Diesel vs. Gasoline (Petrol) Engines: Differences, Pros and Cons

Differences between a diesel and a gasoline (petrol) engine: ignition, compression, fuel economy, torque, maintenance and emissions. Advantages and limits of each.

By Diesel Universal technical team2 min read

Key points

A diesel ignites the fuel by compression; a gasoline engine does it with a spark.
Diesels usually offer more torque at low revs and better fuel economy per mile.
A modern diesel depends heavily on its injection system and fuel quality.
The right choice depends on use: heavy work and long trips favor diesel; short urban trips sometimes favor gasoline.

At first glance a diesel engine and a gasoline engine look alike: cylinders, pistons, crankshaft. The fundamental difference is how the mixture is ignited, and almost everything else follows from it: fuel use, torque, maintenance and the kind of use each suits best. (In the UK and many other countries, gasoline is called petrol; this article applies to both.)

How does each engine ignite?

Gasoline engine: draws in a mixture of air and gasoline, compresses it moderately and a spark plug produces a spark that ignites it.
Diesel engine: draws in only air, compresses it much more (high compression ratio), so the air becomes extremely hot. Then it injects fuel at high pressure and the fuel ignites on its own.

That is why a diesel needs stronger parts and a highly precise injection system. See types of diesel injection pumps and what is Common Rail.

Comparison

Aspect	Diesel	Gasoline
Ignition	By compression (auto-ignition)	By spark (spark plug)
Compression ratio	High	Moderate
Torque at low revs	High	Lower
Fuel use per mile	Usually lower	Usually higher
Maximum engine speed	Lower	Higher
Sensitivity to fuel quality	High (water and dirt damage the injection)	Lower in this respect
Emissions most controlled	NOx and particulates (EGR, DPF, SCR)	CO, hydrocarbons and NOx (catalytic converter)
Typical use	Heavy work, highway, hauling and towing	Urban and mixed use, light vehicles

Advantages of diesel

High torque at low revs: ideal for hauling, towing and climbing grades.
Lower fuel consumption on the highway and under load.
Durability: diesel engines are built for demanding work and long distances.
Longer range per tank.

Limitations of diesel

Costly injection system, very sensitive to fuel quality. See water in diesel fuel.
Emissions systems (EGR, particulate filter, SCR) that require a certain type of use and maintenance. See what is a DPF, EGR valve problems and AdBlue and SCR.
Cold starting is more demanding, with glow plugs. See diesel glow plugs.
Less suited to very short trips, which do not let the engine or the particulate filter reach temperature.

What suits which use?

Lots of highway, loads, towing or heavy work: diesel is usually the natural choice.
Short urban trips and low annual mileage: a gasoline engine can be more practical and avoid particulate filter problems.
Machinery, trucks and buses: diesel is practically the standard because of its torque and durability.

A mistake to avoid

Never put gasoline in a diesel, or diesel in a gasoline engine. Gasoline lacks the lubricity of diesel and destroys pumps and injectors in a short time. If it happens, do not start the engine: drain the tank and lines before using it again.

Frequently asked questions

What is the main difference between a diesel and a gasoline engine?+

How the mixture is ignited. A gasoline engine uses a spark plug to produce a spark; a diesel compresses air only until it is very hot, injects the fuel and it ignites on its own.

Why does diesel use less fuel?+

Because it has a higher compression ratio, which makes better use of the fuel's energy, and because diesel fuel contains more energy per liter than gasoline.

Is a diesel more expensive to maintain?+

It depends. Injection system components are more expensive and sensitive to fuel quality, but diesel engines are usually very durable if properly maintained.

Can gasoline be put in a diesel?+

No. Gasoline does not lubricate like diesel and can seriously damage the pump and injectors. If it happens, do not start the engine and have the tank drained.

Diesel Universal technical team

Laboratorio Diesel Universal S.A.S. Diesel injection laboratory in Barrancabermeja, Colombia. It diagnoses and calibrates injection pumps, injectors and turbochargers on an electronic test bench.

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