| At the background of energy-saving, light-vehicles with traditional engines are facing unprecedented survival pressure with increasing stringent emission regulations and fierce competition from electric vehicles. The mainstream power of light-vehicles is gasoline engine, so it is imperative to reduce the fuel consumption of gasoline engine. EGR has been one of important technologies to improve engine performance relying on its clean, efficient and easily realized features. Not only the pumping loss and NOX emission are reduced at partial load with gasoline engine applying EGR, but also knocking is inhibited at high load. Besides, engine compression ratio can be increased and ignition timing can also be advanced with EGR, which improve the engine’s fuel efficiency. But the main problem for gasoline engine applying EGR is that the air-fuel mixture will be over-diluted, leading to lower flame-propagating speed and weaker combustion stability, even misfire. As a concequence, further application of EGR on gasoline engine is restricted.The main goal of applying high EGR rate is to reduce the pumping loss during intake stroke for gasoline engine worked in stoichiometric air/fuel ratio mode so as to improve fuel efficiency at partial load of the engine. In order to explore effective ways to enhance EGR tolerance, homogenous EGR experiments with many different coupling strategies were conducted on a 1.4T GDI engine. Results showed that the engine torque obtained in both high-loop and low-loop EGR circuits increased first and then decreased with EGR rate at partial load. The maximum EGR rate improving engine performance was very low, 15% and 10%, at speed of 1500 rpm and 2500 rpm. Contrasting the engine performance in different EGR circuits, it can be found that transient responsive performance and high EGR rate were realized easily by high-loop, but with poor EGR uniformity for each cylinder and low efficiency of turbocharger. Un-cooled EGR with lower gas density can reduce the diluting effect on combustion, at the same time, mixture temperature can also be increased, which helped the fuel evaporation and mixture combustion. Flame propagation velocity and EGR tolerance can be increased by burning oxygenated fuel due to increasing C and O reaction probability. However, the amount of air required for combustion and throttle opening were reduced burning oxygenated fuel, which decreased the saving of pumping loss for gasoline engine partial load. Tumble flow was breakup at the end of compression stroke and was converted to turbulent motion, promoting the transfer of momentum, mass and heat. So flame propagation velocity with homogenous EGR can be quickened by stronger tumble motion, especially for high EGR rate operation. Greater torque was generated when EGR that produced by burned rich mixture was introduced into cylinder. The main reason was that the unburned HC and CO in EGR gas can be used as a kind of new gas fuel and participate in the combustion. Besides, owning to the fast combustion reaction rate of HC and CO, the constant volume of combustion was increased for higher EGR rate.Although the negative effects of EGR on GDI engine combustion at partial load can be reduced by applying hot EGR, burning oxygenated fuel, enhancing tumble motion, and changing EGR composition, the effects of all of the above strategies were limited. Given the reasons, a new stratified EGR strategy induced by exhaust gas backflow was present based on tumble-orientated intake port gasoline engine in this paper. Intake valves and exhaust valves were opened at the same time during intake stroke reflowing exhaust gas to cylinder. The exhaust gas rotated around tumble motion and kept stratification with air before ignition timing. The quantity and position of backflow exhaust gas can be controlled proactively by regulating the second open parameters of exhaust valves and tumble flow intensity, thus decreasing exhaust gas concentration around spark plug and reducing negative effects on combustion.Calculation and optical measurement were conducted together to investigate this new stratified EGR strategy and it’s affected factors that decided the backflow exhaust gas location in cylinder. A tumble-orientated intake port gasoline engine computing platform was established using AVL Fire software. The main research contents included in this paper are as follows: exhaust valves secondary opening parameters, tumble motion intensity, and adaption for different operations. The exhaust valves secondary opening parameters can be specified into secondary opening timing, secondary opening duration, and secondary opening lift. Calculated results showed that only exhaust valves opened at the middle and later intake stroke could allow the backflow gas rotated around tumble motion and layered distribution formed. Instead, if the exhaust valves were opened in earlier stage of intake stroke, there was no stratified phenomenon. Longer exhaust valves secondary opening duration introduced more exhaust gas, while also weakened tumble motion intensity. The longer the exhaust valves secondary opening duration was, the weaker tumble motion became. So it can be used to slow down the backflow exhaust gas rotating speed. Secondary opening lift of exhaust valves was not the higher the better in spite of acquiring more EGR quality per unit time. The higher lift reduced pushing effect of tumble motion leading to more exhaust gas diffusing