Eficiência Ciclo Otto
- Created by
- Renato Passos, Eng. de Software
- Reviewed by
- Renato Passos, Eng. de Software
Last updated: Apr 18, 2026
Formula
η Otto
About this calculator
The Otto Cycle Efficiency Calculator determines the thermal efficiency of an internal combustion engine operating on the Otto cycle, common in gasoline engines. Efficiency is calculated using the formula η = 1 − 1/r^(γ−1), where r is the compression ratio and γ is the specific heat ratio of the working fluid (typically air, with γ ≈ 1.4). The result is expressed as a decimal or percentage, indicating the fraction of fuel energy converted into useful work.
The operation is straightforward: input the compression ratio (r) and the specific heat ratio (γ). Typical compression ratios in gasoline engines range from 8:1 to 12:1. The calculator applies the formula directly, ignoring losses from friction, incomplete combustion, or heat transfer, providing the ideal cycle efficiency. This helps understand the theoretical maximum performance limit of the engine.
Use this calculator to compare different compression ratios and see how small changes affect efficiency. For example, increasing r from 8 to 10 raises efficiency from about 56% to 60% (with γ=1.4). It is useful for engineering students, engine enthusiasts, and professionals looking to optimize designs or understand the fundamentals of engine thermodynamics. Remember that actual efficiency is always lower due to mechanical and thermal losses.
Cautions: the formula assumes an ideal gas with constant specific heats and a reversible adiabatic process. In practice, γ varies with temperature and gas composition. Additionally, real efficiency depends on factors like effective compression ratio, knock, and volumetric efficiency. Do not use this result as an exact prediction of fuel consumption, but as a theoretical reference for conceptual designs.
Frequently asked questions
What is the maximum theoretical efficiency of the Otto cycle?
The maximum theoretical efficiency is given by η = 1 − 1/r^(γ−1). With γ=1.4 and typical compression ratios (8 to 12), efficiency ranges from 56% to 63%. Higher values require very high compression ratios, which may cause knocking.
Why is actual efficiency lower than calculated?
Actual efficiency is lower due to losses from friction, incomplete combustion, heat transfer to cylinder walls, finite combustion time, and pumping losses. The ideal formula ignores these losses.
Can I use γ different from 1.4?
Yes. For air, γ ≈ 1.4. But if the working fluid is a gas mixture (like combustion gases), γ can be lower (about 1.3). The calculator allows you to input any γ value.
What happens if the compression ratio is too high?
Very high compression ratios can cause knocking (engine ping), damaging the engine. In practice, there is a maximum limit for each fuel. For regular gasoline, r rarely exceeds 12:1.
Does this calculator work for diesel engines?
No. Diesel engines operate on the Diesel cycle, with a different efficiency formula. Use the Diesel cycle calculator for those engines.