AIR STANDARD CYCLES
Most of the power plant operates in a thermodynamic cycle i.e. the working fluid undergoes a series of processes and finally returns to its original state. Hence, in order to compare the efficiencies of various cycles, a hypothetical efficiency called air standard efficiency is calculated. If air is used as the working fluid in a thermodynamic cycle, then the cycle is known as “Air Standard Cycle”.
To simplify the analysis of I.C. engines, air standard cycles are conceived. Assumptions
1. The working medium is assumed to be a perfect gas and follows the relation pV = mRT or P = pRT
2. There is no change in the mass of the working medium.
3. All the processes that constitute the cycle are reversible.
4. Heat is added and rejected with external heat reservoirs.
5. The working medium has constant specific heats.
Otto Cycle (1876) (Used S. I. Engines)
This cycle consists of two reversible adiabatic processes and two constant volume processes as shown in figure on P-V and T-S diagrams. The process 1-2 is reversible adiabatic compression, the process 2-3 is heat addition at constant volume, the process 3-4 is reversible adiabatic expansion and the process 4-1 is heat rejection at constant volume.
The cylinder is assumed to contain air as the working substance and heat is supplied at the end of compression, and heat is rejected at the end of expansion to the sink and the cycle is repeated.
Process
0-1 = suction
1-2 = isentropic compression
2-3 = heat addition at constant volume
3-4 = isentropic expansion
4–1 = constant volume heat rejection
1-0 = exhaust
Diesel Cycle (1892) (Constant Pressure Cycle)
Diesel cycle is also known as the constant pressure cycle because all addition of heat takes place at constant pressure. The cycle of operation is shown in figure 2.4
(a) and 2.4 (b) on P-V and T-S diagrams.
The sequence of operations is as follows :
1. The air is compressed isentropically from condition ‘1’ to condition ‘2’.
2. Heat is supplied to the compressed air from external source at constant pressure which is represented by the process 2-3.
3. The air expands isentropically until it reaches condition ‘4’.
4. The heat is rejected by the air to the external sink at constant volume until it reaches condition T and the cycle is repeated.
The air standard efficiency of the cycle can be calculated as follows:
is also > 1,therefore for the same compression ratio the efficiency of the diesel cycle is less than that of the otto cycle.