Evaluation of MFB50 is very useful for combustion control, since it gives an evaluation of the combustion process effectiveness. Real-time monitoring its value enables to detect for example the kind of combustion that is taking place (useful for example for HCCI applications), or could provide important information to improve real-time combustion control. While it is possible to determine the position where the 50% of mass burned inside the cylinder is reached using an in-cylinder pressure sensor, this work proposes to obtain this information from the engine speed fluctuation measurement. In-cylinder pressure sensors in fact are still not so common for on-board applications, since their cost will constitute an important portion of the whole engine control system cost. Engine speed measurement is instead already performed in modern engine control systems and therefore being capable of obtaining MFB50 related information from this signal means obtaining it at approximately no additional costs.
The MFB50 estimation procedure presented in this paper is based on the measurement of the engine speed fluctuations and it mainly consists in two separated steps. As a first step, a torsional behavior model of the powertrain configuration is developed. The engine-driveline torsional model enables to estimate the indicated torque frequency components, from the corresponding components of the instantaneous engine speed fluctuation. This estimation can be performed cycle by cycle and cylinder by cylinder. As a second step, the analysis of the relationship between MFB50 and the phase of the frequency components over an engine cycle allowed defining the final estimation algorithm that reconstructs the MFB50 starting from the instantaneous engine speed fluctuation analysis. The developed approach has been applied with success to a diesel engine mounted on-board a vehicle.
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