Engine Tuning
Tips
Most
of what I have to present here pertains to 2-stroke cycle Glow Ignition
Engines, easily the most used in our hobby.
However, much of this information is applicable to 4-stroke cycle and
gasoline fueled engines as well. Two
stroke engines are mechanically very simple, but THERMODYNAMICALLY, they are
quite complex. 4-stroke engines are
mechanically more complex, but their means of converting fuel into rotary
motion are a bit easier to follow. Most
of the gasoline engines in use for models are 2-stroke engines, but 4-stroke
engines are coming into use for the larger aircraft as well. It should be noted that most
larger glow engines CAN be converted to gasoline operation if desired.
Glossary of Terms
Things that you might like
to know
Displacement
- How much volume is swept by the top of tthhe piston, this is how big your
engine is; .40 = 4/10 of a cubic inch, .90 = 9/10 of a cubic inch. It can be expressed in Metric terms as cubic
centimeters, i.e. .60 = 10 cc
Mixture
- Refers to the ratio of air and fuel beiinngg consumed by the engine. If 15 parts air and 1 part fuel is just
right, then 14.5 parts air and 1.5 parts fuel is a RICHER mixture, the opposite
would be LEANER i.e. 15.5 parts air and
.5 parts fuel. The amount of air that
goes into the engine a function of the several things, RPM, volumetric
efficiency , air pressure, etc., but mostly HOW BIG THE INTAKE IS. The amount of fuel to be mixed with the air
is regulated by the needle valve or valves as the case may be.
Torque
- The amount of rotary force the engine ccaann produce.
RPM
- How fast the engine is turning
Horsepower
- The Mathematical product of the RPM &ammp;ampp;; Torque divided by a constant Torque X RPM / 5252 = HP (can be expressed in
4-cycling
– A very misunderstood, non-scientific term used to describe the condition when
a 2-stoke engine is running so rich of a mixture that it is not firing cleanly
every revolution. (makes a burbling sound)
TDC
- Piston is at the top of its stroke
BDC
- Piston is at the bottom of its stroke
Compression
ratio - Volume of space in the cylinder at TDC compared to the volume of space
in the cylinder at BDC (mechanical compression ratio) the dynamic compression
ratio is a bit more elusive as it changes with the port (or valve) timing and the RPM of the engine.
This
has ruined more engines than crashes have.
An engine will run better 15% too rich than it will 5% too lean. (and longer too)
If
you hold your airplane up at an angle say between 45 degrees and vertical, it
should speed up some, but not begin to lose RPM after a few seconds. You should be able to run out the entire tank
holding the plane in this position. If
you cannot, One of the following is the problem:
1. Engine mixture TOO LEAN, (most of the time
this is the problem)
2.
Tank is too low (tank should have centerline even with or only about
3/8 of an inch below the center of the carb.
3.
Tank is too BIG or too FAR from the engine
When
you make needle adjustments, turn the needle in small increments and let the
engine stabilize, then, if needed, move it a bit further. When setting the high speed needle, do not
peak it out and then fly. I don’t even
max out the needle on my racing planes. Rich is always better.
If
you need muffler pressure to keep your engine running, there is something else
wrong. Probably a tank
position problem.
The
fuel tank should not be more than about ½ full for this. If it is full, it will cause
you to set the mixture too lean as it gravity feeds the carb.
This
can get really confusing as both the carb. opening and the
mixture will have an effect on the idle speed (2000 RPM is very good, 2500 is
good. 3000 is acceptable most of the time.)
I
like to set my throttle servo/linkage etc. so the barrel is only open about
.030 to .060 (1/16) of an inch or less.
1.
Start the engine and let it warm up
2.
Close the throttle to the idle position and pay close attention to the
sound and apparent speed of the engine.
3.
If it idles fast (a sort of buzzing sound) it is too lean. Enrich the mixture in small amounts and allow
the idle to stabilize. Repeat this
until it idles at a satisfactory speed.
Smaller engines usually will not idle down as much as larger engines.
4.
If is slows down to a nice burble, but continues to get slower and
slower, it is too rich of a mixture and needs to be leaned in small amounts
until it will stay at a steady speed for almost the entire remaining fuel.
5.
If the only reliable idle seems too fast, reset the idle position of
the barrel to a smaller opening and go through the entire process over again
When
you open the throttle, the engine SHOULD accelerate cleanly to full throttle,
some do, most do not. Generally a slight
crackling and blue smoke will result and then the engine will “clear out”, if
it burbles, smokes, stumbles and quits, it is too rich or the tank is too full
or too high. If it just quits, it is too
lean. Remember, the engine has its
maximum fuel draw when the throttle is closed to idle position. Just about every make engine has
characteristics of its own as well as individual characteristics for the
particular engine. The high and low
speed needle valve, are not SUPPOSED to affect each other, but they often do;
sometimes to the point that you have to start the whole process over if you
make an adjustment. OS, Webra and MVVS are probably the easiest to get to idle
well, but there are so many new makes and “House Brands” now it can be hard to
predict which will be the easiest to handle.
Any of the engines will idle if
you play with them long enough. If your
engine is really giving you fits; suspect an air leak (usually around the
throttle barrel) or a Glow Plug problem.
Some engines just don’t like plug A or B, but run fine on C. Just because an engine RUNS does not mean
that the Glow Plug is OK. If the RPM
drops off noticeably when you disconnect the starting battery, the plug is
probably in need of replacement. Another
“trick” if an engine does not want to Idle; disconnect the muffler pressure if
you have it connected, you may be surprised at how well the engine runs on just
suction. Only engines with a really big
bore carburetor need fuel fed to them.
If
you have to lean out your engine excessively to get your plane in the air to
the point that it sags off peak RPM while in flight, get a bigger engine. You should be able to see a slight contrail
of smoke behind your plane in flight (bright days), actually the more the
better, to a point.
You
really can’t talk about tuning engines without some mention of propellers. Now this is where it gets spooky and some
things happen that at first blush don’t seem to make sense. Like how does that racer go so fast on that
little prop, or the theoretical top speed with 6 inches of pitch is 90 mph, how
come the plane went 110?? If you care,
ask me.
A
couple of things to remember, these are generalities, however:
1.
More pitch = more speed
2.
More diameter = more efficiency
3.
Both of the above reduce RPM
The
law of diminishing returns tends to come in here, as you only have so much
power with which to turn more pitch and or diameter.
I
encourage experimentation with props, but the following are good places to
start and if the plane doesn’t at least fly OK with the following engine prop
combinations; you probably have the wrong size engine on your plane. Or a very sick engine.
.10
start with a 7X4
.15
start with a 7X4, 7X6, 8X4
.20
start with a 8X6, 9X4
.25
start with a 9X4, 9X6
.40
start with a 10X6
.45
start with a 10X6, 11X5, 11X6
.60
start with a 11X7, 11X8, 12X6
For
higher drag planes (like a biplane) use more diameter and less pitch, for
sleeker, combat and high speed stuff, use less diameter and more pitch.
I
can’t recommend modifying props unless you KNOW what you are doing, there are
so many prop sizes to pick from there should not be much of a need to cut on
them.