The Diesel Engine

This is a simple guide to understanding how the FUEL DELIVERY EQUIPMENT works in a modern diesel engine, what the various pieces of equipment look like, and what can go wrong. The fuel injection equipment (FIE) manufactures spent a great deal of time, energy and skill designing equipment for a specific function. Generally these items last for an incredibly long period of time. However when they do become worn or damaged the consumers are inclined to place blame with the manufactures for supplying sub standard components, especially when they receive the costly repairs bill.

What many consumers are not aware of though is that the fuel they use in their vehicle can have a major impact upon the life of their engine.
This document will hopefully make the diesel driver aware of what the fuel filters function is and why its so important to have it replaced at the factory recommended intervals, and most importantly why we should always use good quality fuel.

The Diesel engine is also commonly referred to as the compression ignition engine, which describes how the fuel ignites and the engine works. Up until Dr Rudolph Diesel invented his engine in 1892, the only engine in use had been the petrol engine. The petrol engine of that period was somewhat temperamental due to the fact that you have to have a mixture of petrol and air in the chamber and then ignite the mixture by means of an electrical spark. NO spark no engine power. Dr Diesel recognized this shortcoming and designed a highly efficient engine that required no electrical gadgets to keep the motor running. Once started his motor ran by compressing light oil until the point of self-combustion. He had designed this engine with the intention of making farmers self-sustainable. Instead of having to travel a long way into the local town to purchase petrol for his tractor or generator, the farmer would be able to set aside some of his crop and make it into fuel for this new motor. When the diesel engine was shown to the world press at the 1900 Paris trade and technology fair, they were amazed. The reason for that was the engine ran on peanut oil and corn oil. (The birth of BIODIESEL?)

Many years have past since those early days and there have been great advances in diesel technology. We have even witnessed diesel powered racing cars winning at the famous Le man 24Hour, absolutely pulverizing the petrol-powered competitors. Modern generation diesels have several key advantages over petrol-powered cars.

  • Better fuel economy: due to their high volumetric efficiency, diesel engines can consume 30 – 60% less fuel than petrol engines of the same power.
  • Power: Diesels produce more torque and power at lower engine speeds than petrol motors of similar capacity.
  • Durability: Diesel engines are designed and built to last longer than petrol motors and often exceed 500 000 km and require less parts replacement and maintenance.
  • Exhaust gas emissions: diesel contains a higher energy content per liter than petrol, allowing diesel to perform the same amount of work while burning significantly less fuel than the petrol motor and producing lower CO2 emissions.
  • Reliability: water can cause a short circuit and stop a spark plug from firing, no problem for the diesel as there are no spark plugs, if you can start it, the engine will continue to run. (Excluding some modern electronically controlled diesels)

There are basically two types of modern fuel injection systems. The basic direct or indirect fuel injection system, and more recently the high-pressure common rail injection method. They still have parts that perform the same basic function, but there is a technology leap in terms of efficiency. There has been a movement from mechanically controlled to electronically controlled systems. Regardless of the type in your vehicle, they both require high quality finely filtered diesel fuel for their operation.

The first diagram shows the typical fuel injection system that is in most current trucks, pick-ups and cars. Fuel is sucked from the fuel tank and is passed through a fuel filter system. This is the engines first and only line of defense, and it is the filters task to remove any sludge, sediment, rust or other contaminants that have found their way into the fuel tank. Vehicles with this type of system are commonly fitted with 20-micron filters manufactured from tightly woven cellulose material. This filter is designed to mechanically prevent any abrasive material from entering the sensitive fuel injection system. I have seen people remove filters as they kept getting blocked and caused slow fuel delivery to the engine. We have also seen vehicles with damaged injectors and yet the owner has chosen to run with the same filter for two or three times the recommended mileage. A filter can only filter a set amount of sediment before blocking and becoming useless. If a fuel contains excess water it may cause rust in the filter housing itself, in which case the filter will not perform as intended.

