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The Basics of Diesel Combustion – Old Style Injectors

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We have spent quite a bit of time lately talking about the “newest”, and the “latest”, and the “next generation”, but how about those of us who are still using the old-style injectors? Since there are plenty of these still running the roads, let’s take a few minutes to look at what exactly the older injectors do, what can go wrong, and the symptoms that show on your vehicle.

Some of you are enthusiasts and techies, so our next post will drill down a bit, and get more specific, but this post will cover the basics, and what you need to know as the average owner of an old-style injector diesel.

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What Does the Older Diesel Injector Do?

On older systems, the fuel pump is the component in the system that controls timing and injection quantity, so basically, the injector has one job, and one job only. The job of the older injector is to deliver fuel into the cylinder. Comparing to the new injectors, this seems like a relatively simple task. Having said that, there are a few things to look out for, and a few things that can go wrong. Here are the critical parts the old injector plays, as well as some problems and symptoms to look for.

Fuel Atomisation and Diesel Combustion.

Critical to proper combustion, your fuel has to be “busted up” into the smallest particles possible to generate the optimum surface area-to-volume ratio for complete and fast fuel burn. Internal high-pressure leaks inside the injector can cause poor atomisation, and may never show any visible signs as the leaked-off fuel is returned to the tank. These sealing surfaces must be reconditioned using special equipment and CANNOT BE DONE BY HAND.

Opening Pressure and Closing Pressure and Diesel Injection.

Opening pressure of the older injector is critical, as it delays the start of injection until the point where the piston in the injection pump has built up enough power to be able to inject the fuel into the cylinder with enough pressure to effectively atomise. Another way to think of this is as “closing pressure”, which is even more critical. If closing pressure gets too low in a worn injector, you are injecting fuel well after the piston down-stroke, causing heat, black smoke, and high fuel consumption.

Fuel Contamination and Older Diesel Injectors.

For the old injectors with Pintle -type nozzles, a build-up can happen on the “spray-surface” of the needle, changing the atomisation pattern, causing no end of grief. Also, fuel contamination can simply jam the internal components, meaning the injector cannot completely close or open. This means the opening and closing pressure (above) of the injector is now effectively zero.

When Older Diesel Injectors Fail – Warning Signs.

Black smoke is caused by poor fuel atomisation, creating a scenario where the fuel-to-air ratio is not sufficient for the fuel to burn quickly and completely, and this incompletely burned fuel comes out as black smoke.
High fuel consumption is also caused by poor atomisation, due to slower burning of the fuel. Slower burn means that it is burning at the least efficient part of the piston stroke, which means to get the required power, you have to inject more fuel.

Next post, drilling down into the combustion process.