23297880 Cylinder Head Valves (1)

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Inlet & Exhaust Valves&

Actuating Mechanism



Overview

Inlet and exhaust valves play an important role in the gasexchange process of 2-stroke and 4-stroke internal combustion

engine.

Inlet and exhaust valves of medium speed engines are subject

to severe operating conditions.

Inlet valves do not generally present any serious problems as

they are cooled by incoming charge air.

However, exhaust valves are subjected to higher temp and theirlife is limited by deposits and high temp corrosion from sodium &

vanadium compounds.

It is very important to keep valve seat temp below 450

o

C.



Objectives

Function of cylinder head valves (inlet & exh)

Working condition of cylinder head valves

Design features

Construction features

Problems of exhaust valve

Factors influencing exhaust valve life

Valve actuating mechanism

Valve maintenance



Why 4-stroke Engine has Multiple Valves ?

Multiple valves are employed to give larger valve areas

than single valve

Larger valve areas allow larger gas flow and better

flow pattern

Improve performance of gas exchange process

Less back pressure on piston during exh stroke reduces

loss of useful work of the crank shaft

Heat dissipation problem with single valve (exh)



Mechanically Operated Exhaust Valve



SULZER RTA

EXHAUST

VALVE



MAN B&W

EXH VALVE



Working Condition of Exh V/V

Passing large volume of burnt gas in short time

Working under high temp condition

Due to repeated heating and cooling causing thermal

stress and possibility of hardening

Hot and cold corrosion due to burning HFO of poor quality

Valve bouncing and vibration

Valve tampering and wear



Valve Stress

Mechanical stress

- at flame face centre in radialdirection

- at blend radius circumferentially

Thermal stress

- at valve seat area

Combined stress

- at valve neck



Effect of Stresses

Valve leaking and burning

Dishing-in of valve head

Crack at centre of flame face

Cracks at the seat

Valve breakage at neck causing heavy damage



Damaged

Exh Valve

Spindle



Exhaust valves

High Temp Undesirable

Reduces volumetric efficiency

Excessive deposits take place

Causes valve burning & valve failure



Avoid large valves

Difficult to accommodate

Destroy compactness of combustion chamber

Small inlet valves can induce turbulence

Larger the valve, higher will be the temperature

Exhaust valves



Valves – design feature

Valve Diameter

Usually exh v/v diameter is smaller than inlet v/v

as it opens against higher pressure

assists in keeping cool

better performance



Exhaust Valve – design feature

Material

Inserted v/v seat

Cooling

– v/v spindle

– v/v guide

– v/v seat

– v/v housing



Exhaust Valve Problems

V/V bouncing

Seat tampering & wear

Hot Corrosion

Cold corrosion

Valve Burning



Exhaust Valve ProblemsDue to …

Fuel quality (ignition)

Impurities in fuel oil

- catalytic fines

- S, V, Na compound

Combustion

Leaky valve due to seat distortion

Cylinder power in excess of design rating



Factors Accelerate Valve Failure

Mechanical dents

Spallation of salt layer

Sealing face corrosion



High Temperature

Corrosion Damage

of Exhaust Valve



Low Temp Corrosion

Due to the presence of Sulphur in Fuel oil

Counter Measures

Correct CW outlet temp

Coating in gas duct

Thicker gas duct wall

Cooling water by-pass



High Temp Corrosion

Due to presence of ….

SodiumVanadium

Which produces corrosive flux at high temp.

E h V l



Exhaust Valve

High Temp Corrosion



High Temp Corrosion

Countermeasures

limit fuel oil vanadium concentration

minimise fuel oil and charge air sodium concentration

avoid excessive exhaust gas temp.

use of Nimonic v/v spindle

v/v stem coated with Cermet using HVOF process

v/v lid facing combustion chamber is coated with Inconel 625



Counter Measures

Design

Maintenance

Operation



Design

Heat shield

Cooling

Use of seat ring

Surface treatment of seat and spindle

Limiting v/v temp by efficient bore cooling arrangement

Valve rotating device

Minimising v/v bouncing by hydraulic operation

Heat and corrosion resistance material

Efficient scavenging

Optimising v/v timing



Exhaust Valve With W-seat



W-seat

W-seat enables

to achieve

lower temp

in the v/v seat

and increase

v/v life & TBO



Maintenance

Areas requires special attention:

Valve stem

Valve seatGuide bush

Valve cage

Coil spring / air springRotocap / vanes

Actuating mechanism



Maintenance

Tappet Clearance

Necessary to allow for thermal expansion of valve spindle at

working temperature and ensures positive closing of valve.

Clearance should be set at cold condition of engine.

Excessive clearance will cause the valve to open late and

close early with reduction of max lift, causing hammering and

damage.

Insufficient clearance will cause the valve to open early and

close late with increased max lift.

It may prevent valve from closing, causing loss of power and

v/v burning.



Shrouding of valve

• Shrouding occurs due to excessive manual grinding

• If v/v is harder than seat, shroud builds up in the seat

• If seat is harder than the v/v, shrouding occurs in v/v

• Normally shroud builds up in both valve & seat

• Even small amount of shrouding can seriously affect the air

and gas flow across a valve



Different angle of

Valve seat and

Valve head

Improves

Valve Life



Operation

Fuel Quality

Engine Load

Scavenging

Turbocharging

Engine Power BalancingEngine Cooling



Valve Actuating Mechanism

Mechanical

– rocker arms & push rod

– double valves may be actuated from a single push

rod in several ways…

forked rockers (master & slave)

twin rockers and a yoke actuating two valves

– disadvantage of increased wear on spindle & guide



Valve Actuating Mechanism

Hydraulic

– Common in X-head engines but used in 4-stroke engines also

– Replaces push rod and mechanical coiled spring

– No transverse thrust results in less guide wear

– Controlled closing of valve reduces tampering and valve-seat wear

– Valve rotation by vanes ensures well balanced thermal & mechanical

stress and uniform valve seating

– Reliable operation

– Reduces maintenance cost with increased TBO



SULZER RTA

EXHAUST

VALVE



Valve Rotation

Prolongs valve life

Minimises risk of local overheating and valve distortion

Prevents deposits and built up of vanadium on the valve seat

Prevents hammering on the same spot

Valve rotation is achieved by:

Rotocap Unit

Valve Stem Spinner



Rotocap Valve Rotator



High Exhaust temperature

High CW temperature

Hissing Noise

Low Pcom and Pmax

Smoky exhaustT/C surging in extreme case

Indication of Leaky Valve

23297880 Cylinder Head Valves (1)

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