MAHLE fuel filters—we take care of any dirt left over

Modern fuel injection systems require an extremely clean, homogeneous and free-flowing fuel supply. However, contaminants can get into the fuel during the production and transport process and need to be filtered out. MAHLE fuel filters reliably remove contaminants. Thanks to high-quality filter media, the fuel system is protected from the smallest contaminants and therefore from corrosion damage. This protects the engine and assures efficient operation of the vehicle. The required constant fuel supply is achieved with pressure regulation and recirculation of the surplus fuel from the injection pump to the tank.

Pulsation damping compensates for the pressure fluctuations caused by the fuel pump. Our fuel filters comply with the correspondingly high safety standards of the automobile manufacturers and ensure secure sealing even in the event of accidents.

However, an important consideration for optimum functioning of fuel filters is that filters are replaced within the maintenance intervals stipulated by the automobile manufacturers.

In order to prevent paraffin separation and clogging of the filter at low ambient temperatures, pre-warming with electric heating elements or recirculation of fuel that has been warmed by the engine is used in our fuel filters.

For newer generations with even higher requirements, MAHLE has opted for two separate filter stages during water separation.

The first filter phase consists of a cellulose filter medium with an untreated melt-blown contact surface for increasing the contaminant absorption capacity. This melt-blown contact surface optimally causes the many small droplets to coalesce into larger ones. The particulate filter in the shape of a pleated star even agglomerates the finest water droplets.

The second filter phase consists of a water separator, whose hydrophobic fabric with a mesh width of 25 µm divides the barely stable emulsion, thus separating the water. As this occurs on the clean side of the filter, it is referred to as clean-side water separation.

MAHLE fuel filter module with spin-on fuel filter

Spin-on fuel filters are designed for fast and easy replacement. Depending on their type, they are supplied with a thread to connect a water drain plug or water level sensor. For replaceable filter elements, only the dirty filter cartridge is replaced. These filter elements are also available in versions that can be completely incinerated easing the burden on the environment.

What have a Liebherr excavator of the A/R series, a MAN truck 20.225 and a Mercedes-Benz coach O550 Integro in common? A certain fuel spin-on filter: the KC 102. What is so special about the filter that is listed for an amazing 736 applications in the catalogue? And why is it so often used in combination with the KC 102/1 in MAN vehicles?

The two filters share the task of fuel filtration in commercial vehicles. One of the filters is equipped with a heating element that prevents the dreaded paraffin separation of the diesel fuel. However, the heating element requires space. The designers decided therefore to delegate the water separation and drainage—to the KC 102, which comes without heating as standard. Both fuel spin-on filters work therefore in parallel, side-by-side in a screw-on housing. Compared with an assembly in series, this offers two advantages—first, a slightly reduced flow resistance … and second, and more important: even if one filter fails completely, the engine is still supplied via the second filter.

Perfect team work: the KC 102/1 and the KC 102 work together as KKC 109/1—at one screw-on housing. The small difference: the KC 102/1 (left) has no water drain plug. Otherwise, the filters are of the same design.

The problem of paraffin separation is well-known: at temperatures below zero centigrade, paraffin crystals are formed in the diesel fuel—the fuel converts into gel and flocculates. The flocculate is carried to the fuel filter, where it clogs its surface and blocks the filter. The fuel can no longer flow to the injection pump, the engine is no longer supplied … and stops running.

There are indeed modern diesel fuels for winter use that are cold-resistant; for instance, the winter diesel available in Germany remains liquid down to about -23 °C thanks to special additives. However, to assure vehicle operation also at extremely low temperatures, fuel filters with heating elements are increasingly used. PTC heating elements are the safe and modern solution for fuel heating. The abbreviation PTC stands for “Positive Temperature Coefficient”. These have the characteristic that the electric resistance of the heating element increases with rising temperature. As the current decreases disproportional when a set temperature limit is reached, there is no danger of overheating. This makes heating units with PTC elements especially safe.

The heating element from below: view of the fuel heater with PTC element.

Both filters are resistant to so called biodiesel, or to be more precise: FAME (RME) according to EN14214. FAME stands here for fatty acid methyl ester—which are compounds of a fatty acid and methyl alcohol.

