Steve wrote:
Sport Pilot wrote:

The model
diesel engines will not turn as fast as the gas or methanol/nitro
engines because the fuel (ether and kerosene) will not burn as fast.


I say again: there is no magic rule that says "diesel burns slower" than
gasoline. Increasing the boost pressure can increase the burn rate AS
MUCH AS YOU WANT. Its just not done most of the time, because no one
WANTS a 7000 RPM engine when a 4000 RPM engine is available to do the
same job. People only build 7000 RPM engines when there's no other way
to get the power.

This is because the fuel burns slower,
not because of the cycle, because all, the spark ignition gas engine,
the glow plug methanol/nitro engine, and the compression ignition
engine, are actually otto cycle engines.

No, that's already been covered. Diesel (true diesel with either direct
or pre-chamber injection) is a subtly different cycle than the Otto
cycle. Its constant-pressure combustion instead of constant-volume
combustion.

I was not quite correct if I said desiel fuel burns slower, it is
however made to ignite better with desiel engines so that the fuel
ignites spontanously when injected. The fuel burns slower because it
is injected over a period of time. However, I may have been correct
about the model fuel. There are diesel conversion heads for model glow
engines. This replaces the head and glow plug with a head with a
varible compression. You adjust the timing by adjusting the
compression. The fuel is a mix of kerosene, ether, amyl nitrate, and
oil. This fuel will ignite at a much lower temperature than true
desiel fuel. The ether and amyl nitrate are the componets which ignite
at a low temp. Though ether is extremely volatile it is not a very
high energy fuel and I think it burns slowly. These converted engines
will turn large propellers at slower speeds than the glow engines they
were converted from.

Another example of a high energy fuel that limits speed is
nitromethane. Because it is a monopropellent the racers can pretty
much inject as much as their ignition can ignite, except for
hydrolocking. That is if more fuel is injected than the combustion
chamber volume the engine will blow up. But nitro burns slowly so RPM
is more or less limited to just over 10,000 RPM loaded. The slow
burning is why you see huge flames coming out the exhaust.