Posted: 12/16/13 07:11 PM
In an effort to simplify what actually happens inside an engine, Let's "take a walk" inside a typical engine, just like the one you might have in your car. We will discuss valve events, piston position, overlap and centerlines. Although we can't explain cam design in such a small space, we might be able to clear up some of the most misunderstood terms and make clearer what actually happens as the engine goes through its four stroke cycle. We will graphically illustrate the relationship between all parts of the engine and try to help you understand how the camshaft affects the power of the engine. Put on your walking shoes, open your eyes and get ready for a good look inside this engine.
We begin with the piston all the way at the top with both valves closed. Just a few degrees ago the spark plug fired and the explosion and the expansion of the gasses is forcing the piston towards the bottom of the cylinder. This is the event that actually pushes the crankshaft around to create the power and is referred to as the "power stroke". Each "stroke" lasts one half crankshaft revolution or 180 crankshaft degrees. Since the camshaft turns at half the speed of the crank, the power stroke only sees one fourth of a turn of the cam, or 90 camshaft degrees.
As we move closer to the bottom of the cylinder, a little before the piston reaches the bottom, the exhaust valve begins to open. By this time most of the charge has been burned and the cylinder pressure will begin to push this burnt mixture out into the exhaust port. After the piston passes the true bottom or Bottom Dead Center (BDC), it begins to rise back to the top. Now we have begun the exhaust stroke, another 180° in the cycle. This forces the remainder of the mixture out of the chamber to make room for a fresh, clean charge of air fuel mixture. While the piston is moving toward the top of the cylinder, the exhaust valve quickly opens, goes through maximum lift and begins to close.
Now something quite unique begins to take place. Just before the piston reaches the top, the intake valve begins to open and the exhaust valve is not yet fully closed. This doesn't sound right, does it? Let's try to figure out what is happening here.
The exhaust stroke of the piston has pushed out just about all of the spent charge and as the piston approaches the top and the intake valve begins to open slowly, there begins a siphon or "scavenge" effect in the chamber. The rush of the gases going out into the exhaust port, will draw in the start of the intake charge. This is how the engine flushes out all of the used charge. Even some of the new gases escape into the exhaust. Once the piston passes through Top Dead Center (TDC) and starts back down, the intake charge is being pulled in quickly so the exhaust valve must close at precisely the right point after the top to keep any burnt gas from reentering. This area around TDC, with both valves open is referred to as "overlap". This is one of the most critical moments in the running cycle, and all points must be positioned correctly with the TDC of the piston. We'll look at this much more closely later.
We have now passed through overlap. The exhaust valve has closed just after the piston started down and the intake valve is opening very quickly. This is called the intake stroke, where the engine "breathes" and fills itself with another charge of fresh air/fuel mixture. The intake valve reaches its maximum lift at some defined point (usually about 106 degrees) after top dead center. This is called the intake centerline, which refers to where the cam has been installed in the engine in relation to the crankshaft. This is commonly called "degreeing". We will talk about this later also.
The piston again goes all the way to the bottom and as it starts up, the intake valve is rushing towards the seat, with the piston in hot pursuit. The closing point of the intake valve will determine where the cylinder actually begins to build pressure, as we are now into the compression stroke. When the mixture has all been taken in and the valves are both closed, the piston begins to compress the mixture. This is where the engine can really build some power, or combustion pressure. Then, just prior to the top, the spark plug fires and we are ready to start all over again.
The engine cycle we have just observed is typical of all four stroke engines. There are several things we have not discussed, such as lift, duration, opening and closing points, overlap, intake centerline and lobe separation angle.
