Running In the Engine
The object of running an engine in is to get the engine to the point where
all the rubbing surfaces are nicely mated to each other at all temperatures
likely to be attained (by the engine) while causing as little wear to the
engine as possible in the process. The benefits of doing this properly are
twofold; the engine would be a 'better' engine throughout it's life and that
life should also be extended.
The method used to achieve this is simple. Run the engine very rich and
lightly loaded at first and gradually increase the amount of work the engine
is allowed to do, at the same time gradually increasing the temperature that
the engine is allowed to attain, by judicious use of the main needle valve.
This gradual process is spread over approximately the first hour of the engine's
life and at the end of this hour it should be ready for its first full speed
run.
All running-in is done with the throttle fully open.
Ringed Motors
Start by using one of the smaller propellers recommended by the manufacturer
and a minimum of 20% of castor oil in the straight or low nitro fuel.
Make the first tankful absolutely
soggy-rich,
keeping the glow-plug lead attached if necessary to keep the engine running.
Keep all runs of short duration with a few minutes cooling down time between
each.
For the next few runs set the main fuel needle to give a very fast
four-stroke with just the occasional hint of
two-stroking.
Allow engine to run for 30 seconds and then close throttle or richen up needle
to slow engine for 20 or 30 seconds cooling period, open the throttle again
for 30 seconds then allow to cool again. Gradually increase the length of
the full throttle run over two tankfuls of fuel. At the end of these two tankfuls
lean engine out to the point where the engine is two-stroking and four-stroking
(ratio about 60/40) and repeat previous procedure of short runs, gradually
being increased in length for the next two tankfuls. The next stage is to
lean the engine out until a full
two-stroke
is attained, but make sure it is only one notch from the
four-stroke
coughing, i.e., still set richer than maximum rev's. Repeat
above procedure of short runs gradually increasing in length for two
tankfuls.
The engine is now ready for full speed running, but still needs the first
couple of tankfuls to be short runs gradually increasing in length.
The engine is now run-in for normal use, but if the engine is to be used
under more harsh conditions (e.g. with a tuned pipe), then it will need further
running-in under those conditions using the above short-run procedure.
The reason for everything being done in small and gradual steps is that
the facing surfaces have to be mated at gradually increasing pressures.
Now that the engine is run-in check all screws and bolts for security
and if you have to tighten any cylinder head bolts, remember to tighten a
little at a time and in diagonal rotation.
Running in ABC engines
An 'ABC' engine is one with special liner and piston metallurgy, e.g.,
the piston is aluminium (A), the liner is brass (B), and the brass is chrome
plated (C). Generally these are performance orientated engines.
When an ABC engine is warmed up the liner, made of brass, will expand
more than the piston which is made of aluminium. Consequently, as the engine
reaches working temperature the piston seal would not be very satisfactory.
The manufacturers have taken note of this and taken steps to counteract this
undesirable state of affairs by making the liner and piston the correct sizes
for when the engine is hot. This means that when cold the piston is a very
tight fit at the top of the liner, to the point where some make a light groaning
noise when forced over Top Dead Centre (when the piston is at the very top
of its travel, or TDC for short).
Very little running-in can be accomplished with these engines, as the
cylinder temperature must be raised to full working temperature as quickly
as possible to avoid excessive piston wear due to the very tight fit at lower
temperatures.
My usual method is to use about 5% extra castor oil in the fuel and set
the engine for just below full speed running (throttle fully open, main needle
just a little bit rich), and run the engine in short cycles of approximately
30 secs. full speed and 5 secs. at 1/3 speed, for the first 15 minutes. This
keeps the cylinder temperature up and the slow running should give time for
any hot spots to cool down somewhat. For the next 45 minutes use the engine
normally but keep it just a touch rich (just 2 or 3 clicks).
Setting the main needle
This is a most important setting as not only does it set maximum power
but it also controls the running temperature of the engine and from there
the length of the engine's life, the life of the glow-plug and the overall
reliability of the engine's running in flight or elsewhere. Engines don't
very often cut out in flight because they are set slightly too rich, but they
most certainly do when set too lean.
It takes about five minutes to learn the drill for correct needle valve
setting so it's worth taking the trouble for the long term benefits gained.
