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The
compression ratio on model engines actually controls the ignition
timing which is also effected by the nitro content of the
fuel.
A glow engine is similar to a diesel engine. The compression
ratio in model engine can be from as low as 7.5 to 1 to a
high of 9 to 1. The compression ratio varies with the type
of fuel, altitude, port timing, and exhaust configuration.
Higher performance engines hold very close tolerances to assure
the proper combustion point.
The
basic rules are as follows:
If
you engine is overheating...
It could be due to the compression ratio being too low for
the fuel you are using. Lower nitro content require higher
compression ratio. If the compression is too low the firing
of the combustion charge will be too late or retarded creating
poor burn and thus allowing a burning fuel charge to be released
out the exhaust. This elevates the temperature of the engine
and lowers the performance.
If
you increase the nitro content the combustion will be advanced
and can produce a more powerful and cooler running engine.
or
You
can increase the compression ratio by removing a head shim
(see also How to set squish
band).
NOTE:
No all engines use or require head shims. Many sport engines
only have a metal to metal (head to cylinder) fit without
head shims because they are usually run on less than 10-15%
nitro fuel which is more forgiving than higher nitro fuels.
On
the other hand if your compression ratio is too high the engine
will detonate. This the same as the ignition being too advanced
in you car or if you run too low of an octane rating. Like
when you car engine pings with regular gas.
If
you want to run higher nitro fuel to get more power you may
have to lower the compression ratio. You may need to add head
shims. Detonation is basically the fuel charge exploding before
the piston reaches the top of the stroke so the piston forces
the explosion to be contained without expanding. It has no
place to go because the inertia of the crankshaft is forcing
the the piston to move thru its proper course. Thus you get
a ping which is the sound of all the clearances in the connection
rod being driven together squishing the oil film out of the
bearing. If you have detonation you can destroy your piston
as the crown (or top) of the piston is subjected to extreme
heat and pressure because of the trapped exploding gasses.
Engines
are designed to have the combustion expansion drive the piston
down the cylinder, not have it trapped.
Note
the higher the nitro content is not like a higher octane gasoline
rating. Higher octane gasoline requires higher compression
ratio as it actually burns slower than regular gas and requires
more compression to ignite properly. Nitro actually lowers
the compression ignition point of glow fuels.
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