Fuel Under Pressure

Diesel fuel injectors make things work

Published in the April 2019 Issue April 2019 Tech Corner, Feature

By Eric Brisbon

Fuel injectors are, without question, one of the most significant components of an engine’s functionality, as the reputation of a diesel engine relies heavily on the performance of the injector and the reputation of the OEM which produced it.

Simply put, an injector is a device that takes a specified amount of raw pressurized fuel (or in many cases does the actual pressurization), and then forces the fuel through small holes which atomizes that fuel in the combustion chamber. 

When your injector is performing correctly, you have the accurate amount of atomized fuel to cause ignition. Done incorrectly, the engine will inform you pretty quickly that it’s very unhappy. Experience suggests that injectors are likely the culprit.

Injectors come in many shape and sizes. Worldwide, many diesels still utilize pump line nozzle systems (PLN) for their fuel management. A separate fuel pump pressurizes and meters the fuel, sends it through a high-pressure line to the fuel injectors, which then atomize it into small droplets. 

Similarly, there are also many different types of unit injectors which incorporate the high-pressure pump and fuel metering into the injector housing itself and drive it with an engine camshaft. Other pressure means such as engine oil amplify the pressure charge inside the injector, as with HEUI. Furthermore, common rail injectors get their high-pressure fuel from a central “fuel rail” (pressure plenum feed), by a fuel pump which feeds the injector and meters the charge electronically. 

Hybrid systems exist; however, the basic tenants of taking a known quantity of pressurized fuel and forcing it through small holes to atomize it remains the same.

There is an obvious question about what the difference between a diesel injector and one found in any other type of engine. Based on physics, the answer is nothing. Any time you take a liquid and force it under pressure through an orifice, you fundamentally have an injector. In diesels, however, the criteria for accomplishing that task is very different.

Modern diesels use fuel pressure of 1500-2500 Bar. In some case this is double or triple the injection pressure of a few decades ago. Fuel injectors must be able to seal this kind of pressure and operate over millions of cycles at very consistent flows values with little degradation in overall performance. If systems start to degrade with time, overall performance and emissions will suffer.

As an example, let’s look at a current common rail set-up. 

There are a few basic elements that we discussed such as a high-pressure pump that feeds a “common rail” which provides high pressure fuel to the individual injectors at each cylinder. The injectors have control valves that meter the fuel which are controlled by the FICM (Fuel Injection Control Module).

Fuel also serves more than one purpose with these systems. Note that high-pressure and low-pressure lines run to-and -fro constantly circulating fuel from the tank with excess being returned. This loop serves as a cooling function for the injectors and other portions of the system which absorb heat from operation.

In addition, the injector and high-pressure pumps are also fuel lubricated, consequently filtration is paramount to keep the fuel clean once it finds it way to an injector. It only takes microns of contamination to clog an orifice or stick a needle.

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