To improve sealing between dirty and clean side the KC 102 has an outer sealing ring and an additional inner sealing ring, which sits directly at the thread between screw-on housing and filter end-plate.

The combination: inner and outer sealing ring for optimum seal between dirty and clean side.

In order to protect the injector nozzles from damage by water (such as cavitation), the filters are not only equipped with water separation at the dirty side, but have also an additional water uptake volume of 80 cm³—this provides sufficient reserves to last until the next water drainage.

The filter duo KC 102/1 and KC 102 is used in many MAN vehicles. In addition, the KC 102 is used in other commercial vehicles such as HGV’s from ERF, tractors from Fendt, excavators and wheel loaders from Liebherr as well as coaches and trucks from Mercedes-Benz. In order to ensure perfect working of the filter, regular replacement at the specified intervals is recommended by the vehicle and engine manufacturers.

Fuel preheating—the effective solution against paraffin separation
Experienced diesel drivers are familiar with the problem: when the temperature falls below zero, the vehicle is in danger of stopping. This is due to the tendency of the light oil used as fuel to form wax-like paraffin from -7 °C and below. The fuel takes on a gelatine-like consistency and flakes form. These flakes float towards the filter and clog its microporous surface. It may only take a short time until the fuel supply to the injection pump is interrupted. The result: the engine looses power—and then it stops; an annoying state of affairs.

Much has been done meanwhile to avoid this dilemma. First there is the 'insider tip' of mixing petrol into the diesel fuel. However, this can only be accepted in absolute emergencies, as the added petrol lowers the Centan number, which is essential for the ignition quality and thus for cold starting. Moreover, some diesel engines will not tolerate such mixtures. A widespread solution was the use of additives that could be added by the driver. However, it was necessary to add these already before the onset of the cold. Moreover, this runs the risk that the additives could affect performance and service life of the engines.

Nowadays, the oil companies are responding to this problem and sell so-called 'winter diesel' during the period from early November until the end of February. This diesel remains liquid down to -20 or -22 °C. However, if the temperatures drop even lower or if wind chill takes effect, trouble is still looming.

The optimum solution: A heat source for the diesel
The obvious solution is to supply the fuel at its optimum temperature. Two methods are available on the market for this purpose: firstly, electrically controlled warm-up with a small additional heat source—and secondly, heating by return flow, which is using the heat that is available anyway in the fuel flowing back from the injection pump.

This is because in modern high-pressure injection systems, high temperatures are generated in the return flow of the fuel. This heated-up fuel can then be added again to the feed line. The rate of recirculation can reach up to 100%. The response characteristics and the available amount of energy of these preheating systems result in high efficiencies.

Bimetal or wax element—the deciding difference
In conventional types of control, bimetals are used. However, this can cause problems, especially in the first minutes after a cold start, as a bimetal-preheating valve switches over instantly. Very cold fuel is then let into the filter, which is therefore again in danger of clogging.

With continuous control via a wax element (as used in the MAHLE KL 229/3, for instance), the temperature of the fuel flowing to the engine becomes a controlled variable. In this way, the recirculation temperature can be optimised. In simple terms, the fuel temperature control works similar to a single lever mixer tap at the domestic washbasin: the ratio between warm and cold fuel is controlled continuously and matched to the requirements.

The wax element can be understood as a small hydraulic cylinder, which is filled with a particularly heat sensitive wax that expands when heated and then moves the control piston according to the temperature (see box). It can therefore respond to slightest temperature changes, offering significant advantages.

However, the KL 229/3 offers even more benefits. This is because the preheating system is here integrated into the filter—an economical design that also improves crash safety, as the amount of connections is reduced.

MAHLE inline fuel filters with integrated continuous temperature control are fitted, for instance, in the VW Touareg 2.5 l R5 TDI from 01/2003 onwards, in the VW Transporter 1.9 l TDI from 04/2003 onwards and in the VW Transporter 2.5 l TDI from 04/2003 onwards.

Temperature control via wax element
from -35 °C to +115 °C.

In position warm (closed), the fuel heated up by the highpressure pump is directly returned to the tank. In position cold (open), the hot fuel is added to the fuel coming from the tank.


