Fuel Injection in Diesel Engines

Older engines make use of a mechanical fuel pump and valve assembly which is driven by the engine crankshaft, usually via the timing belt or chain. These engines use simple injectors which are basically very precise spring-loaded valves which will open and close at a specific fuel pressure. The pump assembly consists of a pump which pressurizes the fuel, and a disc-shaped valve which rotates at half crankshaft speed. The valve has a single aperture to the pressurized fuel on one side, and one aperture for each injector on the other. As the engine turns the valve discs will line up and deliver a burst of pressurized fuel to the injector at the cylinder about to enter its power stroke.

The injector valve is forced open by the fuel pressure and the diesel is injected until the valve rotates out of alignment and the fuel pressure to that injector is cut off. Engine speed is controlled by a third disc, which rotates only a few degrees and is controlled by the throttle lever. This disc alters the width of the aperture through which the fuel passes, and therefore how long the injectors are held open before the fuel supply is cut, controlling the amount of fuel injected.

This contrasts with the more modern method of having a separate fuel pump (or set of pumps) which supplies fuel constantly at high pressure to each injector. Each injector then has a solenoid which is operated by an electronic control unit, which enables more accurate control of injector opening times depending on other control conditions such as engine speed and loading, resulting in better engine performance and fuel economy. This design is also mechanically simpler than the combined pump and valve design, making it generally more reliable, and less noisy, than its mechanical counterpart.

Both mechanical and electronic injection systems can be used in either direct or indirect injection configurations. (see below)

Indirect injection

Main article: Indirect injection

An indirect injection diesel engine delivers fuel into a chamber off the combustion chamber, called a prechamber, where combustion begins and then spreads into the main combustion chamber.

Direct injection

Distributor pump direct injection

The first incarnations of direct injection diesels used a rotary pump much like indirect injection diesels, however the injectors were mounted directly in the top of the combustion chamber rather than in a separate pre-combustion chamber. Examples are vehicles such as the Ford Transit and the Austin Rover Maestro and Montego with their Perkins Prima engine.

The problem with these vehicles was the harsh noise that they made and particulate (smoke) emissions. This is the reason that in the main this type of engine was limited to commercial vehicles (the notable exceptions being the Maestro, Montego and Fiat Croma passenger cars). Fuel consumption was about 15% to 20% lower than indirect injection diesels which for some buyers was enough to compensate for the extra noise.

This type of engine was transformed by electronic control of the injection pump, pioneered by Volkswagen Audi group with the Audi 100 TDI introduced in 1989. The injection pressure was still only around 300 bar, but the injection timing, fuel quantity, exhaust gas recirculation and turbo boost were all electronically controlled. This gave much more precise control of these parameters which made refinement much more acceptable and emissions acceptably low. Fairly quickly the technology trickled down to more mass market vehicles such as the Mark 3 Golf TDI where it proved to be very popular. These cars were both more economical and more powerful than indirect injection competitors of their day.

Common rail direct injection

Main article: Common rail

In older diesel engines, a distributor-type injection pump, regulated by the engine, supplies bursts of fuel to injectors which are simply nozzles through which the diesel is sprayed into the engine's combustion chamber.

In common rail systems, the distributor injection pump is eliminated. Instead an extremely high pressure pump stores a reservoir of fuel at high pressure - up to 1,800 bar (180MPa) - in a "common rail", basically a tube which in turn branches off to computer-controlled injector valves, each of which contains a precision-machined nozzle and a plunger driven by a solenoid.

Most European automakers have common rail diesels in their model lineups, even for commercial vehicles. Some Japanese manufacturers, such as Toyota, Nissan and recently Honda, have also developed common rail diesel engines.

Different car makers refer to their common rail engines by different names, e.g. DaimlerChrysler's CDI, Ford Motor Company's TDCi (most of these engines are manufactured by PSA), Fiat Group's (Fiat, Alfa Romeo and Lancia) JTD, Renault's DCi, GM/Opel's CDTi (most of these engines are manufactured by Fiat, other by Isuzu), Hyundai's CRDI, Mitsubishi's D-ID, PSA Peugeot Citroen's HDI, Toyota's D-4D, Volkswagen's TDi, and so on.

Unit direct injection

This also injects fuel directly into the cylinder of the engine. However, in this system the injector and the pump are combined into one unit positioned over each cylinder. Each cylinder thus has its own pump, feeding its own injector, which prevents pressure fluctuations and allows more consistent injection to be achieved. This type of injection system, also developed by Bosch, is used by Volkswagen AG in cars (where it is called Pumpe Düse - literally "pump nozzle"), and most major diesel engine manufacturers, in large commercial engines (Cat, Cummins, Detroit Diesel). With recent advancements, the pump pressure has been raised to 2,050 bar (205 MPa), allowing injection parameters similar to common rail systems.