The most common systems are the vapour sequential injection and liquid injection systems.    The Vapour sequential injection system injects the fuel in a gaseous phase but aligns the gas injections to what the original electronic gasoline controls of the car command.   The Liquid injection system exactly replicates gasoline systems and injects the fuel in a liquid phase without prior vaporisation. This enables more precise metering of the fuel and an injection timed to the opening of the intake valves.