamics point of view.
- Such an energy balance will help us to understand the flow of energy input to the system and its utilization. Consequently, it will help us to understand the important performance parameters of an L.C. engine.
- Figure A and B shows the block diagram of energy flow through an I.C. engine.
- According to law of conservation of energy, the energy can neither be created nor destroyed. It can only be converted from one form to another.
- Therefore, there must be energy balance of rate of energy input and its output.
- In an engine, the air and the fuel is burnt inside the cylinder. As a result the chemical energy of fuel is converted into heat. If me is the rate of fuel burnt of calorific value (C.V.). then the rate of energy input to the engine becomes :
Qi = mf x C.V.
(b) Rate of energy output :
- The engine cannot convert the whole input energy into work according to Kelvin-Planck statement of second law of thermodynamic.
- Out of the total rate of energy input, a part of the energy is lost (Qi) to cooling water in exhaust gases and in radiation and convection.
- The reminder of the energy is used to push the piston. The rate of energy available to push the piston is called indicated power (I. P.). This energy is utilized to move the piston in the cylinder. Therefore,
I.P. – Rate of energy input, Qi – Rate of energy losses, Qi
- The input power to piston is transmitted through the connecting rod to the crankshaft. Due to motion of piston in the cylinder and rotating parts in bearings etc, a part of energy supplied to push the piston is lost in overcoming the friction, pumping etc.
- The algebraic sum of all the energy losses due to friction in cylinder, bearings and pumping losses are called frictional power (F.P) losses or mechanical losses.
- The remainder of the energy available at the output shaft is the useful mechanical energy. The mechanical energy or the output energy available at the crankshaft is called Brake Power (B.P.).
∴ B.P. = I.P. – F.P.
Engine Performance :
The engine performance is indicated by the term efficiency. Various efficiencies used in LC. engine practice are defined below.
(i) Mechanical efficiency :
It is defined as the ratio of brake power to indicated power.
Therefore, Mechanical efficiency, ηm = B.P. / 1.P.
(ii) Thermal efficiency : Thermal efficiency of an engine is the indicator of conversion of heat supplied into work energy. It is either based on I.P. or on B.P. accordingly, we have two types of thermal efficiencies,
(i) Indicated thermal efficiency, ηi
(ii) Brake or overall thermal efficiency, ηb or ηo
Let, mf = Fuel consumption in kg/s
C.V. = Calorific value of fuel, kJ/kg
Indicated thermal efficiency, efficiencies
ηi = I.P. / mf x C.V.
Brake or overall thermal efficiency.
ηb or ηo = B.P. / mf x C.V.
Volumetric efficiency :
- It is the measure of the degree to which the engine fills to its swept volume. It is defined as the ratio of actual mass of charge inducted during suction stroke to the mass of charge corresponding to swept volume of the engine at atmospheric pressure and temperature. Accordingly,
- Volumetric efficiency,
ηv = Mass of actual charge inducted / Mass of charge corresponding to swept volume at atm. p and T
Alternately,
ηv = Actual volume of charge inhaled at suction conditions / Swept volume
- Swept volume of the engine is determined by known dimensions of the cylinder i.e. by its bore and stroke.
- Actual volume of mixture inducted is determined by measurement of fuel and air flow rates at the known speed of the engine during the test.
- The volumetric efficiency puts the limit on the amount of fuel that can be burnt. hence, on its power since the output of engine depends on the amount of air inducted.
Reduced volumetric efficiency causes the reduction in power output.