Written by the Fiveable Content Team • Last updated August 2025
Written by the Fiveable Content Team • Last updated August 2025
Definition
The Otto cycle is a thermodynamic cycle that describes the functioning of a typical spark ignition piston engine, commonly found in automobiles. It consists of two isochoric (constant volume) and two adiabatic (no heat transfer) processes.
Course connection
Topic 15.2: 15.2 The First Law of Thermodynamics and Some Simple Processes
The Otto cycle includes four stages: isochoric compression, adiabatic expansion, isochoric heat rejection, and adiabatic compression.
The efficiency of the Otto cycle depends on the compression ratio and the specific heat ratios of the working fluid.
The thermal efficiency formula for an ideal Otto cycle is given by $\eta = 1 - \frac{1}{r^{\gamma-1}}$, where $r$ is the compression ratio and $\gamma$ is the specific heat ratio.
In real engines, deviations from the ideal Otto cycle occur due to factors like friction, non-instantaneous combustion, and heat losses.
The P-V diagram of an Otto cycle forms a distinct loop representing work done during each stage.
Related terms
Compression Ratio: The ratio of the maximum to minimum volume in a cylinder during a complete engine cycle. Higher ratios generally lead to better efficiency.
A process in which no heat is transferred to or from the working fluid. In an ideal gas undergoing an adiabatic process, $PV^{\gamma} = \text{constant}$.
A measure of how well an engine converts heat energy into work. For cyclic processes like the Otto cycle, it’s calculated as $\eta = \frac{W_{out}}{Q_{in}}$.