1. Field of the Invention
The present invention relates to desmodromic valve and cam systems for internal combustion engines with poppet valves. In particular, the present invention relates to a rockerless desmodromic valve and adjustable cam system which utilizes splitting cam lobe assemblies with internal follower grooves. The present invention also relates to camshafts which have replaceable cam lobes providing various duration/timing adjustability options.
2. Background of the Invention
Most conventional internal combustion piston driven engines utilize valve trains to induct an air/fuel mixture into the cylinders and to expel the burned air/fuel mixture from the cylinders. Typically, each cylinder is assigned at least one poppet intake valve and at least one exhaust poppet valve. The valves are driven open by cam lobes on a camshaft that push against the valves to open the valves as the camshaft rotates. In a different manner, the valves are forced closed by springs concentrically disposed around the stems of the valves.
For example, in a typical four-stroke engine, an intake valve is opened by the force of a cam lobe while the piston goes down inducting an air/fuel mixture into the cylinder (I.E., induction stroke). Next, the intake valve closes by force of a spring while the piston moves upward. This compresses the air/fuel mixture (I.E., compression stroke). With all the valves closed so that the combustion chamber is sealed tight, a spark is then produced by a spark plug which ignites the air/fuel mixture wherein the rapidly expanding hot gases force the piston downward with great energy creating power (I.E., power stroke). The exhaust valve then opens by force of a cam lobe and as the piston moves back up it expels the burned air/fuel mixture (I.E., exhaust stroke).
For the conventional combustion engine with poppet valves to run efficiently, the valves must open and close with great precision. Their ability to tightly seal when closed must be nearly perfect. This timing aspect is controlled by the cam, which either directly, or through a rocker mechanism, pushes the valve open at the correct time. This manner in opening the valves has proven to be highly effective.
However, closing the valve by the force of the spring has its disadvantages. Most notably, the use of springs to close the valves utilizes/consumes engine power. The springs in an engine induce excessive tension into the valve train because they continuously force the valve mechanism against the cam lobes as the camshaft rotates. Another disadvantage is that because the cam mechanism cannot afford to have any ‘bounce’ from the springs, the cam profile has to be somewhat gentle, I.E., it must gently push the valve, but never shove it. This means the valve must open slowly like a water faucet—not quickly like a light switch, for example. Another disadvantage is that when the motor is turned at high RPM's, the valves can “float” and hit the piston. Valve float happens when the speed of the engine is too great for the valve springs to handle. As a result, the valves will stay open and/or “bounce” on their seats.
To overcome these disadvantages, innovative desmodromic valve trains have evolved over about the last century; however, in a very slow technological pace and in most applications with little or limited success. The term “desmodromic” arises from the two Greek words: “desmos” (controlled or linked), and “dromos” (course or track). A desmodromic system is also known as system that provides “positive valve actuation” wherein both strokes are “controlled”. In other words, desmodromic valves are those which are positively closed by a leverage system or follower, rather than relying on the more conventional springs to close the valves. Typically, a desmodromic valve operating system utilizes a camshaft that controls both the opening and closing of the valve.
Desmodromic valve trains have several advantages over conventional spring closed valves trains. A first major advantage is that in a desmodromic valve system there is almost no wasted energy in driving the valve train. In other words, the constant force that the springs exert on the lobes of the camshaft is removed. Another advantage is that because there is no tension and no possibility of “bounce” in the desmodromic system, the cam profiles can be as steep as the engine designer wishes them to be. This desirable aspect allows the engine to be more powerful and more flexible. Thus, the manufacturer can use more radical cam grinds or profiles for better performance. Another advantage is that when the motor is turned at high RPM's or even over-revved, the valves are still controlled, whereas when the valves are returned by springs the valves “float” and hit the piston.
Nevertheless, even though desmodromic valve trains have the aforementioned advantages, they have had limited success in large scale commercial applications due to reliability issues, complexity of design, and valve train binding to name a few reasons. The most relevant prior art is now herein discussed below.
U.S. Pat. No. 4,711,202 to Baker [hereinafter “BAKER”] teaches a direct acting cam-valve assembly. BAKER discloses a double cam designated 60 fixed to an engine driven camshaft 26. The double cam forms a following track composed of internal or inner cam 64 and 63a, and an internal or outer cam 64 and 64a. The valve stem 11a is connected to a cam follower 30 which has a pair of roller followers 56 transversely disposed on the top distal tip of the cam follower 30. The followers are retained within the following track. During engine operation, as the camshaft 26 is rotated, the roller followers 56 engaging the inner cams 63, 63a will operate to effect opening movement of the poppet valve 11 from the closed position to the open position via a hydraulic lash adjuster 40. However, upon continued rotation of the camshaft 26, the roller follower 56 engaging the outer cams 64, 64a will pull the cam follower 30 back up and, via the force of spring 70 it will also move the poppet valve toward the closed position.
Although the BAKER reference discloses a highly refined desmodromic solution, it does have some disadvantages. The main disadvantage of the BAKER desmodromic system is its complexity. The head requires at least a cavity 16, guide bore 15 an oil galleries 50, 53, standing pads 18, and longitudinal extending bores 24. All of these aforementioned features add significant machining costs to the manufacture of the head. Thus, additional complexity to the head greatly adds costs to the entire BAKER desmodromic system. Additionally, the spring 70 still induces some tension into the valve train.
U.S. Pat. No. 1,644,059 to Holle [hereinafter HOLLE] discloses a desmodromic type valve actuating mechanism in FIGS. 4–6 in which an internal cam 29 and external cam 31 are attached to shaft 1a. A roller mechanism 26–28 attached to a connecting rod 25 which is further connected to linking yoke which in turn is attached to the valve stem 14. A spring 24 is concentrically disposed around the valve stem in a compressed manner. The spring 24 acts to hold the valve fully closed or seated. However, the drawback of HOLLE is that it provides no solution on how the system may be integrated into a modern heavy duty cast head. Additionally, the spring 24 still induces some tension into the valve train.
It would be advantageous to provide a desmodromic valve and cam system which does not depend on springs to return the valve head closed to fully eliminate any binding tension that the springs typically induce into the valve train system. Moreover, it would be advantageous to provide a desmodromic valve and cam system which is simple to manufacture and of which utilizes few parts. An ideal desmodromic valve and cam system could either be integrated into modern engines having specially designed heads, or retrofit onto existing heads that are already on internal combustion engines. Furthermore, it would be desirable to provide a desmodromic valve and cam system which would have interchangeable cams. With such a feature, various cams having varying profiles, durations, etc. could be utilized on the same system. Moreover, it would even be more desirable to provide cams of which the timing could be either individually advanced or retarded by merely choosing the position on which the cam lobe is installed onto the camshaft. Such features would provide a wide array of adjustability in regards to being able to tune the engines performance characteristics.