If you look at the similarities

Transcription

1 ANATOMY OF THE THE VALVE TRAIN FIGURE 1 If you look at the similarities between a human and an engine, I would compare them like this - The oiling system is the heart; it transfers vital fluids (oil) to the critical areas of the engine. Without it the engine would not live very long. The intake system is the lungs, it takes oxygen and mixes it with fuel to ultimately have its energy converted to power once inside the engine. This then makes the piston, rod, and cranks the muscles of the engine. They take the energy from the lungs and convert it into mechanical work; ultimately what turns your tires. Much like a person, all of these operations are needed to survive, but a sound brain is still the most important of them all. The valve train is the brain of the engine. It tells the engine when to breathe in and when to breathe out. It also classifies the engine as to what characteristics it will have. Will it make a lot of torque or will it make high rpm horsepower? You could call this its By Bob Markiewicz For this article, I have called out the lobe as the area of the cam that does the lifting of the valve. The lobe itself can be broken down into many different areas including the base circle which is where the lifter rides when no lifting is being done on the valve. temperament or attitude. Without the valve train the engine wouldn t know how to think. If the engine isn t told exactly when to breath, or open and close the intake and exhaust valve, it will not function properly- if at all. The camshaft is the deciding factor as to what and when the valve opens. The breakdown of the valve timing is an article within itself. I would like to go over at this time what parts are related to the brain of the engine and some considerations that go into their design. Before we go too far, I would like to talk about inertia so you can understand how it relates to your valve train components. Inertia is defined as, a property of matter in which it remains at rest or in uniform motion in the same straight line until acted upon by some external forces. In other words, things in motion like to stay in motion. You see this every time you try to turn your race kart; it tries to keep going straight and takes a high counter acting force to make it change direction. Everyone also knows that they wouldn t want their tires filled with lead because the higher the mass of an object the higher the inertia, and the more it resists a change in direction, making it slow to accelerate, and just as hard to stop. You are probably wondering why this relates to the valve train. Hopefully, by keeping these thoughts in the back of your mind, objects in motion want to stay in motion, and the higher the mass (weight) the more an object will resist a change in direction, you can see how it could affect the valve trains function. Starting at the base of the valve train, the camshaft is the intelligent part of the system. The camshaft is gear driven off of the crank at 1/2 its speed. So for every two times the engine turns the camshaft turns only once. This is done to achieve the 4 cycles needed for the engine to work. It is supported by two plain bearings on each end. The cam lobes, which the lifters ride on, are what ultimately lifts the valves. Because the events of the cam are measured from the crankshaft the cycles are based off of 720deg (even though the cam only makes 1 revolution in this time ). When measured at the crank, the valves are lifted for about ~225deg of this 720deg cycle. This is the time the cam spends on its lobes, the rest of the time the cam is on the base circle where the valves are not being lifted. (Figure 1) To recap, the cam spins at half the speed of the crank and is responsible for the actuation and timing of the valves motion. The cam grinder has the ability to change when, how fast, and how far the cam opens the valves. By doing this, the engine s power output and power band can be adjusted to match FIGURE 2 Just by changing a camshaft a dramatic increase in power can be had. An engine and all its components must work as a team to build good power. A big cam in a mild engine will result in poor power output. Likewise, a mild cam in an extreme engine will be a waste of all the other components. Camshaft selection is a balancing act that many have made careers out of. 32

