Valve Gear

The valve gear of an internal combustion engine provides timely admission of the fresh charge into the cylinders and exhaust of spe gases from them. For this purpose the valves at definite moments open and close the intake and exhaust ports in the cylinder head, through which the cylinders communicate with the intake and exhaust manifolds.

The valve gear consists of timing gears, a camshaft, tappets, push rods, rockers with fasteners, valves, springs with fasteners and valve guides (see Fig. 7).

Fig. 7 valve gear of overhead-valve V-type engine

The timing gears in most engines are housed in a special case fitted at the front end of the engine. These are necessary to transmit rotation from the crank-shaft to the camshaft, fuel injection pump shaft, and to the oil pump, and other mechanisms. The gears are made of steel and use helical teeth to reduce noise.

Camshaft serves to open the engine valves positively and timely, in a definite sequence, and to control their closing against the return action of the valve springs.

The shaft is made integral with its cams and bearing journals. Each cam controls a single valve, either intake or exhaust. The cam profile of the exhaust-valve cams is more obtuse than that of the intake-valve cams, hence the exhaust valves stay open for a longer period than the intake ones do. Inn some automobile engines, the camshaft is made integral with fuel pump eccentric wheel and oil pump drive gear. The cam on steel camshafts is induction hardened.

The camshaft bearings are lubricated with oil supplied under pressure from the main gallery in the cylinder block. In most engines, the front end of the camshaft carries the timing gear. The camshaft gear may be fabricated from steel, cast iron or fabric-base laminate. Sandwiched between the end faces of the timing gear boss and the camshaft front bearing journal is the spacerring which is encircled by the thrust plate bolted to the front wall of the cylinder-block-and-crankcase unit to locate the camshaft axially. The ring is 0. Ito 0.2 mm thicker than the plate, and it is this difference that determines the camshaft end play.

The tappets are to transmit the force from the camshaft to the push rods. The tappets are small cylindrical bores receive the push rods. The tappets are made of cast iron or steel and located in the guides, which may be made integral with the cylinder block or removable as in the engine. With the engine running, the tappets continuously rotate about their axes for uniform wear. The rotation is ensured by a convex surface of their bottoms and a slanted surface of thd cams.

The push rods transmit the force from the tappets to the rocker and are made as steel stems with hardened tips or duralumin tubes with spherical steel tips press-fitted at both ends. The push rod tips bear against the tappet hollow at one end against the spherical surface of the rocker adjusting screw at the other.

The rockers transmit the force from the push rod to the valve. The rockers are made from steel and are installed on a hollow fulcrum a bronze bush is press fitted into the rocker hole to reduce friction. The hollow fulcrum (rocker shaft) is supported by standards on the cylinder head. End play of the rocker is prevented by a coil spring. The rocker arm is a bell crank made of steel. At the middle of the rocker arm, there is a boss with a bore into which is pressed the bushing. A hardened curved pad is provided on the end of the rocker (on its longer arm) where it contacts the valve stem tip, while a threaded hole is machined in its other end to receive the adjusting screw used to set the valve clearance, the clearance between the rocker contact pad and the valve stem tip, so that the valve will be tightly pressed against its seat when hot. The rocker arm freely oscillates about the rocker-arm shaft sunnorted by a series of pedestals, or brack-est, which are bolted to the top deck of the cylinder head.

The valves are used to open and close the intake and exhaust ports in the engine periodically, depending on the position of the pistons in the cylinders and engine firing order.

In the engines, the inlet and exhaust ports are made in the cylinder heads and terminate in inserted valve seats of heat-resistant cast iron

A valve consists of a head and stem. The valve head has a narrow chamfer of 45’ or 30’ referred to as valve face. The valve face fits tightly against the seat, which is achieved by grinding.

The heads of the inlet and exhaust valves are of different diameter. For better engine breathing, the inlet valve has a larger diameter than the exhaust one. As the valves are not equally heated in the running engine, they are made from different materials. The inlet valves are made from chromium steel, the exhaust valves are of silchrome heat-resistant steel.

The cylindrical stem of the valve has a recess at the upper end for fastening the valve spring. The valve stems slide in the castiron or cerametallic valve guides. The valve is pressed against its seat by a steel cylindrical spring.

The spring has a variable coil pitch to lessen the tendency of the spring to vibrate or “flutter” at high speeds.

One end of the spring rests on a seating collar arranged on the cylinder head, the other end bears against the spring retainer. The spring retainer is held on the valve stem by 

two tapered locks whose inner shoulder enters into the recess in the valve stem.

To decrease oil penetration along the valve stem to the combustion chambers, the seating collars are fitted with rubber rings or the stems are provided with rubber caps. More uniform heating and wear of the valve are ensured with “free valves”, i.e. the valves rotating during the operation of the engine.