Measurement Errors in Real-Time IMEP and ROHR Evaluation 2008-01-0980
Combustion control is one of the key factors to obtain better performance and lower pollutants emissions, for diesel, spark ignition and HCCI engines. This paper describes a real-time indicating system based on commercially available hardware and software, which allows the real-time evaluation of Indicated Mean Effective Pressure (IMEP) and Rate of Heat Release (ROHR) related parameters, such as 50%MFB, cylinder by cylinder, cycle by cycle. This kind of information is crucial for engine mapping and can be very important also for rapid control prototyping purposes.
The project objective is to create a system able to process in-cylinder pressure signals in the angular domain without the need for crankshaft encoder, for example using as angular reference the signal coming from a standard equipment sensor wheel. This feature can be useful both for test bench and on-board tests. In order to gain reliable results or acceptable precision on ROHR-related parameters (ROHR peak & 50%MFB, for example) a high sampling rate is required for the in-cylinder pressure. Since the angular reference signal can have low angular resolution (6 degrees with a typical sensor wheel), the in-cylinder pressure signal sampling rate must be higher than the crankshaft signal frequency. The choice for this application has been to use a high sample rate on a time base for the cylinder pressure signal, performing the transformation from the time domain to the angular domain (necessary in order to evaluate the indicating parameters) by means of an interpolation algorithm.
The use of standard automotive crankshaft position referencing sensors (and sensor wheels), however, increases the risk of angular errors in the conversion from the time to the angular domain: transducers intrinsic delay, analog filters delay, TDC referencing error could cause large errors on RHOR and IMEP. Furthermore, if the in-cylinder pressure signal mean value is reconstructed by means of the Polytropic Index Pressure Referencing methodology, errors on the pressure samples angular position would lead to a wrong estimation of the pressure signal mean value. This would introduce an offset in the pressure trace, affecting ROHR results.
The paper shows how these considerations can be taken into account in the implementation of the algorithm of IMEP and RHOR evaluation. In order to correct IMEP and ROHR calculations, some parameters need to be identified: the position sensor delay, the TDC actual position and the sensor wheel teeth unevenness. Algorithms implementing these functions have been integrated within the main application. Once the correction-parameters are estimated, they are directly used for IMEP and RHOR evaluation.