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In a modern diesel engine, a high fuel injection pressure is achieved by a common-rail system. Therefore, it is important to understand the effects of fuel properties on engine performances because a diesel fuel could deteriorate inside an injector at such severe conditions. The test methods so far basically use the fuel with pro-fouling agent to form deposit on injector. In this study, a novel test procedure was developed to evaluate the effect of the use of the fuel with and without zinc contaminant on injector performance. With Zn doped European specification B7 fuel (7% biodiesel) as a reference, the test result showed that an engine torque decreased almost lineally over time, and the overall torque drop was 9% after 300 hours. The investigation of the dismantled injector after the test revealed that the deposit was not formed on the sliding parts of the injector, but on the nozzle hole surface.

Investigation of both intake valve deposit (IVD) formation tendency and the effect of surrounding conditions (e.g. atmospheric temperature) on IVD formation was conducted using poly aromatics-rich gasoline in a port fuel injected (PFI) engine. The IVD mostly consisted of carbonaceous deposit and this was shown to be derived from the heavier poly aromatic fuel molecules and engine oil. Analysis also indicated that some metals in the deposit structure came from additives of lubricating oil and the wear and/or corrosion within the engine. In this engine test, there was a significant effect of room temperature where higher room temperatures correlated with higher levels of IVDs. These results illustrate the importance of the surrounding condition in order to understand the IVD issues, higher IVDs weights will result under hotter driving (e.g. in summer) condition.