2 FIGURE 3 The lifter is very smooth and hard because it has to survive extreme force where it meets the cam. This eliminates wear. For proof, try scratching an old lifter with a screwdriver. Chances are the screwdriver won t even scuff it. FIGURE 4 Edge loading a lifter can wipe out a cam in a matter of minutes. Always make sure that the lifter head you are using is big enough to ride on its face to avoid this disastrous situation. FIGURE 5 The lifter takes the eccentric shape of the cam and turns it into the linear motion needed at the valve. As the cam rotates the lifter is moved up and down in its bore. what the driver needs or what the class structure allows. The actual timing of the cam warrants its own article in the near future. (Figure 2 - two cam compared) The next question that needs to be answered is how the cam, which resides in the crankcase of the engine, lifts the valves in the head of the engine. The lifter is the first component that the cam lobe contacts. (Figure 3) The lifter has the highest stress concentration of any engine part at the point where it meets the cam. For this reason great considerations must be given to the lifter s surface finish and hardness. If the lifter is not smooth or hard enough it will destroy itself and the cam in a matter of minutes. The head of the lifter must also be big enough so that the cam lobe always rides on its face and never rides off its edges. This is why high performance cams with aggressive lobes require a bigger lifter to keep the lifter from digging into the side of the cam. Figure 4 does a good job of showing how this issue can occur. The lifter rides in a precision bore located in the block which allows it to take the eccentric shape of the cam and make it into the linear motion needed at the valve. (Figure 5) The lifter takes this movement and directly transfers it to the pushrod. The push rod is basically just a connector link between the lifter and the rocker arm. Its function is to precisely transfer the motion of the cam to the rocker arm. Any deflection that happens during this operation will result in an unpredictable motion transmitted to the valve, and not what was designed into the cam. Keeping the push rod stiff is essential to minimize this deflection. This should be an easy task, but here is where the inertia comes into play. By making the push rod stiffer, in most cases, it also gets heavier. And, as we know, objects in motion want to stay in motion, so the heavier the pushrod is the more likely it will want to keep moving up when the cam has reached the top of its lobe (nose). If the inertia force of the push rod (and other related components) surpasses the force of the valve spring, which Wanna Race? For more than 50 years, the Hoosier brand has been collecting wins on tracks around the globe. Select from a full range of sprint and super kart tires engineered for the specific needs of competitive kart racers. 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3 The push rods are relatively simple but have the essential job of transferring the motion from the lifters to the rocker arms. FIGURE 7 FIGURE 6 The picture on the left shows the proper motion of the pushrod; everything is stable and moving inline. The picture on the right shows what happens when the pushrod is not stiff enough to do its job. The flexing of the push rod reduces the available lift at the valve, ultimately reducing the power output. is the opposing force that keeps everything following the cam, the valve will be moving in a motion different from the cam s intention. Consideration must be taken to keep the pushrod stiff so it doesn t deflect, but also light so its inertia is kept low. By doing this, we ensure that the designed cam motion is transferred to the valves accurately. (Figures 6 & 7) The push rods are kept in position with guides which are also used to center the rocker arm and keep everything moving squarely to the valve. These guides are made from a special plastic and sit in the head plate. (Figure 8) The push rod actuates the rocker arm. The rocker arms function is to change the direction of the push rod motion so it can be used to lift the valve in the opposite direction. The rocker arm also has the ability to amplify the motion of the cam. By changing the position of the center pivot point, the ratio of the rocker is changed. If the center is moved closer to the push rod than the valve, then it increases the ratio of movement at the valve. For example, a small block Chevy has a ratio of 1.5:1 so for every inch the pushrod moves the valve will move 1.5 inches. The Animal, on the other hand, was designed with a ratio of 1:1, which means when the push rod moves.25 on one side the valve will also move.25 on the other. The actual FIGURE 8 function of the rocker arm may be a whole other article, but much like the push rod the main design consideration falls under stiffness and weight. The rocker arm also has inertia, actually a rotating inertia that will try to overcome the opposing valve spring force. Once again, we want to keep the rocker arm stiff, but also light. If the rocker arm is allowed to flex, then its reproduction of the cam motion becomes unpredictable, and can hurt power and maximum engine speed. But, The pushrod guides keep everything inline. By supporting the pushrods they also keep the rocker arm straight on the valve. 34

