Effective Filtration Of Even Tiny Water Droplets

Even high-quality diesel may contain traces of water, whether from moist air entering through the tank opening or from condensation in the large tanks used to store the diesel in gas stations.

However, advanced common rail diesel injection systems with pressures of up to 2,500 bar have higher requirements than simple mechanical systems in terms of the purity of the fuel. Tiny dirt particles as well as water drops can damage the components of the fuel circuit, which in the worst case can result in system failure.

Another challenge is the composition of modern diesel fuels, which have changed greatly in recent years: they contain very little sulphur and have higher proportions of biofuel and additives. This results in the formation of very small and stable water drops in the diesel fuel and requires a new water separation technology.


With the three-stage water separators, car workshops and wholesalers are offering their customers a new filter element which achieves extremely high efficiency over its entire service life. It separates more than 93% of the water in fuel when new, and its separation efficiency is still over 90% at the end of the service interval.

The benefit for the vehicle owner is therefore clear: “Whether corrosion or cavitation effects in injectors, valves, or the electric injection pump, this innovative separation technology from MANN-FILTER greatly reduces damage that can result in system failure,” says Lena Ketschau, Product Manager Fuel Filters in MANN+HUMMEL’s Automotive Aftermarket business unit.

Reliable performance is even more important if the vehicle is used in countries where the fuel has high water content.


If water enters the fuel circuit, it is reduced to small drops in the low-pressure fuel pump. After several fuel cycles, drop sizes smaller than 1/10th the diameter of a human hair are possible.

The innovative PU 9005 z and PU 10 011 z diesel fuel filters from MANN-FILTER are designed for the challenging job of removing these very small and finely distributed water droplets from the diesel fuel. Fuel flows from the outside to the inside of the three-stage filter element (pressure-side filter operation).

  1. In the first stage, the filter medium reliably removes solid dirt particles from the diesel fuel.
  2. The second filter stage uses a coalescer fleece. It retains even the finest water droplets from the fuel/water emulsion and combines them to form much larger drops.
  3. In the third stage, a hydrophobic sieve ensures that the enlarged drops do not enter the injection system. Gravity causes the separated water to collect in a water collection chamber in the filter module. A sensor signals as soon as the water needs to be manually drained or siphoned. Metal-free elements mean that both filters are completely incinerable and therefore can be recycled to protect the environment.


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