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Effects of the variation of valve lift, duration, and phasing on the emissions and fuel economy of a multi-cylinder, four-valve SI engine
Published June 09, 1999 by Institution of Mechanical Engineers in United Kingdom
Mechanisms that allow the control of camshaft phasing are becoming relatively commonplace on current design of 4-valve spark ignition engines. These are employed to reduce the compromises that have to be made in camshaft design in order to have acceptable stability and emission levels at part-load conditions combined with high volumetric efficiency throughout the speed range at full load. Camshaft phasing is more beneficial on 4-valve than 2-valve engines because the increased flow area gives higher volumetric efficiencies at full load, but making full use of this potential will lead to both loss of low- speed torque and also excessive residual gas concentration at part load, unless some change in phasing is applied. There are four possible strategies for camshaft phasing: intake camshaft only, exhaust camshaft only, intake and exhaust camshafts phased equally (Dual Equal), and intake and exhaust camshafts phased independently (Dual Independent). Leone et al., investigated each strategy in turn and reported on their relative advantages. An alternative approach was taken by Seabrook et al., in which the experiments were designed using statistics such that the effect of any variation in phasing could be predicted from the results of a relatively low number of selected test points.
An obvious development as designers strive to add further refinement to 4-valve engines is to introduce some method of altering camshaft life and/or camshaft duration as well as, or instead of, phasing. This has already been achieved in production using a mechanism that switches from a camshaft designed for part load to one with higher lift, increased duration, and different phasing for high speed and load operation. Ideally the settings would be continuously variable rather than having a two-position arrangement, and this has been conceived for duration alone through the adoption of an ingenious mechanism which varies the speed of camshaft rotation. Wilson et al., have described the application of an electrohydraulic valve actuation system which allows complete variation of valve events; their system provides an excellent tool for test bed development work but is impractical for production. Efforts have been continuing for years to develop electromechanical and electrohydraulic systems for production, and may well succeed soon.
The work reported here describes an experimental investigation to evaluate the potential benefits in terms of reduced emissions, improved stability, and lower fuel consumption of being able to optimize phasing, lift, and duration for a range of low speed and load conditions. The work was performed on a modern design of 4-valve, V-8, engine and used the statistical approach to the design of the experiments which has been described. The method by which the changes to valve events would be achieved on a production engine was not a subject of this investigation, and so the changes were made simply by having a range of different camshafts. It was intended that the results would quantify the benefits of having a continuously variable system as compared to a profile-switching arrangement. The investigation has been divided into two phases, the first covering symmetrical valve event strategies and the second asymmetric strategies (i.e., one inlet valve following a different camshaft profile to the other). It is the first phase reported here.