Engine Fuel Control System

Apparatus for controlling fuel supply to a fuel injected engine having an electronic engine management system (1) that supplies a primary injector control signal (8) to each of primary fuel injectors (2), the apparatus comprising an emulator (11) that emulates the electrical characteristics of a primary injector, a fuel control switch (10) that switches the primary injector control signal (8) from the primary injector (2) to the emulator (11) when an alternative mode is selected, a monitor (16) that monitors the primary injector control signal (8) to produce a monitor signal, and a controller (4) that processes the monitor signal to derive an alternative control signal which is used to control the primary fuel supply or a mixture of the primary fuel and a secondary fuel, to the engine. Preferably the primary fuel supply is modulated by controlling injector control pulses. A secondary fuel may also be injected (3) and controlled by the controller (4).