Formula & Calculator
Overall Propulsion Efficiency
Combined efficiency of a jet engine, equal to the product of thermal efficiency and propulsive efficiency.
Interpretation
Overall propulsion efficiency: η_o = η_th · η_p, the product of thermal and propulsive efficiencies. It gives the overall efficiency of converting fuel energy into useful thrust power. Example: η_th=0.4, η_p=0.8 → η_o=0.32.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| η_o | Overall efficiency | |
| η_th | Thermal efficiency | |
| η_p | Propulsive efficiency |
What it means
The overall propulsion efficiency is the product of the thermal efficiency (how well the engine converts fuel heat into kinetic energy) and the propulsive efficiency (how well that kinetic energy is used to produce thrust). This is the key metric for fuel economy in aircraft. Higher η_o means less fuel consumption for a given thrust. It is used in mission analysis to estimate fuel burn and in comparing different propulsion concepts. Understanding η_o is essential for assessing overall aircraft performance and environmental impact.
Worked example
Overall Propulsion Efficiency – Two Examples
Real‑World| Parameter | Value |
|---|---|
| η_th | 0.45 |
| η_p | 0.60 |
| Parameter | Value |
|---|---|
| η_th | 0.50 |
| η_p | 0.65 |
Common mistakes
- Overall propulsion efficiency: η_o = η_th · η_p.
- Product of thermal and propulsive efficiencies.
- Indicates how well fuel energy is converted to useful work.
- Typical overall efficiency for modern turbofans ~30‑40%.
Applications
Overall propulsion efficiency, η_o = η_th·η_p, combines thermal and propulsive efficiency to give the total efficiency of the propulsion system. It represents the fraction of fuel energy converted into useful thrust power. Engineers use this to compare different engine cycles and to optimise propulsion systems. By maximising overall efficiency, aerospace engineers can reduce fuel consumption and operating costs, making aircraft more sustainable. This metric is central to aircraft performance and environmental impact analysis.
- Propulsion system performance assessment
- Trade‑off studies between thermal and propulsive efficiency
- Engine cycle selection and optimisation
- Fuel consumption and emissions reduction strategies
- Comparison of alternative propulsion concepts
Frequently Asked Questions
It is the combined efficiency of a jet engine, equal to the product of thermal efficiency and propulsive efficiency. It measures how well the engine converts fuel energy into useful thrust power.
ηth = thermal efficiency (Brayton cycle)
ηp = propulsive efficiency
It determines the fuel consumption and range. Maximising ηo is the goal of engine design.
For a high‑bypass turbofan at cruise, ηo ≈ 0.35–0.40 (35–40%).
- Adding thermal and propulsive efficiencies instead of multiplying them, which greatly overstates overall efficiency.
- Using the wrong values for ηth and ηp (e.g., using ideal instead of real).
- Confusing overall efficiency with thermal efficiency.
If ηth = 0.45 and ηp = 0.70, then ηo = 0.45 × 0.70 = 0.315 (31.5%).
Increasing bypass ratio improves ηp but may slightly reduce ηth; the net effect usually improves ηo.
Designing for high ηth (high pressure ratio, high turbine inlet temperature) may increase exhaust velocity, reducing ηp. The optimal balance gives maximum ηo.
TSFC = 1/(ηo·QR), where QR is the fuel heating value. Higher ηo gives lower TSFC.
It would be the product of electrical and propulsive efficiencies, which can be very high (e.g., ηo > 0.8 for some electric propellers).