What are the Air-Standard Cycle Assumptions of the Reciprocating IC Engine?

In order to fully understand the concept of air standard cycle assumptions of the reciprocating internal combustion engine, there needs to be a brief background before its subtle explanation.

In real applications, the comprehensive study of the reciprocating internal combustion engine takes into account certain complex features, which are mentioned below for the sake of understanding:

  1. Ignition and combustion of the air-fuel mixture in the cylinder and the presence of inevitable irreversibilities in the form of friction, thermal and pressure gradient, and so on.
  2. Heat transfer between the gases produced during the combustion process and the cylinder walls.
  3. Energy is required to charge the cylinder during the intake process and eject the gases produced during the exhaust process.

To take into account the effects mentioned above, the modeling of internal combustion engines requires computer simulation, which is performed at the advanced level of research. However, in the thermodynamic examination of the power cycles of the IC engines, there are certain theoretical simplifications or idealizations to assist the analysis.

Based on such assumptions, one of the methods is called the cold air-standard analysis, which aids the study of IC engines only qualitatively. It has certain characteristic elements, which are described below:

  1. The working fluid would be an ideal gas in a fixed quantity.
  2. There would be no combustion process as such. Rather, it would be replaced by a heat transfer process from an external source.
  3. A constant volume heat rejection process would replace the exhaust process with the piston positioned at the bottom dead center.
  4.  All processes comprising the cycles would be internally reversible par excellence.
  5. The values of specific heats (c) would be taken constant at their ambient temperature conditions.
  6. The composition of the charge would be considered unchanged during the heat addition process, and so on.

One of the implications of the cold air-standard analysis is that although it simplifies the thermodynamic study of IC engines quite comfortably, the values of mean effective pressures and those of temperatures estimated during different phases of the cycle deviate considerably from those of the actual cycle.

In the forthcoming articles, the cold air-standard analysis will be performed in the study of Otto, diesel, and dual cycles of IC engines.