Why does Otto Cycle Engine calls a Constant Volume Cycle?


Constant Volume Cycle- 

Otto Cycle was introduced in practical form by a German scientist Otto, in 1876, although it was described by a French scientist Beaude Roches in 1862. The engines operating on this cycle are known as Otto cycle engines. The petrol engines operate on this cycle. 

An I.C. engine does not undergo a cyclic change but it is assumed here that the working medium is pure air which does not undergo any chemical change. The air is simply heated and cooled to undergo a cycle. It is also assumed that the ideal indicator diagram is strictly followed, in the figure. The ideal Otto cycle consists of the following operations :

    1-2 Adiabatic compression.
    2-3 Heat addition at constant volume.
    3-4 Adiabatic expansion.
    4-1 Heat rejection at constant volume.

At point 1, the air in the cylinder is initially corresponding to 

P1V1,  and T(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.

At point 3, the hot body is removed and the air is expanded adiabatically during the expansion stroke, up to point 4 corresponding to P4, V4, and T4. A cold body is then brought in contact with the cylinder end such that the pressure drops at constant volume, corresponding to the conditions P1, V1, and T1. Here V4
= 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.

    2-3 Heat addition at constant volume. 
    3-4 Adiabatic expansion.
    4-1 Heat rejection at constant volume.

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 are 

P1,
V1, and T1
. The piston compresses the air adiabatically from Vto 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 :

    1. No heat is supplied or rejected during the expansion.
    2. Work of some nature must be done by the expanding gas.
    3. The expansion is assumed to be frictionless.

In an adiabatic expansion 

PV
= constant.

Thus, higher efficiency can be achieved by increasing the compression ratio. The lesser the difference between T1 and T„ the more the efficiency reaches close to the Carrot efficiency but in that case, the power developed is reduced. The Carnot cycle has the highest possible efficiency and consists of two isothermal and two adiabatic operations. 

Particularly for air standard efficiency the value of y is to be taken as 1.4. This efficiency is more sensitive to compression ratio r. If r is increased from 2 to 4, the efficiency is increased from 0.24 to 0.424 ; and if r is increased from 10 to 20, the efficiency increases from 0.6 to only 0.695

Characteristics of the Otto Cycle- 

The efficiency depends on:

  1. Compression ratio, r.
  2. The ratio of the specific heats, y
The net work done in the Otto cycle can be expressed in terms of pv.

 

Assumptions:

The assumptions made in deriving the formula for the air standard efficiency of the Otto cycle are as follows :

1. The working fluid is not subjected to any chemical reaction. It is simply heated and cooled and used over and over again.

2. There is no heat loss during the cycle.
3. The compression and expansion are strictly adiabatic.
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.
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