In this blog, I am going to discuss one practical problem which comes in day to day work onboard. This is also important for the exam as well as a knowledge point of view being an engineer. So, I am trying to explain all possible method to find out the TDC (Top Dead Center) of the auxiliary ( diesel ) engine.
When checking the tappet clearance or rocker clearance of auxiliary engine, we have to make sure that the piston is at TDC. For this markings are provided on the engine flywheel, but being a good engineer we should also know the other methods for the inspection of the camshaft and the fuel pump etc. So before proceeding further let’s know why we want to know TDC of the piston.
What is the need to find the Top Dead Center TDC of the piston?
While overhauling or in case of some routine maintenance of four-stroke marine auxiliary engine, we must know the position of the particular unit piston for some work, e.g. when we are checking the tappet clearance of the engine we should make sure that piston of that unit should be at the top dead center (TDC).
The flywheel markings are the most common method used by the engineers to find out Top Dead Center (TDC) but other than this there are also various methods to find out, verify or to calibrate the marking. For a four-stroke engine, the piston will be at TDC twice in one complete cycle(four strokes ) at injection(firing) as well as for exhaust.
To find Top Dead Center (TDC) of any unit of the Four-stroke engine, these are possible methods we can try on board :
- Flywheel Method:
Most commonly used method is the flywheel method because it is the simplest method to know which unit is at Top Dead Center (TDC). There will be marking on flywheel for each unit. Every unit will have two markings on the flywheel, cause each unit will go to TDC two times in one complete cycle of four-stroke, one will be firing TDC and other will be non-firing TDC. The unit at Top Dead Center (TDC) will have both the inlet and the exhaust valve closed and hence the piston rod will be free to move, this condition shows the firing TDC. If the push rod for inlet and exhaust valves are not free i.e. valves are not close this shows TDC but not firing TDC or not TDC(if marking on the flywheel is wrong). The push rod of the unit at firing TDC would be loose and can be turned by hand easily. This method is only useful in a working generator which we have just stopped to check the tappet clearance. Once we have removed the rocker arms arrangement for any reason the spring height and the push rod freeness, then checking will give an error and will misguide.
- Stud Method –
This is a most accurate method to find the position of the piston without opening up the cylinder head. In this method take out the injector of that unit. Now take a long stud(normally M8 stud), it should be able to go inside the cylinder through injector pocket provided on the cylinder head. And also the thread of this stud should be as same as the groove of the piston top, provided for taking the piston out. Now connect stud with the piston. In next step start turning the engine, the stud will go up and down as piston movement. When the piston is close to TDC(you will come to know through stud movement), Mark the stud at top of the cylinder head and also at the flywheel. Start turning the engine, the sud will go up and will start coming down after piston reached TDC. Then again the top marking of the stud comes to the same place as marked before i.e. at cylinder head top, mark it on the flywheel. Now you have two markings on the flywheel. The midpoint of these two markings will give you the exact position of your TDC.
- Dial Gauge Method-
This is a little complicated, in this method we have to remove the fuel injector and from the opening, a dial gauge(depth gauge) is put inside. Then engaged the turning gear and turn the engine slowly. Initially, the pointer of the dial gauge will move in one direction and then stop and after that, it will start moving in the opposite direction. The moment the pointer of the dial gauge stops and changes its direction of movement is the Top Dead Center (TDC) of the piston of that unit. This method is not used commonly to find the TDC but we can use this method to calibrate the flywheel marking or verify the marking after a major overhaul.
One thing we have to note that in the four-stroke engine has two rotations of the crankshaft there is one firing TDC where the injection and the combustion take place. The second is non-firing TDC is when the exhausting of the flue gases take place. While taking tappet clearance to make sure unit in at firing TDC. All the above methods can be used to verify marking on the flywheel. In four stroke engine flywheel, marking will show each unit at TDC two times in one revolution. But to find out the firing TDC from these two markings, below methods can be used.
- Fuel Pump Method –
To find out the firing TDC of piston this method can be used i.e. with the help of fuel pump plunger position. The fuel pumps have marking on the body as well as on plunger which can be seen from cam case cover. As the plunger goes up and down the plunger mark will match with the fuel pump body marking. There are two marks on the pump body, the top one is called the injection point and the bottom one is called as the set point mark. the fuel pump plunger will match with set point at all times, except the time of injection. At the time of the injection plunger, the mark will match with the injection point. If you know both the TDC position of that unit, then at one out of these two positions the mark on the plunger of the fuel pump will be at the injection point. This is your firing TDC of that unit. In a diesel engine, the start of injection is the injection TDC where both the inlet and the exhaust valves are closed. In this injection Top Dead Center (TDC) we can check the tappet clearance. But this is not actual TDC, injection starts 4-5 degree before TDC, depending upon maker and engine design.
- Camshaft Method
In this method, we have to open the camshaft window of the engine and look at the camshaft. The cam of the engine has a base circle, and cam which is working face. If the roller of the follower is at the base circle, then the particular valve is closed. When both the exhaust valve and the inlet valve follower are on the base circle, then the unit will be at firing Top Dead Center (TDC).
- Valve Spring Method
This method is used to confirm the firing TDC of that unit. To confirm that the unit is at firing TDC or not check the valve springs. If the valve spring is not compressed (very slightly loose) the piston of that unit is at firing TDC because both the valves are closed and there is less tension on valve spring. We used this method commonly while taking tapped clearance. This method will not work while we had removed rocker arm connection during overhauling.
- Push Rod Method
This is the method similar to the valve spring method, in this, we have to check the push rod is free to turn or not. If pushrod is free to turn means that the valve is closed and that means the piston of that unit is at firing TDC.
All the above three methods the main aim to find out whether the suction and exhaust valves of that unit is open or not. If the piston is at TDC and suction-exhaust valves are shut, that means that is the firing TDC of that unit.
- Spill Timing Method
As the name suggested, in this method we have to watch the spilling of fuel from the fuel pump. In this method, the delivery valve of the fuel pump along with the delivery valve spring is removed. And in the place of high-pressure pipe put a “U” shaped pipe. Now slowly start turning the engine with turning the gear, slowly brought near TDC. At one TDC we will observe that oil start spilling from the pipe because the oil started entering the pump from the inlet port which is uncovered by the plunger. That will be the injection/power stroke of piston and piston will be at firing TDC.
This method is not used to find while taking tappet clearance, we use this to find out the start of injection, in case we are dealing with the problem of thermal power balance of the engine.
Please do read about indicator cards of a 2-stroke engine.