The Otto cycle is the idealised thermodynamic cycle that describes a spark-ignition (petrol) engine. It consists of two adiabatic processes (compression and expansion) and two constant-volume processes (heat addition during combustion and heat rejection during exhaust).
Intake (fuel-air mixture drawn in), compression (piston squeezes the mixture adiabatically), power/expansion (combustion pushes the piston down), and exhaust (burnt gases pushed out). In the idealised Otto cycle these map to adiabatic compression, isochoric heat addition, adiabatic expansion, and isochoric heat rejection.
The ideal thermal efficiency is η = 1 − 1/r(γ−1) = 1 − r(1−γ), where r is the compression ratio and γ is the heat-capacity ratio (about 1.4 for air). It depends only on the compression ratio, not on the amount of heat added.
A higher compression ratio means the gas is expanded over a larger volume range, extracting more work per unit of heat. Since η = 1 − 1/r(γ−1), increasing r always raises efficiency, which is why high-performance engines use high compression ratios.
Knock (auto-ignition of the fuel-air mixture before the spark) limits compression ratio in petrol engines. Higher-octane fuels resist knock and allow higher ratios. Typical petrol engines use 8:1 to 12:1; diesel engines, which compress only air, reach 14:1 to 22:1.
An adiabatic process happens with no heat exchange with the surroundings. For an ideal gas it follows PVγ = constant. The fast compression and expansion strokes are modelled as adiabatic because they occur too quickly for significant heat transfer.
The enclosed area of the PV loop equals the net work done by the engine per cycle. A larger loop means more work output. Efficiency is that net work divided by the heat added during combustion.
γ = Cp/Cv is the ratio of specific heats at constant pressure and constant volume. For diatomic gases like air it is about 1.4. It governs how steeply pressure changes during adiabatic compression and expansion.
In the Otto cycle heat is added at constant volume (spark ignition). In the Diesel cycle heat is added at constant pressure (compression ignition). Diesel engines use higher compression ratios and are generally more efficient for that reason.
Real engines lose energy to friction, heat transfer through cylinder walls, incomplete combustion, pumping losses, and finite-time processes that are not truly adiabatic or isochoric. Real petrol engines reach 25 to 35 percent, below the ideal Otto value.
Watch a four-stroke petrol engine piston move through one complete thermodynamic cycle while the matching point traces a pressure-volume (PV) diagram. The Otto cycle idealises the spark-ignition engine found in most cars, and its efficiency depends only on the compression ratio.
Adiabatic legs: P·V^γ = constant. Efficiency:
η = 1 − 1/r^(γ−1) = 1 − r^(1−γ), with compression ratio
r = V₁/V₂ and γ ≈ 1.4. Net work equals the
enclosed PV-loop area: W = Q_in − Q_out.
Nikolaus Otto built the first practical four-stroke engine in 1876. Because efficiency depends only on the compression ratio, doubling the heat added does not change η at all — it only makes a bigger loop with proportionally more work and waste heat.