2 stroke heli engines an over view and some thoughts.
Why 2 Stroke?
The name 2 stroke comes about due to the engine only having a compression and
exhaust cycle, unlike a 4 stroke which has an inlet, compression,
ignition and exhaust cycle.
OS Engines state:
“Glow engines are a
hybrid between diesel and gasoline engines in regards to their mode of
operation. In a glow engine, the catalytic action of the platinum glow plug
coil is what ignites the fuel-air charge rather than a spark or the heat due to
compression of the mixture in the chamber. As a consequence, the compression
ratio used tends to be higher than in a gasoline engine but lower than a diesel
engine.”
Crankcase: This is the outer shell, which is
made of aluminium, within which all other parts of the engine reside or are
fixed to. The crankcase surprisingly acts as a reservoir or pressure vessel for
the fuel/air mixture which we will come to later.
Head: Or cylinder head, again made of aluminium,
this is the area of the engine where the gases (air) and heli
fuel is first compressed against and then ignited. It also acts as a heat sink,
dissipating excess heat from the engine via it’s metal
fins.
Liner: Also known as cylinder or barrel. The liner
is a steel cylinder that the piston moves up and down within. It has holes cut
in its sides, known as transfer ports, at strategic places that are designed to
bring in the fresh air/fuel mixture and expel the spent fuel (exhaust gases)
These transfer ports are staggered, the reason for which I will explain when
covering the subject of piston (or at least I hope to!)
Glow plug: Vaguely similar in looks to a very small and dumpy
spark plug, its job is to provide a source of ignition to the air/fuel mixture.
This is where the similarity between the two ends.
Whereas a spark plug generates a spark that jumps across from the centre to
ground electrodes. The glow plug glows red hot, at least the coil within the
plug does. The spark plug is timed to generate a spark at the optimum point in
the compression cycle (when the piston is at the top of the cylinder), however our trusty glow plug glows all the time when the
engine is running (hopefully!). The glow plug doesn’t just suddenly burst into life, it needs a little help to get going by way of an
electrical current from the glow starter. However once the engine is started
the glow starter can be removed and the glow plug’s heat is sustained by the
combustion of the air/fuel mix.
Piston: The pistons main task is, to put it very simply,
translate explosive force to rotary motion. How the hell does it do that?
Compressed air/fuel mixture inters the cylinder via the inlet port on the
cylinder. The fuel doesn’t just think “oh there’s a space, lets go fill that”
it needs some help and it gets it in two ways. Firstly the fuel tank is
pressurised by exhaust gases and secondly the piston actually creates a small
vacuum on it’s compression stroke that causes the
air/fuel mix to be drawn in behind the piston on it’s compression stroke. It’s
not the easiest thing to visualise but if we can start where the glow plug
causes combustion. At this point the piston is at the top of it’s
travel and closest to the head. The Fuel and air in the cylinder have been
compressed, combined with the heat of the glow plug, the mixture ignites. The
resulting explosion drives the piston downward. As the piston moves downward,
it compresses the air/fuel mixture within the crankcase. As the piston nears
the bottom of its stroke, the exhaust port is uncovered. The pressure in the
cylinder drives most of the exhaust gases out of cylinder.
As the piston finally
bottoms out, the intake port is uncovered. The piston's downward movement has
pressurized the mixture in the crankcase, so it rushes into the cylinder,
displacing the remaining exhaust gases and filling the cylinder with a fresh
charge of fuel.
Conrod: Made
from aluminium with brass inserts at each end. (Beware, the brass inserts have
oil-ways which can easily be blocked by debris, always check them when you have
the engine apart) The name is short for connecting rod. So what does it
connect? Well its job is to connect the piston to the crankshaft. As the piston
moves up and down in the barrel, the reciprocating motion is transferred by the
conrod to the crankshaft. This reciprocating motion
is converted to rotary motion as the crankshaft turns.
Crankshaft: The crankshaft as well as being the output shaft and
driven by the piston, acts as a kind of valve for the air/fuel mix. The
crankshaft has a cut-out section in its shaft, which as it rotates, allows fuel
from the carburettor to be drawn/forced in to the
crankcase.
