Combustion road vehicles: Engineering methods

Petrol, diesel, CNG and LPG engines compared head to head

What this tab covers The engineering methods that can do this conversion, compared on efficiency, scale, economics, limitations and reliability.

Three engine families power most road vehicles: spark-ignition petrol, compression-ignition diesel, and spark-ignition engines adapted for CNG or LPG.

Comparison

Method	Efficiency	Scale	Economics	Limitations	Reliability
Petrol car (spark ignition)¹³⁸⁷¹⁰	Peak engine 40%; 18-25% fuel-to-wheels on the combined cycle	Most of the roughly 75% of new cars sold with engines in 2025 (calculated)	55.87-98.54 US cents per mile to own (US, 2025)	Knock limits compression; poor part-load efficiency; tailpipe CO2 69.3 kg per GJ of fuel	Mature; exhaust catalyst and engine wear are main issues (no failure data in sources used)
Diesel truck and bus (compression ignition)⁴⁵⁶⁹¹⁰⁷¹⁸	43-44% (2013-2014 production engines); 50-51% in the first SuperTruck demonstrators; 53.09% Weichai (2024); 55% Cummins SuperTruck II test engine	About 91% of 2025 truck sales were not electric (calculated from 9% electric), most of them diesel	EUR 0.98-1.15 per km total cost for European trucks of model year 2030, 16-22% above battery electric (calculated)	NOx and particulates need costly aftertreatment; CO2 74.1 kg per GJ	Heavy-duty engines are designed for very long lives (no figure in sources used)
CNG vehicles¹²⁷	Similar to petrol engines (no separate figure in sources used)	India: about 788,000 CNG three-wheelers, cars, buses and goods vehicles registered in April 2024 to January 2025 (calculated sum of official figures); 7,395 stations	Cheaper fuel per km than petrol where gas prices are regulated (no checked figure)	Bulky tanks; refuelling networks are sparse in most countries; unburnt methane is a strong greenhouse gas	No data in sources used
LPG vehicles (autogas)⁷	Similar to petrol engines (no separate figure in sources used)	No vehicle count in sources used	No figure in sources used	Tank space; restricted parking in some places	No data in sources used

Diesel engines are the most efficient combustion option and dominate heavy freight, where their extra cost is paid back by fuel savings. Petrol engines are cheaper, quieter and cleaner in local pollutants but waste more fuel, especially in town. CNG and LPG are mainly chosen for fuel price and lower CO2 per unit of energy (19% and 9% less than petrol, calculated from 56.1 and 63.1 against 69.3 kg/GJ), not for higher engine efficiency. All combustion vehicles share the same thermodynamic ceiling, which electric drives avoid.

How each method works

Petrol car (spark ignition)

Fuel and air are mixed, compressed (13 to 14 times in Toyota's most efficient engines) and lit by a spark. A throttle controls power, which wastes energy at light load.

Diesel truck and bus (compression ignition)

Air alone is compressed until it is hot enough to ignite injected diesel, so there is no throttle and compression can be higher. Turbochargers, high-pressure injection and aftertreatment (particulate filter, urea SCR) are standard.

CNG vehicles

Methane stored as a high-pressure gas in steel or composite cylinders is fed to a spark-ignition engine; many cars are bi-fuel with petrol.

LPG vehicles (autogas)

Propane and butane are stored as a liquid at modest pressure and vaporised into a spark-ignition engine; usually converted petrol cars and taxis.

Sources

Where the Energy Goes: Gasoline Vehicles US DOE and US EPA (fueleconomy.gov), 2024
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
2006 IPCC Guidelines for National Greenhouse Gas Inventories, Vol. 2, Ch. 3 Mobile Combustion, Table 3.2.1 IPCC, 2006
Your Driving Costs 2025 fact sheet AAA, 2025
A total cost of ownership comparison of truck decarbonization pathways in Europe: fact sheet ICCT, 2023
Global EV Outlook 2026: Executive summary IEA, 2026
Lok Sabha Unstarred Question No. 1420: CNG stations and CNG vehicle registrations (Ministry of Petroleum and Natural Gas reply) Government of India, Lok Sabha, 2025
2022 Annual Merit Review: Cummins/Peterbilt SuperTruck II US Department of Energy, Vehicle Technologies Office, 2022