Saturday, 31 October 2015

What are Air Standard Cycles and its Assumptions?

The internal combustion engine works on the sequence of separate process intake, compression, combustion, expansion and exhaust. It does not operate on thermodynamic cyclic because it involves an open system. The working fluid (fuel and air mixture) enters into the system in intake stroke and leaves it in exhaust stroke.  The working fluid does not complete a cyclic process.  To analysis this system we considered it as an close cycle where the working fluid bring at initial condition by injecting the new charge and exhausting burn charge. However the accurate analysis of internal combustion engine is complicated.  So to analyze the performance of engine it is idealized as the closed cycle in which only air is the working fluid and heat is supplied by the reservoir.  This standard cycle is known as air standard cycle. 

Following assumption made in actual cycle to analysis as air standard cycle. 

1. The working fluid is perfect gas. 
2. There is no change in mass of the working medium. 
3. All the process that constitutes the cycle is reversible. 
4. Heat is assumed to be supplied from a constant high temperature source and not from chemical reaction during the cycle. 
5. There are no heat losses. 
6. The working medium has constant specific heats throughout the cycle. 




The air standard cycle is often used because of the simplicity in getting approximate answer to the complicated process. To define different engine different air standard cycle are used. These are 

Types of air standard cycle:

1. Carnot Cycle:

It is refer as the ideal cycle of air standard cycle. It represents as a standard of perfection and engine can be compared with it to judge the degree of perfection. It gives concept of maximum work output between two temperature limit. 

2. Stirling Cycle:

Stirling engine works on the Stirling cycle. This engine used in automotive industries and marine industries.

3. Otto Cycle:

The petrol engine works on the Otto cycle. This engine used in bikes, scooter and small petrol cars. 

4. Diesel Cycle:

Diesel engine works on diesel cycle. This engine used in heavy vehicle like truck, bus and cars.

5. Ericsson Cycle:

The Ericsson cycle does not find practical application in piston engine but is used by a gas turbine employing a large number of stages with heat exchanges, reheaters etc.

6. Dual Cycle:

The dual engine works on dual cycle. This engine used in automobile industries. 

7. Atkinson Cycle:

It is an ideal cycle of Otto engine exhausting to a gas turbine.

8. Brayton Cycle:

Brayton cycle is the theoretical cycle of gas turbine.


Today we have discussed how the air standard cycle term arrived and why we used air standard cycle to analyze an engine. We will discuss analysis of these cycles in next article. If you have any query ask by commenting below. 

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