The theoretical cycle of any heat engine using air as working substance is known as air standard cycle. The processes used for the operation are reversible. The theoretical cycle efficiency of any engine using air as working fluid is known as air standard efficiency. It is of two types:- 1. Otto Cycle 2. Diesel Cycle.
Type # 1. Otto Cycle (Constant Volume Cycle):
Otto cycle is the basic cycle used for spark ignition (SI) engines.
Following are the assumptions:
(a) Working fluid is the air which obeys gaseous law.
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(b) Specific heats are constant.
(c) Compression and expansion processes are adiabatic.
(d) All processes are reversible.
Figure 4.23 shows the theoretical cycle on p-V and T-S diagram.
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It consists of four processes:
(a) Process 1-2: Isentropic compression
(b) Process 2-3: Constant volume heat supply
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(c) Process 3-4: Isentropic expansion
(d) Process 4-1: Constant volume heat rejection
Considering m kg of air.
Q1, heat supplied at constant volume = mcV(T3 – T2)
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Q2, heat rejected at constant volume = mcV(T4– T1)
Work done = Q1 – Q2
= Heat supplied – Heat rejected
Type # 2. Diesel Cycle (Constant Pressure Cycle):
The cycle was invented by Dr. R. Diesel in 1897. The heat transfer in a diesel engine takes place at constant pressure. A diesel cycle is the basic cycle using diesel as the fuel. Figures 4.24(a) and 4.24(b) show the cycle on p-V and T-S diagrams, respectively. The assumptions are same as in Otto cycle.
A diesel engine consists of four processes:
(a) Process 1-2: Isentropic compression
(b) Process 2-3: Constant pressure heat supply
(c) Process 3-4: Isentropic expansion.
(d) Process 4-1: Constant volume heat rejection
Considering m kg of air.
Q1, heat supplied at constant pressure = mcp(T3 – T2)
Q2, heat rejected at constant volume = mcV(T4 – T1)
Work done = Q1 – Q2
= Heat supplied – Heat rejected
= mcp(T3 – T2) – mcV(T4 – T1)
Substituting the value of T2, T3, and T4 from Eqs. (4.3), (4.4), and (4.5), respectively, in Eq. (4.2).
