We now know we need to pump a spray of fuel under high pressure into the combustion chambers of our CI engine. So what equipment do we need to achieve this? Generally we would use at least one high-pressure pump and a device called an injector for each cylinder of the engine.
CI engine injectors come in a myriad of sizes, shapes and designs but all have at their end a specialised nozzle through which the fuel is sprayed. Even nozzle designs may be one of very many types, but they all aim to achieve a uniform, very fine spray of fuel when they operate and to stop and start that spray instantly, with minimal leakage or ‘dribble’. Usually this is performed by a finely machined ‘needle’ which uncovers and covers the tiny orifices through which the fuel is forced at very high pressure.
More modern injectors are two-spring types. In these, the needle opens a small amount at a lower pressure and then fully at a higher pressure. This gives a ‘softer’ start to combustion and a reduction in the typical diesel ‘rattle’ at idle and part-throttle.
The precision and accuracy of modern high-pressure injectors is truly mind-boggling. They are manufactured under super clean conditions to avoid any contamination. If you were to pull the needle out of a modern injector nozzle with your fingers, have a brief look at it and then put it back, you could then throw that hundred dollars or so worth of nozzle into the rubbish bin. The minute amounts of body oil and acid on your skin would have marred the surface of the needle enough to make it inoperable! This is the main reason why diesel specialists agree on the importance of always maintaining clean fuel filters.
High-pressure pumps also come in a wide variety of designs but again, they all have a common aim: to generate a precisely metered high-pressure pulse of fuel and deliver it to the right cylinder injector at the right time. Further, as requirements for performance (kW / Litre of engine capacity) and emission control increase, CI injections systems are constantly evolving to produce ever higher injection pressures.