The compression ratio in the ideal air standard cycle is the ratio of:

Concept:

Otto cycle:

The air-standard Otto cycle is the idealized cycle for the spark-ignition internal combustion engines.

The Otto cycle 1-2-3-4 consists of the following four process :

• Process 1-2: Reversible adiabatic compression of air
• Process 2-3: Heat addition at constant volume
• Process 3-4: Reversible adiabatic expansion of air
• Process 4-1: Heat rejection at constant volume

The thermal efficiency of the Otto Cycle:

\({\eta _{otto}} = 1 - \frac{1}{{{r^{\gamma - 1}}}}\)

Compression ratio:

The compression ratio is given by the ratio of volume before compression to volume after compression i.e., It is the ratio of maximum volume to minimum volume.

\(Compression\;ratio\left( r \right) = \frac{{{V_1}}}{{{V_2}}} = \frac{{{V_c} \;+ \;{V_s}}}{{{V_c}}}\)

The air standard efficiency of an Otto cycle is \(1 - \frac{1}{{r_c^{\gamma - 1}}}\)

Important Points

The air standard efficiency of a Diesel cycle is \(1 - \frac{1}{{{r^{\gamma - 1}}}}\frac{{\rho^\gamma - 1}}{{\gamma \left( {{\rho} - 1} \right)}}\)​