During the maintenance of the marine engines we have to ascertain that the piston is at TDC to check tappet clearance.
Though markings are provided on the flywheel, the marine engineer must know the other methods like inspection of camshaft and fuel pump window.
What is the need to know the TDC Position of Piston
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 during the checking the tappet clearances of the engine it is
important to know which unit is at TDC.
Referring the flywheel would indicate
two units, but only one can be at injection TDC.
So
which one is it? In this article the various methods to find out the position
of the piston would be discussed.
Some are very simple and conventional methods
but some are little complicated but never the less very important when you
require an independent method to find the TDC.
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 check visually.
The unit at
TDC would be having 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.
Flywheel Marking
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 and as the plunger
goes up and down, so does the mark on the bottom spring holder.
On the body of
the fuel pump there are cut marks which start the start of injection.
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 TDC we can check the
tappet clearance.
It must be noted however 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 TDC.
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.
Dial Gauge
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, acceleration and the dwell period.
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 also the unit is at TDC.
It
must be remembered that as a four stroke engine has two rotation 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 take 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 the springs are loose at 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 than it can
cause errors.
Push Rod Method
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.
Spill Timing Method
This is a very accurate and tedious method and generally used to
check the start of injection of the fuel pump.
It will also give you the
injection TDC.
It is not used in tappets checking process but used to find the
start of 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.
Thereafter a special
"U" shaped pipe is put in place of the high pressure pipe.
After this
operation the engine is turned by the turning gear and 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 and the oil quantity will reduce.
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 and if you observe the fuel cam you
will find the follower is no longer at the base circle.
As this method involves the spilling of fuel, it is called as
the spill timing method.
Conclusion
There are various methods by which the TDC can be found.
The
simplest and the most straightforward method is the reference of the flywheel,
but sometimes the flywheel itself is changed and you need to calibrate the new
one.
Sometimes to check if the slippage of the crankshaft has taken place you
need two methods, especially where the crankshaft does not have marks to
indicate slippage.
Thank you for reading this article....
Have
any suggestions and questions regarding this.
Feel free to share and
ask us via the comments section!
Remember to share this article with others on
Facebook, Twitter & Google Plus.
Sharing
is caring...
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 TDC we can check the
tappet clearance.
It must be noted however 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 TDC.
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.
Dial Gauge
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, acceleration and the dwell period.
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 also the unit is at TDC.
It
must be remembered that as a four stroke engine has two rotation 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 take 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 the springs are loose at 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 than 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.
Spill Timing Method
This is a very accurate and tedious method and generally used to
check the start of injection of the fuel pump.
It will also give you the
injection TDC.
It is not used in tappets checking process but used to find the
start of 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.
Thereafter a special
"U" shaped pipe is put in place of the high pressure pipe.
After this
operation the engine is turned by the turning gear and 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 and the oil quantity will reduce.
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 and if you observe the fuel cam you
will find the follower is no longer at the base circle.
As this method involves the spilling of fuel, it is called as
the spill timing method.
Conclusion
There are various methods by which the TDC can be found.
The
simplest and the most straightforward method is the reference of the flywheel,
but sometimes the flywheel itself is changed and you need to calibrate the new
one.
Sometimes to check if the slippage of the crankshaft has taken place you
need two methods, especially where the crankshaft does not have marks to
indicate slippage.
Thank you for reading this article....
Have any suggestions and questions regarding this.
Feel free to share and
ask us via the comments section!
Remember to share this article with others on
Facebook, Twitter & Google Plus.
Sharing is caring...
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