Mike Rapoport wrote:


"Bill Daniels" wrote in message
...

"Mike Rapoport" wrote in message
ink.net...
Water injection does not increase efficiency, it lowers it. The
water
goes
in as a liquid and goes out as a gas. The energy to do that comes
from
burning fuel. It will always take more fuel to produce a given
amount of
power with water injection than without. Water injection does
allow
higher
MP or higher compression so the engine can produce more power.
Piston engine fighters used it for more peak horsepower and some
turbines use it for the same purpose but it definately come at the
price of higher fuel
burn
per horsepower.

Mike

Hmm, consulting my ancient copy of Ricardo's "High Speed Internal
Combustion
Engines", Sir Harry said that water injection can be substituted
for any excess fuel consumed for the purpose of reducing cylinder
temperature and/or
increasing detonation margin. Further, evaporation of the water
reduces the
intake charge temperature so as to reduce pumping losses. He goes
on to say
that, while there is energy lost to evaporating the water droplets,
the overall fuel economy of an aircraft engine at max power setting
will be improved by use of water injection particularly if the
compression ratio has
been increased to take advantage of the increased detonation
margin.

Bill Daniels


Yes, water injection can replace fuel used for cooling. I was not
precise
enough in my wording. In the case of using water injection at lower
power settings (where excess fuel for cooling is not used)
efficiency will be
reduced. I used water injection in a Corvette that had 11:1
compression to
stop detonation. It worked but power was definately less than with
high octane gasoline and without water injection.

That's because you weren't running a lean mixture at high manifold
pressures while the water was going in. An integral part of ADI
(Anti Detonant Injection) systems on the big reciprocating airplane
engines was that the mixture would be leaned much closer to
stochiometric, and specific power and fuel burn would increase.
For example, the Wright R3350-32WA used on the P2V Neptune patrol
airplane, and on most Constellations and DC-7s, burned 45 lbs/minute
at max Dry Power (277 BMEP/2900 RPM), 'bout 3400 HP. The equivalent
info with ADI operating was 34 lbs/min at 301 BMEP/2900 RPM, giving
3700 HP. These engines generally ran on 115/145 Octane fuel.

While you can make water injection work by just dumping water in,
you don't get the full benefit unless you can adjust the fuel flow
properly.

--
Pete Stickney
Java Man knew nothing about coffee.