The formulation and content of an engine’s fuel obviously has a major effect on how the engine behaves and what comes out of the exhaust. The relative proportions of different substances in the fuel, whether they are natural constituents, impurities or deliberate additives, can have either a positive or a negative effect on performance and emissions so engine design has to be carefully adjusted to work best with the fuel formulation likely to be used. It is therefore very important that fuel formulation does not vary too far from that expected, as this will affect both driving performance and pollutant emissions.

Fuel specification becomes even more critical with the increasingly complex and sophisticated emission control equipment now fitted to vehicles. Each of these devices needs a high quality fuel with a predictable formulation so that it can be designed and calibrated to perform correctly.