The introduction of a large amount of EGR gas into cylinders is effective for reducing exhaust emissions in diesel engines. As combustion characteristics under the condition of higher EGR rate are highly influenced by the amount of intake charge gas and its composition. It requires a precise control method for preventing the increase of NOx and particulate matter (PM) in spite of the transient conditions of supercharging and tolerance of turbocharger.
In this study, a new control method was developed based on the observed oxygen concentration of charge gas and excess air ratio (stands for the oxygen concentration of burnt gas) which are the main physical factors governing NOx and PM formation. In a conventional method, the intake airflow rate is controlled to meet a target value determined in advance in the stationary engine operating condition. But the oxygen concentration and the excess air ratio of the charge gas were not controlled to the target value under the variance and transient conditions of supercharging.
As a result of experiments, the amount of NOx emission was found to be approximately proportional to the charge oxygen concentration, which was also comprehended by the relationship between flame temperature and oxygen fraction of the charge. The amount of PM was affected not only by the charge oxygen concentration but also by the over all excess air ratio of the charge gas.
In the new control method, some required physical values were estimated as follows: (a) The oxygen concentration of the charge gas was estimated from EGR rate. Oxygen concentration of EGR gas and air, and transportation of EGR gas from exhaust valves to intake valves was also taken into consideration. (b) The EGR rate was calculated from the estimated charge air and the amount of total charge gas estimated from the pressure and temperature of mixed gas inside the intake manifold. (c) The EGR gas temperature was calculated from the conservation of intake and exhaust energy and heat exchange at the EGR cooler and intake manifold.
NOx fluctuation under the tolerance of turbocharger was almost eliminated with the new method, and at the same time the fluctuation of PM was also reduced to one-third compared to that of the conventional method. And NOx increase during supercharging delay was barely observed for the new control method.