to the centre of combustion chamber, which went against initial flame formation and propagating. Fixed exhaust valves secondary opening parameters were not fitted for different engine speed, and optimizations of exhaust valves secondary opening parameters were necessary for better stratified EGR distribution. Backflow exhaust gas quality was obviously increased by low differential pressure between intake and outlet port. Exhaust valves secondary opening parameters should be optimized for high inlet-outlet pressure drop, but not for low differential pressure. The backflow exhaust gas location in chamber can be controlled away from spark plug by adjusting tumble motion intensity.To further study backflow exhaust gas distribution characteristics in-cylinder and verify the accuracy of the calculated results, a PLIF measurement system was established on a single cylinder optical engine. This PLIF system mainly included optical engine, Nd: YAG laser, ICCD, mirrors for generating planar laser, zoom UV lens, and devices for producing trace gas. Three exhaust camshafts were modified for valves secondary opening, 1# camshaft realizing exhaust valves opened at later stage of intake stroke, 2# camshaft realizing exhaust valves opened at earlier stage of intake stroke, and 3# camshaft realizing exhaust valves opened with higher lift and longer duration. Besides, different inlet-outlet pressure was realized by adjusting throttle open angle and different tumble intensity was realized by customized gasket. Experimental results of 1# camshaft showed that backflow exhaust gas location in cylinder was closed to exhaust side at 180oCA BTDC, then the backflow exhaust gas rotated along with tumble motion to the bottom right corner and piston top as piston traveled up, reaching to inlet side of cylinder at last. So it can be concluded that backflow exhaust gas rotated with tumble only when it was introduced at later stage of intake stroke, and kept stratified distribution with fresh air. Results of 2# camshaft showed that homogenous air and EGR was formed at earlier stage of intake stroke. Results of 3# camshaft showed that higher lift and longer duration was able to increase EGR quality and slow down rotating range of backflow gas, eliminating over-rotation phenomenon. Backflow gas entered into cylinder rapidly and brought stronger gas impacting with higher inlet-outlet differential pressure, making it difficult to control the location of backflow exhaust gas. The distribution of backflow exhaust gas in chamber can be controlled actively by enhancing the intake tumble motion intensity which increased the rotating rate of backflow gas. We can see that the above EGR distribution laws of PLIF measurement were the same with the calculated results, which proved the accuracy of computation.Although the stratified EGR distribution can be obtained with 3D calculation and PLIF measurement, the merits of applying this strategy for gasoline engine were eventually evaluated on the improvement of combustion performance. The equivalence ratio case of exhaust valves secondary opening duration 70-180 oCA ATDC and maximum lift 5mm that capturing higher EGR quality and presenting good EGR stratified distribution was calculated preliminarily. Calculated results showed significant combustion improvement, peak cylinder pressure of stratified hot EGR was higher than 33.94% for stratified cooled EGR and 79.81% for homogenous cooled EGR. Combustion performance difference generated by backflow exhaust gas distribution was also studied. Backflow exhaust gas distribution in cylinder should be as far as possible from the spark plug, thus brought larger combustion space and shorter propagating distance for the mixture. And it was better for backflow exhaust gas to be centralized distributed, preventing thicker mixed boundary layer between exhaust gas and air at the end of compression stroke. To compare engine combustion and emissions performance of equivalence ratio mode and that of fuel injection mode, the spray characteristics of original injector were studied at the beginning. High injection pressure showed longer spray penetration under the same backpressure. The spray penetration of injection pressure 9MPa case was 22.6mm and 50.6mm longer than 5MPa and 1MPa at backpressure 0.1MPa. But different injection pressure showed nearly same spray angle(about 40-50 oCA). Under the same injection pressure and different backpressure, spray development was impeded with the increase of gas pressure in constant-volume bomb. The shape of spray became compact, and the spray penetration was shortened. But the spray angle still kept the same. Based on the calibrated spray model, stratified EGR coexisting with stratified fuel vapor was accomplished by matching injection strategy and intake tumble motion. Compared with equivalence ratio case of exhaust valves secondary opening duration 70-180 oCA ATDC and maximum lift 5mm, fuel in-cylinder injection mode had no large difference on combustion performance, but knocking index was reduced by over two orders of magnitude. Analysis from the cylinder pressure data and visualization flame images showed that in-cylinder combustion mean peak pressure of backflow exhaust gas stratification operation was higher than 18.6% for homogenous EGR operation with the same EGR rate, and exhibited lower cyclic variations and shorter ignition delay. |
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