In the scope of the cluster of excellence “Tailor-Made Fuels from Biomass” new biofuels are developed. To ensure safe and reliable functioning of the injection system operating with the new fuels, the tribological characteristics of the fuel candidates have to be investigated.

The biofuel candidates which have been studied so far tend to have a lower viscosity compared to diesel [1]. This has an enormous impact on the efficiency of common-rail piston pumps. For low viscosity fuels the volumetric losses become the dominant factor. These losses are influenced by the geometric parameters of the pump, the operating conditions and the rheological characteristics of the fuels. Regarding the geometric parameters, the gap height in the piston-cylinder-contact is the predominant factor. In modern common-rail pumps the nominal gap height is in the range of 2–3 μm [2]. A further reduction of the height is limited by tolerances of the manufacturing process and the risk of the piston getting stuck in the cylinder due to different temperature gradients and consequently different thermal expansions of piston and cylinder.

Besides the nominal gap height, the high pressure in the lubricating film in operation leads to an expansion of the gap. If this expansion can be limited or even avoided, a significant reduction of the leakage losses will be possible. In the scope of this paper an approach to a gap compensation of the sealing and lubricating contact between piston and cylinder is presented. Based on a detailed study of the state of the art design, including efficiency measurements of pumps and EHD-simulation, a modified piston design is investigated and optimized. The results show a great potential for efficiency improvement of common-rail pumps, especially if operated with biofuels, which provide low viscosities.

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