As a result of the increasing request to reduce pollutant emissions and improve efficiency in modern Internal Combustion Engines, it is important to know a high number of quantities that are representative of the combustion process. One of the most important parameters to estimate on-board is the angular position where 50% of fuel mass injected over an engine cycle is burned (MFB50), because it provides information about the effectiveness of combustion (useful, for example, in HCCI combustion control). MFB50 can be evaluated using in-cylinder pressure sensors, nevertheless they would cause engine control systems cost to rise. The MFB50 estimation algorithm presented in this work is based on engine speed measurement, that can be performed using the same 60-2 toothed wheel already present on-board for other control purposes. For the above reason, this approach is not only compatible with on-board applications, it also requires no additional costs. The developed method mainly consists of 2 parts. The first one is to determine the relationship between the engine speed harmonic components and the corresponding indicated torque harmonic components, expressed as a Frequency Response Function representative of the engine-load system’s torsional behavior. After having estimated the indicated torque harmonic component, the following step is to estimate the value of MFB50 cycle by cycle. This is done analyzing the relationships between the indicated torque harmonic components phase and the angular position where 50% of the fuel injected over the engine cycle is burned. The procedure has been applied to an L4 turbocharged Diesel engine mounted on-board a vehicle, obtaining an estimation accuracy adequate enough to feedback a control algorithm for optimal combustion positioning over an engine cycle.

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