Internal-combustion engines – Poppet valve operating mechanism – With means for varying timing
Patent
1993-07-21
1995-02-07
Wolfe, Willis R.
Internal-combustion engines
Poppet valve operating mechanism
With means for varying timing
123 9044, F01L 124, F01L 134
Patent
active
053868064
DESCRIPTION:
BRIEF SUMMARY
This invention relates to cam mechanisms for controlling valve means, and relates in particular to cam mechanisms for controlling the inlet and/or exhaust valves of an internal combustion engine, with the aim to improve the power, torque, fuel economy and emissions output of said engine.
Cam design in an internal combustion engine is frequently the result of compromises between differing requirements of the combustion chambers of such an engine at different engine speeds and loadings.
For example, in a high output, multi-valve, spark ignition engine having a four-stroke cycle and which is designed to operate at high engine speeds, it is generally desirable to provide cams controlling the inlet valves which have a long valve opening period, in order to maximise the combustible charge drawn into the combustion chambers during the suction strokes of the engine. This has the advantage of improving the volumetric efficiency of the engine, thereby increasing the maximum power and torque outputs of the engine. Cams providing a long valve opening period are referred to herein as long duration cams.
However, if such an engine is operated at speeds below that at which maximum torque is developed, since the inlet valves are open for a relatively long period, some of the combustible charge drawn into each combustion chamber on its suction stroke can be forced back through the valve before it closes. This effect clearly reduces the volumetric efficiency, and hence the output, of the engine. It also causes uneven engine idling and low speed operation, and also makes exhaust emissions more difficult to control.
It is therefore desirable to provide a cam mechanism for use only at low engine speeds which has a relatively short operating or opening period, i.e. a short duration cam lobe.
There have already been a number of proposals for variable valve timing devices in which means is provided for varying the duration of the opening of the valve in an internal combustion engine.
For example in U.S. Pat. No. 4,727,831 a pair of adjacent valves are controlled to operate together by means of rocker shafts and cams. The two valves are normally driven from the camshaft by two low-speed cams operating on separate rocker arms for each valve but a separate third rocker arm is mounted between the two aforesaid rocker arms and is arranged to be driven by a high-speed cam. When it is desired to operate the valves via the high-speed cam the third rocker arm is selectively interconnected to the other two rocker arms so that the valves are both driven via the third rocker arm.
In U.S. Pat. No. 4,475,489 a valve is driven either by a first rocker arm driven by a high-speed cam or a second rocker arm driven by a low-speed cam and means is provided to move the two rocker arms between operative and inoperative positions whereby the valve is driven by either of the rocker arms. There is an overlap between the high-speed and low-speed positions where both rocker arms are driving the valve in order to overcome the problem that if there is no overlap both of the rocker arms will be at intermediate positions at which an undesirable impact takes place between the valve and the rocker arms.
We aim to provide an alternative variable cam mechanism which avoids the problem caused by taking both of the rocker arms out of engagement one after the other and the problem of U.S. Pat. No. 4,727,831 where all the rocker arms are in contact continuously with the cams which means that the cams have to provide power to drive all the rocker arms all the time.
Accordingly the invention provides an internal combustion engine having valve means and a cam mechanism for controlling the valve means comprising camshaft means having first and second cam means mounted thereon for rotation therewith and first and second finger followers having first and second follower means respectively arranged to follow the surface of the first and second cam means, the first and second finger followers being rockable about fulcrum means which holds the finger followers in direct or ind
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Allen Jeffrey
Hurr William J.
Group Lotus Limited
Lo Weilun
Wolfe Willis R.
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