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On Tue, 16 Oct 2007 09:52:38 -0700, (Harry
Andreas) wrote:

In article , "Ed Huntress"
wrote:

"Harry Andreas" wrote in message
...
In article , "Ed Huntress"
wrote:

I don't recall the details on water injection, which has been used on
aircraft and racecars for 50 years or more to prevent knock, but a little
voice in my head tells me that the effect of water injection is to slow down
combustion, which results in higher unburned hydrocarbon emissions.

I recall reading about aircraft in the 1930's that used it...

Water injection is a way to increase the density of the air that's going into
a piston engine. Higher density in a fixed volume yields more mass in the
combustion chamber, which results in higher BMEP which is more efficient.
The water also partially counteracts the heat of compression by vaporizing
which lowers the inlet temperature, which is also more efficient.
Most important from a practical POV:
Because the charge temp is lower, peak combustion temp is lower, so it
allows the use of lower octane fuel for a given compression ratio.

IIRC, combustion speed has little to do with formation of unburned HC or
NOX (unless you're thinking of knocking, or pre-detonation).
Unburned HC and NOX are primarily functions of combustion temperature.
Higher temp reduces unburned HC
Lower temp reduces NOX

If you
don't raise compression ratios to the degree that water injection (or
methanol injection, or ethanol injection) is required, all you do by
injecting water is to make the engine behave as if it's running too rich.

In a fully compensated fuel injection system that's true. The system only
sees higher mass flow and does not know it's partly water, so it injects
the full amount of fuel required for that mass flow, and there's not enough
free oxygen to fully combust all the fuel.

Water injection was used in various aircraft until the 1960's.
According to the A.F. tech training it cooled combustion chambers and
allowed more power to be generated. An R-4360, for example, was rated
at 3500 H.P dry and 3750 H.P with water injection.

The fluid injected was a mixture of water and menthol and fish oil.
the menthol was to prevent the mixture from freezing at altitude and
the fish oil prevented corrosion.

Bruce in Bangkok
(brucepaigeATgmailDOTcom)

John

wrote:

On Tue, 16 Oct 2007 09:52:38 -0700, (Harry
Andreas) wrote:

In article , "Ed Huntress"
wrote:

"Harry Andreas" wrote in message
...
In article , "Ed Huntress"
wrote:

I don't recall the details on water injection, which has been used on
aircraft and racecars for 50 years or more to prevent knock, but a little
voice in my head tells me that the effect of water injection is to slow down
combustion, which results in higher unburned hydrocarbon emissions.

I recall reading about aircraft in the 1930's that used it...

Water injection is a way to increase the density of the air that's going into
a piston engine. Higher density in a fixed volume yields more mass in the
combustion chamber, which results in higher BMEP which is more efficient.
The water also partially counteracts the heat of compression by vaporizing
which lowers the inlet temperature, which is also more efficient.
Most important from a practical POV:
Because the charge temp is lower, peak combustion temp is lower, so it
allows the use of lower octane fuel for a given compression ratio.

IIRC, combustion speed has little to do with formation of unburned HC or
NOX (unless you're thinking of knocking, or pre-detonation).
Unburned HC and NOX are primarily functions of combustion temperature.
Higher temp reduces unburned HC
Lower temp reduces NOX

If you
don't raise compression ratios to the degree that water injection (or
methanol injection, or ethanol injection) is required, all you do by
injecting water is to make the engine behave as if it's running too rich.

In a fully compensated fuel injection system that's true. The system only
sees higher mass flow and does not know it's partly water, so it injects
the full amount of fuel required for that mass flow, and there's not enough
free oxygen to fully combust all the fuel.

Water injection was used in various aircraft until the 1960's.
According to the A.F. tech training it cooled combustion chambers and
allowed more power to be generated. An R-4360, for example, was rated
at 3500 H.P dry and 3750 H.P with water injection.

The fluid injected was a mixture of water and menthol and fish oil.
the menthol was to prevent the mixture from freezing at altitude and
the fish oil prevented corrosion.

