Strain Gage Based Instrumentation For In Situ Diesel Fuel Injection System Diagnostics

Dynamic start of injection (SOI) is identified as one of the key injection parameters that needs to be measured during engine operation in order to study ignition delay and its impact on the qualitative and quantitative aspects of the combustion process in the direct injection diesel engine. Application of traditional SOI measurement techniques, based on needle lift sensors, to unit injection systems presents challenges. Hence, a strain gage measurement technique which is readily available to engineering students and researchers has been adopted to determine SOI for unit injection systems. A strain gage was installed on the rocker arm acting on the unit-injector plunger, and the injection pressure was calculated from the force and the diameter of the plunger. Start of injection was determined from the profile of the injection pressure as a function of crank angle, and the known injector opening pressure. A series of tests was performed on a heavy-duty turbocharged diesel engine, and measurements from 20 consecutive cycles were statistically analyzed. Very small cycle-to-cycle variations in the SOI determined from the injection pressure signal indicate that the strain gage technique is reliable and suitable for rigorous combustion analysis.