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In our November Bulletin we
discussed some items and some corrective measures. This bulletin lets
get down to some basics on the operation of the two stroke engine in
regards to how it effects us. One of the more serious problems I see as
a service center is that of piston damage. This can be classed as a
serious stoppage of the engine. This can also be of a serious nature if the engine quits just as you get airborne. It usually takes the bulge
out of your wallet (that's if the government hasn't already done the job).
An
engine is much like our body. IF a TRAINED technician with a good
knowledge of the product inspects the failed part he can quite often suggest the cause
of the failure. Remember it important to find the
cause for if you don't it will most likely do it all over again.
You
probably know the old saying "if life isn't worth living it might
be the liver", and as my Doc says it may be because of a disease or
you may be tipping the bubbly too much. Well us guys in the service
Industry also have roughly the same saying its How is the engine
operated AND How is it Maintained.. So lets look at some of the
piston problems, keeping in mind as to how we may prevent some of these
problems.
PISTON PHYSICAL DAMAGE
(1) A part Failure unrelated to the
piston. This is where parts of the unrelated Failed part goes through
the ports and cause physical damage to the piston.
(2) Hole in the piston crown top). This
can be caused by either pre-ignition or detonation. (more on this
later).
(3) Piston to cylinder wall seizure Metal
transferring From the piston to the cylinder wall).
(4) Worn out - too much clearance between
the cylinder liner and piston this will not usually cause an engine
Failure).
With Out getting too technical lets go
over some of the Four items and see what some of causes of each and how
we can prevent them From happening.
(1) - Usually
and I say usually failed parts going through the engine caused by the
parts not being replaced before they fail. All parts have a life expectancy. However in our business some times its
difficult to lust
arrive at a life expectancy do to the varying conditions. However. over
the years that Ultralighting has been around a good guestimate has come
to the Fore. Rotax has published a Very good Maintenance Plan along with
a Daily Inspection and Preflight check. You may be unsure of a particular engine problem give
us a call - sometimes we can help you sort out the problem before it
becomes a BIG problem
(1) - Pre ignition and Detonation are in many ways
much the same-BUT DIFFERENT (Confused ?). In pre-ignition the compressed
Fuel mixture is ignited BEFORE the spark plug does. The Fuel burn
time is longer therefore more heat is generated on the piston crown with
the possibility of a hole appearing in the center of the piston!
Some of the causes of pre-ignition are
excessive carbon on the piston which can glow and start the ignition process
early. Under propping the engine causing over heating . Stuck
piston' rings - remember the piston rings have TWO purposes.
1 to seal
combustion pressures
2. to transfer heat from the piston
to the cylinder walls For cooling purposes. Usually this is not a problem
we run into too much.
(2) - Detonation however is a problem we do
run into with more Frequency. Old or rotten Fuel produces less power, however
this is not the main culprit. Old Fuel especially with oil added) looses some
of its octane qualities resulting a lower number than
what the engine requires. Remember Fuel mixture is Burned not EXPLODED
In the cylinder. IF the Fuel mixture explodes physical damage is done to
the piston rendering it useless and a costly repair. Compression ratios
and octane is beyond the scope of this text. However the Factory has
done this For and all you have to do is FOLLOW THE INSTRUCTIONS.
92 pump octane now marked on most gas pumps) should be used on all Rotax
engines. There are again other reasons
however rotten. old and under octane Fuel is the main culprit in our type
of operation.
(3) - Piston Seizure; OF all the piston problems
this is probably the most common AND THE EASIEST TO CONTROL BY THE OPERATOR.
First you have a piston made of aluminum. Second you have a cylinder
that the piston (made of aluminum) Fits into and this the cylinder
liner) is made of cast iron. Unfortunately these two items do not have
the SAME EXPANSION RATE. Therefore this is where the problem
lies. Lets say you start up the engine and away you go. Now the piston
warms up Faster and expands quicker than the cast iron liner. When this
happens due to the lack of clearance between the cylinder and piston you
have what we call a COLD STICK or COLD SEIZURE.
This is where metal to metal
contact is made and as a result a Failed engine.
The second problem is
where you are doing touch and go landing procedures. You are on cruise
on the down wind and most likely on the base leg, the engine is up to
normal operating temperature piston and cylinder liner at normal temp
and normal clearances). Now you make a turn onto Final. What happens
when you turn on to final ? Well First you start to descend the engine
is now idling, lack of power - less heat - cylinder cools down reducing
the normal clearance between piston and cylinder. You touch down apply
Full power For take off and BINGO! the piston expands more rapidity than
the cylinder and you have metal to metal contact. So what can we do
to help this condition - give it a couple of Full power burst to help
regulate the above condition. The same can happen if you shut off the engine For a short time, then get
in apply Full power. VERY SIMPLY to help this condition ALWAYS
take a Few minutes to warm up the engine to normal operating temperature
BEFORE TAKING OFF
(4) Worn out - Too many hours before overhauls.
Remember there is nothing wrong in saying
'I don't know". What is wrong is saying "I don't know and
don't care to Find out. Nobody knows all the answers. Good luck and Happy
Flying. Reg's Air Cooled Engines. |
