DJA3032 CHAPTER 7

CHAPTER 7:VALVE AND COOLING VALVE

byMOHD SAHRIL BIN MOHD FOUZI, Grad. IEM (G 27763)

DEPARTMENT OF MECHANICAL ENGINEERING

© MSF @ POLITEKNIK UNGKU OMAR

(DJA3032) INTERNAL COMBUSTION ENGINE

General Objective:

To understand the structures and types of valves

Specific Objectives:

At the end of this unit you should be able to:

1. describe the types of valves2. draw and label the different parts of a valve3. explain types of materials used in valves4. explain the process of coating a valve5. explain the function of an interference angle valve6. explain the purpose and operation of valve rotation 7. explain the heat zones and working temperature of a valve.8. explain how working temperature is maintained by sodium cooled valves.9. explain the purpose and operation of DOHC engine.



Introduction In this chapter we are to discuss about valves in an engine. Valves can either be in

the block, or in the cylinders. Valve location is used to classify an internal combustion engine.

Types of valve location for the internal combustion engine

Valves in the block (flat head), are also called L Head engine, because the cylinder and combustion chamber are in the shape of an inverted ‘L’.

Engines with valves in the block have the intake valve on one side of the cylinder and the exhaust valve on the other side, these are called T Head engines.

Valves in head ( overhead valve), are also called I-Head engine because the cylinder and combustion chamber are in the shape of an ‘I’.

One valve in head ( usually intake ) and one valve in block, are also called F head engine; this is less common.



Arrangement of valve

FIGURE 7.1: ARRANGEMENT OF VALVE



Valve Parts

FIGURE 7.2: PARTS OF VALVE

A valve has several parts and they are in the tip or rocker arm contact area, valve spring retainer lock grooves, valve stem, fillet, face, margin and valve head. The head of the valve is the part that is inside the combustion chamber.

The valve can be designed with different head shapes such as :

i. oval headii. flat headiii. concave headiv. recessed head

The more metal on the head of valve, the more rigid the valve is.

Less metal means the valve will be able to conform to the seat more effectively. These valves are said to be elastic.

Rigid valve does not seat that well. Elastic valves seat better, but they may not last long.



Flat head Concave head Oval head Recessed head

FIGURE 7.3: TYPES OF HEAD VALVES

Valve Materials

Intake and exhaust valve materials include :

i. alloy steel valves with aluminized face and chrome stemii. silchrome valve with aluminized faceiii. austenitic steel with aluminized face and chrome stemiv. SAE 21-2steel with nickel-plated facev. Satellitevi. Sodium-filled stems ( for valve cooling ).



Valve Protection

Corrosion tends to cause pitting on valve seat faces, as a result of which the hot gases start to leak through, eventually burning a channel locally through which compression is lost .

Due to this, coatings are applied for protection against corrosion. Materials that are used for coatings include alloy containing :

i. nickelii. manganeseiii. cobaltiv. chromiumv. siliconvi. molybdenumvii. tungstenviii. titanium



However these materials are costly. Due to that, a much economical coating system has been developed . This coating system is called ‘Aldip process’ in which application of an aluminum coating is used.

The process is :

i. Aluminum material is sprayed in the form of a paste after valve seats have been finished – machined and the stems rough grounds.

ii. The component, in a jig, are then dipped for a few seconds in a molten flux bath at 7600 C.

iii. Later it is removed from bath before surplus aluminum is blown off.

The outcome is a smooth permanently adhering coating of aluminum on an iron alloy underlay. No further finishing is necessary.



Valve Seat

Valve seat is defined as a circular surface that is machined into the cylinder block or head. There are two types of valve seats;

i. integralii. insert

The integral type is cast directly as part of the cylinder head , the insert type uses a metal ring as the seat.

It is pressed into the cylinder block and ground to the correct angle. Insert valve seats can be made from cast iron, hardened cast iron, hi-chrome steel, and satellite (very hard steel).

Valve seats are ground to a specific angle for correct operation. They are either 30 o or 45 o . An interference angle is very common when grinding the valve.



FIGURE 7.4 : AN INTERFERENCE ANGLE

Figure 7.4 shows the valve is ground to 44o while the seat is ground to 45o. This interference angle tends to cut through any deposits that have been formed on the seat.

Valve Rotators Valve rotators are used on certain engines. If valve are rotated a small amount

each time they are opened, valve life will be extended. This is especially true when using leaded fuel.



Below are the effects of using valve rotators :

i. minimize deposits of carbon on the stem of the valve,ii. keep the valve face and seat clean,iii. prevent valve burning caused by localized hot spots,iv. prevent valve edge distortion,v. help to maintain uniform valve head temperatures,vi. help to maintain even valve stem tip wear, andvii. help to improve lubrication on the valve stem.

There are several types of valve rotators. Most operate on the principle that as the valve spring compressed, small balls inside the rotator roll up an inclined surface. This action causes the valve rotates slightly as it is being compressed.

FIGURE 7.5 : VALVE ROTATOR



FIGURE 7.5: ROTORCAP VALVE ROTATOR OPERATION

Valve Temperature and Cooling

FIGURE 7.6: ZONES OF HEAT

Exhaust valve temperature may reach approximately 704o

C to 815o C. This means that they are in fact running red hot. Good heat transfer, therefore, is essential.

It is important that valve can be fully seated when they are closed. The exhaust valve is closed approximately two – third of the time while the engine is running.



It is during this time that a large part of the heat is transferred from the valve head to the seat. The heat from the seat is transferred to the engine coolant.

Figure 7.6 shows the zones of heat and working temperature of the exhaust valve. The remaining heat transfer takes place from the valve steam to the valve guide then to the engine coolant.

How Sodium Cooled Valves Can Maintain Working Temperature.

Some exhaust valves also use a metallic sodium inside the stem. The sodium becomes a liquid at operating temperature.

Valve movement causes the liquid sodium transfers heat from the head of the valve to the valve stem and then to the valve guide, which helps the valve head to run cooler. Figure 7.7.

Intake valve temperature is considerably lower than exhaust valve temperature.

In coming air-fuel gases cool the intake valve while the valve is open. The valve seat has a great deal to do with good heat transfer.



FIGURE 7.7: SODIUM-FILLED STEM IMPROVES COOLING

FIGURE 7.8: DOUBLE OVER HEAD CAM WITH FOUR VALVES PER CYLINDER

Overhead Camshaft Overhead camshaft (OHC) assembly procedures vary considerably between

SOHC and DOHC, split or full circle cam bearings.

OHC may operate the valve directly through bucket-type cam followers, through pedestal-mounted cam followers, or by rocker arms.



On DOHC engines with four valves per cylinder, there is one camshaft for intake valves and one for exhaust valves. Now, most engines which have DOHC, operate without rocker arm, that means the engine is less inertia from the valve linkage. The engine produces good performance. Figure 7.8shows DOHC.





DJA3032 CHAPTER 7

Automotive