A system for supply of LPG/ammonia for direct-injection petrol or diesel engine includes an electronic control unit where the control unit governs a petrol/diesel pump, a supplementary petrol/diesel pump, a petrol/diesel-delivery solenoid vale, LPG/ammonia-return solenoid valve, LPG/ammonia pump, a supplementary LPG/ammonia pump, a LPG/ammonia-delivery solenoid vale and a LPG/ammonia injector, such that only one of LPG/ammonia and petrol/diesel is fed to the engine at any point in time, and a combination of i) LPG and petrol, ii) LPG and diesel, iii) ammonia and petrol, and iv) ammonia and diesel is never fed to the engine.