Internal-combustion engines – Poppet valve operating mechanism – With means for varying timing
Patent
1996-10-10
1997-11-11
Lo, Weilun
Internal-combustion engines
Poppet valve operating mechanism
With means for varying timing
123 9022, 123 9039, F01L 1300, F01L 126, F01L 3122
Patent
active
056852649
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a cam mechanism for the cylinder head valves of an internal combustion engine.
Cam mechanisms are well known. In conventional cam mechanisms a camshaft is provided with several cam lobes and the cylinder head valves of the engine are each driven by one of the cam lobes of the camshaft. In such conventional engines, the cylinder head valves are each driven by one cam profile only for all engine speeds and loads. This is disadvantageous since a cam profile best suited for high speed, high load operation of an internal combustion engine is not suited for low speed, low load operation and vice versa.
There have been a number of proposals for cam mechanisms which enable a valve to be driven by two different cam profiles, a first cam profile at low engine speeds and loads and second cam profile at higher engine speeds and loads. One example of such a system is described in WO-A-91/12415. In this published application a cam mechanism is shown which has a camshaft with first and second cam lobes mounted thereon for rotation therewith, the first and second cam lobes having different cam profiles. First and second finger followers are provided with first and second follower means respectively arranged to follow the profiles of the first and second cams. The first and second finger followers are rockable about fulcrum means comprising a fixed fulcrum in the form of a rocker shaft and a movable fulcrum. The first cam and the first follower means of the first finger follower together form a first short duration cam mechanism for low speeds of the engine, the movable fulcrum being in a first position at low speeds in which the second finger follower is out of engagement with the second cam. At higher speeds the movable fulcrum is moved to a position in which the second finger follower brings a second cam follower into abutment with the second high lift cam and the controlled valve is moved under the action of the cam of higher lift, the second cam.
In one embodiment shown in the prior art document WO-A-91/12415 two side by side valves are controlled by a cam mechanism which comprises two finger followers which sandwich a third finger follower. The two outer finger followers engage cam profiles for low speeds of the engine, the cam profiles being both low lift profiles, but the profiles being different to each other. At high speeds, the third inner finger follower engages a high lift cam which has a lift higher than both of the other two cams and thus both valves are controlled by the lift of the high lift cam.
The prior art specification WO-A-91/12415 teaches that the low lift cam can be replaced by a circular lobe, which imparts no lift to the control valve and therefore provides valve deactivation.
In the prior art system each valve can be controlled by only two different profiles, a profile for low speed operation and a profile for high speed operation. Whilst this is an improvement on the position in conventional engines where one profile is used for all engine speeds and loads, it is preferable to allow more than just two operating conditions of the cam mechanism, so that the valve motion can better meet the requirements of the engine throughout a range of engine conditions.
In EP-A-0213758 FIGS. 7 to 9 show a cam mechanism for controlling a pair of valves which has three operating conditions. In the first operating condition one valve is driven by a finger follower which engages a low lift cam, whilst the other valve remains deactivated since the finger follower that abuts it engages a circular raised portion. In the second operating condition both valves are driven by the low lift cam. In the third operating condition both valves are driven by a high lift cam. Thus one valve has three operating modes, low lift, high lift, deactivated, whilst the other has just two operating modes.
In EP-A-0276533 FIGS. 14 and 15 show a cam mechanism in which a camshaft has a low lift cam, a high lift cam and an annular raised portion. A pair of cylinder head valves are driven by the cam mechanism which ha
REFERENCES:
patent: 4799463 (1989-01-01), Konno
patent: 5386806 (1995-02-01), Allen et al.
Allen Jeffrey
Miller Michael
Lo Weilun
Lotus Cars Limited
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