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Air Standard Cycle: Part – 1

written by: Haresh Khemani • edited by: Lamar Stonecypher • updated: 8/9/2008

To carry out the analysis of the heat engines, the concept of air standard cycles was conceived. In these cycles certain mass of air is considered to be working in the thermodynamic cycle.The two most commonly used air standard cycles are Otto cycle and Diesel cycle.

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    Introduction

    The thermodynamic cycles like Carnot cycle, Stirling cycle, Brayton cycle are closed cycles as the working fluid keeps on circulating inside the cycle. In these cycles the addition of heat takes places externally, hence they also called external combustion engines. The most commonly used practical engines used in our day-to-day life are internal combustion engines for our vehicles, motorcycles, construction machineries etc.

    The engines in which combustion of fuel takes place inside the engine are called as internal combustion engines or IC engines. The IC engines comprises of the piston and cylinder arrangement with suction and exhaust valves.

    In the beginning of the internal combustion cycle, the air-fuel mixture is inducted inside the cylinder. When the fuel is combusted lot of heat is released which makes the engine cylinder very hot. To avoid the deterioration of the engine, it is very important to cool the engine either with water or with external air. After combustion of fuel the pressure of gases becomes very high which pushes the piston producing the work. Thereafter the burnt gases are released to the atmosphere as exhaust gases and the fresh air-fuel mixture is inducted inside the cylinder. Since the burnt air-fuel mixture does not moves into the cycle again and fresh air-fuel mixture is inducted at the beginning of each cycle, the internal combustion engines are said to be working in non-cyclic process.

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    The Concept of Air Standard Cycles

    No matter whether the engine works in cyclic or non-cyclic process, its analysis is very important to find out the amount of fuel consumed and work produced by it. To carry out the analysis of the heat engines, the concept of air standard cycles was conceived. In these cycles certain mass of air is considered to be working in the thermodynamic cycle. The addition and rejection of heat is considered to take place with the external reservoir and ideally all the processes are reversible.

    The internal combustion engines are considered to be working on the principle of air standard cycles. The two most commonly used air standard cycles are Otto cycle and Diesel cycle. The Otto cycle corresponds to four stroke gasoline or petrol engines also called Spark Ignition (SI) engines. The Diesel cycle corresponds to four stroke Diesel engines also called as Compression Ignition (CI) engines.

    In the next part of the article we shall see Otto and Diesel cycles.

    Next: Part-2 of this article

    Next: Part-3 of this article

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    Reference

    Book: Engineering Thermodynamics by P K Nag