Diesel Fuel Injection Systems

In fuel-injected spark ignition [SI] engines, fuel is always injected into the air charge well before ignition takes place. This is necessary because the liquid or gaseous fuel must be thoroughly mixed together with air into a combustible mixture and then be ignited by the electrical arc generated by the sparkplug. If the ratio of air to fuel is not reasonably close to 15:1 in the vicinity of the sparkplug, the mixture will not ignite at all and a miss-fire results.

Compression ignition [CI] engines always inject the fuel charge directly into a combustion chamber in the engine. Fuel injection and ignition are inextricably tied together in compression ignition [CI] engines. Recall that CI engines only work because they compress the air charge so that it is hot enough to instantly ignite the fuel charge as it is being injected. The combustion of the fuel begins at the instant it begins being injected (within a couple of milliseconds) into the combustion chamber full of very hot air (more than 400 ºC).

This means the timing of ignition is intimately tied to the fuel injection process. So, the fuel injection system of a CI engine is responsible for regulating both the quantity of fuel to be injected and the timing at the very start of combustion. Many ingenious techniques have been developed to achieve both these tasks with admirable accuracy, long before the advent of sophisticated electronic controls.

As you can see, diesel and petrol/gasoline engines go about the task of releasing energy from their fuels in quite different ways. For more general information on this topic see our Why Diesels are Different article.

There are many factors that will determine how much fuel is being injected. To help understand how much fuel is being injected, a few calculations must be observed. We will use a 1997 Land Rover as an example.

SI engine (petrol/gasoline) injection systems typically run at pressure of 2 to 3 bar (30 to 40 psi). In contrast CI (diesel) engines employ injection pressures of at least 350 bar (~5000 psi) and possibly in excess of 2000 bar (more than 29,000 psi) – quite a bit different to petrol/gasoline systems! This explains why CI injection systems are so solidly built and piped-up with strong steel tubing, etc.

Petrol/gasoline readily vaporises in the air stream, entering the engine’s cylinders and in contact with the hot cylinder head surfaces, to form an easily ignitable air/vapour mixture. On the other hand, to instantly ignite the much less volatile diesel fuel in the hot air charge of a CI engine, it is necessary to spray it into the combustion chamber in extremely small droplets. And to achieve this, extremely high injection pressures are required – the higher the better, in general.

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.

All mechanical and many electronically-controlled CI injection systems use a piston or plunger pump. These are a type of positive displacement pump, in which a plunger slides in a cylinder to push a volume of fuel out of the pump to an injector, similar to a hypodermic syringe. If you have a plunger and pump that’s of a fixed size, then it will always pump the same volume of liquid with each stroke. Since this is true, then how can the volume of fuel which is injected into the engine be varied over the wide range necessary? In general, the ‘spill’ method is used.

In-line Pumps

The ‘traditional’ style of injection pump is the in-line pump. They have been used for many decades and are still commonly found on agricultural and stationary diesels. They are also found in many older model diesel road vehicles such as the Toyota Landcruiser 2H diesel. They are typically capable of generating injection nozzle pressures up to about 750 bar in light road vehicles – towards the lower end of the range that is required these days.

Axial Piston Distributor Pumps

The most common type of single-plunger injection pump is the axial piston distributor pump. The pertinent word here is ‘distributor’. Just as many SI engines have (or at least, used to have) a distributor to distribute the high voltage pulses to each spark plug in the firing order of the engine, a distributor-type injection pump distributes high-pressure fuel pulses to each injector in the firing order of the engine. In fact, in some older engine designs, where the same basic engine block was utilised for both petrol SI and diesel CI versions, the diesel distributor pump sits near-vertically up on the side of the block, in the same position as the petrol engine’s ignition distributor. The Land Rover 2.25L diesels from the late 1950s to the early 1980s are an example of this design, using Lucas CAV injection pumps. See the image below.

It's quite likely that the most common form of this type of pump is the Bosch VE (Verteiler) type. They have been manufactured in the millions by Bosch and, under licence, by many other manufacturers including the Japanese builders Nippon Denso and Diesel Kiki. Mechanically-controlled VE type pumps can generate nozzle pressure up to about 1200 bar.

It’s pretty evident that most light road vehicle CI engine designs of the 1990s utilised VE type pumps, either mechanically-controlled or, in more recent years, versions with various electronically-controlled functions.

VE type pumps are used in: Land Rover Tdi, Mitsubishi 2.8 TD Pajero/Shoguns, Isuzu/Holden 2.8TDs, Toyota 4.2 1HZ/1HD, Nissan Patrol TD42/T and many other engines.

The Nissan TD42Ti engine (TD6 Patrol) uses a variant of the VE type pump with added electronically controlled timing advance (Covec-T) and some late Land Rover 300tdi engines used the Bosch VE-EDC (Electronic Diesel Control) version with fully electronic control of both fuel quantity and timing.

Radial Piston Distributor Pumps

While the axial piston injection pump is capable of moderately high nozzle pressures, it is limited by the fact that the axial plunger design places enormous axial (thrust) loads on the pump shaft bearings. To overcome this, Bosch developed the radial piston pump.

To attain even higher nozzle pressures, unit injector systems were developed. Instead of a ‘central’ high-pressure injection pump and high-pressure pipes feeding individual cylinder injectors, this system combines a small high-pressure pump into each injector assembly. A common low pressure feed pump supplies each unit injector with fuel.

Each cylinder of an engine with a UIS system has an extra lobe on the cam shaft. This lobe acts on the unit injector (either directly or through a rocker arm) to operate it’s high-pressure plunger. The regulation of fuel delivery quantity may be achieve by a common rack system as in in-line pumps. Many Cummins heavy duty diesel engines used a unit injector system with an ingenious totally-mechanical system to vary both delivery quantity and timing.

While high-pressure injection systems such as radial piston pumps and unit injection systems have greatly improved the precision and efficiency of diesel injection, they still suffer a fundamental limitation. The actual injection process into each engine cylinder is limited to a single event for each power stroke. That is, for each power stroke, the injector opens (at the optimum timing) and stays open until the ‘correct’ fuel charge for the current load on the engine has been delivered. It then closes until the next power stroke.

To further improve the emission control and fuel efficiency of a CI engine it becomes desirable to spread the injection process over longer periods of the power stroke. This means being able to open and shut each injector several times during each delivery cycle. Modern Common Rail (CR) injection systems allow this.