Scott Dorsey wrote:
They are used (beyond their designers' wildest dreams, in fact) every
year at the Reno National Championship Air Races ;-)
Advancing the timing wasn't the goal, raising the BOOST PRESSURE from
supercharger or turbochargers was the goal. Advancing timing is
counter-productive beyond a certain point because combustion is just
pushing "the wrong way" on the piston. But raising boost pressure is
advantageous in many ways. The problem is that thanks to good ol PV=nRT,
compressing the intake charger with a super- or turbo-charger raises the
temperature of the intake charge. Too high an intake temperature results
in detonation regardless of the ignition timing, and can also result in
manifold explosions. Aftercoolers (air-to-air or air-to water) help, but
are limited in how much heat they can reject and also add weight to the
aircraft. Water injection in WWII aircraft engines primarily reduced the
temperature of the intake charge to a manageable level because the
evaporating water absorbs a LOT of heat (latent heat of vaporization.)
Running the engine excessively rich also does the same thing using the
fuel itself to absorb heat, but the drawback is that over-rich
combustion produces less power than stoichiometric combustion.
In addition, the water adds inert gas to the combustion cycle, and that
slows down the flame front and reduces the peak temperature in the
cylinder and further suppresses harmful detonation of the type where a
pocket of charge in a remote corner of the cylinder self-ignites after
the primary flame-front starts increasing cylinder pressure. Modern
automobile engines (since the late 70s) actually use a metered dose of
their own exhaust gas (exhaust gas recirculation or EGR) to do this
(slow combustion, reduce peak combustion temperature) as well as to
reduce nitrogen oxide emissions.
The big drawback to water injection is maintenance. Corrosion in the
plumbing, pumps, valving, etc. is a big problem. Also, the water/ethanol
mixture has to be replenished along with fuel and oil and it has to be
PURE water which requires reverse-osmosis, distillation, or at least
deionization. Otherwise deposits build up in the system and in the
engine. Water injection really isn't practical for cars, especially
since most drivers can't even be persuaded to check their own oil these
My understanding is that the water/alcohol is fully vaporized well
before it passes through the intake valve(s).
Did they ever? Ah, for the good old days when the guy at the pumps always
asked if I wanted the oil checked...
Derek Broughton wrote:
Yeah. The summer of 1975...
I miss that. I'm sure some folks DON'T miss the occasional crook at the
pumps who made sure to put the stick in "short" so it read a quart low,
the guys who dropped antacid tablets in the battery and said that the
foam was due to the alternator overcharging the battery, etc. etc. etc.
Gladly those guys were a small minority, but they were out there!
Ah, yes. Those were the days when I didn't ever have to check my oil -
every 100 miles it needed another quart.
Well, me neither, but it does come as a huge surprise these days when
Two could play at that game. I remember when you could reset the
counter on the gas pump ever $.00 just by flipping it off and on.
That way a fillup only ever costed $.00
Oh wait a minute, it only cost $.00 to fill up then anyway. Sorry...