Abstract: In a diesel engine fuel injection system, a fuel injection pump draws fuel from a reservoir, delivers a portion of the fuel to the engine, and directs excess fuel through a return line back to the reservoir. A supply pump delivers fuel from a tank to the reservoir to maintain a constant volume of fuel in the reservoir. A fuel metering unit measures the rate of fuel delivery by the supply pump. An additive pump feeds an additive through the return line to the reservoir where the additive mixes with the fuel. The fuel metering unit controls the additive pump to proportion the rate at which additive is fed to the reservoir to the rate at which fuel is delivered by the supply pump, and the amount of additive delivered to the engine is thereby proportioned to the amount of fuel consumed by the engine.