3. RUNNING (IN) THE ENGINE
Before I sent the engine, it has been fully checked by me, but it has not run.
Running in is at first necessary to get the dust out of the
engine. Even though all parts have been ultrasonically cleaned, only by running
the engine, all the dirt will come out or find a place in the engine where it
is not in the way of moving parts.
To run the engine clean, it has to run rich 3 times appr. 2 minutes, preferably
(but not really necessary) without pipe on a standard flying prop shortened
to 85% of its original diameter.
After this it should run appr. 2 minutes rich (now with the pipe) at the same prop, 4- stroking at appr. 26000 rpm, a couple of times leaned out to just over 30000 rpm for 3-5 seconds. After running on the pipe for 7-10 seconds more, rpm will increase to 31000 - 32000 or more due to the higher gas temperature in the pipe, but we usually don’t do this. I don't like too long ground running at high load when the engine is new.
The test prop has to simulate conditions in flight. Rpm’s in the air of 31500-33000, depending of pipe type. A “standard” flying prop, shortened to 85% of it original length gives a bench-test prop that simulates the engine load in flight quite well.
I'm convinced that in a properly built engine (what the MBPROFI
is supposed to be), rotating parts, like the ball bearings, big-end, connecting
rod, are run in after a few seconds and should be absolutely uncritical. The
difficult part is the piston in the cylinder.
The piston, when new, gets thermal load on it's surface, both by mechanical
friction and from the reflected (hot) pulse-wave of the pipe. The mechanical
friction has to do with the fit of the piston in the cylinder as it was made.
The piston has to "wear" itself a bit on to the cylinder surface without
getting a too high surface temperature. If a too high temperature occurs, it
will show "shining spots" at certain points, a sign of a local change
of material structure in the basically unstable high 30% silicon aluminium alloy.
Let's say the material melts locally on the surface and a kind of change of
the crystal-structure occurs.
To prevent as much as possible a high piston temperature during
running-in the following helps:
• run rich, fuel is cooling and may give lubrication
• run high rpm to make sure that the back pressure from the pipe is "late".
This helps to give less filling of the cylinder (less energy and pressure in
the combustion chamber) and the hot pressure pulse will not press gases back
through the transfer ports into the crankcase, which builds up a lot of heat
in the engine quickly.
So, when you start flying, start with a relatively small prop (ground rpm > 29000) for the first 5 flights and keep ground running time to the absolute minimum. Start the engine from rich and turn the needle slowly in, easy into resonance, but keep relatively rich, and release quickly. Air running for the first flights should be slightly four stroking.
Then take the cylinder off and check the piston.
If there are some shining spots, take them away using wet and dry paper 800-1200.
This should be done very lightly and carefully, especially at the "ring",
appr. 2.5 mm from the top of the piston where it seals.
Small shining spots at the position of the 2 small bridges in
the main transfer ports may be neglected. They seem irrelevant and will disappear
after more running.
Clean the piston (see next chapter).
If you have disassembled the connecting rod from the piston for cleaning (see
for disassembly and assembly of connecting rod chapter 5), note the following
for assembly of these parts:
• the front-side of the connecting rod has the lubrication
holes (the side where the fuel comes from); the front of the big-end side has
also a radius in the bronze bush to fit the radius between the crank-web and
the crankpin.
• the long skirt of the piston is at the back side (to prevent sub piston
induction).
• Check if the cir-clip is well in the groove.
If the piston shows scratches in certain places, check the top
side of the ports for too sharp edges. If this seems to be the case, use wet
and dry paper 800-1200 around your finger to round the ports coming in from
the bottom side. Never touch the cylinder surface with the sandpaper at more
than 1 mm above the exhaust port.
The cleaning of the cylinder after this is important, see further chapter 6.
If there are signs of detonation or broken plug wires, make the top of the piston
smooth again (do not round the edges) with wet and dry paper 800-1200. Again:
CLEAN!
Now you start to really use the engine. Find a prop that runs between 28500 and 29500 on the ground. Try to keep ground running to a minimum, peak rpm less than 3 seconds, then immediately rich again. Try to get the engine in resonance by turning the needle from rich all the time. Always start the cold engine with a needle setting 1/2 or 1 turn rich!
Make more flights and lean each time a bit until you think the
setting is right. At this point the engine is supposed to be run- in.
Check the piston after let's say 15 - 20 flights. If still present, take away
the last shining spots and fly again, immediately at full power.
If you had to work on it, check again after 15 - 20 flights. If the piston looks
still bad, send piston and cylinder to me and I will do what is necessary.
Once a piston looks good, the sealing ring is pretty well round and not too
many plugs has gone through it, our experience is that it should last for a
very long time.
When an engine runs well and without detonation, carbon will start to build
up on top of the piston. Clean the top of the piston every 20-25 flights, too
much carbon will make the engine lazy.
After 50- 70 flights or so, you have to start checking the play of the piston pin in the piston and looking at deformation of the piston (significant changes of shape of the sealing ring are a first indication).
It is difficult to say when a piston is worn. If you are in doubt and the performance
of the engine is going of the hill, just send me the piston and cylinder to
check what the problem can be.
Be aware of the fact that wear highly coincides with high temperatures and these
don't happen in the air normally, but during ground running at peak power with
flying props. So:
• Try to prevent this "test"running as much as possible, use
it only to check shortly if the engine will come into resonance: never lean
it out completely. When you know your props, testing an engine by finding peak
ground rpm is quite useless; only air rpm counts.
• Try to start your engine in competition as late as possible, try to
turn the needle to its right setting just before release and keep it as rich
as possible until that moment.
• Use preferably props that give 28500 or more rpm on the ground. 28000,
depending on the pipe of course is possible, but is quite hard for the piston,
certainly when held for 20 seconds prior to start. If you want to fly even bigger
props, use water or fuel injection directly into the pipe (10-15cm distance
from the exhaust) on the ground to bring the resonance rpm down . The pipe is
made acoustically "longer" that way, because at lower temperature
the speed of the shock-wave in the pipe decreases, and there will be far less
heat build-up in the engine; it will run very relaxed! After switching the water
supply off, it will take less than a second to bring it back to the normal rpm
for take off.
Running (in) the MB engine