Design And Experimental Research On Diesel Engine EGR Systems Based On Step Actuator

With the fast development of the automotive industry, the number of theautomobile expands sharply, from which the pollutant emissions affect theenvironment and human health and have already caused great public concern.Especially in the past decade, stricter and stricter emission regulations were issuedfor all over the world successively because of human environmental consciousness.The NOx emission from the diesel engine is equivalent to the SI engine, while theemission of the particulate matter from the diesel is obviously higher. So greatemphasis on the limitation of these two kinds of pollutants from the diesel has beenplaced in emission regulations universally.EGR, exhaust gas recirculation, is to recycle the exhaust gas partially into theinlet system to increase the mass fraction of inert gases. Since the inert gas does notparticipate in the combustion, and is higher in thermal capacity to absorb morecombustion heat, and thus depresses the peak combustion temperature. Meanwhile,EGR reduces the oxygen concentration in the fresh air and depresses the NOxemission. However, when EGR is introduced, the particulate matter, CO and HCemissions may increase, and the feul economy may deteriorate, too. Therefore, thetrde-off between reduction of the NOx emission and increase of the other emissionsor the deterioration of the feul economy must be considered when EGR is used.In general, three kinds of EGR are used on the diesel engine, includingmechanical, magnetoelectrical and electronical controlled kinds. The pressuredifferences between intake and exhaust pressure are used to modulate the EGRratio in mechanical kind. However, the range it can control is small and itscontrolling freedom is limited. The mechanism of the second one is to switch on oroff the EGR valve through controlling the vacuum. Although the control circuit issimple, there are only two states of the tube: on and off. So the control accuracy isrestricted. The electronic controlled system can control the EGR ratio preciselyaccording to the different condition of the engine. The overlarge ratio maydeteriorate the emission and fuel economy. On the other hand, the control resultsare not dramatic with a small EGR ratio. So a method of controlling the EGR ratioaccurately is studied.According to GW2.8TDI turbocharged diesel engine, a control system basedon linear step actuator is designed including the hardware and software to reducethe NOx emission. Also bench tests has been done. The input signals consist of thespeed and the accelerator sensor. Experiments included the acquire of the MAP ofthe optimal EGR ratio and the ultimate validation tests. A serial of constant speedtests were conducted in the former one and the best EGR ratio is selected accordingto the general performance of the engine. In the latter one, The ECU could regulatethe position of the valve automaticly related to the engine condition.Results show that the system can work effectively and reliably. It could improvethe overall performances of the engine greatly in part load, although it may causehigh smoke emission. Therefore, the EGR-MAP should be regulated further.