Characterisation and Model Based Optimization of a Complete Diesel Engine/SCR System

In order to make efficient use of a Diesel engine equipped with an SCR system, it's important to have a complete system approach when it comes to calibration of the engine and the aftertreatment system.

This paper presents a complete model of a heavy duty diesel engine equipped with a vanadia based SCR system. The diesel engine uses common rail fuel injection, a variable geometry turbocharger (VGT) and cooled EGR.

The engine model consists of a quasi steady gas exchange model combined with a two-zone zero dimensional combustion model. The combustion model is a predictive heat release model. Using the calculated zone temperatures, the corresponding NO_x concentration is given by the original Zeldovich mechanism. The SCR catalyst model is of the state space type. The basic model structure is a series of continuously stirred tank reactors and the catalyst walls are discretized to describe mass transport inside the porous structure. Implicit methods are used to solve the ODEs which allow long time steps and high computational efficiency.

The combined engine-SCR model is a useful tool to characterize the system. A number of critical operating sequences are studied; step response from steady state low load conditions to high load and vice versa and also cold start experiments. Model based optimization is used aiming at achieving Euro VI emission levels while minimizing the brake specific fuel consumption (including Urea cost) by simultaneously optimizing the engine and SCR control parameters.