Fuel injector The fuel is delivered by the fuel pumps to the fuel injectors or fuel valves. For the fuel to burn completely at the correct time, it must be broken into tiny droplets known as atomisation. These tiny droplets should penetrate far enough in the combustion space so as to mix with oxygen. The temperature of the tiny droplets rise rapidly to above their self ignition temperature as they absorb heat from the hot compressed air in the cylinder. Thus 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 spring loaded needle valve. Fuel under pressure from the fuel pump is fed down the injector body to a chamber in the nozzle just above where 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.

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 small holes in the nozzle, where it emerges as ‘atomised spray”. At the end of delivery, the pressure drops sharply and the spring closes the valve smartly. Note: the closing pressure of the needle valve is much lower than the opening pressure.

Older loop scavenged engines had a single injector mounted centrally in the cylinder head. In modern uniflow scavenged diesel engines, the exhaust valve is centrally mounted in the cylinder head. Two or three fuel valves are mounted around the exhaust valve in the cylinder head. The pressure at which the injector opens is adjusted by adjusting the load on the spring. This pressure is specified by the manufacturer and is around 250 bar. After the initial opening, the pressure rises further and is much higher (may be around 600 bars). Some injectors have internal cooling passages in them extending into the nozzle through which cooling water or oil is circulated. This is to prevent overheating and burning of nozzle tip.

In modern diesel engines, injectors 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 cam follower is on the base circle or when the engine is stopped. Apart from cooling the injector, recirculating the fuel when engine is stopped, keeps the fuel at correct viscosity for injection by preventing it from cooling down

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 must be changed in line with manufacturer’s recommendation, overhauled and tested. Springs can weaken with repeated operations leading to injectors opening at lower pressures than designed. The needle valve and seat can wear, which together with worn nozzle holes, will lead to incorrect atomisation and dribbling.