Bruce in Bangkok
(brucepaigeATgmailDOTcom)

Im more familiar with the R2800 CB16 (Military R2800 W97) that had
water injection. The alcohol water mix was called ADI fluid, anti
detonation injection fluid. As you advanced the throttles and passed an
absolute manafold pressure of 52 inches the sustem would lean out the
engine and inject the ADI fluid into the engine. With the ADI fluid
injection you could go to an absolute manafold pressure of 56 inches.
The ADI prevented detonation with the increased manafold pressure. The
problem was that you needed the hot fuel with 145 octane rating that
was hard to come by at most airports.
The common practice was to keep one set of tanks with the high octane
fuel for takeoff and quickly switch over to the 100LL fuel after backing
off the throttles.

John

[email protected]

On Tue, 16 Oct 2007 22:55:36 -0400, John
wrote:

wrote:

On Tue, 16 Oct 2007 09:52:38 -0700, (Harry
Andreas) wrote:

In article , "Ed Huntress"
wrote:

"Harry Andreas" wrote in message
...
In article , "Ed Huntress"
wrote:

I don't recall the details on water injection, which has been used on
aircraft and racecars for 50 years or more to prevent knock, but a little
voice in my head tells me that the effect of water injection is to slow down
combustion, which results in higher unburned hydrocarbon emissions.

I recall reading about aircraft in the 1930's that used it...

Water injection is a way to increase the density of the air that's going into
a piston engine. Higher density in a fixed volume yields more mass in the
combustion chamber, which results in higher BMEP which is more efficient.
The water also partially counteracts the heat of compression by vaporizing
which lowers the inlet temperature, which is also more efficient.
Most important from a practical POV:
Because the charge temp is lower, peak combustion temp is lower, so it
allows the use of lower octane fuel for a given compression ratio.

IIRC, combustion speed has little to do with formation of unburned HC or
NOX (unless you're thinking of knocking, or pre-detonation).
Unburned HC and NOX are primarily functions of combustion temperature.
Higher temp reduces unburned HC
Lower temp reduces NOX

If you
don't raise compression ratios to the degree that water injection (or
methanol injection, or ethanol injection) is required, all you do by
injecting water is to make the engine behave as if it's running too rich.

In a fully compensated fuel injection system that's true. The system only
sees higher mass flow and does not know it's partly water, so it injects
the full amount of fuel required for that mass flow, and there's not enough
free oxygen to fully combust all the fuel.

Water injection was used in various aircraft until the 1960's.
According to the A.F. tech training it cooled combustion chambers and
allowed more power to be generated. An R-4360, for example, was rated
at 3500 H.P dry and 3750 H.P with water injection.

The fluid injected was a mixture of water and menthol and fish oil.
the menthol was to prevent the mixture from freezing at altitude and
the fish oil prevented corrosion.

Bruce in Bangkok
(brucepaigeATgmailDOTcom)

Im more familiar with the R2800 CB16 (Military R2800 W97) that had
water injection. The alcohol water mix was called ADI fluid, anti
detonation injection fluid. As you advanced the throttles and passed an
absolute manafold pressure of 52 inches the sustem would lean out the
engine and inject the ADI fluid into the engine. With the ADI fluid
injection you could go to an absolute manafold pressure of 56 inches.
The ADI prevented detonation with the increased manafold pressure. The
problem was that you needed the hot fuel with 145 octane rating that
was hard to come by at most airports.
The common practice was to keep one set of tanks with the high octane
fuel for takeoff and quickly switch over to the 100LL fuel after backing
off the throttles.

John

It was ADI on the R-4360's too. I just used the phrase "water
injection" because the other guy did. And you are correct that it
allowed an increase in manifold pressure before detonation took place.
The explanation in Tech training was that the water vaporized and
cooled the combustion chamber thus allowing more air and fuel to be
burned, thus more power.

Since I was in the A.F. at the time 115/145 (lean/rich octane values)
was the standard fuel, none of this richman/poor man fueling :-)

Bruce in Bangkok

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