Research On Influence Of Partial Heating Of The Intake Port And Fuel Strategy On Engine Performance During Starting

Cold-start is a very important process to gasoline engine, reducing emissions during gasoline engine cold-start plays a key role in reducing the overall emissions from the gasoline engine. The emission is usually high at the cold-start stage of the PFI engine because the cold environment limits the fuel evaporation. As a result, a large amount of fuel is required to overcome these negative factors and produce an ignitable in-cylinder mixture. Even so, limited by the improper fuel injection strategies, excessive fuel injection could not guarantee that proper mixtures are prepared for all cylinders. Consequently, the occurrence of misfire and incomplete combustion will deteriorate the cold-start emissions. For these reasons, this paper researched the cold-start process from two aspect: improving the mixture preparation and optimizing the fuel injection strategies during the gasoline engine cold-start stage.The effect of the wall temperature on the fuel film thickness was firstly studied by the method of laser induced fluorescence. It is found that after increasing the wall temperature from 26℃ to 60℃,90℃ and 120℃,the fuel film thickness decreased significantly. The maximum thickness of the fuel film decreased by 19%, 38% and 53%, respectively. And the fuel film area decreased as well from 5.66cm2 to 2.82cm2, 1.69cm2 and 1.55cm2. Based on the results of the fundamental research, a method of partial heating of the intake port end is proposed. The purpose of the partial heating method is to heat the surface of the intake port end for promoting the fuel film evaporation. To achieve a high efficient partial heating, a partial heating system was designed and produced by using electro-thermal film. The partial heating system was tested under different heating powers. The test results suggest that the partial heating system could increase the surface temperature efficiently and quickly. With 60 W and 80 W heating power, increasing the surface temperature from 20℃ to 110℃ only needs 70 seconds and 50 seconds, respectively.A cranking-start test bench was set up for testing the effect of the partial heating system. The test bench can control the cranking start process precisely and, therefore, realize high repeatability cranking start process. The influences of the partial heating on the preparation and combustion of the mixture were experimentally tested at different surface temperatures. The experiment results show that, partial heating could improve the fuel evaporation and increase the mixture concentration effectively. As a result, fewer amounts of fuel are needed for producing an ignitable mixture at the cold-start stage. With the effect of partial heating, the minimum fuel pulse width for producing an ignitable mixture was reduced by 25% from 40 ms to 30 ms. Under 100℃ surface temperature, the fuel transport efficiency was almost doubled. In addition, in the partial heating tests, mixtures were found to burn faster, appropriate delay of the ignition timing could obtain better IMEP output.The fuel injection strategies also have a significant influence on the mixture preparation. An independent fuel injection strategy was developed in this paper, the purpose of this strategy is to provide proper injection timing the suitable fuel pulse width in order to obtain high quality and uniform mixture for all the cylinders in the shortest time after the cranking start. Four typical initial piston positions were discussed and verified by engine stop tests. Then, researches were carried out to find out the differences between different initial piston positions. It is found that cylinders with different initial piston position would experience quite different intake stroke owning to their initial phase. For example, cylinders starting from the middle of the intake stroke have very weak intake flow, fuel injection could hardly produce ignitable mixture, even excess fuel is injected. By varying the injection timing and fuel pulse width, The effects of the initial piston position on the mixture preparation were investigated in both the cold mode and hot mode. And the principles of the independent fuel injection strategy were summarized for both the cold mode and hot mode.The independent fuel injection strategies were then tested in the hot-mode for simulating the HEV engine stop-start process. The experiment results suggest that the independent fuel injection strategies have significant effect on improving the performance and emission of the cold-start process. With the advanced fuel strategies, proper and uniform mixtures could be obtained in all cylinders. Misfire and incomplete combustion were successfully eliminated. With 24 ms and 36 ms fuel pulse width of the first injection, shorter and more stable engine starts were achieved, compared to the original fuel strategy, the independent fuel injection strategy with 24 ms and 36 ms fuel pulse width reduced the starting time from 0.684 s to 0.639 s and 0.658 s, respectively. Moreover, better emissions were also realized. With proper fuel pulse width for the first cycle, the average HC emission from 2nd cycle to the 10 th cycle was reduced by 43%.