Direct injection diesel engines have injectors mounted at the top of the combustion chamber. The injectors are activated using one of two methods - hydraulic pressure from the fuel pump, or an electronic signal from an engine controller.

Hydraulic pressure activated injectors can produce harsh engine noise. Fuel consumption is about 15–20% lower than indirect injection diesels. The extra noise is generally not a problem for industrial uses of the engine, but for automotive usage, buyers have to decide whether or not the increased fuel efficiency would compensate for the extra noise. The noise level of modern common rail direct injection diesel engines with multiple injections per combustion cycle is however comparable to those of their gasoline counterparts.

Electronic control of the fuel injection transformed the direct injection engine by allowing much greater control over the combustion.