Fuel Injector

The fuel is delivered by the fuel pumps to the injectors. For the fuel to burn completely at the correct time it must be broken up into tiny droplets in a process known as atomisation. These tiny droplets should penetrate far enough into the combustion space so that they mix with the oxygen. The temperature of the droplets rise rapidly as they absorb the heat energy from the hot air in the cylinder, and they ignite and burn before they can hit the relatively cold surface of the liner and piston.

Fuel injectors achieve this by making use of a spring loaded needle valve. The fuel under pressure from the fuel pump is fed down the injector body to a chamber in the nozzle just above where the needle valve is held hard against its seat by a strong spring. As the fuel pump plunger rises in the barrel, pressure builds up in the chamber, acting on the underside of the needle as shown. When this force overcomes the downward force exerted by the spring, the needle valve starts to open. The fuel now acts on the seating area of the valve, and increases the lift. As this happens fuel flows into the space under the needle and is forced through the small holes in the nozzle where it emerges as an "atomised spray". At the end of delivery, the pressure drops sharply and the spring closes the needle valve smartly.

Older loop scavenged engines may have a single injector mounted centrally in the cylinder head. Because the exhaust valve is in the centre of the cylinder head on modern uniflow scavenged engines the fuel valves (2 or 3) are arranged arround the periphery of the head.

The pressure at which the injector operates can be adjusted by adjusting the loading on the spring. The pressure at which the injectors operate vary depending on the engine, but can be as high as 540bar.

Some injectors have internal cooling passages in them extending into the nozzle through which cooling water is circulated. This is to prevent overheating and burning of the nozzle tip.

Injectors on modern engines do not have internal cooling passages. They are cooled by a combination of the intensive bore cooling in the cylinder head being close to the valve pockets and by the fuel which is recirculated through the injector when the follower is on the base of the cam or when the engine is stopped.

As well as cooling the injector, recirculating the fuel when the engine is stopped keeps the fuel at the correct viscosity for injection by preventing it from cooling down. The animation opposite shows the principle on which one system operates.

Fuel injectors must be kept in good condition to maintain optimum efficiency, and to prevent conditions arising which could lead to damage within the cylinder. Injectors should be changed in line with manufacturers recommendations, overhauled and tested. Springs can weaken with repeated operation leading to the injector opening at a lower pressure than designed. The needle valve and seat can wear which together with worn nozzle holes will lead to incorrect atomisation and dribbling.