外文翻譯--優(yōu)化活塞行動改進(jìn)的發(fā)動機(jī)性能-其他專業(yè)(參考版)
2025-01-23 09:55本頁面
【正文】 thus, the frictional losses are directly related, to the square ofthe velocity. These losses are not the same for all strokes. The high pressures in the power stroke make its friction coefficient higher than in the other strokes. The intake stroke has a contribution due to viscous flow through the valve. The function we have optimized is the maximum work per cycle. Because both fuel consumption and cycle time are fixed, this also is equivalent to maximizing both efficiency and the average power. In finding the optimal piston motion, we first separated the power and nonpower strokes. An unspecified but fixed time t was allotted to the power stroke with the remainder of the cycle time given to the nonpower strokes. Both portions of the cycle were optimized with this time constraint and were then bined to find the total work per cycle. The duration t of the power 10 stroke was then varied and the process was repeated until the work was a maximum. The optimal piston motion for the nonpower strokes takes a simple form. Because of the quadratic velocity dependence of the friction losses, the optimum motion holds the velocity constant during most of each stroke. At the ends of the stroke, the piston accelerates and decelerates at the maximum allowed rate. Because the friction losses are higher on the intake stroke, the optimal solution allots more time to this stroke than to the other two. The piston velocity as a function of time is shown in . 11 The power stroke was more difficult to optimize because ofthe presence of the heat leak. The problem was solved by using the variational technique of optimal control theory (8). The formalism yields the equation of motion of the piston as a fourthorder set of nonlinear differential equations. These were solved numerically. The resulting motion is shown in Fig. 1 for the entire cycle. The asymmetric shape of the piston motion on the power stroke arises from the tradeoff between friction and heat leak losses. At the beginning of the stroke the gases are hot, capable of 12 yielding high efficiency, and the rate of heat loss is high. It is therefore advantageous to make the velocity high on this part of the stroke. As work is extracted, the gases cool and the rate of heat leakage diminishes relative to frictional losses. Consequently the optimal path moves to lower velocities as the power stroke proceeds. The solutions were obtained first with unlimited acceleration and then with limits on acceleration and deceleration. The latter situation yields a result familiar in other contexts under the name of turnpike solution (9). The system tries to operate as long as possible at its optimal
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