Research engine systems

To conduct the research, a custom-built boost air-exchange system including EGR was designed and implemented on the test engine. The layout of the experimental air exchange system is shown in Figure 3.2. The air-exchange system was developed specially for the purpose of this study. The flexibility provided by the air-exchange system allowed independent control of air and EGR levels in the engine.

Figure 3.2 Schematic of gas flow in the research engine

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Two 25 litre high-temperature surge tanks were used to reduce the pulsations in the intake and exhaust streams. These tanks, which were manufactured of stainless steel by Echo Engineering, were certificated pressure vessels for pressure up to six bar absolute pressure and temperature up to 650°C. The size of the tanks was defined using a 1-D engine simulation which showed the system to be able to reduce the pulsation in both the intake and exhaust systems to an acceptable level (i.e. <

±0.01 bar in the inlet pipe; < ±0.05 bar in the exhaust pipe). Appendix A1 shows the effects of different sizes of the surge tanks on the intake and exhaust pressure pulsations with the comparison of simulation results. Since it is a single cylinder engine, a reservoir was desirable to store the exhaust gas from the previous cycles and to supply it to the following cycles. Exhaust samples for emissions analysis were drawn from downstream of the surge tank, as shown in Figure 3.2. Although sampling downstream of the surge tank resulted in some ‘ageing’ of the exhaust gas species, it avoided sample biasing caused by pressure pulsations and inhomogeneity in the exhaust gas stream if sampled from immediately downstream of the exhaust port. Downstream of the exhaust surge tank (shown in Figure 3.2), an electrically controlled butterfly valve was used to control the pressure in the exhaust system.

This was used to exert a specified back-pressure which would normally have been generated by turbocharger and aftertreatment systems within a vehicle exhaust system.

EGR system

The EGR system (shown in Figure 3.2) was developed for providing fully controllable EGR rate and temperature. The EGR gas flow was driven by setting the exhaust back pressure to be a higher level than the intake pressure. The EGR rate was controlled by adjusting the exhaust back pressure valve and the EGR valve positions simultaneously. A proportional valve was installed in the coolant circuit to control the coolant flow rate through the EGR cooler, thereby controlling the EGR temperature.

For most of the work reported here, uncompressed ambient air (at test-cell temperature) was used as the fresh-air intake; as a result, the temperature at the intake manifold could be controlled only with the EGR temperature. The actuators for the exhaust back pressure valve, EGR valve and coolant flow valve were fully electrically controlled through a programmable logic controller (PLC) unit coupled with transistor driver modules.

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The custom-built PLC system was used to achieve closed loop control of the exhaust back pressure, EGR valve position and EGR temperature. The controller logic was developed using the Moeller EASY-SOFT V6.22 Pro software. The pressure in the exhaust tank was sent to the PLC as a feedback signal or a proportional-integral-derivative (PID) control algorithm; from this, the controller generated an analogue output to command the valve position via a DC motor. The EGR valve had a valve position feedback which was used as the feedback signal for the closed loop control of the EGR valve position. The EGR temperature was controlled based on temperature measurement downstream of the EGR cooler; another PID algorithm was used to generate the valve position command, which was then sent to the proportional valve in the control circuit. The schematic of the closed loop controllers and their application are shown in Figure 3.3.

Figure 3.3 Schematic of experimental control system

Sampling gas

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The maximum EGR rate achieved through adjusting the EGR valve position was about 45%, and then the exhaust back pressure was increased to get higher rates of EGR. With the EGR valve fully open, 3 kPa of increase in exhaust back pressure generated ~70% EGR.

The controllers had been tuned to achieve satisfied control accuracies on exhaust back pressure, EGR rate and intake charge temperature. Through tuning the PID gains and improving the control hardware circuits the following control accuracies were achieved: the back pressure was controlled with an error range of ±1kPa; the EGR rate control accuracy was ±0.5%, i.e. for a 60% EGR demand, the actual EGR rate was within the range of 60±0.5%. The intake charge temperature control was within an error range of ±3°C.

Boost system

A highly flexible boost system was developed to provide a wide range of intake charge pressures and temperatures. The supercharged air system was designed by another researcher of the research group, Asish Sarangi. A production supercharger driven by an electric motor was used to provide a wide range of boost pressure for the whole engine operation map. An inter-cooler and an electric heater were used to control the intake air temperature to the desired value.