ANS) Piston inspection on ships is part of the engine planned maintenance schedule (PMS) carried out to ensure the components is within the allowed tolerances. There are two methods of inspection: when the piston has been removed from the liner or inspection through the liner scavenges ports.
a)Piston Removed for Inspection:-
This examination will be under taken in a modular format, since the piston can be divided into various components.
Piston Crown
Check for any burning at top part of the piston.
Check any wear at the sidewalls of the crown and on ring grooves.
Check for any cracks at top due to the thermal and mechanical stress, check also for high temperature corrosion.
Check any signs of hot corrosion at the top surface and acidic corrosion at the lower part.
Piston Rings and Grooves
Check for the free movement of the piston rings. Check the ring clearance / groove clearance. Inspect for any wear, stepping and for scuffing. Piston Skirt and Side-wall
Check for any rubbing marks.
Inspect for any wear down of wear rings. Cooling Water Passage
Check for any scale due to poor water treatment. Choking due to high temperature.
Finally inspect the locking bolts; wires, studs and ‘O’ ring condition
b)Maintenance Schedule:-
Periodic inspection has to be done when the engine is not running. It can be carried out as above or by entering the scavenge space and inspecting the piston and piston rings through the scavenge ports, with the piston brought in line by rotating the engine via a turning gear.
Overhauling the piston as per Planned Maintenance Schedule (PMS).
Monitoring of the condition of the piston and the piston rings by the compression curve of the indicator diagram through process analysis.
The images shown below show examples of two means of inspection.
Piston removed for closer inspection
Emergency Repair of Piston Crown:-
Once the above checks have been carried out, what actions can be taken if some values or observations are out with the specifications? Given below is a list of common faults that might be found during inspection and means to make temporary emergency repairs.
Gauge piston crown and ascertain shape and wear-down. If it is beyond recommended limits, replace the piston if
there is a spare available. If not, rebuild the engine and proceed to the nearest port at reduced revolutions and arrange replacement. The crown head should not be welded except in a dire emergency- and even then only by an experienced welder. Remember that modern diesel engine pistons have a special lining of high temperature alloy on the top of the crown. This measure improves resistance to corrosion as well as to high combustion temperatures that the piston top is exposed to.
Examine the crown for fractures or cracks, and if found the piston should be changed. If no spare is available these can be welded to manufacturer’s specifications; using the correct alloy welding rods, again as a means to proceed to the nearest port at reduced revolutions for a replacement.
Dismantled piston rings should be kept in sequential order so as not to interchange the rings when re-fitting to the piston.
Once repairs are complete, replace the piston rings and check for normal butt clearance.
If the butt clearance is more or less than the normal range, then replace the piston rings with new set of piston rings.
Note: It would be an extraordinary predicament to be in where as a Chief Engineer you sailed without main engine piston spares. However, strange things happen at sea, maybe the spares have been already used, and you're awaiting delivery of replacements.
If any of the above repairs are carried out, it is imperative that a close watch is carried out on the appropriate cylinder with the exhaust temperatures closely monitored as well as the piston cooling medium temperatures.
Q23) WHAT IS PURPOSE OF TAPPET CLEARANCE & HOW IT IS DONE?
ANS) Tappet clearance is a space between the top of the valve stem and the rocker arm. Its purpose is to allow for some mechanical expansion and lengthening of the valve stem and push rods as the engine warms up. This clearance is also called valve lash.
If insufficient (lower clearance) valve lash is set when the engine is cold the valves will not properly close when the engine warms up or early opening of the valve.. If too much lash is provided (additional clearance) then even after the engine warms up there will be some clearance, which will result in lost motion. Lost motion mean that as the cam tries to open the valve the push rod and rocker arm moves to first take up the clearance before touching the valve to open the valve. The result is late opening of the valve.
When checking tappet clearance on marine engines, we have to ascertain that the piston is at TDC. Though markings are provided on the flywheel, the marine engineer must know the other methods for this like inspection of the camshaft and the fuel pump window.
During the maintenance of a 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?
Flywheel Method: -
The flywheel is the simplest method to know which unit is at TDC. If the flywheel shows two units, simply open the bonnet covers and checks visually. The unit at TDC will have both the inlet and the exhaust valve closed and hence relaxed springs; the other unit would have both the arms of the rocker arm at different levels. In addition the push rods of the unit at TDC would be loose and 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 a 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 and the push rod freeness check would lead you nowhere and misguide you.
Dial Gauge Method: -
In this method the fuel injector is taken out and from the opening a dial gauge is put inside. Then the turning gear is engaged and the engine turned over. The pointer of the dial gauge will move in one direction and then stop and start in opposite direction. The moment the pointer of the dial gauge stops and changes its direction of movement is the TDC of the unit. This method is not normally 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 and the camshaft inspected. The cam of the engine has a base circle, and acceleration and 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 and the inlet valve follower are on the base circle, then the unit is also at TDC. It must be remembered that as a four-stroke engine has two rotations of the crankshaft there is one injection TDC where the injection and the combustion take place. The second time the piston is at TDC is when the exhausting of the flue gases takes place. It is very important to identify the combustion TDC, as tappets have to be adjusted at that point.
Cam Profile
Crankcase Method: -
In this method the crankcase doors are opened up and 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 strong doubt about the other methods.
Valve Spring Method: -
This is not an independent method but is used in 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 springs are loose is the one at TDC. The caution is that this method is useful for an engine in use. If you have removed the rocker arms during the overhaul and thereafter you want to use this method then it can cause errors.
Push Rod Method: -
This method is like the spring method and you check that the push rods are free to turn. The unit at TDC will have loose springs. The care that must be taken is that it should be used along with the flywheel method and should be used in a working engine. By a working engine, I mean the engine that was running and has been stopped for tappets adjustment. Loosen the lock nut of the rocker arm.