Which Fuels Can Be Used in an Internal Combustion Engine?

#Why coal fails on every count

It cannot be metered or atomised. You cannot inject a solid through a nozzle or mix it homogeneously with air in a manifold.

It burns far too slowly. Coal burns from the surface inward. Even the powdered coal used in a power station takes one to two seconds to burn out — and an engine at 2,000 rpm gives it roughly fifteen milliseconds.

It leaves ash. Between 5% and 45% of coal's mass is incombustible mineral matter. Abrasive ash circulating in an engine would score the bore, jam the rings and wreck the valves in minutes.

The only way to run an engine on coal or wood is to convert the solid into a gas first — gasification, producing carbon monoxide and hydrogen. Wood-gas vehicles genuinely existed during wartime fuel shortages, and they worked, but the gas generator was a large hot appliance bolted to the vehicle, and the power output was poor.

#The fuels that do work

Fuel	Engine type	Notes
Petrol	Spark ignition	Needs high octane
Diesel	Compression ignition	Needs high cetane
CNG	Spark ignition	Very knock-resistant, clean
LPG	Spark ignition	Clean, easy to store
Ethanol	Spark ignition	High octane, blended widely
Biodiesel	Compression ignition	Good cetane, from plant oils
Hydrogen	Spark ignition	No carbon in the exhaust

#The octane and cetane split

This is where people get tangled, and it is worth being clear because the two properties run in opposite directions.

A spark-ignition engine compresses a fuel-air mixture and then ignites it with a spark at a chosen moment. If the mixture self-ignites early from compression heat, you get knock, which destroys engines. So petrol must resist self-ignition — that is what a high octane number means.

A compression-ignition engine injects fuel into air already heated by compression, and wants it to ignite immediately. So diesel must self-ignite readily — that is what a high cetane number means.

Same word structure, opposite requirement. It is also why putting petrol in a diesel tank does real damage: the ignition behaviour is entirely wrong, quite apart from the lost lubrication.

CNG's high effective octane, around 120, is a genuine engineering advantage — it allows higher compression ratios, which recovers much of the power that a converted CNG engine otherwise loses.

#Hydrogen in a piston engine

Worth knowing because it comes up as its own question. Hydrogen burns very well in a modified spark-ignition engine, with wide flammability limits and fast flame speed. There is no carbon in the fuel, so the exhaust contains no carbon dioxide, no carbon monoxide, no soot and no unburnt hydrocarbons — only water vapour, plus nitrogen oxides formed from the air at high flame temperature.

The obstacles are entirely upstream: hydrogen has to be manufactured, compressed to 700 bar or liquefied at −253 °C, and distributed, and it embrittles many metals. The engine is the easy part.

#Gas turbines are far less fussy

A related question asks which fuels suit a gas turbine, and the answer is: most of them. Natural gas, kerosene and aviation turbine fuel, diesel, naphtha, and increasingly hydrogen blends.

The reason is architectural. A gas turbine burns fuel continuously in a combustor, rather than in timed explosions synchronised to a moving piston. Nothing depends on the fuel igniting at exactly the right crank angle, so octane and cetane simply do not apply. What matters instead is clean, steady combustion and no ash or contaminants to erode the turbine blades — which is why aviation kerosene is specified so tightly.

#The compact answer

Coal is not used in internal combustion engines. It is a solid, so it cannot be injected or mixed with air inside a cylinder, it burns far too slowly for the milliseconds available, and it leaves abrasive ash that would damage the engine. Petrol, diesel, CNG, LPG, ethanol, biodiesel and hydrogen are all used, because they are liquid or gaseous, burn rapidly and leave no solid residue.