You have seen that the intake valve must be opened for the intake stroke,both valves must remain closed during the compression and power strokes,and the exhaust valve opens during the exhaust stroke.The designer must design a device to open and close the valves at the proper time.
The shaft will have a egg-shapeed bump called a cam lobe.The cam lobe is machined as an integral part of the shaft.This shaft is called a camshaft.The distance the valve will be raised,how long it will stay open,and how fast it opens and closes can all be controled by the hifht and the shape of the lobe.
As you will see later ,it is impractical to have the cam lobe contact th eend of the valve stem itself.You have placed the camshaft some distance above the end of the valve stem.
When the camshaft is turn,the lobes will not even touch the valve stem.The lifter is installed between the cam lobe and the valve stem.The upper end rides on the lobe and the lower end almost touches the valve stem.The lifter slides up and down in a hobe bored in the head mental that separates the valve stem from the camshaft.
You have developed a method of opening and closing the valves.The next problem is how and at what speed to tuen the camshafts.Each valve must be open for one stroke .The intake valve is open during the intake stroke and remains closed during the compression,power,and exhaust strokes.This would indicate that the cam lobe must turn fast enough to raise the valve every fourth stroke.
You can see that it take one complete revolution of the cam lobe for every fourth strokes of the piston.
Remember that four strokes of the piston require two revolutions of the crankshaft.You can say that for every two revolutions of the crankshaft,the camshaft must turn once.If you are speaking of the speed of the camshaft,you can say that the camshaft must turn at one-half crankshaft speed.
If the crankshaft is turning and the camshaft must turn at onee-halfcrankshaft speed,it seems logical to use the spining crankshaft to turn the camshaft.One every sample way to drive the camshaft would by means of gears and a belt.One gear is fastened on the end of the crankshaft,and the other is fastened on the end of the camshaft.The large camshaft gear drive the smaller crankshaft gear through the belt.
If,for instance,the small gear on the crankshaft has 10 teeth and the large gear on the crankshaft has 20 teeth,the crankshaft will turn the camshaft at exactly one –half crankshaft speed.
auther:kellywww.sinodiesel.com
