Thursday, May 12, 2011

Engine Valves

 Engine valves

Fig 2.8

Fig 2.7
A valve, with its part identified, is shown in figure 2.7. It has two main parts: the stem and the head. The valve is fitted to a port and ahead with its face providing a gas-tight seal against the seat in the port. This type of valve is known as a poppet valve are a mushroom valve.
A portion of the cylinder head with an intake and an exhaust is shown in figure 2.8. This can be used to identify the various parts including valves, springs, seals, guides and valve seat inserts.

The intake valves are larger than the exhaust valves. This is because the intake air that is being taken into the cylinder through the intake port is at a low pressure, while the gases that are being forced from the cylinder through the exhaust port are at much higher pressure. The larger intake port and valve opening are designed to assist intake air flow.

Number of valves per cylinder
Some engines have two valves for each cylinder-one intake and one exhaust. Some engines have three valves for each cylinder-two intake and one exhaust. Other engines have four valves for each cylinder-two intake and two exhaust.
Two intake valves provide Better breathing. The two allow larger intake passages and a freer flow into the cylinder, so that the cylinders receive a better charge. This increases the engine’s volumetric efficiency.
Similarly, two exhaust valves enable the engine to be designed with larger exhaust passages. This provides a freer flow of exhaust gases, from the cylinder and so there is less gas residue.
  •         The term breathing refers to the engine’s taking in air or air-fuel mixture.

Valve seats and guides
The valve ports in the Cylinder-Head have seats on which the valves rest when they’re closed and this forms a gas tight seal. The seats are met a tight rings (inserts) that are pressed into recesses that are cut in the head. Inserts are made of a special iron alloy that is designed to withstand the high temperatures of the gases that pass through the exhaust ports. The valves operate in valve guides that are a form of cast iron bush. The guides are pressed into holes bored through of the Cylinder-Head into the valve of ports. Guides can be removed and replaced if they become worn.

Valve springs

Fig 2.9

The valves are normally held on their seats of one or two coil springs which are compressed between the top of the cylinder and a retainer on the valves stem. The retailer is held on the end of the valve stem by conical shaped collects (figure 2.9). These are also known as Cotters, Keepers are keys.
The pitch of the coils of valve springs is often closer at the bottom of the spring than at the top. Springs can also be made of wire with the specially shaped section. The purpose of these variations is to keep the valve on its seat when it closes and prevent valve bounce. A simple spring could have resonance that would allow the valve to bounce on its seat under certain operating conditions. For this reason, two springs, an inner and an outer (As can be seen in figure 2.8), are some times used.
Prince can also be tapered, with the top coils being of smaller diameter than the lower coils. This is done to reduce the mass of the spring that actually moves when the valve is opened and closed.


Valve-stem Seals
Oil seals are fitted to the valves stem or to the valve guides. These prevent excessive oil from passing down between the Valve stem and its guide into the combustion chamber. Oil seals fitted to the tops of the valve guides can be seen in figures 2.8 and 2.10.
The action of the valve stem seal is shown in figure 2.11. The coil spring on the outside holds the sealing edge against the valve stem, while the angle at the top of the seal forms a small reservoir of oil to lubricate the stem and guide.
Fig. 2.11
Some oil is needed for the valve stem lubrication, but too much oil passing through the guides will cause problems. Oil will be burnt and carbon deposits will form in the intake valve ports and on the valve heads.
Valve components in fig. 2.10 are shown below:
1. Cotters, 2. Spring retainer, 3. Valve spring, 4. Valve stem
-seal,  5. Spring seat, 6. Exhaust valve, 7. Intake valve
  • Worn valve guides will also cause excessive oil consumption and smoke from exhaust.

Reasons for Valve-stem Seals
The intake valve is more likely to pass oil through its guide than the exhaust valve. This is because the intake port has low pressure that tends to suck the oil in. the exhaust port has a higher pressure that tends to keep the oil out.
Oil will pass the intake valves in the seals are a worn, or if the valve guides are worn and there is excessive valve-to guide-clearance. Also, more oil will tend to pass through the guides when the engine is operating under light conditions are no load conditions, such as men traveling downhill. Under these conditions, the intake manifold pressure will be much lower than atmospheric pressure and this will cause more oil pass down through the valve guides.

Valve temperatures
Intake valves pass air or air-fuel mixture and so run at a cooler temperature than exhaust valves. The exhaust valves are in the path of the hot gases that pass through the exhaust ports and so the heads of the exhaust valves become very hot.
Fig. 2.12
Figure 2.12 shows the typical temperature pattern of an exhaust valve. During operation, the stem transfers heat to the guide, so the stem is the coolest part of the valve. That had near the face of the valve transfers heat to valve seat, so that is the coolest part of the head. The valve seat and guide, in turn, are cooled by the coolant into water-jackets that surround the valve ports. The valves temperatures show the importance of correct valve seating. If the valve does not sea properly there will be a smaller contact area, through which heat transfer can take place and the face will over heat. Local hot spots will reach very high temperatures and the edge of the valve will burn.
The exhaust-valve seat is also subjected to extremely high temperatures, and so for this reason, exhaust valve seats in many engines are fitted with heat resistant to alloy inserts.

Sodium-cooled Valves
Fig. 2.13
Some engines have sodium cooled valves. These have hollow stems that are partly filled with metallic sodium. This melts at about 90°C and so becomes liquid- at engine operating temperatures.
As the valve moves up and down in its guide, the liquid sodium is thrown around inside the Valve-stem.
While doing this, sodium absorbs heat from the hotter part of the valve near the head and transfers it to the cooler part of the valve at the stem.
The cylinder-Head in figure 2.13 shows valve with hollow stems. The intake valve stem is hollow to reduce its mass. The exhaust valve stem is also hollow but it contains sodium for cooling. Sodium valves can often be recognized by their stems which are larger in diameter than normal valve stems. Sodium is a highly reactive element that is safe, while it is contained within the hollow stem.
  • Old sodium filled valves should not be tempered with and should the disposed of in a safe manner.

Valve face angles
The faces of most valves are ground at an angle of 45° to the Valve-stem, although angles of 30° have been used for some intake valves. In some engines, that intake valves have 30° face and angles and the exhaust valves have to 45°. The valve seats are often ground to the same angle as the valve face, but some manufacturers use an interference angle as shown in figure 2.14. With this the valve-face and seat are ground to slightly different angles. This Vary with different engines and the interference angle may be 0.5° or as much as 2°. A typical specification is 45° for valve at face and 44° for the valve seat.
An interference angle is provided by an engine manufacturer to allow for quick bedding in off the valve face to the seat on new engines, and is not always used when reconditioning valve and seats.

Valve rotation
Fig. 2.15
Valve rotation refers to the action of the valve turning a little as it opens and closes, so that it gradually rotates and does not always seat in the same place.
The valve rotation has a number of advantages: it produces a slight wiping action, which tends to keep face and seat free of carbon; it helps to prevent sticking into the guide and it distributes the heat around the valve seat. All these help to increase valve service life.
Valves have a natural tendency to rotate and this is aided in different ways. One way is to have the rocker arm slightly offset to the valve-stem as shown in figure 2.15. This causes the valve turned slightly each time it is opened. Another method is to have positive valve rotators on the exhaust-valves. These are sometimes used on larger engines. The rotor is similar to a small thrust ball race, which is fitted either under are on top of the valves springs. The balls rest on small ramps and each time the valve is opened, the balls are forced to move up the ramps. This has a positive action which causes the valve to rotate a slightly as it is opened and closed.

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