【正文】 ). This period of time is known as valve overlap. The closing of the exhaust valve laps over the opening of the intake valve. During this time, the first of the new mixture pushes the last of the burned gases out the exhaust valve. Valve overlap is held to a minimum on turbocharged engines. This prevents the intake charge from being blown out the exhaust. Valve Operation To coordinate the fourstroke cycle, a group of parts called the valve train opens and closes the valves (moves them down and up, respectively). These valve movements must take place at exactly the right moments. The opening of each valve is controlled by a camshaft. Overhead camshaft (OHC) valve train The cam is an eggshaped piece of metal on a shaft that rotates in coordination with the crankshaft. The metal shaft, called the camshaft, typically has 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 an exhaust stroke. As the camshaft continues to rotate, the high spot moves way from the valve mechanism. As this occurs, valve springs pull the valve tightly closed against its opening, called the valve seat. Valves in modern car engines are located in the cylinder head at the top of the engine. This is known as an overhead valve (OHV) configuration. In addition, when the camshaft is located over the cylinder head, the arrangement is known as an overhead camshaft (OHC) design. Some highperformance engines have two separate camshafts, one for each set of inlet and exhaust valves. These engines are known as dual overhead camshaft (DOHC) engines. Pushrod valve train The camshaft also can be located in the lower part of the 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 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 seesaw. The downwardmoving side of the rocker arm pushes on the valve stem to open the valve. Because a pushrod valve train has additional parts, it is more difficult to run at high speeds. Pushrod engines typically run at slower speeds and, consequently, produce less horsepower than overheadcamshaft designs of equal size. (Remember, power is the rate at which work is done.) Valve Clearance When the engine runs in pression stroke and power stroke, the valves must close tightly on their seats to produce a gastight seal and thus prevent the gases escaping from the bustion chamber. If the valves 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 clearance is needed in the operating 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 metallic tapping noise. Camshaft Drive Mechanism Each cam must revolve once during the fourstroke cycle to open a valve. A cycle, remember, corresponds with two revolutions of the crankshaft. Therefore, the camshaft must revolve at exactly half the speed of the crankshaft. This is acplished with a 2:1 great ratio. A gear connected to the camshaft has twice the number of teeth as a gear connected to the crankshaft. The gears are linked in one of three ways: (1) Belt drive A cogtype belt can be used.