The quality of diesel is not always the same. In Europe the prevailing standards ensure a largely uniform quality of diesel, but outside central Europe the quality of diesel fuel can fluctuate. In fact, around the world, the exact quality of diesel is sometimes difficult to define. The differences in quality mainly depend on components such as the quantity and size of the dirt particles which can damage the injection system. The amount of biodiesel content, which can clog the diesel particle earlier in case of too high blending ratio. And the water content which can lead to corrosion or cavitation effects on the injectors, valves or injection pump. This in turn leads to an increase in fuel consumption and pollutant emissions. Depending on their seriousness, the accompanying effects of wear can lead to a complete system failure.

But how does water enter the diesel fuel?

Up to 7 % biodiesel is currently added to diesel in Germany and Europe and in the U.S. even up to 20 % is common. Biodiesel increases the solubility of water in diesel, in particular as the temperature increases. This water does not represent a problem for the injection components as long as it remains in its dissolved form. However, when the fuel cools, e.g. when the vehicle engine is switched off, the solubility level is exceeded and in a second phase free water is present. This can be likened to a mixture of salad oil and water. The composition of diesel fuel has also changed in recent years. Nowadays numerous additives are mixed into the fuel , which lead to a very stable emulsion of microscopically small water droplets. These droplets are naturally not desired in the highly complex engine technology and have to be filtered out before the diesel is injected into the engine.

The water droplets are very small, partly with a size of 4 to 10 micrometers (in comparison a human hair on average which is roughly 60 micrometers) and this makes the filtration process for today’s diesel fuel considerably more complex. MANN+HUMMEL has therefore developed a three-stage water separation which removes almost all of the free water from the diesel under real driving conditions and therefore meets the high requirements of modern diesel engine and injection system manufacturers.

three-stage water separationLet me briefly explain the principle of the three-stage water separation. Diesel flows through our filter from the outside to the inside. The filter medium (first stage) removes the solid dirt particles. The second stage consists of a coalescence medium. In principle, this medium collects the fine water droplets and binds them into larger drops and they become heavier. The third stage consists of a sedimentation gap. This gives the larger water drops an opportunity to reach a water accumulator supported by gravity. Downstream a water-repellent screen fabric prevents the entry of water droplets in the direction of the injection system. Filters using this principle have already been fitted to numerous passenger cars in series and are also available for many models in the aftermarket.

Around the world, the quality of diesel is often substandard. This may be due to inappropriate storage of the diesel, local climatic conditions with a higher level of humidity, or because the diesel has been adulterated before it reaches the market. The resulting considerably higher water content will then quickly push even the best filter to its application limit. We therefore asked ourselves the question of how we could further improve our patented three-stage system to ensure its function even under the most extreme conditions.

Up to now it was sufficient for passenger cars using diesel with an acceptable quality level to fit a filter in the pressure line between the feed pump and the high pressure pump. With lower quality diesel or with heavy duty trucks and construction machines which consume a considerably higher volume of diesel, these would – as mentioned above – quickly reach their application limits. In this case additional pre-filters are used. These are usually fitted in the suction line between the tank and lift pump which, for example, could result in completely different pressure conditions. The filter requirement is to generate the lowest possible pressure drop with optimum filter performance. We therefore started a development project to transfer the three-stage water separation from the main filter to the pre-filter, without a significant increase in pressure drop.

It was then that we received an enquiry from our plant in China. A vehicle and engine manufacturer established in China was searching for a pre-filter for its trucks division with specifications which corresponded almost exactly to the objectives of our development work. I therefore packed our results and some prototypes, traveled to China, and presented the current status of the development to our Chinese colleagues. The meeting was very fascinating for me because we all spoke very openly and honestly about the requirements and possibilities. The Chinese colleagues recognized the advantages and opportunities of the new product and were able to impress their customer with its potential. The first series order for a three-stage pre-filter was in the bag.


We naturally had to adapt our prototypes to the customer requirements in the engine compartment and develop the product to be ready for series production. Due to our international development network and the ambitious participation of all those involved we were able to conclude the process in just a few months. The new pre-filter is currently being produced in our Chinese plant and delivered to the customer.

The Bund in Shanghai

Naturally here in Germany there is also close cooperation with well-known manufacturers and towards the end of the year the new filter will find application in local models. All in all it was an exciting and inspiring experience for me to see how our global network was used to develop a product in Germany which was then used for the first time somewhere else in the world.