Combustion road vehicles: Science and limits
Where three quarters of the fuel goes, and why
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 typical petrol car gets 18-25% of its fuel energy to the wheels on a mixed test cycle; the rest leaves mostly as heat. The best heavy-duty diesels turn over 50% of fuel energy into crankshaft work.
The conversion, step by step
CombustionChemical (petrol, diesel, CNG or LPG) → Hot, high-pressure gas · about 97% (3% of fuel left unburnt)2Expansion on pistons
Heat → Mechanical (crankshaft) · engine losses 68-72% of fuel (combined cycle), mostly heat to radiator and exhaust12Accessories
Mechanical → Electrical and hydraulic services · 4-6% of fuel1Gearbox and driveline
Mechanical → Mechanical (wheels) · 3-5% of fuel lost1End to end, petrol car
Chemical → Kinetic · 14-20% city, 22-30% highway, 18-25% combined1End to end, heavy-duty diesel engine
Chemical → Mechanical (crankshaft) · 43-44% (2013-2014 production); 53.09% (Weichai, verified 2024)45
The science
Burning fuel in a closed cylinder makes hot gas that pushes a piston; the crankshaft turns the push into rotation. The more the charge is compressed before burning, the more of its heat can become work. Diesels ignite fuel by compression alone and run at higher compression than petrol engines, which use a spark and must avoid knock. At light load a petrol engine's throttle restricts the air, adding pumping losses. Rejecting much heat to exhaust and coolant is required by the second law, not a design fault. CNG and LPG engines are spark-ignited; their gain is lower carbon per unit of energy (56.1 and 63.1 kg CO2 per GJ, against 69.3 for petrol and 74.1 for diesel).
Limits set by science
| Limit | Value | Why it exists | Best today |
|---|---|---|---|
| Ideal Otto cycle | 64% at a compression ratio of 13 (calculated: 1 - 13^-0.4)3 | Ideal efficiency is 1 - 1/r^(gamma-1), with r the compression ratio and gamma 1.4 for air; for hot combustion gas (gamma about 1.3) it is about 54% (calculated). Wall heat loss, friction and finite burn time cut real engines further. | Toyota 2.5 litre Dynamic Force engine: 40% peak thermal efficiency at r = 13, 41% at r = 14 in the hybrid version |
| Carnot bound | about 88% (calculated: 1 - 300/2,500, for an assumed peak gas temperature of 2,500 K and surroundings at 300 K)5618 | No heat engine can beat 1 - Tc/Th. The gas is this hot only briefly, so this is a loose upper bound; the ideal cycle is the better guide. | Best diesels reach 53-55%, about 60% of this bound (calculated) |
| Heavy-duty diesel frontier | 55% brake thermal efficiency (share of fuel energy delivered at the crankshaft)5618 | Shown on a US DOE SuperTruck II test engine at a 65 mph cruise point, using waste heat recovery. | Cummins 55% (2021, test engine); Weichai 53.09% (2024, verified by TUV SUD and CATARC) |
| Part-load penalty | idling alone is 6% of fuel in city driving (3% combined)1 | Engines are most efficient near full load; in town a car uses a small fraction of its power. | Stop-start and hybrids remove idling |
Sources
- Where the Energy Goes: Gasoline Vehicles US DOE and US EPA (fueleconomy.gov), 2024
- Fact #709: Engine energy use, where does the energy go? US Department of Energy, Vehicle Technologies Office, 2012
- TNGA powertrain: 2.5-litre Dynamic Force engine Toyota Motor Corporation, 2016
- The ever-improving efficiency of the diesel engine ICCT, 2015
- Weichai unveils diesel engine with 53.09% thermal efficiency DieselNet, 2024
- SuperTruck II team reaches brake thermal efficiency goal Heavy Duty Trucking (truckinginfo.com), 2021
- 2022 Annual Merit Review: Cummins/Peterbilt SuperTruck II US Department of Energy, Vehicle Technologies Office, 2022