The drill is to start the engine on low throttle. Once started, open the
throttle fully and set engine to maximum speed. Leave the engine to warm through
thoroughly at top speed for a minute or so then adjust the needle to see whether
an improvement in RPM can be achieved. From this established point richen
up slowly until a small but definite drop in RPM is noticed, with the
engine still two-stroking. Lift the nose of the model up vertically
and if the small loss of RPM is regained then you should have a good flight
setting. If the model has a tendency to go rich or go lean in flight then
an extra allowance will have to be made for this on the final setting.
The aim of this is to give maximum power from the engine when it is needed
most, either when the model is climbing or when turning sharply and will also
give sweet and cool running during level flight, the best of both worlds.
Four strokes
The previous sections are written specifically for two-strokes, but exactly
the same principles apply to a four-stroke, just some of the symptoms are
slightly different.
When running-in from new use a fuel with 20% castor oil for the first
hour. A four-stroke cannot be made to four-stroke when rich, it's already
doing it, but what does happen is the engine misfires in a rather uneven manner,
the richer the setting the more pronounced the misfire. The main needle, after
running in, is set in exactly the same manner i.e., just on the slightly rich
side of maximum RPM., when the engine is thoroughly hot.
If the main needle is set too lean the engine may slow down with the.
exhaust exhibiting a more leaden note than usual and may progress to the point
where the engine stops with a bang due to 'detonation' and throws the propeller
off in flight, or on the ground and perhaps at whoever is standing in the
way.
'Detonation' is like early ignition but the difference is that instead
of the mixture burning progressively from the glow-plug outwards, the temperature
and pressure in the cylinder are such that the whole fuel charge ignites at
the same time, before TDC, stops the piston dead, and blows it back the other
way.
'Knocking' or 'Pinking' are like a slightly milder form of detonation
that happens late enough in the compression stroke for the flywheel effect
of the propeller to get the piston over TDC and therefore for the engine to
keep running. Always close the throttle when this happens as it overstresses
the engine.
The main defence against these phenomena is to run the engine a little
richer. This works in two distinct ways, first the richer mixture will keep
the engine a little cooler, thus making the conditions for commencement of
'knocking' harder to attain and secondly and very importantly, a rich mixture
is much less prone to detonation than a lean mixture. These two effects work
together, either for you or against you depending whether you set the main
needle rich or lean.
The low speed needle is adjusted in the same manner as a two-stroke except
generally speaking it needs to be noticeably richer than a two-stroke, to
ensure a good pick-up when the throttle is opened and also on some engines
to stop 'knocking', and the consequent risk of throwing the propeller, whilst
the engine is picking up speed.
Setting the low speed
The need for a low speed adjustment on the carburettor is because as the
throttle is closed it lets less air through into the engine and to keep the
fuel air mixture within combustible limits the fuel flow. has to be altered,
it's too fussy to leave to chance.
Most manufacturers have opted for a two needle configuration or something
which works in a similar manner.
To adjust this low speed mixture start the engine, warm it up and make
sure the main needle is properly set. Connect the glow-plug lead to energise
the plug and slow engine down by gradually closing the throttle until the
engine starts to run badly. At this point adjust the slow run needle to give
smoothest running characteristics just a little on the rich side of fastest
setting. Having made this adjustment slow the engine further until it runs
badly again, now adjust slow needle until engine runs smoothly once more and
continue this step by step procedure until the desired tickover has been reached.
Now remove the plug lead and repeat the whole process. If the engine cuts
dead in the middle of adjustments it's usually because it is too lean.
Having reached a slow tickover we have to check if the engine will pick
up properly. Open the throttle to full speed as fast as a servo would move
it. If the engine picks up but splutters a little whilst doing so, the low
speed needle is a little on the rich side. If the engine appears to miss and
then picks up suddenly the low speed needle is slightly lean, and if the engine
cuts dead when the throttle is opened richen the low speed needle 1/4 turn
and try again.
If the engine starts to pick up pretty well and then cuts at about 1/3
speed or so, try opening the main needle two or three clicks.
On many engines there has to be a compromise or two to get the low speed,
midrange, top speed and pickup to 'gel' into a useable whole, and usually
the compromise is that somewhere in the range the carburettor has to be set
a little richer than would be considered ideal.
What happens when an engine is set
too lean?