4 if it is made too heavy, its inertia will have to be counteracted with higher valve spring force, which increases internal friction and stresses on the valve train parts. (Figure 9) Steve Swartz of Comp Cams had this to say about the topic. Many will assume that the weak link in the valve train is the pushrod. Fact is a pushrod that remains straight is one of the systems most dependable components. Forces travel only in one plane over a straight line. The true weak link is the rocker arm. Forces are constantly shifted from one end to the other as it pivots on the rocker stud. Check the pushrods for straightness at every FIGURE 9 This illustrates how changing the pivot point of the rocker arm closer to the pushrod increases the lift ratio at the valve. ANATOMY OF THE ANIMAL FIGURE 10 The rocker adjuster ball is the pivot point for the rocker arm. Also, by turning the adjuster up or down you can adjust the static clearance of the valve train or lash. rebuild and replace them when in doubt. The rocker arm pivots on the rocker adjustor ball that is mounted to a stud. The rocker ball is threaded to allow it to be adjusted up and down to adjust the lash. A set screw is then used to lock the ball in place for operation. (Figure 10) Now we are finally to the valve itself. The valve has a hard job; it has to survive in an extreme environment. Both the intake and exhaust valves are exposed to the heat and pressure of combustion on the chamber side. The exhaust has an extra hard job, because it also has to deal with the hot exhaust gases flowing past its stem while going out the exhaust port; this makes it one of the highest demanding jobs inside the engine. The valve design considerations include weight, heat stability, hardness, and fatigue resistance to name a few. The weight of the valve is said to be the most critical mass of the valve train and greatly influences high rpm reliability. This is why most high rpm engines use titanium valves instead of steel. Titanium can reduce the valves weight by as much as 30% compared to steel, but comes at a cost of about 5 times that of a steel valve. The high cost is why titanium is not allowed in most of what we do, but high end racing like NASCAR and F1 use it exclusively. The valves main job in life is to seal the chamber so pressure can be made on the compression stroke when the piston is ascending up the bore. If air is allowed to leak past the valves, compression will be lost and power will be reduced. The term valve job refers to re-facing the valve and the seat (its mating surface) so a good seal can be made. Over time the extreme heat and pounding of the valve can cause this seal to be disrupted; FIGURE 11 A valve job is the resurfacing of the seat in the head as seen here, and may require the valve face to be reground or having the valve replaced. By having a positive seal in the chamber you maximize horsepower by eliminating any leakage when compressing the air/fuel mixture. JULY NATIONAL KART NEWS 35

5 so always make sure your engine builder includes a quality valve job when blueprinting and rebuilding your engine. (Figure 11) The standard Animal valves are actually very FIGURE 12 The arrows represent airflow coming into the intake port and around the valve. This represents how much the valve is truly involved with the airflow of an engine. It also shows how adding a higher lift cam, to get the valve further from the seat, can help in getting more airflow into the engine. good and seldom cause problems; actually I have never seen one outright fail. Because of the class rules, to pass tech inspection you are better off replacing the valve s than grinding them. This should eliminate any chance of being disqualified because of a wrong angle or too small of a margin. The final consideration of the valve is how it responds to airflow. The valve shape and size is critical to maximizing the airflow into and out of the engine. Airflow is what horsepower is all about. Maximizing airflow is too in-depth for this discussion, but just keep in mind the importance at an affordable price! FIGURE 13 The green arrows indicate the direction of force and motion in the valve train. The red arrow represents the counteracting force of the valve spring. When the cam reaches the top of its lobe, the valve spring must then push against the cam to keep all the components following its profile. Too little spring force will cause the components weight to overcome the spring, and the valve to no longer follow the cam. becomes too high, it will cause undo stress to the other components. Most importantly it will cause you a HP loss due to an increase in friction. It then becomes a balancing act of getting just the right amount for the application. (Figure 13) Make sure your engine builder checks your valve springs and replaces them often. They are cheap assurance that your engine is running at top RPM and that it will finish a racing weekend. Finally, the retainers and keepers are what keeps the spring attached to the valve. They also have to be rigid enough to not fail under these harsh conditions. And, because they are connected directly to the valve, their weight is also very important. Most stock retainers are made from sintered steel, but most aftermarket retainers are either aluminum or titanium to reduce their weight. (Figure 14) As you can see the brain of your engine has many functions and considerations that must go into its design. But, by understanding each component s function you can more easily diagnose problems at the track and be able to better communicate them to your engine builder. For example, if your engine is not spinning the rpm that it once was, the valve spring might have gone soft and warrant replacement. By having a good knowledge base you can also make sure that your engine builder is giving you the most for your investment by suggesting what you want done come rebuild time. of the valve on performance when considering your next engine. (Figure 12) The valve springs are another highly stressed component of the engine. Their job is to keep the valve following the cam. Engine speed and valvetrain weight are the two main considerations, but there are many others when picking the right valve spring. The higher the valve train weight, and the higher the engine speed, the higher the spring force must be to overcome the inertia of these components. Unfortunately, if the spring pressure Choose from our designs or we can customize a design for you FIGURE 14 The pushrod, retainer, and lifter on the left represent production steel components. On the right are lighter aftermarket pieces made from aluminum and titanium. The lighter components allow the engine to rev higher before the cam starts to lose control of the valve. 36