EGR Control Strategy

Hannu Jääskeläinen

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Abstract: Commercial EGR systems utilize open or closed loop electronic EGR control to provide precise EGR rates and proper A/F ratios in order to achieve their NOx reduction targets while minimizing the PM and fuel economy penalties. Even more precise A/F ratio control is possible with variable geometry turbochargers.

Introduction

During the engine design stage, an EGR control strategy has to be developed to meet not just NOx reduction targets, but to ensure PM emissions meet design targets, that fuel consumption does not increase excessively and that vehicle performance meets customer expectations. In most modern diesel engines, it is not sufficient to simply control the amount of EGR flowing to the combustion chamber; rather the EGR control strategy must fit in and work well with the entire engine management system. Since EGR can affect the availability of oxygen for the combustion process, EGR control strategies are typically tasked with two critical objectives: (1) ensure that the air-to-fuel (A/F) ratio—or more specifically the oxygen-to-fuel ratio is suitable at all engine operating conditions to meet performance and emissions objectives and (2) that the total mass of oxygen trapped in the cylinder is appropriate for the fuel injection quantity. Thus the EGR control system is an integral part of the air management system.

The strong interaction between EGR control and air management is the primary issue from a control point of view. In many cases, the control and calibration of the EGR valve and the primary air management actuator (for example the VGT vanes) is via two separate closed control loops that can have significant negative interaction unless special measures are taken to minimize or prevent such interactions.

Control of HPL EGR Systems. In this paper, we focus on control strategies typical for high pressure loop EGR systems. Figure 1 shows the basic components for a modern diesel engine’s air management system, including sensors, with a HPL EGR system [2516]. This hardware arrangement would be typical for many modern light- and medium-duty diesel engine applications. The inlet air passes through an air filter and mass airflow sensor before it enters the turbocharger compressor. The compressed air is then cooled by the air-to-air intercooler, and mixed with the cooled EGR gases. The state of this compressed and heated air/EGR mixture is sensed by the MAT (manifold air temperature) and MAP (manifold absolute pressure) sensors just before it enters the cylinders. The exhaust gas pressure is measured by the exhaust backpressure (EBP) sensor before it exits through the turbocharger. In this example, the variable geometry turbocharger is called an Electronic Variable Response Turbocharger (EVRT) and uses an electronic control valve to control oil pressure to position the vanes and determine the effective size of the turbine housing to meet backpressure requirements.

[schematic]
Figure 1. HPL EGR system for medium-duty diesel engine

2003 Ford Powerstroke

For heavy-duty applications, the air management system for an engine with HPL EGR would be similar; the primary difference being that a mass air flow sensor would not be used.

Sensors. In order to control EGR with the required precision and accuracy, signals from a variety of sensors such as those shown in Figure 1, are required. Real sensors that measure common parameters such as temperature, pressure and flow or virtual sensors can be used. Some common sensors used for EGR control include:

Additional information on sensors can be found in the literature [2514][2515].

The reader is referred to Controls for Modern Diesel Engines for clarification of terminology used in this paper and some basic control concepts.

Acknowledgements

We extend out thanks to Chris Atkinson, Atkinson LLC, for reviewing this paper and providing valuable comments.

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