The valve mechanism opens or closes the intake valve and exhaust valve at the proper timing in order to draw the air-fuel mixture into the cylinder and discharge the combustion gas into the outside.
1. Valve opening and closing system
The rotation of the crankshaft is transferred to the camshaft via the timing chain (timing belt), rotating the cam. The number of teeth on the camshaft sprocket (pulley) is double that of the crankshaft so that the camshaft rotates once for every two rotations of the crankshaft. As the camshaft rotates, the cam forces the valve to open or close.
Valve and Related Parts
The intake valve opens during the intake stroke to draw the air-fuel mixture. The exhaust valve opens during the exhaust stroke to exhaust the combustion gas. Both valve closes during the compression and combustion strokes to keep the combustion chamber airtight. Since the valve is exposed to high temperatures and high pressure, it is made of a special metal. Generally, in order to increase the amount of the intake air, the valve diameter of the intake valve is larger than exhaust valve. In order to keep the valve and the valve sheet airtight, the angle of the valve face is generally set at 44.5 or 45.5. Valves are pushed to the closing direction by springs and cams operation cause the valve to move down along the valve guide bushing inside the cylinder head.
3. Valve spring
The valve spring is a coil spring that applies tension in the closing direction of the valve. Most engines have one spring per valve, however some engines use two springs per valve. In order to prevent the valve from vibrating when the engine is running at high speeds, uneven pitch springs or double springs are used.
Valve springs have the natural frequency. When the number of the valve opening and closing and the natural frequency match to vibrate together, the waviness vibration may occur irrelevant to camshaft operation. This condition is called surging and it can be a cause of abnormal engine noise as well as damage to the valve spring or interference between the valve and piston.
Asymmetrical type of uneven pitch springs is installed with the wider pitch toward the top.
4. Valve seat
The valve seat is press-fitted into the cylinder head. When the valve closes, the valve face and valve seat fit closely together to make the combustion chamber airtight. The valve seat also transfers heat from the valve to the cylinder head, serving to cool the valve as well. Since the valve seat is exposed to high temperature combustion gas, and repeated contact with the valve, it is constructed of a metal that excels in resistance to heat and wear. When the valve seat wears, it can be ground by the carbide cutter or replaced. In recent years, lasers have been used to weld a wear-resistant alloy valve seat layer directly to the cylinder head making the valve seat and the cylinder head one unit on some engines. With this type of laser clad valve seat, replacement is impossible.
Valve seats are generally shaped like a 45 cone in order to fit with the valve face. The valve seat contact width is generally from 1.0 mm to 1.4 mm. The wider the contact area of the valve seat, the greater the cooling effect will be, however air-tightness may suffer since carbon intrusion is likely. In contrast, the narrower the contact area of the valve seat, the lesser the cooling effect and the lesser the likelihood of carbon intrusion will be.
5. Valve guide bushing and oil seal
The valve guide bushing is generally made of cast iron and is press-fitted into the cylinder head. It acts to guide the movement of the valve so that it closely fits the valve seat and valve face. The contact surfaces of the valve guide and valve stem are lubricated with engine oil. To prevent excess oil from entering the combustion chamber, a rubber oil seal is fitted on the topside of the valve guide bushing.
Valve stickingoccurs when the valve stem in the valve bushing guide stops moving smoothly or stops moving completely. This occurs when the amount of clearance between the valve stem and the valve guide bushing is too small or when they are not sufficiently lubricated. If the valve stem oil seal is broken or hardens, engine oil will enter the combustion chamber and be burned. This can cause excessive oil consumption.