A Simple Diesel Engine Air-Path Model to Predict the Cylinder Charge During Transients: Strategies for Reducing Transient Emissions Spikes

Simple air-path models for modern (VGT/EGR equipped) diesel engines are in common use, and have been reported in the literature. This paper addresses some of the shortcomings of control-oriented models to allow better prediction of the cylinder charge properties. A fast response CO₂ analyzer is used to validate the model by comparing the recorded and predicted CO₂ concentrations in both the intake port and exhaust manifold of one of the cylinders.

Data showing the recorded NO_x emissions and exhaust gas opacity during a step change in engine load illustrate the spikes in both NO_x and smoke seen during transient conditions. The predicted cylinder charge properties from the model are examined and compared with the measured NO_x and opacity. Together, the emissions data and charge properties paint a consistent picture of the phenomena occurring during the transient.

Alternative strategies for the fueling and cylinder charge during these load transients are investigated and discussed. Experimental results are presented showing that spikes in both NO_x and smoke can be avoided at the expense of some loss in torque response. Even if the torque response must be maintained, it is demonstrated that it is still possible to eliminate spikes in NO_x emissions for the transient situation being examined.