A two-stroke reciprocating piston engine. The engine principally consists of an expansion cylinder and a compression cylinder, and an expansion chamber and a combustion chamber. When pistons of the expansion cylinder and compression cylinder move from a bottom dead center to a top dead center, gas in the expansion cylinder is compressed in the expansion chamber, gas in the compression cylinder is compressed in the combustion chamber, a water spray nozzle sprays water towards the high temperature and high pressure gas in the expansion chamber, and when the water encounters the high temperature and high pressure gas, the water is rapidly vaporized. At the same time, an oil spray nozzle sprays a fuel into the combustion chamber, a spark from a spark plug ignites the combustible gas of the combustion chamber, the high temperature and high pressure gas produced by the combustion pushes open a valve of the combustion chamber.

An internal combustion engine injector mounting is configured to i) secure an injector (36) to an internal combustion engine body so that the injector can inject a heated aqueous fluid and a fuel into a combustion chamber of the internal combustion engine, and ii) hold a catalyst so that the heated aqueous fluid and fuel are injected into the combustion chamber by passing through the catalyst. The catalyst is contained within the mounting to protect against oxidation by combustion processes that, in use, take place in the combustion chamber.

An internal combustion engine injector mounting is configured to i) secure an injector (36) to an internal combustion engine body so that the injector can inject a heated aqueous fluid and a fuel into a combustion chamber of the internal combustion engine, and ii) hold a catalyst so that the heated aqueous fluid and fuel are injected into the combustion chamber by passing through the catalyst. The catalyst is contained within the mounting to protect against oxidation by combustion processes that, in use, take place in the combustion chamber.

Systems and methods for reducing NO.sub.x emissions are provided, comprising: adjusting an amount of water injected into an intake manifold responsive to an oxygen concentration, temperature and pressure in the intake manifold; and heating the injected water if humidity is higher than a threshold. Water injected into the intake manifold decreases the temperature of, and dilutes the oxygen content of intake gases thereby decreasing NO.sub.x emissions.

An internal combustion engine comprises an engine block defining a cylinder having a longitudinal axis A. A piston is arranged slidably within the cylinder and an impeller is arranged at one end of the cylinder. The impeller is rotatably mounted on a shaft, which extends out of the cylinder and which is driven in rotation by rotation of the impeller. The engine further comprises an anti-rotation formation to prevent the piston rotating about a longitudinal axis of the cylinder and a swirl-inducing vane arranged on the face of the piston which faces the end of the cylinder at which the impeller is arranged. Combustion gas generated by combustion of a fuel in the cylinder between the piston and the impeller is caused to swirl by reaction with the swirl-inducing vane and the swirling combustion gases, in turn, cause the impeller to rotate.