Injection pump for a 12-cylinder diesel engine

An Injection pump is a device that injects fuel into the cylinders in a diesel engine. The injection pump was traditionally driven by the crankshaft via gears, chains, or a toothed belt, which also drives the camshaft. In a four-stroke conventional diesel engine, it rotates at half the speed of the crankshaft. 

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The timing of the injection is such that fuel is only injected just slightly before the compression stroke’s top dead centre. The pump belt of gasoline engines can also be driven directly from the camshaft. Some systems can inject as high as 620 bar (8992 PSI).


The pump must be able to inject positive fuel into very high-pressure environments. This creates a great deal of pressure, typically 15,000 psi (100 MPA) on older systems. Diesel systems should be handled with care as escaping fuel can easily penetrate skin and clothing and cause serious medical complications that could lead to an amputation.


Inline diesel injection pump

An in-line layout was used for diesel fuel injection pumps in the past. This arrangement consisted of a series of cam-operated injection tubes in a straight line. It was more like an inline engine. The pistons have a constant stroke volume. Throttling (i.e. injection volume) can be controlled by rotating the cylindrical against a cutoff port that aligns with the helical slot within the cylinder.

Each cylinder can be rotated simultaneously to change its injection volume, allowing the engine to produce more or lesser power. Large multi-cylinder engines, such as trucks, construction machines, and agricultural vehicles, still benefit from inline pumps.

Distributor diesel injection pump

The rotary pump, also known as a distributor pump, was designed for use on light trucks and cars. The rotary pump or distributor pump uses one injection cylinder that is driven by an axial cam plate. It injects fuel into individual fuel lines using a rotary distribution valve. The Bosch VE pump, in its later versions, adjusts the timing of injection to match crankshaft speed.

This allows for greater power at higher crank speeds and smoother, less expensive running at slower revolutions. Some VE variants have a pressure-based system that allows the injection volume to increase over normal to allow a turbocharger or supercharger-equipped engine to develop more power under boost conditions.

Inline diesel metering pump

Every injection pump has a governor that cuts fuel supply in the event of crankshaft rpm exceeding the engine’s limit. Diesel engines are heavy and can cause catastrophic damage if over-revved. Wearing engines, which are often poorly maintained and worn, can eat their lubrication oils through worn crankcase ventilation systems. This causes the engine to ‘run away and increase its speed until it is destroyed. Most diesel engines don’t have any throttle valves to regulate air intake.

The latest types

To comply with international emission directives, and increase efficiency and economy, mechanical pumps are being gradually phased out.

 In the 1990s, pumps that utilized electronic control units to manage some functions of the rotary pump were used as an intermediate step between full electronic control and electronic control. However, they were still mechanically controlled by the engine and timed. 

These pumps were first introduced by the four- and five-cylinder VW/Audi TDI TDI engines. They were then replaced by Unit Injectors. 

These pumps were used to improve injection control and refinement of car diesel engines, as they switched from indirect injection to more efficient but less refined direct injection engines in the 1990s. ECUs can adjust the damping of hydraulic engine mounts to improve refinement.

BOSCH VP30, VP37 and VP44 pumps are examples. There has been a wide-scale shift to electronic unit direct injection systems and common rail diesel systems. These systems allow for higher pressures, finer control over injection volumes, and multiple stages of injection compared to mechanical systems.

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