On engines with diesel direct injection, the fuel is injected directly into the combustion chamber. Direct injection diesel engines are currently the most efficient form of combustion engine with the highest degree of thermal efficiency. Various methods are used to optimise the combustion process in conjunction with diesel direct injection. Alongside the pump-line-injector technology, established solutions also include the common-rail and pump-injector systems. The highest injection pressure is currently achieved with the pump-injector system where the pump and injector together form a single unit. This set-up generates the maximum injection pressure of up to 2,400 bar at each cylinder separately. The injection system is integrated into the cylinder head and pressure is built up mechanically with the aid of the camshaft. Multipoint injection is used for even more efficient combustion of the fuel/air mixture. Depending on the rev speed and engine load, modern diesel engines may use up to four injection processes in all, comprising double pilot injection, the main injection process and single post-injection. Pilot injection pre-injects no more than 1 to 2 cubic millimetres of fuel into the combustion chamber but this is enough to reduce combustion noise considerably and minimise the levels of nitrogen oxide contained in the exhaust gas. Volkswagen offers diesel direct injection technology based on the pump-injector system in all of its model series.