Because internal combustion engines need “clean” fuel to function properly, the fuel from the tank is always filtered.
Impurities in the fuel can enter the engine in several ways:
Contamination that has settled inside the fuel tanker may be swirled out during filling.
There may also be impurities in the engine or vehicle’s fuel tank or pipe system.
It is important that these dirt particles are filtered out upstream of the combustion process in order to protect the engine and/or specific components.
In an internal combustion engine, the fuel is fed to the combustion process via carburetors or injection systems, in some cases under high pressure. Some of the gaps through which the fuel must flow are very small and must not be clogged or damaged by dirt particles. Were this to happen, clean combustion and proper functioning of the engine would no longer be guaranteed. In the worst case, the engine could stop running altogether or lose power.
What happens if the fuel filter is clogged?
Unlike oil filters, fuel filters are not equipped with bypass valves. Therefore, a completely clogged filter would cause the engine to stall and/or not start at all. The designers have thus eliminated the risk of harmful dirt getting into the combustion chamber and damaging the fuel system and thus the engine.
Therefore, the filter must be replaced at regular intervals (as specified by the manufacturer) to prevent problems.
Some industrial machines are equipped with pressure gauges that make it possible to identify a clogged fuel filter. However, if this problem occurs in a passenger car, the engine will simply stop or no longer start.
Fuel filters for gasoline and diesel engines
Most gasoline fuel filters used to be simple, transparent strainers that were clearly visible in the engine compartment and were installed between the fuel tank and the carburetor. You could therefore see at a glance how much dirt the filter had already collected and easily change it if necessary.
With the advent of modern technology, gasoline filters had to meet ever more challenging requirements. The injection pressures in common rail system are constantly increasing. To protect the injection nozzles from wear, a filter fineness of less than 3 µm is required for the main fuel filters. Modern gasoline fuel filters are typically installed in the car’s underbody or under a panel in the engine compartment and are therefore usually not visible to the user. Consequently, filter changes are generally carried out by trained mechanics during vehicle inspections.
Diesel fuel filters used to be filter units that were installed in metal housings. Here, too, the injection pressures have increased steadily in recent years, and the required filter fineness is usually less than 3µm. In the case of large diesel engines, for example in the truck sector, there is often an additional pre-filter that intercepts the coarse particles and thus lessens the load on the main filter. Optimum filtration results are ensured by carefully matching the pre-filter to the main filter.
A water separator is installed on the pre-filter.
In most passenger cars, this is integrated in the main fuel filter. Water settles in the lower part of the filter due to its higher density and is typically removed via a drain plug when the vehicle is serviced.
Why is it necessary to remove water from diesel?
Water in diesel fuel leads to cavitation at the valves under high pressure, which can result in significant consequential costs. The vapor bubbles created during cavitation will destroy the surfaces of the affected component due to the large pressure differences. In the worst case, this can lead to a total failure of the component and thus to a shutdown of the engine.
How does water get into the fuel?
As the fuel is compressed to achieve the required high pressures, it heats up. Only a certain amount of this highly compressed fuel is actually needed for the combustion process; the rest is returned to the tank, warming its contents. As a result, the fuel tank is significantly warmer at the end of a long trip than at the beginning.
The air temperature in the tank, which has also risen due to the rising fuel temperature, begins to cool down when the engine isn’t running. The colder it gets, the more condensed water ends up in the tank. As a result, if the tank is large, as in a truck, a significant amount of water is added to its contents every day. In passenger cars, due to their much smaller tanks, this has a much smaller impact.
A truck’s fuel tank contains a much larger space in which water can accumulate – which must be drained from time to time. In a car, it is sufficient to drain the water during servicing.
In many countries, the maximum water content in the fuel is stipulated in the relevant standards. This limit only applies at the time of the vehicle’s handover to the customer. In other words, the facts described above are not in violation of any standards, but are simply the result of physics or thermodynamics.
Is there any point in installing a finer fuel filter than the type specified by the manufacturer?
Some users think they are doing their engine some good by using an even finer fuel filter than the type specified by the manufacturer. This is not true and usually only leads to increased costs for the user because a finer filter typically has a shorter service life if fitted in the same installation space. In most cases, a finer filter will fall far short of the replacement intervals specified by the manufacturer, and the engine will simply stop working.
The manufacturer has designed the fuel filter based on the minimum gap sizes within the engine – and reduced this value even further with a safety margin. Smaller particles than those trapped by the specified fuel filter cannot cause damage in the fuel system because they can slide through the existing gaps without coming into contact with the components. Therefore, it makes little sense to filter them out.
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