Despite controversies: More and more fleet and vehicle owners convert to biodiesel—and more and more biodiesel flows into the fuel tanks. Most of the time it is added to fossil diesel fuel—in European countries up to 7% (B7) of biodiesel is added to fossil diesel by oil companies as standard—but also pure biodiesel (B100) is used more frequently. However, especially in its pure form or alternating with fossil diesel, biodiesel can be more challenging to the vehicle’s fuel system and for instance, can clog the fuel filter due to deposits dislodged from tanks and pipes.

When diesel from fossil origin is used, it leads to deposits in the fuel system. When changing to biodiesel, this acts like a solvent: The old diesel deposits in the tank and in the pipes are released and collect in the fuel filter, which can clog completely due to the sudden, additional load. The possible consequences: the fuel supply is interrupted and the vehicle stops. Experts recommend therefore to replace fuel filters a few hundred kilometres after changeover from conventional diesel to biodiesel, just to be on the safe side.

Also when there are frequent changes between biodiesel and fossil fuel, increased clogging of the fuel filter can be observed. The deposits that are repeatedly caused by fossil diesel are afterwards dislodged by the biodiesel and washed into the fuel filter. This kind of mixed fuelling is therefore not recommended.

Biodiesel cannot only dissolve old deposits, but occasionally also plastic materials used in the fuel system, elastomeres, glues or even metals. In addition, corrosion protections such as zinc plating or resin based internal tank coatings can be affected by biodiesel. When biodiesel reacts with materials used in the fuel system, soap like substances can form and clog the fuel filter. It is therefore strongly recommended to consult the relevant release notes of the vehicle manufacturers before biodiesel is used.

Diesel fuel—whether fossil or from renewable raw materials—contains always a small amount of water with nutrients that provide the basis for the undisturbed growth of bacteria, yeasts, algae and fungi. For biodiesel, the capability to solve water and therefore its holding capacity for water is even greater and water separation in the fuel filter is reduced. As water promotes bioactivity, there is an increased risk that microorganisms may develop that cause the fuel to break down into its basic constituents and lead to corrosion or formation of biomass. When the fuel filter is repeatedly clogged by gelatinous material, this type of “diesel oil pest” can be the reason.

The condensation water in the tank of the vehicle can constantly provide nutrients for the small organisms. In order to contain the growth of such microorganisms, diesel fuel should not be stored for longer time periods. If this cannot be avoided, the tank should always be kept as full as possible—a large air volume favours the formation of condensation water. Also storage in warm environments (for instance, in aboveground tanks exposed to solar radiation as used in farming), promotes the formation of microorganisms.

Biodiesel can also get into the engine oil, mostly via the piston skirt. This can dilute the engine oil with fuel—with the result of reduced lubrication and increased oil temperatures. For vehicles that operate with biodiesel, it is therefore recommended to observe the oil change intervals exactly, or even better, to shorten them.

Generally it can be said that for vehicles running on biodiesel it is essential to comply with the specified change and service intervals for oil and fuel filters. However, it is advisable to halve the usual intervals: fuel filters should therefore be changed after a maximum of 6 month.

Clogged filter paper – the result: collapsed filter element.

Biodiesel—what, how, and why?
The name biodiesel is given to fuels that are obtained from renewable raw materials. In Europe, biodiesel is predominantly obtained from rapeseed oil that is converted into Rapeseed oil fatty acid methyl ester (RME) with the help of methanol—a fuel that has largely the properties of diesel oil. Also fatty acid methyl ester (FME, FAME), Sunflower oil methyl ester (SME) or used oil methyl ester (AME) are biodiesel fuels.

Biodiesel is regarded as a CO2 neutral fuel, since only as much CO2 is released during its combustion as was taken up by the plants from the atmosphere earlier. Environmental experts point out that the low sulphur level and the significantly lower emissions of soot, hydrocarbon and particles as well as the fast biodegradability compared to fossil diesel count as further ecological plus points.

On the other hand, the emissions that are generated during cultivation, production and utilisation (such as carbon dioxide and nitrous oxide) have also to be taken into account. Furthermore, it is regarded as problematic that farm land, which otherwise could be used for food production, is required for biodiesel cultivation.