The glow-plug engine is a semi diesel which means that it is partly the
heat generated by compressing the fuel/air mixture and partly the temperature
of the hot glow-plug element which ignites the charge in the cylinder at the
correct moment. A little thought will let you see that the whole system is
very temperature dependant. No spark to set it off, no injection of fuel to
set it off. Just the combination of these two temperatures.
When an engine is set lean i.e., for maximum RPM at the start of a tankful
of fuel, anything which slows the passage of the fuel through the needle valve
will make the engine run overlean. Some engines will not continue to run in
this condition and so cut dead but the majority will continue to run to some
degree.
As the tankful of fuel gets used the fuel pressure, as seen by the needle
valve, gradually reduces and as a result the engine gets a progressively leaner
mixture as the flight continues. As the engine started set in a lean condition
it must progress into the overlean condition and maybe go so far as to cut
out.
When the engine runs flat out, ignition timing and engine temperature
should be as the designer intended but as the mixture gets into the overlean
area temperatures will start to rise. This is due to over-lean mixtures changing
combustion characteristics in a way that imparts more heat to the surrounding
engine structure and less to useful work.
As a result of this the incoming charge is heated a little more than it
should be and when compressed is at a higher temperature than it otherwise
would be. The secondary effect of the engine structure being at a little higher
temperature is that the glow-plug element is also a little hotter.
As already explained, ignition timing is controlled by the temperature
of the compressed fuel/air mixture combined with the temperature of the glow-plug
element. The net result of both of these being raised is early ignition in
the next cycle.. This early ignition gives just a little more time for even
more heat to be transferred to the engine structure before the hot gasses
are exhausted from the engine and so jacking up engine temperatures just a
little more, resulting in the next cycle jacking up temperatures yet again.
This is the slow build situation that causes engine seizure in some cases
and in other cases runaway early ignition that makes an engine stop quickly
in the air, as though it has seized, but in fact feels perfectly O.K. and
runs O.K. when it has cooled down again.
Many of the modern engines do not reach these extremes but keep running
at a steadier but much higher temperature than they were ever designed for.
These engines run other risks which are much less obvious.
One risk common to ail engines which are run at elevated temperatures
is that the oil gets very hot, thins out too far and so can no longer lubricate
as effectively as it should, resulting in excess wear and shortened engine
life.
These extra stresses may only result in wear being more rapid than necessary
but if it is a regular occurrence it can result in very early bearing failure,
and if it happens to be the con-rod that gives up first the resultant damage
can be very expensive.
With all this in mind, when adjusting the main needle of your engine the
catchphrase should be 'The future of your engine is in your hands'.
A Few General Do's and Don'ts
- Always keep your engine clean and free of dirt.
- Choose the propeller that allows the engine to run in the RPM range that
the designer intended.
- Always filter your fuel from fuel bottle to tank and from tank to engine,
and keep the filters clean.
- Don't wipe model with cloth that sheds fibres, some will undoubtedly
find their way into the needle valve or somewhere just as inconvenient.
- Don't use a damaged propeller, an 11-inch diameter prop turning at 15,000
RPM has a tip speed of 490 m.p.h., and to shed a blade can obviously be lethal.
- Always balance propellers and if possible spinners as there will be less
vibration to affect the engine, model and radio.
- Make sure your glow-plug battery is charged before you go out.
- At the end of a days running 'dry' the engine out by pulling the fuel
line off at full RPM. Then put After-Run oil in the carburettor and ensure
it is dispersed throughout the engine. Raw fuel is corrosive to steel, aluminium
and copper bearing alloys.
- Never store the model nose down in the corner of your garage, going home
in the car, or even while cleaning it at the field, if it is fitted with a
tuned pipe or an extra large silencer. The exhaust residue which collects
in these is highly corrosive and would run straight back into the engine.
- If the engine is badly 'flooded' i.e., liquid fuel in the crankcase turn
the model over in such a manner that the excess fuel in the crankcase runs
up the transfer passage, into the cylinder through the transfer port (make
sure it's open-piston at the bottom of it's stroke), across the cylinder,
out of the exhaust port and out of the silencer
Definitions:
Two-Stroking:
Engine running at optimum RPM with correct.
Four-Stroking:
Engine running at slower RPM with great amount of fuel.
Lean:
Reduced fuel & increased air ratio.
Rich:
Reduced air & increased fuel ratio.
Soggy Rich:
Reduced air & greatly increased fuel ratio (glow plug attached).
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