valves

VALVESVALVESVALVESVALVES

Function• Function of the exhaust valve is used

for expelling the burnt gases from the engine cylinder.

• Function of the inlet valve is used for allowing fresh air from turbocharger or manifold entering the engine cylinder

• Both valves seals gases on compression and combustion periods

• Exhaust valves are subject to arduous conditions, and require regular overhaul. To aid this, exhaust valves are often fitted in separate cages. This allows the exhaust valve to be changed and overhauled without removing the cylinder head. The cages have water cooling passages connected to the cylinder head cooling water.

Valve cages

• The cage is of cast steel. The cooled seats are made from a heat resistant molybdenum steel which may be stellite faced. The exhaust valve may be of a similar material or of a nimonic alloy.

• Inlet valves are subject to much less arduous conditions and are not usually fitted in separate cages.

Valve springs• Two different sized springs are fitted to aid

positive closing of the valves. The reason for fitting two springs are that if one fails, the other will prevent the valve dropping down into the cylinder. The two springs have different vibration characteristics, so the incidence of resonance is reduced. (resonance is where two items vibrate at the same frequency thus the amplitude of the vibration is amplified.)

Rotorcap or turnomat• Exhaust valves are designed to rotate in service.

The reasons for this are to prevent uneven temperatures  so it does not distort and leak by, and to help dislodge any build up of deposits on the valve and seat which may prevent the valve closing properly and lead to "hammering" of the seating faces. A mechanical method is generally used, and this is either the "rotocap" or the "turnomat". Winged rotators or spinners as used on the 2 stroke engine exhaust valves can also be used, but this entails using a ball bearing race between the spring carrier and the cover.

NON-POSITIVE VALVE ROTATOR

Clearance when valve seated

Tappet

Tip cut

Retainer lock

Spring

Valve guide

Valve spindle

Spring cap/retainer

POSITIVE VALVE ROTATOR

Section D-D

D

D

Valve springs

Spindle

Bellville washer

Circlip Ramp

Rotation

Top view

VALVE CLOSED POSITION

VALVE OPENING POSITION

Cotter

Valve cooling

• Only exhaust valve since inlet valve working condition not severe, flowing gases assist cooling

• Hot gases burn valve and seat process is cumulative

• Heat transmitted thru’ seat (at closed position) and some thru’ stem

• Centre of valve head hottest. Problem of cooling thus valve cage obsolete except in large highly rate engines

• 3 common methods of cooling: valve cages cooling, cooled valve, air bled from air manifold and directed to valve another method is sodium-cooled valve

470 463

450

374

360

410400

360

300

320

250

200

Temperature distribution on valves

Spindle

Cooling water inlet

Cooling water inlet

Cooling water outlet Cooling water outlet

Valve housing

Guide bush and spring retainer

Exhaust gas outlet

Water cooled stem guide Water cooled stem guide and seat

Water cooled exhaust valve

• Advantages – Temperature of valve during operation is lower– Due to above maintenance less– Valve life last longer

• Disadvantages– Required flexible connections – troublesome– Cooling medium must be clean as blockage may

lead to distortion results– manufacturing cost higher

Plug

Cooling media inlet Cooling media outlet

Flexible connection

Flexible connection

Plug

Internal tube

Hollow valve stem and head

Water cooled exhaust valve

Water cooled stem guide Water cooled stem guide and seat

600 – 650oC

550 – 600oC

500 – 550oC

450 – 500oC 400 – 450oC

350 – 400oC

Rocker or Tappet Clearances

• Rocker or Tappet clearances refer to the clearance between the top of the valve spindle and the rocker arm. It is to ensure that the valve closes properly when it expands as it gets to operating temperature.

• Clearances are set according to manufacturers instructions, but usually done with the engine cold, and with the push rod follower on the base circle of the cam. (one way of ensuring this is to turn the unit being adjusted to TDC on the power stroke.)

• If the clearance is too small, then not only is there a chance that the valve will not close properly when it comes up to temperature, but it effectively will open early and close late.

