Use of a Laboratory Scale Test to Study Internal Diesel Injector Deposits

Internal Diesel Injector Deposits (IDID) in compression ignition engines have been widely studied in the past few years. Published results indicate that commonly observed IDID chemistries may be replicated using full-scale engine tests and subsequently fuel injection equipment (FIE) operated on non-fired electric motor driven test stands. Such processes are costly, complex and by nature can be difficult to repeat. The next logical simplification is to replicate IDID formation using laboratory-scale apparatus that recreate the appropriate chemical reaction process under well controlled steady state conditions. This approach is made more feasible by the fact that IDID, unlike nozzle hole coking, are not directly exposed to gasses involved in the combustion process. The present study uses an instrument designed to measure thermal oxidation stability of aviation turbine fuels to successfully replicate the deposit chemistries observed in full-scale FIE. The resulting deposit thickness was measured using a novel spectral reflectance technique, which provided quantitative measurement even when the deposit was not visually apparent. The Thermal-Oxidation-Stability-Test successfully responded to fuel chemistries known to affect IDID in the field, including the beneficial effects of high quality deposit control additive. The well-defined environment around the Thermal-Oxidation-Stability-Test heater tube allowed the interaction between surface temperature and chemistry to be rapidly evaluated. Each deposit mechanism was found to have a critical onset temperature, which caused it to form at a different location along the heater tube. More surprisingly, some deposits also demonstrated a maximum temperature, above which they ceased to occur.