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Diesel fuel filtration

In the search for engine performance – whether it is more power, better fuel economy or cleaner exhaust gases – the overriding direction irrespective of fuel type is one of higher fuel injection pressures. In a typical gasoline direct injected engine, the rail pressure will be around 200 bar or more; in a diesel engine with the latest of common rail injection, the pressure in the injectors will be well beyond this, with figures of up to 3500 bar currently quoted.

From a technical standpoint therefore, and since higher pressures must inevitably mean tighter running clearances for the moving parts (pumps, injectors and so on), a diesel fuel filtration system would seem to pose the greater challenge. Add to this changes in the fuel designed to safeguard exhaust after-treatment devices (where fitted) and the progressive movement towards fuels from ‘bio’ sources, and the challenge of fuel cleanliness at the engine becomes Herculean. So in one instance, while the reduction in fuel sulphur to less than 15 ppm helps the after-treatment devices, at the same time it reduces the diesel fuel’s natural lubricity and its consequential ability to tolerate contaminants. And although the addition of fuel components from bio sources can help to regain some of this lubricity, diesel’s affinity with water brings problems of a different nature.

Found in all diesel fuels of one description or another, the presence of water is almost inevitable. Present as a result of poor storage or environmental conditions when the tank is vented to the atmosphere, water can cause engine issues like corrosion or erosion as a result of fuel lubricity deterioration or fuel pump cavitation. Furthermore, water can also be implicated in the build-up of injector deposits as well as fuel filter plugging at the surface interface, where damaging bacteria can grow. In short, water in fuel is bad news but that found in diesel engine fuel – whether as free water, dissolved water or as an emulsion in the fuel – is doubly so.

The mixture of hydrocarbon chains that make up a typical diesel fuel have a natural tendency to attract water. This is because water is a polarised molecule that is readily attracted to those parts of the diesel fuel hydrocarbons that have a similar but opposite charge. Once this amount exceeds the saturation point of the fuel then any excess will fall out, forming an emulsion consisting of small droplets of water suspended within the fuel. The final stage is when these droplets coalesce and, since water (specific gravity: 1.0) is denser than diesel fuel (specific gravity: 0.86-0.90), they will fall to the base of the tank as free water.

At this point it is imperative that the water is removed, which is why most fuel filters designed for diesel engine applications include some kind of water separation system. Placed on the primary side of the fuel system – the suction side of the fuel pump – in this position the fuel pump is not only protected but the water is separated before the action of the fuel pump emulsifies the water-fuel mixture. Removed from the fuel by either stripping it out using a water repellent or hydrophobic medium, or by capturing the fine droplets and coalescing them to form much larger and hence more stable droplets, either way the free water will eventually fall to the base of the filter, from where it will be periodically removed.

To most gasoline engine users the fuel filter is a ‘fit and forget’ device introduced in the fuel line on a ‘just in case’ basis. For a diesel engine a fuel filter is an absolute must.

Fig. 1 - The action of the diesel fuel filter

Written by John Coxon

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