Exhaust valves.doc

Exhaust valves Large exhaust valves are provided with detachable seats made for m molybdenum steel. The main parts of  the valve casing is of cast iron and water cooled, there being no particular strength requirement for this part. It is the seat area which is subject to high temperatures and wear, hence the use of better materials. The seat is detachable in order to allow removable for machining and replacement. Rocker operation of valves presents certain problems; 1. a tappet tappet clearance clearance must must be allowed allowed to suit thermal thermal expansion expansion of the valve valve stem. stem. Insufficien Insufficient  t  clearance may result in the valve not fully closing whilst excessive clearance can effect timing  and the period the valve is open. Hammering will also take place at the tappet face. (The use of  self adjusting hydraulic tappets went some way to alleviating these problems !. " side thrust thrust is imparte imparted d to the valve stem stem when the the valve opens. opens. Thus Thus causing causing wear at the stem bushing and subse#uent leakage $. wear wear takes takes place place at the pushro pushrod d and valve valve cont contact act face% face% addit addition ionall ally y the the rock rocker er beari bearing ngs s re#uire constant lubrication. To avoid these problems hydraulic valve actuation is used there is no tappet and no tappet clearance to set. Thermal expansion is accounted for by allowing the oil to escape at a relief valve on the pump unit. Oil loss is made up at the pump unit from the cam lube oil supply system. The opening face is always axial. ote! The hydraulic pipe must be sheathed to avoid the ris" of fire in the event of pipe failure.

#ith modern fuels, vanadium and other deposits can build up on valve faces leading to damage. These deposits can be hammered into the seating faces. If the valve is rotated and reseats in a diff differ eren entt plac place e then then the the same same dmag dmage e does does not not occu occurr. $ota $otati ting ng the the valv valve e also also preve prevent nts s loca locali lise sed d overheating due to a faulty atomiser.. If the valve is set spinning and is still rotating as it reseats a light grinding action ta"es place. This removes deposits and ensures a good seal. %uch rotation is induced by spinners on the valve stem upon which the escaping exhaust gas acts.. To allow for this effect the

frictional effect of the springs and valve&cover must be removed. The removal of springs means that a closing force by some other means is required. 'ir springing can be used. This consists of a piston fitted to the valve stem below the hydraulic unit. 's the valve opens air below the piston is compressed and this compression provides the upward force of closing the valve. The space above the piston is vented to atmosphere and the pressure below the piston maintained at ( bar from an air supply via a non)return valve. 'n additional advantage with this system is that when the engine is stopped the valves will all close after a short delay. This prevents the flow of cool scavenged air through units which with a roc"er system would otherwise be open.*reventing this allows all cylinder to be equally warm and stops the rotation of the turboblower which can occur. occur.

Springs %prings of sufficient force must be provided in order to ensure that the valve closes when the tappet force is removed. Once the valve is closed, the pressure in the cylinder will increase the sealing force on the valve seat. %prings have natural frequencies and if the engine operating frequency is close to the natural frequency of the spring then vibration will ta"e place and valve bounce will occur. %prings also twist when they are compressed and this causes wear at the landing faces. To avoid problems, double springs may be fitted one inside the other in parallel. These springs must be of different si+e and so have different natural frequencies. alve alve bounce due to spring vibration is thus avoided. The springs are wound in different directions to prevent twist and also to prevent one coil entering the other in the event of brea"age, thereby loc"ing it up. Long springs tend to bow out when they are compressed and this increases the ris" of stress failure. ' solution is to have two springs in series, one above the other and separated by a centre disc which is located via a pivoted arm arrangement so that only vertical movement is allowed. %eries ¶llel arrangements are available. -odern engines use pneumatic springs. Thisi both eliminates the problems of valve bounce , spring bra"age and also the need for rotor caps. 's the valve is free floating spinner vanes fitted on the spindle allow the valve to be rotated by the flow of exhaust gas.

Rotocap The rotocap is a mechanical device which produces valve rotation by a small amount as the valve opens. The valve rotation is about  o when the unit is in good condition. $otation to a new position avoids deposits from being hammered into the seat and repositions the valve thus preventing local overheating. /rictional contact is provided through the springs to the valve cover via the belleville washer which contacts at point ' and 0. 's the tappet force increases to open the valve, the belleville washer is collapsed thus removing that frictional contact. /urther increase in tappet force acts on the spring loaded ball bearings and the ramped slots tend to slide over the ball bearings. These slots are in the valve cover which is connected to the stem thus as the cover moves it rotates the valve. 's the tappet force is removed when the valve closes the belleville washer restores frictional contact and prevents further rotation. %prings return the ball bearings to their original position ready for the next stro"e. Valve closing

Valve opening