New devices and control techniques have been adopted to take advantage of variable valve timing properties to improve engine performance or load control. This paper presents a study focused on engine load control strategies associated with early intake valve closing or late intake valve closing. It...

New devices and control techniques have been adopted to take advantage of variable valve timing properties to improve engine performance or load control. This paper presents a study focused on engine load control strategies associated with early intake valve closing or late intake valve closing. It can be shown that these load control modes can improve the indicated thermal efficiency of the engine as compared with the conventional throttle control. These strategies are sometime called Miller cycle or Atkinson cycle, since the real compression ratio becomes smaller than the expansion ratio. A thermodynamic spark-ignition engine simulation model was employed. The advantage of a simulation model is to conduct parametric studies without the need of complex experimental apparatus. In this way, a deep understanding of the physical phenomena can be achieved and the sole effect of the desired parameter can be shown. So, general trends can be established to reduce time and money in real engine tests. The study was made for two engine speeds. Typical indicated curves are presented, as the indicated diagram and the pumping work associated with each load control strategy. The desired indicated power at a specified engine speed can be obtained with a partial closure of the butterfly valve in the intake system, or with a variable intake valve closure. It was shown that the load control using the intake valve is more efficient than the conventional throttle, since the pumping work is reduced, with a positive impact on the engine indicated efficiency. Results had shown that an increase up to 7% in the thermal efficiency could be obtained for low engine speeds, and a near 9% for high speed, using the early intake valve closing strategy. Late inlet valve closing strategy also presents benefits, but somewhat smaller.

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