VALVE AND VALVE GEARSValve Gear:It designates the combination of all parts, including

the various valves, which control the admission of air charge and the discharge of exhaust gases in four stroke engines, the discharge of exhaust gases in some two stroke engines (uniflow scavenging type), the admission of fuel in air- injection and some mechanical-injection engines, and the admission of compressed air for starting most of the larger engines.

Valve Actuating Gear:It designates the combination of those parts only

which operate or actuate the various intake, exhaust, fuel and air-starter valves, open and close them at the proper moment in respect to the position of the piston and crankpin, and hold them open during the required time.

Valve Timing Gear:It designates the combination of those parts only

which affect and control the moment of opening and closing of the valves with respect to crank and piston position. These parts include cams, camshaft and camshaft drive. The valve gears of diesel engines vary considerably in their construction, depending on type, speed, and size of the engines.

C.13.VALVES AND VALVE GEARS.

Valves - Valves are used to cover / uncover the passage of flow.

Valve Gears – to produce action on valves - combination of parts, including valves which controls the operation of above.

In all 4 stroke engines admission of air charge, discharge of exhaust Gas and in many 2 stroke engines discharge of exh. Gas.

Basic drive - c/shaft drives cam shaft by gears or chain.

Cams on the camshaft lifts push rod, transmits the action to rocker arm to operate the valves for mechanical drive. – for hydraulic drive the cam drives a hyd. Actuator, the oil in turn moves the valve by a piston.

As soon as the closing side of the cam moves under the transmitting mechanism the valve spring starts to return the valve to its seat( closed)

Cam profile

Valve operating gear

Valve requirement

to get fresh air into engine and exhaust gas out

Exh v/v opening (size) is as big as possible for 2 stroke engine – exh open for short duration, so to reduce back pressure

Inlet v/v opening (size) more important in 4 st. engines – to reduce pumping loss and also increase volumetric efficiency.

Some 4 st engines have 2 inlet and 2 exh v/vs. for space arrangement, less v/v opening, cooler valves.

valve construction

. Mushroom- shaped poppet type. Head and stem as one piece

seating edge beveled at a 45* / 30* angle

Inlet v/v – cooler - carbon or low alloy steel

Exh v/v – hotter – silicon-chromium steel ( nickal, chromium)

v/v moves in a removable guide fitted in cylinder head.

Springs holds the valves firmly against the seat.

Valve Requirement

valve construction

• Head of the valve is cooled -conducts heat to seat in cyl.head (water clg). The seat is a removable seat fitted in cyl head with cooling arrangement.

• The clearance between valve and guide – due to excess wear – overheating of valve, carbon forms and sticky, excess oil consumption.

• To make valve and seat faces wear resistance, valve and seat faces are hardened with cobalt-chromium-tungsten (stelite).Seat rings of wear resistant material are also used, in some cases

• valve cages – to make valve seat removal easier, valve and seat as one unit and fitted on cyl head. cage may be separately cooled.

• Some exh v/vs rotated a slight amount each revolution to keep the valve clean (carbon deposits) and ensure even wear between v/v and seat.

Timing gear

• Responsible for actuating the valves at right time with respect to c/shaft (Piston position)

• In 4 st engine the camshaft speed is half the c/shaft speed.

• Chain drive and gear drive.

Valve Constructions

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.)

Valve Springs – more detail

Two inlet v/vs & two exhaust v/vs & their operation

• v/vs in operation

Caged Exhaust valve

Burning Out of Exhaust Valves

Once an exhaust valve does not seat correctly, the high pressure burning gas will pass across the faces of the valve and seat during the power stroke. This will cause the temperature of the valve and seat to rise in this area, weakening the material and distorting the surfaces. The velocity of  the burning gas will erode the surface, allowing more gas to leak by. The temperature of the valve in this area will rise further, leading to further burning and greater distortion. The first indication of a valve burning out will be a rise in the exhaust temperature, which will rapidly increase together with a loss of power from the unit.

Valve cage

Camshafts

• in 4 st engines carries the cams for inlet valve, exh valve & fuel pump.

• in 2 st exh v/v type cams for air starting operation and other aux. Operations. engines carries the exh cams & fuel pump cams.

• Additionally may carry

• construction – forged as one piece including the cams or separate cams keyed on a shaft. In large engines camshaft in sections, with cams either integral or keyed / keyless fitting.

• camshaft is supported by bearings – plain bush or split sleeve.

Camshafts

Pushrods PushrodsGenerally tubes to reduce weight. The lower end contacts the follower which carries a roller( tappet roller) running on the cam. The upper end is fitted with a cup.The end of the rocker arm (fitted with a tappet bolt- end rounded shape)) fits into the cup.

