MOTOR ORAL QUESTIONS & ANSWERS PART-6

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Q – What is tappet clearance and how to take the tappet clearance ?

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Q – What are the different methods with which you get to know out which unit at TDC ?

Ans-How To Check Tappet Clearance

1. Taking all safety precautions.(like Take out from stand by, close the breaker, close priming pump etc)

2. Make sure the piston is on TDC (TO find out this all methods are explained below)

3. Make sure the engine has cooled down.

4. Loosen the lock nut of the rocker arm.

5. Now adjust the tappet clearance between the rocker arm & valve stem by tightening or loosing the nut below the lock nut.

6. Use feeler gauge to adjust the suction valie clearance as .35mm an exhaust valve clearance as .45mm (Exhaust valve clearance is more because it always face the hot gases so there chances of expansion is more)

What will happen if tappet clearance is less:

  • Valve will open early & close late
  • Air induced through inlet valve may leak out. So, less air for combustion.
  • Power will be reduced.
  • Fuel consumption will increase, engine may become unbalanced, exhaust temperature will be very high.
  • In worst condition, valve may remain open, resulting in loss of the compression pressure, burning of exhaust valve, T/C fouling will increase.

What will happen if tappet clearance is more:

i. Valve will open late & close early.

ii. Lesser heat energy to the turbocharger, so reduction in scavenge air & hence power.

iii. No proper removal of the gases.

iv. Hammering of valve stem may cause damage to the valve stem.

How to know which unit piston is at the TDC –

During the maintenance of the four stroke marine diesel engine there are times when we must know whether the particular unit’s piston is at the top dead center or not. For example when checking the tappet clearances of the engine it is important to know which unit is at TDC.

Referring to the flywheel would indicate two units, but only one can be at injection TDC. So which one is it?

In this answer the various methods to find out the position of the piston would be discussed. Some are very simple using conventional methods. Other methods are a little bit complicated, but nevertheless important whenever you require an independent method to find the TDC.

Flywheel Method

The flywheel method is the simplest method to know which unit is at the TDC. If the fly wheel shows two units, simply open the bonnet covers & check visually. The unit at the TDC will have both the inlet & the exhaust valve closed & hence relaxed springs; the other unit would have both the arms of the rocker arm at the different levels. In addition the push rods of the unit at the TDC would be loose & can be turned by hand because of the release of the clearances. There is a word of caution however, this method is only useful in the working generator which you have just stopped to check the tappet clearances. In case you have removed the rocker arms for any reason the spring height & the push rod freeness check would lead you nowhere & misguide you.

Fuel Pump Method

The most accurate method to know the position of the piston without opening up the piston is the fuel pump window. The fuel pumps have a window & as the plunger goes up & down, so does the mark on the bottom spring holder. On the body of the fuel pump there are cut marks which show the start of injection. In a diesel engine the start of injection is the injection TDC where both the inlet & the exhaust valves are closed. In this injection TDC we can check the tappet clearance. It must be noted that the injection TDC is not the absolute TDC as the piston is still some way down depending on the design of the engine. An injection TDC may be around 5 degrees before the TDC.

Dial Gauge Method

In this method the fuel injector is taken out & from the opening a dial gauge is put inside the cylinder. Then the turning gear is engaged & the engine turned over. The pointer of the dial gauge will move in one direction & then stop and start in the opposite direction. The moment the pointer of the dial gauge stops & changes its direction of movement is the TDC of the unit. This method is not generally used in day-to-day practice, but may be used in the calibration of the flywheel if it is not calibrated, or after some repairs.

Camshaft Method

The camshaft window of the engine can be opened up & the camshaft inspected. The cam of the engine has a base circle, & acceleration & the dwell periods. If the roller of the follower is at the base circle, then the particular valve is closed by spring action. When both the exhaust valve & the inlet valve follower are on the base circle, then the unit is also at the TDC. It must be considered that as a four stroke engine has two rotations of the crankshaft there is one injection TDC where the injection & the combustion take place. The second time the piston is at TDC is when the exhausting of the flue gases take place. It is very important to identify the combustion TDC as tappets have to be adjusted at that point.

Crankcase Method

In this method the crankcase doors are opened up & the piston is visually checked whether is going up or down. This is the surest method, but a bit cumbersome. It should be used when you have a doubt about the other methods.

