What is a Diesel Injector Pump?
A Diesel Injector Pump is the device that pumps fuel into the cylinders of a diesel engine or less typically, a gasoline engine. Traditionally, the pump is driven indirectly from the crankshaft by gears, chains or a toothed belt (often the timing belt) that also drives the camshaft on overhead-cam engines ( OHC ).
It rotates at half crankshaft speed in a conventional four-stroke engine. Its timing is such that the fuel is injected only very slightly before top dead-centre of that cylinder’s compression stroke. It is also common for the pump belt on gasoline engines to be driven directly from the camshaft.
Because of the need for positive injection into a very high-pressure environment, the pump develops great pressure—typically 15,000 psi (100 MPa) or more on newer systems. This is a good reason to take great care when working on diesel systems; escaping fuel at this sort of pressure can easily penetrate skin and clothes, and be injected into body tissues with serious consequences.
Earlier diesel pumps used an in-line layout with a series of cam-operated injection cylinders in a line, rather like a miniature inline engine. The pistons have a constant stroke volume, and injection volume (ie, throttling) is controlled by rotating the cylinders against a cut-off port that aligns with a helical slot in the cylinder.
When all the cylinders are rotated at once, they simultaneously vary their injection volume to produce more or less power from the engine. Inline pumps still find favour on large multi-cylinder engines such as those on trucks, construction plant, static engines and agricultural vehicles.
For use on cars and light trucks, the rotary pump or distributor pump was developed. It uses a single injection cylinder driven from an axial cam plate, which injects into the individual fuel lines via a rotary distribution valve.
Later incarnations such as the Bosch VE pump vary the injection timing with crank speed to allow greater power at high crank speeds, and smoother, more economical running at slower revs. 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.
Distributor diesel injection pump
All injection pumps incorporate a governor to cut fuel supply if the crank speed endangers the engine – the heavy moving parts of diesel engines do not tolerate over speeding well, and catastrophic damage can occur if they are over-revved.
Mechanical pumps are gradually being phased out in order to comply with international emissions directives, and to increase performance and economy. Alternatives include common rail diesel systems and electronic unit direct injection systems. These allow for higher pressures to be developed, and for much finer control of injection volumes compared to mechanical systems.
For many home mechanics the diesel injection pump is a bit of a mystery. The Bentley and Haynes repair manuals doesn’t describe its internals, because it’s not serviceable except by a few diesel specialists. Learning some basics of how it works and what its internals are could be of interest to the diesel owner, and the knowledge certainly can’t hurt when troubleshooting fuel injection problems, even if one isn’t about to take the pump apart.
The purpose of the fuel injection pump is to deliver an exact metered amount of fuel, under high pressure, at the right time to the injector. The injector, unlike in a gasoline engine, injects the fuel directly into the cylinder or a pre-chamber connected to the cylinder.
The VE in the name of the Bosch pump used in the VW diesels and many other small diesel engines stands for “Verteiler”, which is German for distributor or divider. The other common kind of injection pump is the inline pump. The difference between them is that the “Verteiler” VE pump has one fuel metering plunger, and a mechanism (the “Verteiler”/distributor) to send the fuel to the right cylinder. The inline pump has one plunger for each cylinder.
The Bosch VE has comparatively few moving parts, but what does move does so in a complex way. The Yanmar pump, which works and looks the same as the Bosch and fuel feed pump. This is a vane pump, just like the vacuum pump on the VW diesel engine. Its purpose is to suck fuel from the tank and deliver it to the metering pump. All the things have to do with the metering, timing and distribution of fuel delivery.
The plunger in the VE pump both rotates along its axis and performs a reciprocating translation in and out. It is the translation that performs the high pressure pumping, while the rotation is responsible for metering and sending the fuel to the correct cylinder.
The cam disk is rigidly attached to the plunger. The drive shaft rotates the cam disk. The cam disk rides on four rollers and has four lobes. Thus for each revolution the plunger will pump four times. Note that with this arrangement the plunger stroke is constant.
The metering (regulation of how much fuel is delivered) is done not by changing the mechanical stroke, but by spilling some of the fuel through spill ports, and thus changing the effective stroke .
This is done by uncovering a spill port under the control sleeve at a particular angle of rotation. The other purpose of the rotation is to deliver the fuel to the correct cylinder. This is done by having four delivery valves (only one shown in the figure), one for every 90 degrees of rotation.
During a full revolution the plunger makes four strokes, one at 0, 90, 180 and 270 degrees. During each stroke the delivery port in the middle of the plunger is connected to a particular delivery valve. During the backward motion of the plunger, the rotation uncovers a fill port, just below the magnet valve (solenoid)), and the plunger barrel is loaded with fuel.
At bottom dead centre the fill port is closed. On the forward pressure stroke fuel is pressurized (to over 120 bar). At this time the Plunger barrel is connected to a particular delivery valve through the channel in the centre of the plunger, and a port in the side. When pressure builds up to the delivery valve opening pressure, the valve will open and deliver high pressure fuel to the injector.
When the desired amount of fuel has been injected the spill port opens (located under the control sleeve), and the pressure quickly drops. This causes the delivery valve to close. During the rest of the stroke fuel is “spilled” through the spill port instead of being injected into the cylinder.
The position of the control sleeve controls at what angle the spill port opens, and thus determines the amount of fuel injected, in other words it controls the metering. The control sleeve is moved in response to a combination of accelerator position and engine speed. The latter is determined by a mechanical governor.
Some other functions of the fuel injection pump are:
- Timing The timing is adjusted in response to engine RPM. At higher RPM s, the fuel pressure from the vane transfer pump is higher. Pressure changes effects a spring loaded plunger, and the resulting movement will move the cam rollers to either advance or retard the timing. There is also a cold start device which advances the idle timing manually.
- Governor A mechanical governor limits the maximum speed of the engine to 4800 rpm in the bus/van application and 5350 rpm in newer passenger cars. It can be seen just above the cam disk.
- Stop A magnet valve or solenoid opens and shuts off the fuel channel between the feed pump and the metering pump.
- Aneroid An air inlet pressure sensor is used to determine maximum amount of fuel delivered on injection pumps for turbo engines. On newer (’89 and later) naturally aspirated engines a similar arrangement is used for altitude compensation.