Valve Gear, Rocker Arm, Valve Clearances

Split camshaft bearingCamshaft – more details

There are several different methods of manufacturing camshafts for medium speed 4 stroke marine diesel engines. On the smaller engines, the camshaft may be a single forging complete with cams.

Alternatively the camshaft can be built up in single cylinder elements, each element made up of the fuel, inlet, and exhaust cam on a section of the camshaft with a flange on each end.

So that the element can be used on any unit in the engine, the number of holes for fitted bolts in the flanges must be sufficient to allow the cam to be timed for any unit on the engine.

For example, on a six cylinder engine, the flanges must have 6 equi spaced holes or a multiple thereof. The cams must be hard enough to resist the wear and abrasion due to impurities in the lub. oil, yet they must be tough enough to resist shattering due to shock loading. The cams are therefore surface hardened using the nitriding process.

Camshaft Photo

On the larger engines it is usual to manufacture the camshaft and cams separately. The nitrided alloy steel cams are then shrunk on to the steel shaft using heat or hydraulic means. Because the cams are fitted progressively onto the shaft, if the bores in the cams were all the same diameter, it would be very difficult, if not impossible, to fit the first cams all the way along the length of the shaft  to the correct position. To overcome this problem the camshaft is stepped, with the largest diameters at the end which has the cams fitted first. The larger bored cams fit easily over the small diameter steps till they reach the correct position on the camshaft.

On the larger engines it is usual to manufacture the camshaft and cams separately.

The nitrided alloy steel cams are then shrunk on to the steel shaft using heat or hydraulic means.

Because the cams are fitted progressively onto the shaft, if the

bores in the cams were all the same diameter, it would be very difficult, if not impossible, to fit the first cams all the way along the length of the shaft  to the correct position. To overcome this problem the camshaft is stepped, with the largest diameters at the end which has the cams fitted first. The larger bored cams fit easily over the small diameter steps till they reach the correct position on the camshaft.

Keys are not generally used to locate the cams as they would act as stress raisers.

Most medium speed engines are unidirectional (i.e they only rotate one way). This is because they either are driving an alternator, or because if they are used as direct main propulsion they tend to be driving a controllable pitch propeller.  In the case where the engine is reversing, then the camshaft has two sets of cams, one for ahead operation, and one for astern.

To reverse the direction of the engine, pressure oil is led to one side of a hydraulic piston which is coupled to the camshaft. The whole camshaft is moved axially and the cam followers slide up or down ramps which connect the ahead and astern cams.

The camshaft is either chain or gear driven from the crankshaft. Because the engine is a four stroke, the camshaft will rotate at half the speed of the crankshaft. (the valves and fuel pump will only operate once for every two revolutions of the crankshaft).

In a case where the cams are shrunk on the camshaft, if a cam becomes damaged and has to be replaced, then it can be cut off using a cutter grinder. Care must be exercised not to damage the camshaft or adjacent cams during the operation

• . The replacement cam is fitted in two halves which is then bolted on the camshaft in the correct position and the timing rechecked

Cut section of a cam

Different cams

•Fuel cam & Exhaust/Inlet cam

Fuel cam for unidirectional medium speed engine.The cam has fast lift for injection and a slow return to avoid shock loading

Inlet or exhaust valve cam for unidirectional medium speed engine.The cam has to open and close the valves smartly without shock loading

Rocker Arms To actuate the valves in the cyl head via cam,cam follower and pushrod tappets.

RA moves at an angle to vertical also some horizontal thrust-on valve stem, causes wear on guide.

Attachment to head by stanchion bolted.

Swings on steel fulcrum pin or pivot / needle bearing.

Contact to v/v stem by roller / screw.(Tappet)

Tappet clearance provided on the valve side – to take care of wear and expansion – to ensure v/v closes firmly. Lubrication of fulcrum and contact points done.

Springs Serves to close valves, made of highly tempered steel wire wound in a spiral coil.

To prevent bouncing the spring is maintained in compression all time.

Rocker Arms

Valve clearance To allow thermal expansion. To be adjusted regularly due to wear. Clearance is required between valve stem and RA, when follower is on base of the cam (v/v closed). If not the v/v will remain partly open.

VALVE CLEARANCES (TAPPET CLEARANCES)

If more v/v will open late and close early , reduces the lift (stroke), and causes noise.

If less open early and close late, increases the lift of the valve.It may prevent the valve from closing completely as it expands To set valve clearance a feeler gauge is used in conjunction with tappet adjustment to manufacturer specification

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.)

Hydraulically actuated Exhaust

valve• No need for RA

• Valve opens by hyd oil pressure

• the actuating gear is equipped with a locking device to retain the roller guide in its top position – so that exh valve can be kept out of operation.