Because of their durability and fuel efficiency, diesel engine passenger vehicles have been growing in popularity, particularly in Europe, where diesel vehicles outsold petrol/gasoline vehicles for the first time in 2006, and where the trend towards diesel is still growing.
Gasoline/petrol engines compress a mixture of air and fuel which is then ignited by a spark plug. The relatively low compression ratios of petrol/gasoline avoid premature detonation during the compression stroke.
Diesel engines are more robust in construction because they operate with much higher compression ratios – high enough to make the compressed air in the cylinder hot enough to spontaneously ignite the fuel when it is introduced into the combustion chamber. Once the fuel is injected, 'ignition lag' creates the familiar diesel knock, a loud clattering sound that has generally been tolerated by drivers of heavy goods vehicles, but is less acceptable to the discerning owners of more luxurious passenger vehicles.
The annoying rattle of a diesel engine has largely been subdued by a combination of engineering and fuel improvements, including injector design and spray pattern, injection timing, compression temperature, and fuel cetane rating.
Fast processors allow precise electronic control of direct injection so that the combustion process can be smoothed to allow the up and down transitions of the pistons to occur without the traditional diesel clatter.
In a modern quiet diesel engine, fuel injection timing and delivery is 'pulsed' – first with a small 'lean' pilot injection to start the ignition process with less noise, quickly followed by a main injection of fuel to develop engine torque and complete the power stroke, finishing with an additional pulse to assist with after-burning pollutants in the converter.
The result of all the combined refinements is dramatically smoothed idle rpm and roughness; far less combustion noise; no smoke, less odor and emissions; and improved fuel consumption.