Gaseous fuel injection system for engines

A gaseous fuel direct injection system (10) which comprises a primary fuel tank (13) for receiving a gaseous fuel such as LPG, and a secondary fuel tank for receiving a reserve fuel supply. The gaseous fuel is delivered to a fuel injector (23) connected to a fuel rail (15). Operation of the fuel injector (23) is controlled by an electronic control unit (ECU). As the gaseous fuel available in the primary fuel tank (13) depletes, the vapour pressure in the fuel rail (15) diminishes. The ECU is able to compensate for the reduced vapour pressure in the fuel rail (15) by increasing the duration of opening of the fuel injectors (23) as necessary in order to maintain delivery of a necessary mass of gaseous fuel by the fuel injector (23). However, at some stage the ECU can no longer compensate for the reduced fuel pressure, at which time it becomes necessary to switch the fuel supply from the primary tank (13) to the secondary fuel tank. The decaying pressure characteristic of the depleting gaseous fuel in the primar