Non-Intrusive Accelerometer-Based Sensing of Start-Of-Combustion in Compression-Ignition Engines

A non-intrusive sensing technique to determine start of combustion for mixing-controlled compression-ignition engines was developed based on an accelerometer mounted to the engine block of a 4-cylinder automotive turbo-diesel engine. The sensing approach is based on a physics-based conceptual model for the signal generation process that relates engine block acceleration to the time derivative of heat release rate. The frequency content of the acceleration and pressure signals was analyzed using the magnitude-squared coherence, and a suitable filtering technique for the acceleration signal was selected based on the result. A method to determine start of combustion (SOC) from the acceleration measurements is presented and validated. In-cylinder pressure (used to calculate heat release rate) and accelerometer data were collected on a 1.9-L compression-ignition direct-injection engine (Z19DTH) over a wide range of speeds (1000-3250 RPM) and loads (2-8 bar IMEP_g), for single- and double-injection strategies, and for fuels of several different cetane numbers (35.5 and 48.5). The relationship between engine block acceleration and the time derivative of heat release rate was verified using the experimental results and indicates that unsteadiness in the rate of heat release is the driving factor for the engine block acceleration signal. The accelerometer-based method developed allows detection of SOC for the main injection with a root mean square error less than 0.75 CAD for the range of conditions tested in this work.