Most cams are rated by duration at some defined lift point. As slow as the valve opens and closes at the very beginning and end of its cycle, it would be impossible to find exactly where it begins to move. In the case illustrated, the rated duration is at .006" tappet lift. In our plot, we use valve lift so we must multiply by the rocker arm ratio to find this lift. For example, .006" x 1.5 =.009". Instead of the original .006" tappet lift, we now use .009" valve lift. If you count the number of degrees between these opening and closing points, you will arrive at the advertised duration, in this case 270 degrees of crank shaft rotation. In this example, it's the same for both the intake and the exhaust lobes, thus making this a single pattern cam. Some cam manufacturers rate their cams at .050" lift. If we again multiply this by the rocker arm ratio, we get .075". we can mark a diagram and read the duration at .050" lift. This cam shows around 224 degrees, standard for the 270H cam. The lift is very simple to determine. You can simply read it from the axis going up. This is the lift at the valve as we said earlier. Sometimes you will hear lift referred to as "lobe lift". This means the lift at the lobe or the valve lift divided by the rocker arm ratio. In this case, it would be .470" divided by 1.5 or .313" lobe lift. The lift is simply a straightforward measurement of the rise of the valve or lifter.
We touched on opening and closing points a little earlier, but now we want to consider them even further. We talked about when these points occur, and how they are measured. As you can see, the valve begins to move very slowly then picks up speed as it approaches the top. It does the same closing, coming down quickly then slowing to a gentle stop. It's kind of like driving your car. If you were to go from 0 to 60 mph in a fraction of a second and stop instantly, you can imagine what that would do to the car, not to mention the driver. It would be much too severe for any valve train to endure. You would bend pushrods, wear out cams, break springs and rockers, and lose all dynamicdesign. The cam would not run to the desired RPM level as you would have all these parts running into each other. As the valve approaches the seat, you also have to slow it down to keep the valve train from making any loud noises. If you slam the valve down onto the seat, you can expect some severe noise and a lot of worn and broken parts. So it is easy to see that you can only accelerate the valve a certain amount before you get into trouble.
Looking a bit further at the timing points, the first one we see is the exhaust opening point. We have all noticed the different sounds of performance cams, with the distinct lopes or rough idle. This occurs when the exhaust valve opens earlier and lets the sound of combustion go out into the exhaust pipes, at very high velocity. It may actually still be burning a little when it passes out of the engine, so this can be a very pronounced sound, breaking the sound barrier.
The next point is the intake opening. This begins the overlap phase, which is very critical to vacuum, throttle response, emissions and especially, gas mileage. The amount of overlap, or the area between the intake opening and the exhaust closing, and where it occurs, is one of the most critical points in the engine cycle. If the intake valve opens too early, it will push the new charge into the intake manifold. If it occurs too late, it will lean out the cylinder and greatly hinder the performance of the engine. If the exhaust valve closes too early it will trap some of the spent gases in the combustion chamber, and if it closes too late it will over-scavenge the chamber; taking out too much of the charge, again creating an artificially lean condition. If the overlap phase occurs too early, it will create an overly rich condition in the exhaust port, severely hurting the gas mileage. So, as you can see, everything about overlap is critical to the performance of the engine.
The last point in the cycle is the intake closing. This occurs slightly after Bottom Dead Center, and the quicker it closes, the more cylinder pressure the engine will develop. You have to be very careful, however, to make sure that you hold the valve open long enough to properly fill the chamber, but close it soon enough to yield maximum effective cylinder pressure. This is a very tricky point in the cycle of the camshaft.
The last thing we will discuss is the difference between intake centerline and lobe separation angle. These two terms are often confused. Even though they have very similar names, they are very different and control different events in the engine. Lobe separation angle is simply what it says. It is the number of degrees separating the peak lift point of the exhaust lobe and the peak point of the intake lobe. This is sometimes referred to as the "lobe center" of the cam, but we prefer to call it the lobe separation angle. This can only be changed when the cam is ground. It makes no difference how you degree the cam in the engine, the lobe separation angle is ground into the cam. The intake centerline, on the other hand, is the position of the centerline, or peak lift point, of the intake lobe in relation to top dead center of the piston. This can be changed by "degreeing" the cam into the engine. There is a recommended intake center-line installation point on each cam card, and it is important to install the cam at this point.
When The Flag Drops.,.
The Bull ***t Stops.,.
P. Engineer, Engine Builder