Valve Spring Method

This is not an independent method out is used in the conjunction with the flywheel method. In this method if the flywheel is indicating two units, you can check the springs of both the units. The unit in which the spring, are loose is the one at the TDC. The caution is that this method is useful for an engine in use. If you have removed the rocker arms during the overhaul & thereafter you want to use this method then it can cause errors.

Push Rod Method

This method is like the spring method & you check that the push rods are free to turn. The unit at the TDC will have loose spines. The care that must be taken is that it should be used along with the flywheel method & should be used in a working engine. By a working engine, I mean the engine that was running & has been stopped for the tappets adjustment.

Spill Timing Method

This is a very accurate & tedious method normally used to check the start of injection in the fuel pump. It will also give you the injection TDC. It is used not in the tappet checking process, but instead to find the start of the Injection when you have power or thermal balance problems.

Basically in this method the delivery valve of the fuel pump along with the delivery valve spring are removed. There after a special “U” shaped pipe is put in place of high pressure pipe. After this operation the engine is turned by the turning gear & slowly brought near the expected TDC.

Soon oil will start spilling out of the pipe because the oil is entering from the inlet port, which is uncovered by the plunger. Keep turning the engine slowly & the oil quantity will decrease. The point where the oil flow just stops is the start of the injection. At this moment the plunger of the fuel pump has closed the inlet port & if you observe the fuel cam, you will find the follower is no longer at base circle.

As this method involves the spilling of the fuel, it is called the spill timing method.

Q – Why Concentric Springs Are Fitted For Auxiliary Engine Head ?

Ans – Concentric springs are helps to open and close the suction and exhaust valve Following are the reasons for Concentric springs in the exhaust and suction valves

1.If one spring fails valve will be held up by the other spring.

2. The thickness of springs can be reduced.

3.Reduce the space required for one large spring.

4 When frequency due to natural vibration of spring is in harmonic with cam shaft speed, then spring may vibrate axially & surge. This can be avoided by using different size spring one inside the other.

Q – what is the material of inlet and exhaust vales ?

Ans- A very good point to note that all the valves are opening inward because the sealing is good.

Inlet valve-It is made up of stainless steel

Exhaust valve – Spindle made up of nimonic alloy & v/v seat  is made of chromium

molybdenum steel, coated with stellite (it has the high temperature hardness)

Exhaust spindle made up of nimonic-Carbon=1%, Ti= 1%, Mo=4%, Fe = 1%, Al = 5%, Cr =15%. C=20% and remaining Nickel.

Q- What is the function of rotocap and working of it ?

Ans – Most important thing is your valve must rotate on the valve seat because it help in

cleaning the seat as we burn large quantity of HFO and due to this large quantity of ash has been generated, so we have to maintain a perfect sealing for that valve must rotate over the valve seat.

When the valve open and afterward it going to close so the spring releases its energy to the washer and from the washer it transmit to the ball and due to this ball try to move, but spring won’t allow it to move, so that energy transfer to the ramp which is fix to the valve and due to this process, the valve rotates.

Note-There will be number of method with which you can check that the valve is rotating or not-

1. At the time of cylinder head overhauling, you can check the valve spindle with the

mallet and before doing this process just put the marking on the valve and body and

after hitting with the mallet both the marking is coincide it means the valve wont

rotate, then you have to overhaul the rotocaps.

Note – The valve normally rotate 5 to 10 degree only not much

Q – How will you get to know out if exhaust valve start leaking ?

Ans – Once an exhaust valve does not seat correctly, the high pressure burning gas will pass across the faces of the valve & seat during the power stroke. This will cause the temperature of the valve & seat to rise in this area, weakening the material & 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 & 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.

Vanadium slag deposits which occur at temperatures above 540°C cause corrosion of the valve surfaces which can lead to exhaust gas blow by. This is combated by effective cooling & the use of suitable materials (stellite and nimonic alloys).

Note – When large amount of gas which have excessive heat and pressure pass through the small opening it create the wire drawing effect and due to this valve seat and valve starts burning and you will get the high exhaust outlet temperature alarm.

Reasons – There will be the number of reasons which are given below –

1. Fuel quality – fuel contain large amount of sodium, vanadium and chromium sulphate which is not good for the exhaust valve

2. If you run the engine on low load for long time then acidic corrosion takes place.

3. High temperature corrosion also takes place – vanadium is liquid and cannot be removed during purification, but alone vanadium could not cause any problem in the engine, but if it mix with sodium (Na) then it will form vanadium pentaoxide. If vanadium pentaoxide forms then it goes to the valve seat and form a hard film on it so the time valve sit on the seat, then valve break this hard film somewhere so the place where film breaks has the perfect sealing and place where film doesn’t break do not have the perfect sealing so, due to this wire drawing effect take place and valve and valve seat starts burning.

