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1、2.5 Valve System The valve system is made up of those parts needed to open and close the valve at just the right time. 2.5.1 Valve System To coordinate the four-stroke cycle, a group of parts called the valve train opens and closes the valves (moves them down and up, respectively). These valves move
2、ments most take place at exactly the right movements. The opening of each valve is controlled by a camshaft. 1. Overhead Camshaft (OHC) Valve Train The cam is an egg-shaped piece of metal on a shaft that rotates in coordination with the crankshaft. The metal shaft, called the camshaft, typically has
3、 individual cams for each valve in the engine. As the camshaft rotates, the lobe, or high spot of the cam, pushes against parts connected to the stem of the valve. This action forces the valve to move downward. This action could open an inlet valve for an intake stroke, or open an exhaust valve for
4、an exhaust stroke. As the camshaft continues to rotate, the high spot moves away from the valve mechanism. As this occurs, valve springs pull the valve tightly closed against its opening, called the valve seat. Valve in modern car engines are located in the cylinder head at the top of the engine. Th
5、is is known as an overhead valve (OHV) configuration. In addition, when the camshaft is located over the cylinder head, the high-performance engines have two separate camshaft, one for each set of inlet and exhaust valves. These engines are known as dual overhead-camshaft (DHOC) engines.2. Push-rod
6、Valve Train The camshaft also can be located in the lower part of engine, within the engine block. To transfer the motion of the cam upward to the valve, additional parts are needed. In this arrangement, the cam lobes push against round metal cylinders called cam follower. As the lobe of the cam com
7、es up under the cam follower, it pushes the cam follower upward (away from the camshaft). The cam follower rides against a push rod, which pushes against a rocker arm. The rocker arm pivots on a shaft through its center. As one side of the rocker arm moves up, the other side moves down, just like a
8、seesaw. The downward-moving side of the rocker arm pushes on the valve stem to open the valve. Because a push-rod valve train has additional parts, it is more difficult to run at high speeds. Push-road engines typically run at slower speeds and, consequently, produce less horse-power than overhead-c
9、amshaft designs of equal size. (Remember, power is the rate at which work is done.)2.5.2 Valve Clearance When the engine runs in compression stroke and power stroke, the valve must close tightly on their seats to produce a gas-tight seal and thus prevent the gases escaping from the combustion chambe
10、r. If the valve do not close fully the engine will not develop full power. Also the valve heads will be liable to be burnt by the passing hot gases, and there is the likelihood of the piston crown touching an open valve, which can seriously damage the engine. So that the valves can close fully some
11、clearance is needed in the opening mechanism. This means that the operating mechanism must be able to move sufficiently far enough away from the valve to allow the valves to be fully closed against its seat by the valve spring. However, if the clearance is set too great this will cause a light metal
12、lic taping noise.2.5.3 Valve Timing The time at which valves open and close (valve timing) and the duration of valves opening is stated in degrees of crankshaft rotation. For instance, the intake valve normally begins to open just before the piston has reached top dead center. The valve remains open
13、 as the piston travels down to BDC and even past BDC. This is intake valve duration. An example of this could be stated as follows; IO at 17BTDC, IC at 51ABDC (or, intake opens 17before top dead center, intake closes 51after bottom dead center). Intake valve duration in this case is 248of crankshaft
14、 rotation. This leaves 129 duration for the compression stroke since compression ends when the exhaust valve begins to open approximately at 51 before bottom dead center. The duration of the power stroke in this case is also 129. Since the exhaust valve is opening at 51 BBDC, this begins the exhaust
15、 stroke. The exhaust stroke continues as the piston passes BDC and move upward to the past TDC. With the exhaust valve closing at 17 ATDC, the duration of the exhaust stroke is 248. It is apparent from this description that the exhaust valve stays open for a short period of time during which the int
16、ake valve is also open. In other words, the end of the exhaust stroke and the beginning of the intake stroke overlap for a short period of time. This is called valve overlap. Valve timing and valve overlap vary on different engines. Opening the intake valve before TDC and closing it after BDC increases the fill of air-fuel mixture in the cylinder. Opening the intake valve early helps overcome the static inertia of the
