1990-02-01

An EGR Control Method for Heavy-Duty Diesel Engines under Transient Operations 900444

Experiments and analyses were carried out to determine the effects of EGR on NOx and other pollutants for heavy-duty direct injection diesel engines under steady state conditions.
Then based on them, optimum EGR control method was examined for effective NOx reduction without causing substantial increases of other pollutants under transient conditions.
A simple EGR control system was developed for trial to achieve almost the same effects of the said method.
Results of experiments with this system indicated that the EGR control method was capable of substantial reduction of NOx mass emission during transient engine operations equivalent to actual driving conditions, with different pay-loads and average vehicle speeds.
REDUCTION of the NOx mass emission from heavy-duty diesel powered vehicles during actual driving operations, is one of the most important demands in automobile technologies. The method to delay the fuel injection timing has been used as a means to achieve the reduction of the diesel NOx emission, but it is necessary to develop some other effective means capable of achieving the drastic reductions to cope with tighter regulations. Of various technologies considered for this purpose, EGR has a relatively high effect on the NOx reduction.
Following problems, however, have been pointed out against the application of EGR to heavy-duty vehicles. Namely, it is difficult to apply EGR to heavy-duty vehicles for the effective reduction of NOx emission, without penalties of substantial increases of other pollutants and the deterioration of fuel economy, as the use of heavy load region where other pollutants are apt to increase with EGR, is more frequent. Problems of wear and erosion in such an engine due to the recirculation of carbon particles and sulfate have been also pointed out. This study, therefore, is aimed at findings an EGR control method for the achievement of drastic reduction of NOx emission during actual driving operations by the application of EGR to heavy-duty diesel vehicles, while suppressing emissions of other pollutants as low as possible.
There are following characteristic features in the use of EGR on diesel powered vehicles. The effect of EGR on the reduction of mass emission becomes higher than the effect on the reduction of concentration, as the exhaust gas flow rate is reduced by the recirculation. On the contrary, the rate of increase in mass emission can be suppressed lower than the rate of increase in concentration. Since diesel engines for vehicles are operated over a wide range of fuel/air ratios, the effect of EGR varies according to different loads.
It is thus presumed that the EGR rate for the effective reduction of NOx emission without penalties of increased emissions of other pollutants is varied with a given load. The optimum control of EGR rate during actual driving operations, therefore, is necessary. Characteristics of the incoming charge oxygen concentration (O2IN) and the exhaust gas oxygen concentration (O2OUT) vary with a diesel EGR [1]*, with a correlation between them as shown in Fig.1. It will be thus quite effective for the determination of the EGR control method during actual driving operations, if it is possible to express the effect of EGR over a wide range of engine loads, using both of the concentrations as the parameters. Therefore, an attempt was made first to find the optimum EGR condition for the effective reduction of NOx emission during transient operations, without causing substantial increases of other pollutants, using the O2IN and O2OUT behaviors of the steady state conditions.

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