| Idle condition is one of the most important running operations in diesel engines. The good or bad of control effect in idle condition will affect the other performances of a diesel engine such as fuel economy and emission. Idle speed is the most important factor that affects fuel consumption and emission performance. High idle speed will improve emission but increase fuel consumption; low idle speed will reduce fuel consumption but worsen emission and fuel combustion, even stop the engine. In idle condition cold running or load shift will induce idle speed changes, and the changes will make the engine running unsteadily or to be worse, even stop. Under the precondition of good emission, a lower speed which can keep the engine running stably is customarily selected as idle speed. Therefor, it is obviously significant to study on the engine idle control, especially on the idle stability control.At present open-loop control is mostly used in diesel engine idle speed control, the basic principle of which is unilaterally control from input to output. There's no feedback in open-loop control so it is unable to amend the speed deviation induced by disturbances. Therefor the control effect of open-loop control in diesel engine is not as ideal as needed. As to closed-loop control in diesel engine idle speed control there are lots of control algorithms and solutions. But because engine idle control system is a high complicated non-linear system, the performance of idle control system is yet far from satisfaction. Accordingly it is still a strong emphasis to seek better control methods.The main study tasks on the diesel engine idle speed control especially idle stability control in this paper are as follows:1. Design hardware & circuit for the electronic control system using INTEL 80C196KC singlechip microcomputer as the hard core. The design is applied to CA498Z diesel engines equipped with VP37 distribution pump. It can collect and process various signals such as engine speed, water temperature from the running engine. It can as well control the output of fuel delivery and injection including quantity and time.2. Programmed control software for the electronic control system with assembly language and programmed system calibration software with Visual Basic. System software was established based on modularization design, and every module was programmed concretely according to different functions. The system software can meet various control requests that needed to keep the engine running normally and in company with the hardware system they conjunctly achieve the conditions that an integrated electronic control system requires.3. After accomplishing the electronic control system, several diesel engine idle control algorithms and their assembly language solutions were discussed detailedly. Three closed-loop control arithmetics based on speed feedback are applied in this thesis, and they are: PID control algorithm, Fuzzy control algorithm and Fuzzy PID control algorithm.4. Some engine start calibration experiments were made, to make sure that the engine can start rapidly and reliably in all kinds of conditions.5. Bench test for the three idle control algorithms were made after the start experiment. The control parameters of the algorithms through the test were gotten, and their actual control effects and their advantage and disadvantage were analyzed. Finally the three control algorithms are analyzed contrastively.Some conclusions can be made as follows through the experimental study on the idle stability in diesel engine:1. Classical PID algorithm can realize the control of idle speed, however it can not meet both the requirements of rapid & smooth passing in transient process and the requirements of stable running in steady idle process. PID segment control can solve this problem partially, but the self-adjust range of its parameters is limited, and programming for segment control is inconvenient, the control effect is still not approving.2. Fuzzy control algorithm can obtain good effect in steady idle process, better than PID control, however, because it is sensitive to speed change, it causes high speed overshoot in transient process. Although the speed declines to the target idle speed rapidly, it needs a long time to become stable. On condition that reducing the relevant fuel change, a steady-state error will be brought to the system.3. Fuzzy PID control algorithm joints the advantages of the two foregoing algorithms. It can get much better control effect with simple program design and flexible control strategy. This algorithm can supply different control parameters according to different speed change conditions, with which engine is able to pass rapidly & smoothly in transient process and run stably without high speed fluctuating in steady idle process. On the whole Fuzzy PID algorithm achieves the requirements that needed in diesel engine idle stability control system.
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