Abstract
In the automotive field during the last decade, owing primarily to increases in maximum engine speed and the almost universal use of overhead valve gear, increasing attention has been focused upon improper valve functioning which causes noisy operation, excessive valve spring surge, valve seat pounding, and exhaust valve burning.
With the conventional design of cams no allowance is made for the flexibility of the push rod and rocker, consequently in general the accelerations imparted to the valve are much greater than is necessary at high speeds when inertia effects build up.
A general mathematical analysis of the conventional three-arc cam is presented, introducing the terms “eccentricity” and “instantaneous radius”, and this is followed by a similar analysis of the new cam profile using the same terms, together with the method of correcting the cam profile for highspeed operation. It is also shown how the new general equations can be used to deduce a variable eccentricity ramp, the purpose of which is to take up clearance at variable velocity.
The effects of the new cam profile on horse-power and thermal efficiency are shown; the effect of the acceleration characteristic on valve spring design also is illustrated, and a comparison shows the reduction in spring weight made possible by the elimination of valve spring surge.
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