Otto Cycle Calculator (Petrol Engine Efficiency, Temperatures and Mean Effective Pressure)

What this tool does

The Otto cycle is the textbook model of a petrol engine: compress the air, add heat at constant volume, expand, reject heat at constant volume. Its efficiency 1 − r^(1−γ) depends only on the compression ratio and the gas, and climbs steeply at first and then flattens, which is why raising a ratio of 8 to 12 pays far more than 12 to 16. Real engines fall well short because of heat loss, friction, incomplete combustion and a working fluid that is not an ideal gas at constant specific heat, so use the result to compare designs, not to predict fuel economy.

Formula

η = 1 − r^(1−γ) · T₂ = T₁ r^(γ−1) · T₃ = T₂ + q_in ÷ c_v · T₄ = T₃ ÷ r^(γ−1) · w = η q_in · mep = w ÷ (v₁ − v₂)

Variables

Symbol	Meaning	Unit
`rc`	Compression ratio	—
`t1`	Temperature at the start of compression	°C
`p1`	Pressure at the start of compression	kPa
`qi`	Heat added per kg of air	kJ/kg
`ga`	Heat capacity ratio γ	—
`EF`	Thermal efficiency	%
`T2`	Temperature after compression	°C
`T3`	Peak temperature	°C
`T4`	Temperature at the end of expansion	°C
`W`	Net work per kg of air	kJ/kg
`MEP`	Mean effective pressure	kPa
`P3`	Peak pressure	bar

Worked example

Compression ratio9
Temperature at the start of compression27 °C
Pressure at the start of compression100 kPa
Heat added per kg of air1500 kJ/kg
Heat capacity ratio γ1.4

Thermal efficiency58.48 %
Temperature after compression450 °C
Peak temperature2540 °C
Temperature at the end of expansion895 °C
Net work per kg of air877 kJ/kg
Mean effective pressure1146 kPa
Peak pressure84.4 bar

Limitations

The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.