Constant Volume Cycle-
At point 1, the air in the cylinder is initially corresponding toP1, V1, and T1 (pressure, volume, and absolute temperature). The piston compresses the air adiabatically during the compression stroke; and at point 2 let the condition of air be P2, V2, T2. The air now occupies the clearance volume of the cylinder. A hot body is then brought into contact with the cylinder end such that the heat is supplied at a constant volume. This increases the pressure and temperature of the air, corresponding to P3, and T3. Here V3 = V2.
= V1. Thus, The air finally returns to its original condition and the cycle is complete. For a given compression ratio, the Otto cycle is more efficient than the Diesel cycle.
The efficiency of the Constant Volume Cycle-
The working medium is pure air which does not undergo any chemical change. It is simply heated and cooled to undergo a cycle, which consists of the following operations —
1-2 Adiabatic compression.
The operation of the Otto cycle is as follows. Let the air is filled in the cylinder and the condition of air (pressure, volume, and absolute temperature) initially at point 1 areP1,
V1, and T1. The piston compresses the air adiabatically from V1 to V2. At the end of compression at point 2, the conditions of air are P2,
V2, and T2. Here the air occupies the clearance volume of the cylinder. Now, the air is heated at constant volume by bringing a hot body in contact with the cylinder. This causes to rise the pressure from 2 to 3. At point 3, the conditions of air are P3,
V3, and T3. Note that V2
= V3. Now, the hot body is removed and the air expands adiabatically from point 3 to 4. At point 4, the conditions of air are P4,
V4, and T4. Now, the air is cooled at constant volume by bringing a cold body in contact with the cylinder. This causes to drop in the pressure from point 4 to 1. At point 1, the air finally returns to its original conditions P1,
V1, and T1. Here V1
= V4. Thus, the cycle is complete.
Note that in an adiabatic process (also known as an isentropic process) no heat is supplied or rejected, i.e., the gas neither receives nor gives out heat. The gas expands thereby doing external work. For an adiabatic expansion, the following three conditions must be satisfied :
In an adiabatic expansionPVY = constant.
Characteristics of the Otto Cycle-
The efficiency depends on:
- Compression ratio, r.
- The ratio of the specific heats, y.
1. The working fluid is not subjected to any chemical reaction. It is simply heated and cooled and used over and over again.
4. The working fluid is a perfect gas following the gas laws and has constant specific heat.
5. The heating and cooling strictly take place at constant volume.
6. The suction and exhaust take place at atmospheric pressure.