Diesel Engine Systems Part 2: High Pressure System

March 7, 2017 - 9:00 AM by Samantha Pudney


This is the second part of our diesel engine systems series. Diesel fuel systems consist of two separate parts, a low pressure system and a high pressure system. Understanding the components and purpose of each is critical to refine your system maintenance standards.

The higher pressure system is equally as important as its counterpart. It ensures that your engine remains free of debris and water contamination. This is the key to the smooth running and longevity of your engine.

The system consists of two major parts, the first of which is the injector pump.

Injector Pump

Fuel is considered to be incompressible. When it is pumped into a closed system the pressure rises rapidly until the break setting of the injector is reached. For non-turbo engines, this is normally 140 times the atmospheric pressure. The fuel then bursts through the injector in a fine automised spray into the combustion chamber.

Automated Pump Timing

The pump timing will need to be set so that the injection starts just before the piston reaches the peak of the compression stroke. Diesel fuel is quite thin, so the fit between the steel pump piston and its housing has to be close in order to stop fuel leakage and pressure loss.

The stroke of the piston, and therefore the amount of fuel delivered at pressure to the injector, is controlled mechanically to control the engine speed. Most pumps incorporate a method of controlling the engine by governing the maximum fuel offered. This delivers constant speed regardless of load.


Servicing Your Injector Pump

Servicing and turning the pump should only be carried out by a qualified professional. The best you, as the owner, can do is to keep the pipe fittings leak-free. Experienced personnel will ensure that the bolts holding the rotary pump to the engine are kept tight.

If not, the timing can be changed and tensioned in accordance with the manufacturer’s instructions, which could differ between models. Always make sure that it doesn’t slip, incorrect timing will result in engine failure or erratic running.


Injectors work like a relief valve. The internal piston is spring loaded, remaining closed as the pressure builds, and opening when the set pressure is reached. This is known as the break setting. Fuel then spurts in fine jets into the engine through the end of the injector.

Maintaining Your Injectors

It is important you determine the precise amount of fuel required to enter the combustion chamber. This can be done by the fuel pump speed control. The injector itself does not control the amount of fuel, but is only responsible for opening and allowing an amount to be pumped in.

While some injectors are able to be serviced, others are just throwaway items. To ensure yours lasts a considerable amount of time (over 1,000 running hours), always follow the maker’s service recommendations.


Sticking Injectors

Sticking injectors may make your diesel engine tricky to start. They could be opening at a pressure higher than the ideal, allowing less fuel into the combustion chamber. If this is the case, they should always be serviced by a qualified engineer or exchanged for completely new ones.

It is possible to take an injector out and then turn the diesel engine over to check if fuel is being squirted through it. However, we advise against this course of action, to instead enlist the service of a credible and qualified professional. Be aware – fuel jets, under high pressure, can easily penetrate the skin or blind an eye. The fuel is also toxic.

Remove the injectors, whether or you believe they are in need of a service, every few years at least to clean the sleeves and prevent them from becoming rusted. Be careful not to drop any debris into the engine and always refit with new sealing washers.

Starting Aids

Most diesel engines need a form of cold starting aid. In some cases, extra fuel is injected directly into the combustion chamber. This has the effect of increasing the compression ratio.

Occasionally, if the diesel engine is fairly old, a small dose of quickstart squirted into the inlet manifold can help. Some manufacturers specifically ban this, as it can build up in the exhaust system and explore, destroying exhaust when the engine starts.

A diesel engine may start more easily in the summer, without needing to use any form of cold start aid. If your engine doesn’t, then it is better to use the cold start procedure to ensure a quick start rather than risk draining the battery by repeated unsuccessful attempts.