Vanadium + Sodium in the ratio of 3:1 forms V205 (Vanadium Pentaoxide)

4. Also valve is damaged if valve do not have valve rotating arrangement like in CFC engine there was the mechanical system is provided so maximum problem takes place over there.

Note – If all other unit valves are okay and only one unit valve burn out then reason is that unit valve will not rotate properly

How to make out that valve burnt out –

1. First make out is that unit exhaust temperature keep on increasing.

2. Excessive cooling of the valve casing is also not good so for that reason we put the orifice at the outlet side which control the flow rate because excessive cooling cause corrosion).

Q – Explain Man B&W exhaust valve with sketch and all terminology related to it?

Ans – Seat – chromium molybdenum seat coated with stellite

Bush-brass or Cast iron

1. It relate at the time of opening because at that time pressure is high.

2. Gas deflector – the purpose of gas deflector is not to allow the exhaust gases to go upward

3. Valve open by hydraulic pressure and close by spring air

4. Spring air at 7 bar (under chamber of air piston is filled with air see in the diagram)

Note – To check that exhaust valve of main engine is rotating or not for that one indicator is there on the exhaust valve whose vertical distance is not the same and every time it keep on changing due to the rotation of the exhaust valve.

In sulzer there was the ball on which there was the strip and from the sight glass you can check that the valve is rotating or not

5. Hydraulic actuator develop pressure at 120 bar to open the exhaust valve so when exhaust valve opens and line pressure drops now the main engine piston start coming up and pressure inside the cylinder start increasing which suddenly close the exhaust valve and exhaust valve hit very badly on the seat so for that reason one throttle valve is given which won’t allow all the hydraulic oil to leak instantly because if this take place then no hydraulic pressure is there on top of the hydraulic piston and cylinder pressure and spring air pressure instantly close the exhaust valve, so for that reason we provide throttle valve.

6. The ship in which cam shaft lube oil pump is separate just make sure before starting the pump, you open the spring air else air piston completely comes down and cover the line from where spring air came into the underside of air piston in that case it is very difficult to close the valve

7. New advancement in exhaust valve –

1. We put some amount of oil underside the air piston for better lubrication of the spindle because it keep on moving up and down

2.Sealing air supply has to be given so that no exhaust gas should go upward as shown in the diagram above.

Note – Throttle valve also have one more function to perform but before that I would like to make you familiar with one more term which is called virtual tappet, it is to be given in main engine exhaust valve because due to high heat of oil which come in contact with valve spindle there may be the possibility that valve will not close fully because of the expanded oil (due to heat). So for that reason little amount of oil has to be released from the system and this thing has been done by the throttle valve, but throttle valve should not release excess oil else no pressure will be there and exhaust valve hit the valve seat because of no hydraulic pressure above the hydraulic piston this term is called bang-bang of exhaust valve if throttle valve is not properly adjusted.

9. At the time of starting banging is there because vent valve take time to vent the exhaust gases or air but if banging is continuous it simply means that your throttle valve is not properly adjusted.

Note – Surveyor mostly ask this question that what is the role of puncture valve which is provided in the valve pump actuator – In 2 stroke engine we want maximum power from the engine that why we open exhaust valve late and also want to close it late but during astern running we want exhaust valve to open early and close early else it was impossible to run the ship in astern because we don’t have any kind of reversing for astern running just as like in for air and fuel, so puncture valve serves the purpose –

1. Puncture valve only work during the astern running of the ship because we perform

the reversing only for the air distributor and fuel pump. So puncture valve delay the exhaust valve opening during the astern running of the engine because we didn’t perform the reversing for exhaust valve so during normal running (Ahead) if valve opens late then, at the time of astern running valve opens early so whatever air comes from the starting air valve that air completely goes out from the exhaust valve and we cannot able to start the engine.

Concept of variable exhaust closing – Concept of variable exhaust closing simply mean to close the exhaust valve little early, some time before even closing of the scavenge ports but the main point is that how to do that – For that we have the solenoid valve when signal comes to it then that solenoid valve opens and release some hydraulic oil pressure from the hydraulic line so because of this exhaust valve comes little upward and now the exhaust valve has to travel the less distance for complete closing with respect to the original distance.

Note – Every early closing will increase the compression pressure by 10 bar that was the reason why concept of variable exhaust closing came into action.