Development of a methodology for engine performance investigation through double crankshaft speed measurement

Optimal combustion control has become a key factor in modern automotive applications to guarantee low engine out emissions and good driveability. To meet these goals, the engine management system has to guarantee an accurate control of torque delivered by the engine and optimal combustion phasing. Both quantities can be calculated through a proper processing of in-cylinder pressure signal. However, in-cylinder pressure onboard installation is still uncommon, mainly due to problems related to pressure sensors' reliability and cost. Consequently, the increasing request for combustion control optimization spawned a great amount of research in the development of remote combustion sensing methodologies, i.e., algorithms that allow extracting useful information about combustion effectiveness via low-cost sensors, such as crankshaft speed, accelerometers, or microphones. Based on the simultaneous acquisition of two crankshaft speed signals, this paper analyses the information that can be extracted about crankshaft's torsional behavior through a proper processing of the acquired signals. In particular, the correlations existing between such information and indicated quantities (torque delivered by the engine and combustion phasing) have been analyzed. In order to maximize the signal-tonoise ratio, each speed measurement has been performed at an end of the crankshaft, i.e., in correspondence of the flywheel and the distribution wheel. The presented approach has been applied to a light-duty L4 diesel engine mounted in a test cell. Nevertheless, the methodology is general, and it can be applied to engines with a different number of cylinders, both compression ignition (CI) and spark ignition (SI).