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Little of the long-recognized potential of variable valve timing (VVT) has yet been realized, mainly because of the many limitations that characterize virtually all the mechanisms that have been proposed for VVT application. These limitations include high cost and complexity, mechanism energy losses, limited range of variability, and high valve impact velocities. We have developed a two-degree-of-freedom mechanism, actuated by two cams operating in series, that appears well-suited for VVT applications. This new mechanism avoids the serious limitations that characterize virtually all other VVT mechanisms. It is relatively simple and it can be implemented using only standard-type major components. It can provide a wide range of valve open duration without excessive mechanical losses or high impact velocities. We examine this mechanism in depth. Experimental verification of the operation of this mechanism confirms the mechanical adequacy of the concept at practical engine speeds.

It has long been known that IC engine variable valve timing (VVT) has the potential for significant improvements in engine performance. Many technical papers have been published on the potential benefits of internal combustion engine variable valve timing and the modes of valve operation that can be used to realize these benefits. Almost 800 US patents have been released on the subject of mechanisms to effect VVT since 1880. An extensive review of this literature and patents was made to determine the benefits obtainable from VVT and what operation modes allow these benefits to be obtained. We conclude that VVT has much potential. The potential gains include 15% or more average fuel economy gains as well as significant improvements in torque characteristics, emission reductions, etc. Nevertheless, little of VVT's potential has yet been realized, since serious mechanical difficulties have limited VVT applications to date.

Mechanisms for internal combustion engine variable valve timing (VVT) have been the subject of almost 800 US patents since 1880. Over 110 of these patents were issued just in the three years 1985 through 1987. Also, many technical papers have been published on VVT mechanisms. We reviewed all of these patents and much of the literature. As a result, we have classified all the mechanisms found into IS basic concepts which we describe and evaluate in this paper. We conclude that VVT has much potential, but that very little of this potential has yet been realized. Serious difficulties have limited the application of variable valve timing to date. Virtually all VVT mechanisms proposed until recently suffer from high cost and complexity, limited variability, and high impact velocities. An improved system is described in a companion paper (1)*.