Valve Train Assemblies This presentation will explore: Valves, Seals and Springs Camshafts, Cams and Bearings Cam Followers, Push Rods and Rockers

Valve Construction Valves are typically called “Poppet” or “Mushroom” valves because of their action and shape. The stem and seating surface (face) are accurately ground to ensure that the valve stem both fits the guide and that the valve face seats precisely and concentrically in the head. To minimize wear under the extreme operating conditions, they are made from heat resistant steel and often coated with a very hard metal alloy called stellite. Stellite valves are often used in engines designed to use unleaded fuel, in order to improve durability without lead lubrication from leaded fuel.

Valve Face Angle Interference angle The valve face angle is the angle between the valve face and the cylinder head. Seat angle Valve face angle The valve face angle is typically either 30º or 45º. Valve Seat Sometimes there is a difference of about 1º between the valve face and seat angles, in order to increase the sealing pressure. This is known as the interference angle. Face angle

Exhaust Valves Exhaust valves are exposed to the higher operating temperatures of combustion gases. The stems are sometimes made thicker to increase heat transfer to the cylinder head. Thicker stem Although not commonly used today, hollow-stem sodium-filled valves provide extra cooling action. At engine temperature, the metallic sodium melts. As the valve moves down, the liquid sodium is thrown down and absorbs heat. When the valve spring forces the valve closed, the hot sodium splashes up and transfers heat to the top of the valve guide and out to the engine coolant. Sodium

Valve Seals Valve seals help keep oil out of the combustion chamber by preventing it from flowing down the guide. Teflon umbrella seals press on to the valve guide and form a seal between the valve and the guide. Umbrella Rubber O-ring seals are also used, fitting over the valve stem. An extra groove is cut in the valve stem to locate the O-ring seal and form a seal between the stem and retainer. This design prevents oil flowing between the cotters and down the valve stem. O-ring Shields are often fitted over the valve springs when an O-ring is used, in order to keep the oil off the valve stem, as it is not fully protected by the O-ring alone. Valve stem Spring shield

Dual Coil Spring Assembly Cotters Dual-coil spring assemblies are used to increase valve closing pressure, especially for high speed (RPM) operation. Spring retainer A spring retainer is used on the top of the spring to hold it in position on the valve, and is held in place by the cotters. Outer spring An oil seal is generally fitted to the top of the valve guide forming a seal between the guide and the valve stem. Inner spring Oil seal A spring seat keeps the spring aligned with the valve stem. Spring seat

Spring open/ closed length Valve Spring Construction Spring tension is quoted in pounds (or kilograms) for specified lengths of compression. Spring open/ closed length Cylinder head Specifications usually give lengths for the spring when the valve is closed or open. The ‘spring free length’ is the expected length of the spring when removed from the engine or supplied new.

Valve Float Loss of spring tension will cause the valve to float, or fail to close correctly at high speed. Shims (accurately machined washers) would have been used in the past to take up this loss in spring height. Today however, any weak spring would be replaced. Shim Valve rotators may be fitted at the top (retainer type) or bottom(seat type) of the spring to prevent carbon buildup and hot spots on the valve face.

Camshaft Camshafts are used to control the opening and closing of the inlet or exhaust valves. They are made from cast or forged steel and precision ground. Camshafts can either be belt or chain driven from the crankshaft. For a four-stroke engine, for every two revolutions of the crankshaft, the camshaft rotates once. Cams A camshaft opens and closes the valves by the use of cams (also known as lobes). The surface area of the cams are hardened, in order to withstand wear. Journals Like a crankshaft, a camshaft has a number of journals, onto which bearings are fitted to allow the camshaft to rotate. Camshafts typically use plain bearings, but roller bearings are sometimes used.

Cam Operation The cams are the part of the camshaft that open and close the valves. Duration Nose As the camshaft rotates, the nose of the cam lobe pushes the valve open. Lift There is generally one cam lobe for each valve. Engine configurations and design dictate how many camshafts and lobes are used. Cam lobes are precision ground profiles used to accurately control valve opening speed and extent (lift).

Valve Timing Cycle During the power and compression strokes, both valves are closed. During the exhaust stroke, the exhaust valve is open and combustion gases are expelled. TDC BDC Compression Intake Exhaust Power During the intake stroke, the intake valve is open and new air/fuel mixture is drawn in. Both valves may be open at the same time, between the exhaust and intake strokes, to provide an overlap. This helps engine “breathing” by slightly increasing the gas flow, both in and out of the cylinders.

Cam Followers - Tappets Cam followers ride on the cams. The diagram, below, shows a basic mechanical follower. Hydraulic cam followers have zero valve clearance (lash) to provide quieter operation. Rocker arm Cam follower Push rod Cam lobe Hydraulic cam follower

Cam Follower Types Mechanical (solid) cam followers have no moving parts. They simply transfer cam action to the push rod. They do, however, require regular valve adjustment due to wear. Mechanical follower Hydraulic followers fill with oil when the engine is running, the pressure lifts a plunger, taking up any play. As the cam then moves the follower, the integral valve is sealed off and the follower then acts in the same way as a mechanical follower. Hydraulic followers are commonly used because they are quiet and self adjusting.

Cam Follower Types Roller cam followers use a small roller on the cam profile in order to reduce cam wear. This type of follower can either be mechanical or hydraulic. Roller follower Cup type followers are fitted directly between the cam and the valve stem. The cam follower moves up and down in a bore machined into the cylinder head. Shims or a screw can be used to provide adjustment for wear in the mechanical design. Cup type followers can also be of the hydraulic type. Cup type follower

Rocker Arms Rocker arms are usually cast or pressed steel. When mechanical followers are used, the rocker arms typically incorporate a screw so that valve clearance adjustments can be made. The rocker arm transfers the upward motion of the push-rod to a downward movement of the valve. A lever advantage may be incorporated where a difference in length between pivot point to valve and push rod provides a lever ratio.

Overhead Valve (OHV) Rocker arm This type of arrangement is also known as a cam-in-block engine. This type of arrangement is still used, but is more common in older engines. With the cam-in-block type many configurations of lifter and rocker arm may be used. Push rod Valve follower

Overhead Cam (OHC) The camshaft is mounted in the cylinder head and usually uses shimmed or hydraulic cam followers. This arrangement is very common in modern automotive engines. Some OHC engines have separate camshafts for intake and exhaust valves.