The far end of the
crankshaft, furthest from the conrod, is the output
shaft which drives the head of your heli or anything
else that’s attached to it.
The crankshaft plays another
vital roll, it doesn’t just drive the output (the head of your heli) it drives the piston back up the cylinder. Without
the crankshaft converting the reciprocal motion to rotary motion and back
again, the piston would be a one bang wonder, literally.
Backplate: The backplate is where you stick your finger to get an
indication of engine temperature. This plate along with its gasket seals the
crankshaft within the crankcase from the outside world. Without it there would
be no pressure or vaccum to keep the engine going.
Carburettor: The carburettors sole purpose in life is to mix fuel
with air and feed it to the engine in the desired ratio and quantity. As the carb arm is opened, the internal cylinder rotates allowing
more air to be sucked in towards the engine. At the same time a fine mist of
fuel is sprayed across it’s path creating the air/fuel
mixture necessary for combustion.
Needle and
mixture screws. What are they,
what do they do? They are in effect needles that limit the flow of fuel. The
needle is generally concerned with low engine speeds and the mixture screw with
higher engine speeds. Adjusting these help to provide the correct amount of
fuel to be mixed with air depending on how far open the carb
is.
Bearings: There are only two bearings to worry about in our
engines luckily. The main bearing or back bearing,
is the one closest to the back plate and the small bearing or front
bearing which is closest to the fan.
The job of the bearings is
simple enough, to provide a smooth and low friction surface for the crankshaft
to turn on. If you didn’t have bearings the crankcase would wear to a useless
pile of aluminium filings within a matter of seconds and the crankshaft would
flap around within it until something broke. None of which would be good.
The heat range indicates the
measure of the plugs ability to dissipate some of the heat produced during
combustion.
O.S. Recommend a colder plug
for higher nitro content fuels as the fuel burns much hotter than low nitro
content fuels. Also a cold plug is recommended for hot days.
“Hot plugs promote better idling and
acceleration. If your engine runs rough or accelerates sluggishly, a hotter
plug will help. Cold plugs produce more power and may improve performance if
your engine runs hot. The downside is rougher idling and more difficulty in
tuning. “
There’s no doubt that
engines need to be run in properly for them to perform at their best and for
the component parts to have a long life. The running in process is designed to
bed in the component parts so that they work smoothly together and so that
there are near perfect seals between the different types
metals within the engine.
There are however differing
thoughts on running in engines. The more traditional thought is less load and
more fuel, this is where I disagree. I say more load and less fuel or at least
normal fuel so that the engine runs in its sweet spot from day one.
My method of running in
engines is to allow them to warm up properly and then just fly it as you would
normally. This does involve messing around with the mixture until the engine is
running smoothly, but that’s no different from any other day. What I try to
avoid is too much fuel and being too gentle, otherwise the cylinder sleeve can
“glaze”. You want the piston and the cylinder sleeve to be a perfect match to
each other when they get up to temperature, this can’t happen if they get
glazed up.
What I have found is that
there’s a certain point during the running in process, when the engine note
suddenly changes and I find that I have to change mixture again to have a
smooth running engine. It’s at this point that I know the engine is run in.
The other thing is that
bearings don’t need to be run in, neither does the conrod
or crank shaft. The running in procedure is concerned only with the piston,
rings and cylinder sleeve.
It doesn’t hurt however when
changing any engine components to add a little oil to them for added protection
until the heli fuel fills the system and coats them
with Klotz oil.
Engine
Care
Personally
I don’t drain the engine or burn off the fuel at the end of a days flying. I
have no idea whether this is right or wrong. It’s one of those things if you
ask 10 people they won’t all have the same answer. They may have anecdotal
evidence that their way this right, but no conclusive documented evidence.
I
do try and keep the engine clean and avoid letting dust get near the carb intake. Corrosion will attack the bearings, so
avoiding damp garages is best, but if like me you leave the engine full of fuel
there may be less chance of water getting to the bearings.