ABSTRACT

Engine components, such as injection systems, encounter a large number of load cycles (N > 107) during their lifetime and are exposed to potentially corrosive media such as fossil fuels. For this reason, corrosion fatigue in the very high cycle fatigue (VHCF) regime has to be taken into account for reliable fatigue design. To reduce carbon dioxide (CO2) emission, fossil fuels are blended with biogenic components. Biofuels are potentially more corrosive than unblended fuels due to the hygroscopic properties, e.g. ethanol, which is added to gasoline fuels.

There is not much known about the corrosion fatigue behavior of high-strength chromium steels in bio-fuels, yet. Indeed investigations show that biofuels reduce the number of load cycles to failure of engine components significantly [1]. Therefore, it is essential to investigate the corrosive impact of fuels with biogenic components by performing corrosion fatigue tests. In the current paper, the impact of corrosion fatigue was investigated on notched and unnotched specimens of stainless 17% chromium steel 1.4016 (X6Cr17), AISI 430 in air and E85 biofuel (gasoline fuel with 85% ethanol added). The results were obtained at a stress ratio of R = 0 with different testing rigs to investigate the influence of testing frequencies (f = 20, 150Hz). The test results represent the basis for a concept that will be able to estimate the impact of corrosion fatigue in the VHCF region.

INTRODUCTION

The amount of renewable sources shall be increased to 10% in biofuels according to the directive 2009/28/EC [2] to reduce the greenhouse gas emissions in 2020 by 6% [3]. During the implementation of E10 biofuel, many consumers have been averse to biofuels, because of the lack of long-term studies in conventional combustion engines [4]. Due to the higher amount of ethanol, the hygroscopic properties of biofuels increase causing an input of different kinds of corrosive substances, e.g. chlorides that are dissolved in water. High alloyed Cr-steels that are used in engine components form passive layers protecting the core material from further corrosion. High chloride contents superimposed by mechanical load can locally damage these passive layers resulting in pitting and possibly intergranular corrosion. These effects have a major impact on the fatigue strength of the materials [5]. The corrosive impact occurs most of the time at the high stressed part of engine components at the surface. This results in a superimposed crack initiation and crack propagation. Therefore, corrosive fatigue tests results are of great interest for such components. Regarding the downsizing trend of the automotive industry by reducing fuel consumption and simultaneously increasing the engine power in terms of material optimization, it is indispensable to consider the effect of corrosion already in an early stage of development. For this reason, corrosion fatigue effects have also to be taken into account during the design process of engine components such as fuel pumps, the fuel distributors, and high-pressure injection valves [6] by choosing the right material and the right design.

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