A physical-based approach for modeling the influence of different operating parameters on the dependency of external EGR-rate and indicated efficiency. 2018-01-1736
ABSTRACTExternal Exhaust Gas Recirculation (EGR) provides an opportunity to increase the efficiency of turbocharged Spark-Ignition Engines. Among the competing technologies and configurations, Low Pressure EGR (LP-EGR) is most challenging regarding its dynamic behavior. Only a part of the stationary feasible potential can be used during dynamic engine operation. This is due to two reasons. One is the inhomogeneous distribution of optimized EGR-rate and EGR-tolerance over the engine load with low values at low loads and vice versa. The other reason is the volume between the EGR valve and the inlet valves. It is large for the configuration with exhaust gas supply in front of the compressor, as it is the case for LP-EGR. Both circumstances are particularly critical during engine load changes. The load changes can be highly dynamic so that time span between EGR valve adjustment and change of the EGR-rate at the inlet valves is comparatively long. As a result, a deviation between the nominal and actual EGR-rate arises in the event of a load step. In addition to disadvantages concerning the efficiency, this can provoke an unstable combustion, especially during negative load steps. In case of excessive deviation, the engine may even misfire.
Nevertheless, it is not possible to simply dismiss the advantages of LP-EGR, which should be used to the greatest extent possible. To do so and to guarantee a fuel consumption-optimized engine operation with no instabilities, a load point dependent limitation of the EGR-rate or alternatively an adaption of the operating point to the actual EGR-rate is crucial. For this purpose a precise knowledge of efficiency and combustion variance is necessary. Since the operating state is including the actual EGR-rate, it has an additional dimension, which is usually resulting in an immense measuring effort.
With the objective to avoid long measuring periods, the given contribution introduces a physical-based approach that is modelling the influence of different engine operating parameters on the dependency of external EGR-rate and indicated efficiency. This implies also the dependency of EGR-rate and combustion variance. Since the model addresses vehicle applications, it only uses input parameters that can be assumed to be known during vehicle operation, such as valve timing, engine load, engine speed, air fuel ratio, intake manifold pressure or ignition timing. Knowing the correlation of these parameters along with the EGR-rate, the indicated efficiency and the EGR-tolerance allows an optimization of the fuel consumption while maintaining the stability limits.
As part of this publication, the measuring program that is used to set up the model is explained and presented. The mathematical formulation is described as well as reasons for the specific choice of the model approach are given. A comparison of model and measurement data is shown and the model quality regarding specific parameter variations is evaluated. Finally the results obtained are discussed under the aspect of usability of the model for the virtual calibration of Spark-Ignition Engines.
Daniel Langmandel, Hannes Orlick, Daniel Haas, Hermann Rottengruber, Franziska Riegger
BMW AG, Otto-Von-Guericke University Magdeburg, TU Berlin
International Powertrains, Fuels & Lubricants Meeting