FUEL INJECTORS

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The function of the fuel injector is to atomise and disperse the fuel evenly within the combustion space, which is achieved by the size, position, and orientation of the injector nozzle. It also acts as a non-return valve to prevent gas blow-back into the fuel system.

In order to achieve satisfactory atomisation at the commencement/end of injection, the valve should not open until a pre-set pressure is reached. This pressure should ensure that the fuel droplet size at the start of injection is not too large, which will increase “slow burning” of the fuel.

Another requirement of the valve is a prompt opening action, to prevent throttling (i.e. pressure loss) during the needle valve opening. This is achieved by using a differential needle valve, which opens with a snap action when the high pressure fuel acts on the full cross section area of the needle. One drawback of using this differential valve, is the lower closing pressure of the valve, but the larger droplets injected at this point will be injected into a hotter cylinder and should ignite and burn relatively fast.

The injector must be cooled in service to prevent softening of the valve and seat, and to minimise expansion of the trapped fuel in the fuel sac. Cooling can be achieved either by separate oil or water system, or more recently using the bore cooling system within the cylinder head to cool the injector.To minimise the need for engines to run on distillate fuels, modern engines are designed to operate on residual fuel even whilst manoeuvring.

For the MAN B&W slow speeds, the fuel injector is provided with a method of spilling/recirculating the hot system oil when the injector is not injecting fuel. This is carried out using a pressure sensitive re-circulation system that only allows the fuel injector to recirc fuel when the pressure is between 2 and 8 bar. Hence once the fuel pump starts to inject fuel, this re-circulation line is closed.