Diesel Engine

Diesels compress air to a much higher ratio than petrol engines, which heats it enough to ignite the fuel when it is injected. Power is controlled by how much fuel is injected rather than by throttling the air, one reason diesels use less fuel at part load. Modern diesels use common rail injection, turbocharging and extensive exhaust treatment: an oxidation catalyst, a particulate filter (DPF), exhaust gas recirculation (EGR) and often SCR with AdBlue to cut nitrogen oxides.

Diesel engines usually use compression ratios far higher than petrol engines. Their high torque at low engine speed makes them popular for heavy vehicles, towing and long-distance driving.

Glow plugs help cold starts by heating the combustion chambers. The control unit meters fuel through high-pressure injectors, and the turbo provides boost for power. Fuel is injected directly into the hot compressed air near the end of the compression stroke. Modern diesels use common rail injection, turbocharging and complex exhaust after-treatment.

Short trips are hard on modern diesels because the particulate filter needs regular hot running to regenerate. Fuel quality, a clean fuel filter and the correct low-ash engine oil matter more than on most petrol engines. Modern diesels are best suited to regular longer journeys, because the particulate filter needs heat to regenerate. Used mostly for short trips, they can develop DPF and EGR problems.