LO 9-3 Describe the features and explain the advantages of overhead camshaft engines

1. Overhead Cam Advantages

Overhead Cam Advantages

Overhead cam engines are now a popular valve train design in diesels with a number of advantages:

1. Simplicity. Pushrods and several other valve train components are eliminated. This also means there are fewer wearing surfaces, with the elimination of parts resulting in fewer adjustments required to maintain valve clearances Figure 22.
2. Easier inspection and service. All the components are easier to inspect and remove. For example, camshaft inspection can be performed visually, and it can be replaced without disassembling the engine front cover, removing the cylinder head, and performing other additional steps Figure 23.
3. More accurate valve train action. Shortening the length of the valve train minimizes its thermal expansion and reduces its inertia. This permits higher engine speeds and closer valve train clearances, resulting in more accurate valve timing. Stiffer and heavier components can be used without the penalty of added mass and length associated with a long pushrod.
4. Optimized intake and exhaust passageways for easier breathing. Because there is no obstruction from push tubes to interfere with intake and exhaust port design, the size, location, and shape of intake and exhaust ports are optimized to improve engine breathing ability.
5. Higher injection pressures. A shorter, stiffer valve train permits a unit injector to be actuated to higher injection pressures (34,000 pounds per square inch [psi; 2,344 bar)]) with more accurate injection timing. When pushrods are used to transfer movement from the camshaft to the injectors, they tend to bend and add considerable inertia to injector actuation. For this reason, overhead cam diesel engines that do not use pushrods will have minimal injection lag, which is an important consideration for emissions reduction.

FIGURE 09-22: The geartrain on the front of this engine synchronizes crank-to-cam rotation and transfers torque to the overhead camshaft, which allows replacement of pushrods.

FIGURE 09-23: Installing the rocker arm shaft on an overhead camshaft MP10 Mack engine using the manufacturer-prescribed lifting tool.