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> Way to go,
Rusty.
>
> I'm no engineer, but I'm a pretty fair troubleshooter
& recognize the
> need to ask the right questions & properly
evaluate empirical evidence.
>
> In the '60s when I paid attention
to auto racing, there were gas
> dragsters & then there were alcohol
dragsters. (then nitro dragsters,
> but that's hardly relevant to
aviation) The alky cars were noticeably
> faster, enough for a separate
class. Indy cars in the '60s & early '70s
> were alky powered. Don't
know if that's still the case. This seems to
> run counter to
'conventional wisdom' I'm seeing here. (Sorry; I'm
> currently reading
'Freakonomics' & couldn't resist.)
>
> The knocks against
alcohol for potential damage to engines/fuel system
> components is deja
vu to unleaded gas in the '70s. Anybody having
> problems with no-lead
gas in their cars today?
>
> Charlie
> (trying for
whitecaps)
Glad to see you back on line, Charlie even if rooting for
that trouble maker, Rusty {:>).
As best, I recall the one advantage to using alcohol
as a fuel is its resistance to detonation enabling much higher compression in
these engines. The alcohol normally used is methanol
rather than ethanol. Pure methanol has been used in indy
cars since the mid-1960s mainly for its high octane
rating of about 135., so less likely to detonate under high compression.
Also as the following extraction will explain better than I, other fuel
characteristics that enable these engines to produce so much power using
methanol. In summary, You can run the engine much richer, alcohol is
better at cooling the charge density, much higher compression piston (and/or
boost), etc., than you can with gasoline.
HOWEVER, the bottom line is it
still takes a much greater quantity of alcohol than gasoline to produce the same
amount of power. (Nitromethane is a bit different story). Now if you
can get a 15:1 or better compression ratio out of your rotor, are prepared to
run a blower or turbocharger and can fit extend capacity fuel tanks to your
bird then you will likely produce more power.. Otherwise, I don't believe
it will.
Ed A
Extracted from Internet Sources:
Now unlike petrol you will find alcohol fuel will
continue to provide increased power for a mixture well above the ideal mixture
strength and you can always tend, therefore, to jet up on the rich
side, and so avoid any possible chance of running into troubles through weak
mixture causing burnt valves and holed pistons.
This larger amount of
fuel compared to petrol and especially as it is a fuel with much higher latent
heat value tends to do two things. The density of the charge entering
the engine is higher than petrol and a greater weight of mixture is therefore
being exploded.
This is a fuel with a large cooling effect
provided by part of it evaporating after it has reached the combustion chamber
and so tending to cool the valves, piston and so on.
Some may well get
into the combustion chamber as liquid, due to the reduction in temperature of
the induction system, pipes, carburetor, etc., and so extending the cooling
effect, in the process counteracting the effect of the high internal
temperature.
In view of this amount of fuel entering the chamber, with
possibly some of it in liquid form, the ignition system must be beyond reproach
since if the spark is weak the mass of fuel will just soak the plug and then at
once ignition troubles arise affecting starting in
particular.
Owing to the use of alcohol a higher compression
ratio can be used with this fuel as compared with petrol, another
consideration is the type of plug used which will be a hotter type than used
before with petrol.
NINETEEN TO ONE
We have just mentioned the
higher possible compression ratio used with alcohol and the limit that can be
used with any particular fuel depends on the tendency of the fuel to
detonate.
As a rough guide the ratio for petrol is limited to
about ten to one, or with certain additives to as much as 12 to one. With
alcohol, however, you can go up to 19 to one or higher in certain
cases. (For all practical purposes however, 14 to one should be
considered the maximum usable ratio in modern short stroke automotive
engines.)
The possible use of a much higher
ratio, of course, means we get a higher power output from the engine, and this,
in fact, is almost the main advantage of alcohol
fuel.
DETONATION
Detonation with alcohol fuel is
really not a problem, but pre-ignition is, or could be unless the mixture is
kept well on the rich side.
The reason for this is that if the mixture is
on the weak side it burns slowly and can still be so doing when the exhaust
valve has opened which then becomes overheated. This in turn ignites the next
charge before the correct time, the whole process becoming a chain reaction
causing even more rise in temperature and so it goes on until the piston holes
and other damage then follows.
The first signs of this process taking
place are a loss of power, a general rise quite quickly of overall temperature,
the head in particular.
To avoid this, run on the rich side always and
use plugs with a good heat capacity.
It might be worth mentioning at this
point that an engine set up correctly for running on alcohol, even though on a
rich mixture, will be found to be (compared to petrol), a much cleaner running
engine inside the cylinder head, and provided the ignition side is up to its job
there will be less fouling of plugs than on petrol.
Regarding Nitromethane:
Note how
with 40 per cent nitromethane mixture the jet size has increased by 1.41, or put
another way by 40 per cent on the diameter, which as mentioned before means an
actual fuel flow of twice the original amount, so by comparison with petrol we now have four times as much fuel
required by the engine.
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