Rust on the fuel filter
(This filter was removed from a vehicle brought into Diesel-Electric center for fuel delivery problems. This rust was contaminating the diesel after the cellulose filter and rust was present in the fuel going through the injection system. This results in damage to the fuel pump and injection nozzels)

A single feed then directs diesel into a high-pressure fuel pump. Here the fuel is pressurized and distributed under equal pressure to the feeder pipes that deliver the diesel to the injector nozzle in each cylinder. It is at this point that fuel quality plays a vital role. Diesel floods this mechanical device and all lubrication inside the fuel pump is now provided by the lubricating properties of the diesel fuel. Sediment in the fuel can act as a grinding paste, sulfur can mix with the water to form a mild sulfuric acid and cause chemical etching, water can cause rust and if the base fuel does not have enough lubricity some parts will simply cease. All this can cause catastrophic failure in the pump.

Damage to engine parts
(The surface of the bearing and cam show rust in the effects of acid etching.)

This pressurized fuel arrives at the injector nozzle. At a set pressure the needle within the injector will open and it will spray a set amount of diesel into the chamber. There are tiny holes at the tip of each injector nozzle that are designed to give an even spread of diesel fuel before it ignites.

Diesel injector
(The tip and holes on a new injector nozzle.)

 Again the fuel quality is of utmost importance. If the viscosity of the diesel is perhaps to thick or to thin then the spray pattern will not be correct and the fuel will not atomize and achieve the correct combustion. Incomplete combustion can cause excessive exhaust smoke and carbon build up in the chamber and around the injector nozzle tips which could further alter the spray pattern or even block the tiny holes at the end of the injectors.

Diesel injector sray pattern    |
(The left image shows fuel being injected from the nozzle. The right image shows fully atomised prior to ignition.)

Finally there is a return fuel line that brings any excess diesel back into the fuel tank. This diesel is now warmed to about 40*C due to being pressurized and mechanically handled. This can raise the temperature of the diesel in the fuel tank to the stage where it may encourage the growth of algae and other microbial forms, especially if there is water contamination of the diesel.

Engines with this technology however are bit noisy, and the fuel pressure is controlled by the engine speed, which results in a variation in the spray pattern. This variation in spray pattern can be a disadvantage and is not an efficient process.

The engine manufactures discovered that an engine would run quieter and more efficientley if they boosted the pressure and fed a constant amount of fuel to the system. This research lead to the development of the COMMON RAIL DIRECT INJECTION system that is found in the latest range of cars. This injection system is pictured below.

Common rail direct injection system

The common rail sucks the diesel from the tank and through the filter. Typically these filters are now 10 or 5 microns. The fuel pumps used in common rail systems use ultra high pressure, often in excess of  200 bar. This fuel passes through the fuel pump and into a small pressure storage reservoir called the common rail, and ensures that ultra high pressure fuel in instantly available when the injectors need it.

Newly developed piezoelectric fuel injectors are able to spray micro bursts of diesel into the cylinder and can inject between a hundred to thousands of times per second, supplying very fine amounts of precisely measured diesel that allows exact control of the combustion process. These new injectors are now computer controlled, and can be connected to other electronic devices in the vehicle that can provide information on driving style, throttle position, ambient temperature and other inputs that when combined can increase the engines efficiency and fuel management. Small amounts of fuel are “pilot” injected into the cylinder ahead of the main combustion and can generate a smoother cycle in the combustion process, eliminating the “clatter” noise found in older diesel motors and also reducing the vibrations.

These motors produce more power, run smoother and provide better fuel economy. In Europe most cars sold have diesel engines. France leads the way and in the luxury car market, 82% of vehicles sold last year were fitted with diesel motors. In South Africa diesel is no longer the fuel farmer’s use. There has been a huge growth of diesel vehicles, especially in the SUV and double cab sector. It has even been stated that most cars of the future will be hybrids that are a combination of small but highly efficient clean burning diesel and electric motors.

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