Internal-combustion engines – Poppet valve operating mechanism – With means for varying timing
Reexamination Certificate
1999-04-13
2001-02-20
Lo, Weilun (Department: 3748)
Internal-combustion engines
Poppet valve operating mechanism
With means for varying timing
C123S090170, C123S090310
Reexamination Certificate
active
06189497
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to camshafts, timing gears, lifter bodies and their associated valve train mechanisms for internal combustion engines. The invention concept, incorporated in several alternative designs, is directed at improving overall volumetric efficiency of an internal combustion engine that is operated over a range of engine speeds. More particularly the invention is an improved camshaft drive, support and housing system which provides a method of adjusting, according to engine r.p.m., the distance a valve is raised from its valve seat (lift). Moreover, the invention provides a method of changing the timing and duration of valve opening and closing, relative to crankshaft and piston position, with a change in engine speed.
DESCRIPTION OF THE PRIOR ART
A significant number of prior art inventions have attempted to solve the problem of achieving greater volumetric efficiency of the internal combustion engine by varying the lift and duration of intake and exhaust valves. The replacement of the camshaft with a system of electrically operated solenoids to open engine valves has been explored. Although these systems may allow precise computer driven control of engine valves, drawbacks in reliability stem from the extreme conditions in the operating environment related to heat and vibration. Solenoid operated valve systems also require external power which poses substantial problems in ensuring long term reliability with circuit connections and circuit components when such a system is called upon to operate in a hostile environment of wide temperature variations, incompatible fluids and vibration. Hydraulic methods of using engine lubricating oil to raise valve lifters at high r.p.m. levels have also been proposed and tested. The amount of change in the lift with this method is not substantial and the uniformity of lift can be affected by differences in oil viscosity. Variances in individual lifter performance, due to manufacturing tolerances or differences in oil temperature and supply pressure, also diminish the effectiveness of this method. Other approaches to improve volumetric efficiency with a variable camshaft design have focused only on changing the advance or retardation of a camshaft, with respect to crank rotation, without addressing a change in valve lift or duration of valve opening.
The submitted invention, which uses a mechanical system of valve lift and duration control, has advantages over previously described methods of valve control. High temperatures, inherent in the operation of an internal combustion engine, do not affect the system. Moreover, operation does not depend on an external power source and is not adversely affected by heat or vibration. In addition, it is believed that the system components of the submitted invention can be manufactured at a reasonable cost with respect to the following described benefits.
An understanding of the principles of volumetric efficiency is necessary to appreciate the advantages of the submitted invention.
Volumetric efficiency is the ratio of the actual fuel/air charge taken into the cylinder during the intake phase in relation to the theoretical amount possible for the cylinder. In general, maximum volumetric efficiency is reached at approximately the same r.p.m. level where maximum torque is reached.
Motor Service's Automotive Encyclopedia,
Edited by Toboldt & Johnson, Goodheart-Wilcox Co. 1968. An engine's volumetric efficiency will decrease with operation at a speed where the lift and duration of valve opening restricts the flow of the air/fuel mixture and exhaust gases.
A practical overview of volumetric efficiency and other design considerations is presented in
Scientific Design of Intake and Exhaust Systems,
Philip H Smith, Rev.Ed 1968. It is generally accepted that the optimum opening point for the exhaust valve at high r.p.m. is set at a point of one hundred ten (110) or one hundred twenty (120) degrees of crankshaft rotation on the power stroke. With only a cursory investigation, it would seem that opening the exhaust valve before the piston reaches bottom dead center would deprive the engine of the opportunity to develop maximum power. It has been found, however, that the early opening of the exhaust valve at high r.p.m.'s provides benefits in scavenging the cylinder of exhaust gases. As stated by Smith:
After the first 110-120 degrees of crank movement from t.d.c. of the power stroke, the working pressure tails off very rapidly; the piston is slowing down and the crank angle (in terms favorable leverage in applying shaft rotation) is becoming progressively less effective. Thus although at first sight shortening the working stroke might seem to be a peculiar method of obtaining more engine power, there is a perfectly logical explanation: it is simply that more is gained from the reduction in exhaust pumping work than is lost by allowing a little of the power impulse to escape.
Page 203.
Smith examines the performance trade-offs of early verses late exhaust valve opening. In acquiring improved elimination of the cylinder at high r.p.m., there is still a loss of pressure through the full power stroke. Moving exhaust gases out of the cylinder requires that the engine must overcome the inertia of the exhaust gas and the tendency of those gases to resist movement. For an engine operating at low r.p.m., more time is available on each exhaust stroke to accomplish the task of exhaust scavenging. At lower r.p.m levels the opening of the exhaust valve can be delayed for additional degrees of crankshaft rotation to be at or near the point that the piston reaches bottom dead center. In this respect, the diminishing force available from the engine's power stroke can be more fully utilized and the task of scavenging the engine can still be accomplished effectively.
The timing for opening the intake valve can also be optimized for a given r.p.m. level in an engine. At high engine speeds the opening of the intake valve before top dead center in the exhaust stroke will assist in filling the cylinder with a fresh air/fuel charge. Again the element of time available for the engine to overcome the effects of inertia on the column of intake air is a factor in choosing the point at which the intake valve is to open.
The point of crankshaft rotation at which the intake valve is to close is also a factor in engine design. Again a compromise is made, relating to the particular point at which the intake valve will close after the piston begins its upward motion to start the compression stroke. As stated by Smith:
This is, of course, made possible by the general refusal of the gas flowing into the cylinder, to reverse its direction simultaneously with the piston reversal at b.d.c. It continues its direction of flow in defiance of the contrarily rising piston, but obviously, if valve closure is delayed too long, the flow will be reversed and the gas pushed back into the induction manifold. . . . The drawback is that with a continually varying throttle opening, as necessitated in normal driving, the inertia of the gas cannot remain constant and thus the tendency or otherwise for reverse flow to occur is dependant on the gas-flow speed, which again depends on throttle opening and engine revolutions. If the closing is delayed for the sake of good power at high rev/min, slow running may be poor because of the uneven distribution set up in the manifold by pressure surges. . . . It has often been found that valve timings aimed at maximum power at peak rev/min undesirably reduce the lower-speed torque.
Page
204
.
Two diagrams, from Smith's book, show the effect of different cam lobe profiles and are presented for review as
FIGS. 1 & 2
.
FIG. 1
shows the resulting crankshaft rotation points of opening and closing of intake and exhaust valves with a cam that is made with a relatively short duration.
FIG. 2
shows the expanded duration of intake and exhaust valve opening for a cam which is ground with a relatively long duration. It would therefore be advantageous, from
Lo Weilun
Woodard Emhardt Naughton Moriarty & McNett
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