Factors affecting diesel fuel degradation using a bespoke high-pressure fuel system rig

Recently, there has been automotive industry-wide impetus to reduce overall diesel vehicle emissions and fuel consumption by increasing fuel injection pressures within common rail systems. Many production fuel injection systems are now capable of delivering rail pressures of 1800-2000 bar with those able to achieve 3000 bar under development. In addition, there has been a gradual increase in the permitted FAME content in EN590 diesel from 5% to 7% with further increases to 10% proposed. With these changes there has been mounting speculation that increasing injection pressures, especially with elevated biodiesel content, could contribute to fuel degradation, deposit formation, fuel filter blocking and corresponding vehicle reliability issues. In this investigation a bespoke, high pressure fuel injection rig was designed and commissioned to mimic conditions representative of those experienced within a modern vehicle engine. The impact of rail pressure, biodiesel content and accelerated testing conditions on the stability of diesel fuel and the deposit formation leading to filter blocking were assessed. Despite the abundance of literature on lab-based biodiesel degradation, under these more realistic operating conditions it was found that biodiesel did not increase the likelihood of deposit formation within the high pressure fuel system, with the same level of filter blocking observed for B0 and B10 blends. This implies that the filter blocking problem caused by on board fuel degradation has the potential to occur broadly in a wide range of different fuels compositions. B10 fuel tested with a rail pressure of 2000 bar resulted in a pressure drop across the fuel filter of 0.5 bar within 12000 minutes (approximately 8.3 days), whilst the corresponding experiment at 1000 bar rail pressure showed no filter pressure increase. When using model (B10) fuel filter blocking was observed at both 2000 bar and 1000 bar rail pressure, however with the lower pressure at a much reduced rate, leading to the belief that the increases in rail pressures toward 2000 bar have a significant effect on the propensity of vehicle diesel filters to block. Measures taken to increase the severity of the test, such as recirculating injected fuel to simulate shear effects, were found to increase the rate of degradation but not change the chemical composition of the solids formed, thus implying that they were valid methods of reducing test durations without introducing new degradation mechanisms. The rig presented here is therefore a suitable accelerated testing system for assessing the behaviour of fuels under higher pressures that will be common throughout the global diesel fleet in the near future