The Optimization of EGR Ratio for Fuel Consumption and Emissions of the Modified Transient Mode in a GDI Engine

Abstract

This study aims to adopt the cooled EGR system to GDI engines for the reduction of pumping loss and NOx. This includes the application, test, and analysis of an FTP-75 cycle of the engine dynamometer prior to testing in an actual vehicle. The difference in thermal efficiency between a gasoline engine and a diesel engine results from pumping loss. The thermal efficiency of diesel engines is still higher than that of gasoline engines since the former involves lower pumping loss. As a gasoline engine controls air volume, it unavoidably involves a higher pumping loss. Pumping loss can be reduced, however, by adopting a cooled EGR system. A way to shorten the test term will be sought by controlling the EC-dynamometer for the automatic operation mode. In the numerical analysis and automatic operation test, such factors as rpm, torque, and fuel consumption rate were compared. The engine rpm was relatively allowable and torque was properly controlled in the range of 80-90%, although it did not indicate the level of instant torque because of the dynamometer’s characteristics. The fuel consumption rate was about 8.9% in consideration of the difference in measurement between numerical analysis and the automatic operation test. A test bench was manufactured to evaluate an EGR performance system. This was conducted by controlling the EGR rate accurately and by determining the flow rate and temperature of EGR valves and coolers precisely. In the duty range of 40%, the flow was insignificant while in the range of 40%~60%, the flow increased in proportion to the duty rate. The effectiveness-NTU method was used to determine EGR cooler effectiveness. It offers some advantages in analyzing the problems in which various types of heat exchangers must be compared to select the best-suited type for a specific heat transfer objective. Its durability and efficiency are recognized as better than those of the other currently available coolers. As for temperature and flow conditions, the efficiency is in the range of maximum 97% and minimum 86%. To control the EGR valve, the values of MAP, Fuel, and rpm of the look-up table are obtained to locate the EGR valve, and then the duty value is determined in reference to the EGR valve duty look-up table. To determine the location of the EGR valve, the results of the bench test were used. The valve location was chosen in consideration of the differential pressure and EGR cooler temperature. In reference to the location and air to fuel ratio, a look-up table is made for engine calibration and provides the duty value. The optimization process was based on the analysis of changes in ignition timing at the given EGR ratio and rpm. As the ignition timing is closer to TDC, the EGR ratio decreased, and as it becomes farther, ignition timing again becomes closer to TDC when the EGR ratio increases from 10% to 15% or higher. The modified transient mode was tested for engine rpm and torque, which were divided to one part that used EGR and the other that did not. The part that used EGR referred to the optimized EGR ratio and optimized ignition timing. Engine rpm was similar regardless of EGR application while the torque showed some variation. The present engine system used λ=1 controlling, which was also applied to the experiment. The fuel consumption and emission of NOx and CO decreased as a result of EGR application. As for the basic condition of the test, the EGR ratio was set to 20% or less, and only middle and low load conditions were applied. For 20% or higher EGR ratio to be applied, more fuel injection had to be involved in the controlling condition of λ=1, which would have increased the fuel consumption accordingly. NOx decreased as a result of EGR controlling, and CO too decreased as the combustion level was lean compared to the present one. The intake pressure and temperature increased upon EGR application. When the intake temperature increased with λ=1 under control, the fuel consumption increased, which required the fuel injection control for corresponding compensation. The fuel efficiency and emission reduction effects were obtained without replacing the current system and the potential of lean-burn effects and additional improvement methods were also presented. In conclusion, the application of EGR brought in 3.63% enhancement as for the fuel consumption and 4.34% as for NOx emission. Since EGR was applied to the existing system, the pumping loss decreased, which reduced both the fuel consumption and emissions.