Aviation propulsion: Science and limits
Why bigger, slower fans are the secret to efficiency
What this tab covers What physically happens in the conversion, step by step, the science behind it, and the hard limits that physics sets on how well it can ever work.A jet engine burns kerosene or sustainable aviation fuel (SAF) in compressed air and turns the heat into a fast jet and a spinning fan. The best engines turn about 40% of the fuel's energy into thrust power.
The conversion, step by step
Combustion in the coreChemical (jet fuel) → Heat in compressed air · near complete1Thermodynamic cycle (compressor, combustor, turbine)
Heat → Kinetic energy of the jet and shaft power to the fan · over 50%, up to about 55% at cruise in the best engines (about 30% fifty years ago)1Propulsion (fan and jet push air backwards)
Kinetic energy of the air streams → Thrust power (thrust x flight speed) · 70-80% for turbofans in service1Whole engine (product of the above)
Chemical (jet fuel) → Thrust power · about 40% today1Aircraft (thrust works against drag)
Thrust power → Passenger-kilometres · about 1.2 MJ of fuel per passenger-km, world average 2019 (calculated from ICCT's 90 g CO2 per passenger-km at 3.16 kg CO2 and 43 MJ per kg of fuel)2
The science
A turbofan runs the Brayton cycle: a compressor squeezes air, fuel burns in it, and turbines drive the compressor and a large front fan. Thermal efficiency rises with pressure ratio and turbine temperature. Propulsive efficiency rises when a large mass of air is accelerated gently, since energy left in the wake is wasted; so bypass ratios have grown from about 2:1 seventy years ago to about 11:1. Turboprops and helicopter turboshafts drive a propeller or rotor through a gearbox; turboprops are about 10% more propulsively efficient but slower, and a hovering helicopter spends power just staying up.
Limits set by science
| Limit | Value | Why it exists | Best today |
|---|---|---|---|
| Carnot limit on the core | about 88% (calculated, illustrative)1 | 1 - Tc/Th with Th = 1,800 K (assumed turbine inlet gas temperature; hot parts must survive 2,200-3,000 F, about 1,480-1,920 K) and Tc = 217 K (air at 11 km): 1 - 217/1,800 = 0.88. A loss-free Brayton cycle gives somewhat above 80%; practical cores about 65-70%. | up to about 55% at cruise |
| Propulsive efficiency | 2 / (1 + jet speed / flight speed); 90-95% judged possible1 | Thrust needs a jet faster than the aircraft; the faster the jet, the more energy is left in the wake. Bigger, slower fans get closer but add weight, drag and noise. | 70-80% at cruise |
| Materials (turbine inlet temperature) | hot-section parts need long life at 2,200-3,000 F (about 1,480-1,920 K); CMCs now rated 2,200-2,400 F16 | Superalloy blades survive only because 20-30% of core airflow cools the hot section, at a cost in efficiency. | CMC parts approaching 2,400 F (about 1,590 K) in the CFM LEAP |
| Overall engine efficiency headroom | 30-35% better than today's best engines judged achievable1 | Overall efficiency has improved about 7% per decade since 1970 through many small steps. | about 40% overall |
Sources
- Commercial Aircraft Propulsion and Energy Systems Research, chapter 2: Aircraft-Propulsion Integration and gas turbine engines National Academies of Sciences, Engineering, and Medicine, 2016
- CO2 emissions from commercial aviation: 2013, 2018, and 2019 ICCT, 2020
- French connection: Air France-KLM will use fuel-efficient LEAP engines GE Aerospace, 2022