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In the conventional homogeneous-charge spark-ignition engine, fixed lift characteristics and timings for opening and closing of the intake and exhaust valves compromise efficiency and maximum power. In order to minimize the compromise, numerous variable-valve-actuation (VVA) systems have been proposed in the past that give adjustable valve operation depending on the engine-operating condition. A literature survey was conducted of the vast array of VVA devices that have been tested and/or analyzed in order to compare attributes of different systems. The review indicates that the chief benefit of phase-shift systems, the ability to extract more power from a given-displacement engine without compromising idle stability, has been demonstrated conclusively. However, assessments of the potential benefits of the devices with variable-lift capability are less reliable because of the limited scope of past studies, lack of effort to separate the effects of different variables, and unreliability of data.
Ahmad et al. (Tue,) studied this question.