Internal Combustion (IC) engines have completely revolutionized transportation, power generation and have perhaps altered the way the society operates forever. Typical IC engines are classified as Spark and Compression ignition engines.

The simplest model for IC engines is the air-standard model, which assumes that:

• The system is closed.
• Air is the working fluid and is modeled as an ideal gas throughout the cycle.
• Compression and expansion processes are isentropic.
• A reversible heat transfer process characterizes the combustion of fuel and air.
• Heat rejection takes place reversibly and at constant volume.

The Otto cycle is used to model a basic Spark Ignition engine, while the Diesel cycle is the basic model for the Compression Ignition engine.

Spark Ignition Engines (Otto Cycle)

The spark-ignition engines are the most common type used in cars. Larger engines operate using a four-stroke cycle, while smaller engines operate on a two-stroke cycle. In a simple four-stroke cycle, a combustible mixture of air and fuel is drawn into a cylinder during the intake stroke, and the temperature and pressure of the mixture is raised during the compression stroke. At near the maximum compression, a spark initiates combustion of the mixture, raising its temperature and forcing expansion. The expanding gases do work on the piston during the power stroke and then the burnt gases are purged during the exhaust stroke. Typically 3000 or more such cycles are repeated in a minute.
 

The Otto cycle is an air-standard model of the actual cycle. In addition to the air-standard assumptions listed above, the combustion process is modelled as a reversible constant volume heat addition process. The four steps of the air-standard Otto cycle are outlined below:

• (1-2) Isentropic compression (Compression Stroke)
• (2-3) Constant-volume, reversible heat addition (Ignition)
• (3-4) Isentropic expansion (Power Stroke)
• (4-1) Reversible, constant-volume heat rejection (Exhaust)