The diesel engine, named after the German engineer Rudolf Diesel, is a type of internal combustion engine that is somewhat different to a petrol engine. Air is compressed first and at around 800°C receives an atomised injection of diesel fuel which creates the ignition. These engines are more efficient than petrol engines of the same power and this results in lower fuel consumption. This article discusses the basic operation of the typical automotive diesel engine and especially, the fundamental differences between diesel and petrol engines.