• Conversely if the clearance is too large, then the valve will open late and close early

Suction and Exhaust Valves

Valve material• Inlet valve

– Good quality steel and can be heat treated• Exhaust valve

– Good strength at high temperature– Retain its properties after repeated heating to working

temperature and cooling in air – no tendency to air harden– Having critical temperature above 800oC– Resist hot and cold corrosion – fuel– No tendency to scaling at high temperature– Must be easily forged or machined– Capable of consistent and reliable heat treatment

Disassembling sequence

• Set the valve spring detaching /attaching tool shown in the figure at right on the cylinder head.

• Turn the handle of this detaching /attaching tool in the direction to press the spring. Incidentally, when disassembling the valve spring or valve rotator without taking out the cylinder head, bring the piston to T.D.C position and carry out the foregoing steps.

Cautions for disassembly check and

reassembly.• At the time of disassembly, carefully check

the valve spring for any existence of crack and corrosion.

• Two valve spring shoe cotters are being used foe each suction/exhaust valve. At the time of reassembly, they should be used by a pair and more over to the same valve. (unless they are handle by a pair, the valve rod might be damaged due to inferior contact of the cotter part .

• When the valve rotator is detached, disassemble it and check its interior too.

Suction / exhaust valve grinding.

• Perform the first time's valve grinding earlier the usual as indicated in Table of Periodical Checking given in section 8 and judging the situation, prolong the henceforth servicing interval.

• Prior to valve grinding, confirm the wear of valve guide and clearance between the valve guide and the valve rod that serve as criteria on valve working, compare these measured values with the corresponding values indicated in Table of Clearance and Wear Limit of major parts given in the manual, and proceed to grinding work upon replacement of the pertinent parts as required.

• Measure the dimension of contact of the seat part of valve and seat ring. And if this measurement is reaching the pertinent limit given, replace the parts involved with the new ones. Correct particularly the valve upon careful checking of it because if the valve head face becomes too thin, it will break and broken pieces will enter not only combustion chamber but the turbocharger, causing serious secondary damage.

• Elaborate execute valve grinding, applying grinding compound of proper quantity to the valve to be ground and using the suction /exhaust valve grinding tool.

Valve problems• Mechanical difficulties or corrosion and

oxidation which lead to burning– Breakage – Valve face and seat wear– Face guttering– Valve pounding or bouncing– Shrouding of valves – Corrosion and burning

Breakage • May occur in valve head. Due to poor design

from strength or heat transfer standpoint. Of from abnormal temperature caused by poor seat contact due to deformation of parts, deposits or corrosion

• May also occur at stem-head junction. Due to heavy leads created by overspeed operation or abnormal pounding created from excessive valve lash or weak valve spring

Valve face and seat wear

• Common with intake valves due to pounding, fretting and scuffing

• Not common with exhaust valve due to better materials and deposits at exhaust valve seat prevent intimate metallic contact between surface

Face guttering

• Not corrosion but high temperature oxidation due to leakage of hot gases

Valve pounding or bouncing

• Caused by too much valve lash and or weak springs

• May caused seat and valve face distortion and reduced valve life

• Bouncing reduced by increasing number of springs instead of one heavy and bigger diameter spring, by improving spring material and by means of hydraulic valve actuating gear

Shrouding of valves• Caused by repeated grinding and

lapping. Valve lift remains constant but effective opening area reduced by amount of shroud

• Affect air or gas flow across valve due to less opening area and loss of stream lined flow

• Removed by seat cutter, machining and special grinding machines

Corrosion and burning

• Serious exhaust valve problems. Corrosion accelerated by some lube oil additives in combination with certain fuel components and by combustion products. Increased by increased valve temperatures

• Corrosion and deposits on valve face and seat are accelerated by high-ash lube oil. Initially deposits make a smooth hard seat. But as deposits thickens, it partially crack off, leakage develops and valve temperature increased. Results is burning and ultimate failure.

• Corrosion on stem and head destroys and roughens surface of material – results in fatigue failure.

MAINTENANCE• Doing overhauls according to the

maintenance schedule or running hours

• Change the vales if necessary (before running hours)

• Taking care to valve tappet clearance

• Correct cooling system• -L.O. pressure -cooling water (fresh water)• Choose correct grinding or lapping

paste

Conclusion • We should taking care about

tappet clearance, valve rotator, valve spring, valve cage, cooling system, valve condition (surface area), valve seat and so on to the valve perform in high performance. And donning maintenance according maintenance schedule.

THE END thank you