EGR Control Strategy

Hannu Jääskeläinen

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• Introduction
• Control Inputs and Outputs
• Controller Structure
• Closed Loop Control Strategies
• System Behavior
• Pumping Loss Considerations
• EGR Related Auxiliary Emission Control Devices

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.

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:

• Temperature Sensors. Intake manifold and EGR temperature can be measured with commonly available temperature sensors. If the engine is equipped with a mass air flow meter, it often incorporates a temperature sensor that can provide a measurement of pre-compressor intake air temperature. In the temperature range of -50 to 150°C, silicon IC sensors are commonly used while for temperatures up to 1000°C, thermistor-type sensors are available. Virtual sensors can also be used to estimate temperature in particularly challenging locations such as at the turbine inlet [2490].
• Pressure Sensors. Many EGR control applications require intake and exhaust manifold pressure, barometric pressure and/or EGR valve pressure drop. Piezoresistive micromachined pressure sensors are common for these applications.
• Flow Sensors. A number of flow parameters are critical for EGR control.
  • Intake air flow at the compressor inlet can be measured with a mass air flow (MAF) sensor—these can use a hot wire, hot-film or thin-film type sensing element. Modern designs are able to detect flow direction to accurately account for flow reversals. These sensors are common for light- and medium-duty applications. However, there are challenges to using these in heavy-duty applications including inadequate durability. Most MAF sensors also incorporate an air temperature sensor.
  • EGR flow can be measured with a flow sensor or a virtual sensor that uses EGR valve position and pressure drop as inputs. Venturi-type flow meters as well as orifice plates have been used for direct EGR flow measurement. Designs based on hot-film anemometry have also been developed [2503].
  • Intake charge flow from the intake manifold to the cylinder can be determined with a virtual sensor based on the speed/density approach, Equation (1) below.
• Position Sensors. EGR valve and variable geometry turbine vane position sensors are also important system signals that can be required for EGR control. EGR valve position sensors can be based on a variety of principles including potentiometers and Hall Effect sensors.
• Gas Composition Sensors. Exhaust Gas Oxygen (EGO) and NOx sensors can also serve a useful function in controlling EGR in diesel engines. Diesel engines operate with excess air so that the switching-type EGO sensors for stoichiometric gasoline applications are not commonly used. Wide range EGO sensors use a ZrO₂ solid electrolyte and oxygen-pump electrochemical-titration to measure air-fuel ratio over a wide range. Sensors to measure NOx are similar but include additional features to first remove excess O₂, then dissociate NOx into N₂ and O₂ and then pump the resulting O₂. Sensors to measure intake manifold oxygen concentration have also been developed.

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

The reader is referred to Controls for Modern 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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