Internal-combustion engines – Particular piston and enclosing cylinder construction – Cylinder head
Patent
1991-02-08
1992-09-22
Argenbright, Tony M.
Internal-combustion engines
Particular piston and enclosing cylinder construction
Cylinder head
123 9027, 123 9023, F01L 104
Patent
active
051487810
DESCRIPTION:
BRIEF SUMMARY
The invention relates to valve arrangements and cylinder heads for internal combustion engines of the overhead valve type in which the opening of pop-pet-type valves associated with the cylinder heads of the engines is caused primarily by the rotation of one or more camshafts, the return of the valves to their respective seats being effected, for example by springs.
It is a known fact in the art that the rapidity and accuracy of the valve actuation are important factors in obtaining the best performance from engines in which the movement of fluids is governed by reciprocatory poppet valves. Rapidity depends on the mathematical profiles used for the cam lobes, and accuracy depends on the rigidity of the elements interposed between each cam and the valve or valves it actuates.
Another known fact is that it is much more difficult to obtain a satisfactory transfer of fluids through the inlet system than through the exhaust system. This is because the transfers depend upon the pressure differential between the interior of the cylinders and the surrounding air. This pressure differential is many times higher during the exhaust period than the inlet period, even if the inlet system is associated with pumps, etc. Thus, design compromises must favour the parameters of the inlet system or side of the cylinder head layout, which chiefly involve the valve sizes, the shape of the conduits or tracts leading from the valve seats to the atmosphere, and the accurate, rapid operation of said valves.
The inlet and exhaust valves, so far as their relative head sizes are concerned are in fact competing for the limited space available inside the combustion chamber defined by the cylinder head. This combustion chamber, for the sake of thermodynamic efficiency, should depart as little as possible, when projected in the direction of the engine cylinder axis, from the actual cylinder bore circle. As a consequence, an increase in the diameter of the or each inlet valve head can be achieved only by reducing the diameter of the or each exhaust valve head.
It is also known that the optimum flow around the valve head and through the adjacent zone of its port defined by the valve seat is obtained when the direction of the flow makes a small angle with the valve stem axis.
In an overhead valve engine, the best known layout is an overhead camshaft layout, in which, to ensure accuracy of valve actuation, between each cam and the associated valve or valves, only one main element is interposed (apart from hydraulic adjusters or other means of adjusting or eliminating valve clearance). That element is a rigid element such as an inverted cup- or bucket-like element, usually called a cam follower, lifter or tappet, which is preferably free, or induced, to rotate slowly in its guiding element about its axis of reciprocation which is generally perpendicular to the rotational axis of the cam shaft. The slow cam follower rotation minimises wear on the cam surface and its cooperating cam follower face and thus allows the use of mathematical cam profiles having the highest acceleration and velocity values compatible with an acceptable service life.
The above considerations have stabilised high performance engine design around the "twin overhead camshaft" layout with inlet and exhaust valves placed at opposite sides of the median plane of the engine cylinder bore or bores and at an angle to each other. The axis of rotation of each of the two camshafts lies substantially in a plane formed by the axes of the associated valves, the bearing surface of each cam is in contact with the upper face of the inverted cup-like cam follower which is, in turn, in contact with the upper end of the stem of the associated valve.
The latter layout originated with racing car engines and is presently increasingly used for normal engines. Valves of a greater diameter for a given cylinder bore diameter can be used, because the inlet and exhaust valves are arranged with their axis at an angle to each other. This increase in diameter helps to obtain more power from the engine.
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Argenbright Tony M.
Macy M.
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