Study On Application Of Start-stop Technology In Gasoline Direct Injection Engine

In order to achieve saving energy and reducing emissions, start-stop technology and gasoline direct injection engine both have great potential for development. Start-stop technology can be realized well in the gasoline direct injection engine (GDI). Consequently, the application of start-stop technology was investigated in gasoline direct injection engine in this paper. Starting process was studied especially, and control strategies of starting process were optimized for making the engine achieved good starting performance, low emissions and vibration. At first, start-stop technology, GDI engine technology, and CFD simulation of its starting process in China and other country were analyzed. Then a Mitsubishi 4G15GDI engine was selected as experimental engine for studying start-stop technology. The drive circuits of spark ignition and fuel injection were designed and developed. Some soft-wares of controlling injection, ignition and idle bypass valve in the starting process were written. Emissions collection system in the start process was designed and erected.The spray process of first cycle in starting process of the GDI engine was simulated by AVL Fire software. Influences of initial position of the compression-stroke piston, injection quantity and injection timing on the first cylinder concentration field, temperature field and flow field were investigated. The piston begins to compress from 60°CA BTDC to TDC, when the mixture is weaker and the piston reaches TDC, no matter when injection timing is, the combustible mixture can be formed near spark plug area. When the mixture is richer and injection timing is far from TDC, pocket of fuel will wet the spark plug that lead to misfire. On the contrary, if the injection timing is close to TDC, it will not lead to misfire and form combustible mixture near spark plug area. The piston begins to compress from 120°CA BTDC to TDC,when the piston reaches TDC, no matter how rich and weak mixture is, and no matter when injection timing is, the combustible mixture can form near spark plug area. The piston begins to compress from 180°CA BTDC to TDC,when the mixture is weaker and the piston reaches TDC, no matter when injection timing is, self-ignition is easy to happen. Both richer mixture and appropriate injection timing can avoid self-ignition.The engine situation after stopped was studied. Fuel rail pressure after engine stopped first decreases quickly and then decreases slowly. Its pressure decreases to the same low pressure as fuel tube within 30 to 45 minutes. The piston in the compression stroke after engine stopped tends to stop at around 80 ? BTDC and hardly stops at around 0~30 ? CA BTDC or 150~180°BTDC. When engine rotational kinetic energy decays to zero, the size of cylinder gas torque determine whether engine will reverse. Two optimatic sensors were installed at a certain phase, which could determine whether the crankshaft will reverse and detect the crankshaft position after engine stopped.The motor drive modes which include start mode of starter, start mode of expansion cylinder injection with starter assisting and start mode of high-speed motor were investigated on the bench test.In terms of the start mode of starter, the compression cylinder's combustion in the first circle was studied. When the compression-stroke piston's initial position is at 60°BTDC, there are misfire region as the injection quantity is greater and lesser. When the compression-stroke piston initial position is advanced to 120～150°CA BTDC, a region of self-ignition occurs and the self-ignition region enlarges gradually as the pistons initial position advances. Careful consideration must be paid to the combustion in the second combustion cylinder otherwise self-ignition is likely to occur in the cylinder. Its self-ignition can be avoided only if appropriate injection quantity and injection timing to be set. Combustion characteristics, starting speed characteristic and emission of the start process were studied. The starting process is subdivided into A, B, C stages by the inlet pressure and speed of start process. The injection timing has a huge effect on the combustion of starting process. As the injection timing is at between the 310°CA BTDC and 260°CA BTDC, the combustion of each stage is relatively stable, misfire is not occurred and the emission of HC is low. As the injection timing is at 330°CA BTDC and 240°CA BTDC, the combustion is bad and misfire happens in many cylinders. Ignition timing also has great influence on the combustion of starting process. As the ignition timing advances, the combustion of stage A advances as well, and the combustion of stage B and C, especially the B stage, become worse. Besides, in terms of start mode of starter,the starting speed is quick as the compression-stroke piston initial position is at around 120°CA BTDC.Compare to start mode of starter, using start mode of expansion cylinder injection with starter assisting can make engine start quickly, especially 105°CA BTDC where the compression-stroke piston initial position is, engine needs only 0.26s for reaching 800r/min.In terms of start mode of high-speed motor, engine can switch steadily from start mode of high-speed motor to the mode of engine catching fire of itself when the dragging speed of motor is at 800~1400r/min. The interim speed and switching mode are steady when the dragging speed of high-speed motor is at 1200r/min和1400r/min, but the HC is higher when the dragging speed is 1400r/min. The start modes of direct injection which include start mode of natural rotation and start mode of reverse rotation were investigated on the bench test.In terms of start mode of reverse rotation, the maximum combustion pressure can be achieved when excess air factor is around 0.84 in compressed cylinder. It is desirable for the compression-stroke piston initial position to be at 80°CA BTDC, which can benefit the expansion cylinder's combustion and working because the expansion cylinder achieves a bigger compression ratio. When the expansion cylinder's excess air factor is between 0.72 and 0.63, its maximum combustion pressure is highest, which can drive the piston to rotate over 360°CA so that engine starts successfully.In terms of the start mode of natural rotation, the highest combustion pressure can be obtained when the excess air factor is controlled around 0.84 in the fourth cylinder. Because of the different valve timing of natural and reverse rotation, the expansion cylinder's piston can hardly pass through the BDC when forward-turning. In terms of the start mode of natural rotation, it is satisfactory for the compression-stroke piston's initial position to be at 105°CA BTDC, which can ensure the compression-stroke piston and the next stroke piton to pass through their own BDC, so that engine starts successfully.In summary, among five start modes mentioned above, the start speed of high-speed motor is quickest, and the engine can reach 800r/min within 0.25s. The engine can reach 800r/min within 0.3s in the start mode of expansion cylinder injection with starter assisting. The five start modes cost lesser time than the traditional start mode. In terms of the emission of HC in the five start modes, the result of high-speed motor mode is relative better than the others', while there is a peak value of emission at the first start stage in the other start modes whose emissions are a little higher.The vibration of start process was studied. In terms of the start mode of starter, the maximum vibration of start process appears during the initial start's stage which is the first two cycle. For the stage of starter dragging, the piston's initial position has great effect on the vibration, and the lowest vibration happens where the piston stops near to the TDC after the intake valve closes. So, the vibration of start process can be reduced by varying the piston's initial position. When the position of initial piston is the same, hot starting (85℃) vibration is somewhat smaller than cold starting (20℃) vibration. Compared with the start mode of starter, the start process vibration of high-speed motor mode is much lower.