#How Inline diesel injection pumps works

04 / April 2022
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How Inline diesel injection pumps works

Seletron Performance

Inline diesel injection pumps, and how they work.


As we have covered the operation of electronically controlled rotary injection pumps, those with radial pistons, and then injector-pump injection systems and later common-rails, someone pointed out that we did not cover inline injection pumps for Diesel engines. We did not do so because the market for cars with engines employing an electronically controlled inline injection pump is so small that it has no impact on our company, which focuses on electronic tuning (Diesel and gasoline).


The first car that comes to mind that uses this electronically controlled injection system is the Mercedes 250td with the 5-cylinder 2500, 4 valves per cylinder, and a 150hp output. We are talking about a car from the 1990s, a direct competitor to the BMW 525tds, and the Audi A6 2.5TDI V6 with the same power as the Mercedes. This model actually mounts an electronically controlled inline injection pump. The problem is that at the time of writing, no others come to mind... in any case, we are here to report how such an injection pump works, so let's start...


This is a type of injection pump that is very suitable for modularity. This means that the pump has a part in which the drive shaft with its minimum-maximum regulator and the centrifugal advance regulator is fitted. After that, the various pumping units can be 3, 4, 5, 6, etc., meaning these pumps are easily adapted to architectures suitable for engines with variable (and many) cylinders.


As with other injection systems, this one is being abandoned today in favor of the common-rail. However, let's have a look at how this type of pump works. We mentioned the issue of early injection: as engine speed increases, the engine needs to advance the injection timing in order to have time to burn off the atomized diesel fuel. The injection pump includes (in fully mechanical versions without electronic management) a centrifugal regulator capable of anticipating (depending on rpm) the position of the camshaft that drives the various pumps by about 15-20°. A second centrifugal regulator acts on the injection delivery to maintain the engine idle and limit the maximum engine speed.


Injection and delivery stage in inline injection pumps


The injection phase in this type of pump is created by the position of the cams located on the camshaft of the pump itself. Each cam controls a pumping element that generates the injection pressure for a single engine cylinder. The delivery is determined by the rotation of an element that acts as a by-pass on the pumping element. With minimum delivery, the by-pass opens after a very short stroke of the pumping element, and with high delivery, the by-pass opens after a longer pumping element stroke.


Excess diesel ends up in a low-pressure diesel recovery channel that is re-sent to the pump inlet. The various mechanisms that regulate (rotate) the diesel delivery are connected to a rack and pinion rod that simultaneously moves all the regulators of the various pumping units (4-6-8-10 etc., depending on the type of pump). This gear rack is moved by two elements: the accelerator pedal and the idle and maximum centrifugal regulator. On turbocharged engines, an additional element on the inline pump consists of a pneumatic valve connected by a small tube to the boost circuit. This system is intended to enrich the injection delivery as the turbo boost pressure increases. This system is similar to that employed in fully mechanically controlled rotary injection pumps.


In electronically controlled inline injection pumps, there is no centrifugal regulator of idle and maximum speed because the delivery control (and thus the idle and rev-limiting speed) is delegated to the ECU (or EDC) engine control unit, which acts on the pump by means of PWM electrical control ( for example, on some John Deere injection pumps). The same applies to any inline injection pumps in which the injection stage is also controlled electronically by the injection control unit. Also, in this case, like with electronically controlled rotary injection pumps (if you want to learn more about this topic, read the article https://seletron.com/it/news/83_funzionamento-centraline-aggiuntive-vp37 that explains precisely how this type of pump works in detail), there is a sensor that detects the diesel delivery position and provides the ECU with the necessary feedback as a feedback loop for precise control of this parameter.


The few chip tuning additional units for inline injection pumps act on the values of this sensor to change the feedback and increase the injection delivery (thus torque and power) accordingly. Other chip tuning unit controls may be on the boost pressure sensor in order to limit peak reading by the ECU or EDC. Therefore, its function is to prevent engine ECU recovery and not necessarily as an active function in increasing performance. Even in the past, this type of injection pump was not widely used in cars. Inline injection pumps were used extensively on heavy-duty trucks and transportation engines in general, but also on EMM (earth-moving machinery) and marine engines, all of which the common-rail injection system later entered.


Well, we hope we have quenched the thirst for curiosity of those who follow us again today. Please remember that we have covered and are covering various technical topics; come back and read us every day!


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