|KYLE’S CORNER – AUTO REPAIR HELP|
VALVE TRAIN & CYLINDER HEADS
Cylinder heads are cast out of iron or aluminum. Cylinder heads bolt onto the top of the engine block and are used to seal the top of the cylinders. They contain the passages that intake and exhaust gases flow through as well as, passages for coolant and oil flow. The intake and exhaust valves are also located in the cylinder heads. On overhead camshaft equipped engines, the cylinder head will also contain the camshaft.
The valves located in the cylinder heads allow the air/fuel mixture to flow into the combustion chamber and provide a means to remove the combustion by-products from the cylinder. Intake and exhaust valves look similar, but the intake valve diameter is usually larger than the exhaust valve. The large circular end of the valve that faces the piston is called the valve head. The beveled edge of the valve that is on the rear side of the valve head is called the valve face. The valve face is held tightly closed against a circular hole in the cylinder head called the valve seat. The valve seat has an edge that is ground at a different angle than the valve face. This is done in order to allow an interference fit between the valve and the valve seat. This interference fit provides a better seal and helps to center the valve in the valve seat when it is closing. The long shaft that extends from the valve head is called the valve stem. The valve stem extends up through a machined guide into the cylinder head. The portion of the valve stem that protrudes from the top of the cylinder head, is retained by a coiled spring and retainer clips. This coiled spring holds the valve closed.
The valves are opened by a rotating camshaft. The camshaft is rotated by a belt or chain connected to the crankshaft. The camshaft contains eccentric lobes that provide the opening motion for the valve when the shaft is rotated. The camshaft lobes are designed to open and close the valves quickly without damaging them. The round portion of the camshaft lobe is called the base circle. The first part of the eccentric is called the ramp or the flank. The tip of the camshaft lobe is called the nose. The opening ramp is cut in an angle to quickly unseat the valve without causing damage. The nose is cut in a measured distance to hold the valve open for a precise time. The closing ramp is cut at an angle to allow the valve to close quickly without damaging the valve.
The amount of time a valve remains open is called the duration. Duration is measured in degrees. The distance that a valve travels when opening is called the lift. The amount of time that the intake and exhaust valves are open at the same time is called overlap. Changes in engine performance, economy, and emissions can be made by using camshafts with different variations of these measurments.
The camshaft does not directly contact the valve stem. Instead, the camshaft contacts a tappet or lifter, sometimes also referred to as follower. The lifter is used to transmit the action of the spinning camshaft lobe to their respective valves. As the camshaft lobe pushes up the lifter, the lifter in turn moves a long rod called the pushrod. The pushrod connects the lifter to an arm that pivots on a shaft or stud. This arm is called the rocker arm. The rocker arm presses directly on the valve stem on one end and contacts the pushrod on the other. The rocker arm is designed to multiply the lifting action of the camshaft and provide a higher lift to the valve. Overhead camshaft engines work in a similar principle, but use less components. By placing the camshaft on top of the cylinder, the need for complex lifter-pushrod designs are eliminated. While overhead camshaft designs vary from manufacturer to manufacturer, the elimination of the cumbersome lifter pushrod designs, allow an engine to produce power more efficiently.
(Kyle has an affinity for Pale Ale and tooling on his 1956 Chevrolet Nomad Station Wagon.)