Instead of a carburetor, a fuel-injection system may be applied for introducing fuel (gasoline, petrol) into the cylinder. It is basically similar to the system employed in a diesel engine, except that with gasoline as the fuel the ignition is initiated by an electric spark. A somewhat higher effective pressure better output can be achieved by injection as compared with a carburetor system. On other hand, the injection equipment is more expensive. In practical life, this method of introducing the fuel is therefore confined to high-output or racing engines.

With the help of injection, the inlet pipe for each cylinder can be designed to give great performance as an individual Oscillation tube. Since the fuel is injected straight into the cylinder, the need to heat the inlet pipe (to prevent condensation of gasoline vapor) is obviated. Consequently, cooler and therefore denser air is drawn into the cylinder, thus improving the volumetric efficiency.

Injection of gasoline starts during the suction stroke. On entering the cylinder, the gasoline vaporizes, and the heat for evaporation is extracted from the air, so that this cools and decreases in volume, thus causing more air to be drawn in and thereby improving the volumetric efficiency.

Fig.1 shows an arrangement in which the injection nozzle is aimed at the hot exhaust valve, which is cooled by the gasoline. During the compression stroke the piston sweeps past the outlet of the nozzle and thus protects it from the high pressure that develops at the instant of combustion i.e. initiated by spark ignition. A different arrangement is shown in Fig.2, in which the injection nozzle is located outside the cylinder, protected from high pressure and temperature. It injects the fuel through the inlet port on to the opened inlet valve and thus into the cylinder.

The measure to improve volumetric efficiency that have been described in the foregoing relate to four-stroke internal-combustion engines which draw in the fuel-and-air mixture by the natural suction developed in the cylinder. Another means of increasing the power output is provided by supercharging. The supercharger is a compressor or blower which supplies air, or a combustion mixture of fuel and air, to the cylinders at a pressure greater than atmospheric. Because of this higher pressure, the air supplied to the cylinders has a higher density and absorbs more gasoline vapor.

This increases the power output, but the gas consumption per horsepower is higher than in a suction-induced-charge engine, and wear and tear becomes more severe. Fig.3 is a partial section through an American V8 engine equipped with a Roots supercharger with three-lobed rotors. The supercharger is usually driven from the crankshaft.