Calculator Inputs
Use the form below to evaluate ideal cycle efficiency and supporting energy metrics.
Example Data Table
| Cycle | Main Inputs | Mechanical Efficiency | Brake Power | Approx. Thermal Efficiency |
|---|---|---|---|---|
| Otto | r = 10, γ = 1.4 | 90% | 250 kW | 60.19% |
| Diesel | r = 18, γ = 1.4, rc = 2 | 88% | 300 kW | 55.98% |
| Brayton | rp = 12, γ = 1.4 | 92% | 500 kW | 50.85% |
| Rankine | h1 = 200, h2 = 240, h3 = 3200, h4 = 2200 | 95% | 1000 kW | 32.43% |
Formula Used
Otto η = 1 − 1 / rγ−1
Diesel η = 1 − [1 / rγ−1] × [(rcγ − 1) / (γ(rc − 1))]
Brayton η = 1 − 1 / rp(γ−1)/γ
Rankine η = [(h3 − h4) − (h2 − h1)] / (h3 − h2)
Supporting output calculations:
- Indicated Power = Brake Power / Mechanical Efficiency
- Fuel Energy Rate = Indicated Power / Thermal Efficiency
- Heat Rejected = Fuel Energy Rate − Indicated Power
- Heat Rate = 3600 / Thermal Efficiency
These equations estimate ideal cycle behavior. Real engines usually perform lower because of friction, irreversibility, heat transfer, leakage, and combustion losses.
How to Use This Calculator
- Select the engine cycle that matches your analysis model.
- Enter brake power and mechanical efficiency for output-side estimation.
- Provide the cycle-specific parameters, such as compression ratio, cutoff ratio, pressure ratio, or enthalpies.
- Click Calculate Efficiency to display results above the form.
- Use the CSV or PDF buttons to export the result block.
- Compare scenarios by changing one variable at a time.
Why Engineers Use Cycle Efficiency Analysis
Cycle efficiency helps estimate how effectively a thermodynamic system converts supplied heat into useful work. It is valuable during concept comparison, classroom exercises, plant studies, engine selection, and preliminary optimization. By linking efficiency with fuel energy rate and rejected heat, designers can better judge expected performance and loss distribution.
This page supports common idealized models, giving a fast engineering reference before deeper simulation or test-bench validation. It is especially useful when screening design choices early.
Frequently Asked Questions
1. What does engine cycle efficiency mean?
It is the fraction of supplied heat energy converted into useful work by an ideal thermodynamic cycle. Higher values indicate better theoretical energy conversion.
2. Why are actual engine efficiencies usually lower?
Real machines lose energy through friction, imperfect combustion, pressure drops, heat transfer, leakage, and component inefficiencies. Ideal formulas do not include those losses.
3. When should I use the Otto model?
Use the Otto model for ideal spark-ignition analysis where heat addition is treated as constant volume. It is common for gasoline-engine teaching calculations.
4. What is cutoff ratio in the Diesel cycle?
Cutoff ratio is the cylinder volume after heat addition divided by the volume before heat addition. A larger value generally lowers ideal Diesel efficiency.
5. Why does Brayton efficiency depend on pressure ratio?
In the ideal Brayton model, raising compressor pressure ratio increases temperature rise during compression and improves thermal conversion, within practical design limits.
6. Why does Rankine use enthalpy values?
Rankine systems are steam-power cycles involving phase change. Enthalpy captures heat and work interactions across pumps, boilers, turbines, and condensers more directly.
7. What does heat rate tell me?
Heat rate shows how much energy input is required per unit of useful output. Lower heat rate usually means a more efficient system.
8. Can I use this calculator for final equipment sizing?
It is best for screening and preliminary studies. Final sizing should use detailed property data, component maps, safety margins, and validated performance models.