LO 9-5 Identify and describe the purpose, construction, and operation of components used in the valve train
1. Valve Train Components
1.11. Valve Actuating Mechanisms
LO 9-05: Identify and describe the purpose, construction, and operation of components used in the valve train. |
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Valve Actuating Mechanisms Transfer of movement from the camshaft to the valves is performed through the valve train. For in-block camshaft engines, the camshaft initiates valve action by transferring force through the cam follower and then to the pushrod, rocker lever, and, finally, the valve. Multivalve setups will use a valve bridge to open two valves with one rocker lever Figure 41. Most of these pushrod-type valve actuating mechanisms will use a roller-type cam follower or lifters and roller rocker levers to reduce friction. This reduction in friction not only increases the durability of the components but also can add 1.5–3% higher power output and reduced fuel consumption. Reducing valve actuation friction is especially important in diesel engines because of very high loading pressure on the camshaft by the exhaust cam followers. This high loading pressure occurs when the exhaust valve opens against cylinder pressure at the end of the power stroke. Cylinder pressures remaining at the end of the power stroke are still high, and tremendous force must be exerted by the camshaft to open the valve and begin the exhaust stroke. The combination of pressure and friction in the exhaust valve train causes premature valve train wear and camshaft failures unless extra features are designed into the exhaust valve train mechanisms. Excessive cam lobe wear is measured using a dial indicator when the camshaft is still in the engine Figure 42. The amount of lift each lobe has is compared against specifications. A worn exhaust will produce an engine misfire or popping sound through the intake air system when the intake valve opens because the exhaust gas cannot leave the cylinder through the exhaust port. |
FIGURE 09-41: Valve bridges actuate two valves with a single rocker lever. Older bridges were adjustable. Newer engines use nonadjustable bridges. |
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| FIGURE 09-42: Camshaft lobe wear is measured either on the engine with a dial indicator or on the bench. | ||||
Roller-type followers and larger-diameter and wider cam lobes are used in combination with exhaust rocker arm ratios to reduce the pressure acting on the camshaft during valve lift, which increases the durability of exhaust valve train components. Roller-type lifters and followers reduce friction between the camshaft and the follower. When hip or flat lifters are used, the cam may have a special angle ground into the lobe to rotate the lifter as the cam wipes along the bottom of the lifter Figure 43. This rotation minimizes premature lifter wear. Offsetting the lifter slightly from the lobe centerline will also cause lifter rotation Figure 44.
FIGURE 09-43: A comparison of a hip and a roller mechanical lifter. The fork used on the roller lifter prevents the lifter from spinning in its bore. |
FIGURE 09-44: Hip valve lifters follow the cam profile and have a centerline slightly offset from the cam lobe centerline. When the cam turns, the lifter rotates on its axis, reducing friction between the lobe and the lifter. |
| Technicians Tip | ||
| Adjustments to valve bridges are generally not performed during scheduled valve and injector adjustments. Whenever they are adjusted, they must never be loosened or tightened while placed on the valve stems. Pulling a wrench to loosen the locknuts will bend the valve stems, causing the valves to stick open. Always remove the bridge and loosen it in a soft-jawed vise. After setting proper clearances on the engine, the bridge is removed again and tightened in a vise. The locknut should be only lightly tightened while on the engine and only firmly torqued while in a vise. |
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