Question about water injection

[tomo pauk]

By looking at power diagram of R-2800-8W/-18W (at America's 100K), the effects of water injection WER are visible at all heights, ie. even above full throttle height. For Allison V-1710-93 (P-63A), the water injection manages to help extracting more HP only under full throttle height.
Was the R-2800 really so super-engineered, or the graph from the book is wrong?

 

[johnbr]

It was well engineered because they spent a ton of money on it.

 

[Shortround6]

Which graphs are you looking at? page numbers please.

 

[billswagger]

.
Was the R-2800 really so super-engineered, or the graph from the book is wrong?

The R-2800 was not any more engineered than other engines but probably well suited for a war time airframe of that era, although use of the R-2800 continued on airliners well into the 1970s.
The introduction of 150/100 gas and water injection allowed higher outputs.
Its also a relatively large radial engine that lent itself well to water injection and high manifold pressure, however even with out an ADI system, the R-2800 still produced 2000-2100 hp with 130/100 gasoline.
Consistency also depended on turbo/supercharging which would also contribute to changes in full throttle height.

The engineering of the engine to handle water injection also took time to develop and understand, mostly dealing with changes in timing and engineering of inlet parts to handle higher pressures. The basic engine was still the same though changes from variant to variant probably differed depending on use, such as parts engineered for turbo or supercharging, reduction gear ratio, prop disc, endurance....etc.

I've only seen a handful of charts that deal with the subject of water injection. They all differ depending on the day the testing took place. I would not question whether a chart is wrong, but what the conditions of the test were and most publishers fail to or lack those details when they choose to include charts.










Bill

 

[tomo pauk]

Bill, I can agree with what you say, but that still doesn't answer my question

Which graphs are you looking at? page numbers please.

Pg.407 (V-1710-93), pg. 510 (R-2800s).

 

[Shortround6]

There can be confusion between military power and "combat power" and WER ratings. Military power, while it may have a 5 minute rating, is not "combat" or WER power. Military power can be used, subject to the time limits and engine temp, in a somewhat unrestricted manner. The use of military power does NOT have to noted in log books or call for special maintenance procedures.
Unlike the Allisons and Merlins it appears ( I could be wrong on this and welcome a correction) that the R-2800 did not have a WER rating without ADI.
It also appears that the navy R-2800s with two stage superchargers had extra supercharger capacity. That is they could supply more air than the military ratings needed. Maybe that is 'over engineering'?
The R-2800 may have been limited by detonation limits (and cooling limits) rather than throttle settings/openings.
The R-28008W with ADI only has a small increase in power over it's military rating. On the order of 10% or so. it goes from 53in manifold pressure to 59in.
The Allison -93 goes from 1325hp to 1500hp dry (13%) to 1820hp (37%) wet. manifold pressure goes from 54in to 61in to 76in. It is little wonder that the Allison could not maintain 76in of manifold pressure at higher altitudes. The Allison also did not have an intercooler. It was depending on ADI for all the cooling of the intake charge as the the super chargers compressed (heated) the air as the plane climbed.
At 22,000ft the air has a pressure of 12.6in and a density of just about .5 . The R-2800-8W at it's military rating of 52.5in is compressing the air 4.16 times but has the aid of the intercooler to lower the charge temperature. The Allison -93 at 21,500 (close enough) was also compressing the air 4.12 times for it's military rating of 1180hp at 52in.
The Allison was also rated at 1150hp at 25,000ft at 50in of manifold pressure which was 4.5 times the surrounding air. This may have been it's "full throttle" hight.

 

[Shortround6]

Pg.407 (V-1710-93), pg. 510 (R-2800s).

As I tried to point out above (perhaps not very well) there may have been differences in the engine set-ups. the lack of the intercooler hurt the P-63 in the 20,000ft an up area because of a lack of charge cooling needed with the high compression ratios needed in the superchargers. The Allison needed a lot of boost to make it's WER settings, the R-2800 didn't need much and may have had it in hand. P&W may have been using the same engine mounted supercharger (second stage on the -8) as they use on the P-47 turbo engines (capable of providing 54in or more of boost at sea level) or the two speed single stage bomber engines.

 

[tomo pauk]

Many thanks, your responses do make a picture more clear

 

[billswagger]

Bill, I can agree with what you say, but that still doesn't answer my question

Typically, WEP rating has a lower FTH unless the throttle arm and inlet is also adjusted. Increased turbo efficiency can also change FTH.
In that case you might have a chart that reveals stats for two different aircraft.
One with out WEP and the other with WEP.

If it were the same aircraft, FTH would be marked depending on the HG setting.

An easy example is a P-40N chart that reveals WER FTH at only 9,200ft (57") but the military rating FTH is at 12,000ft (48").
http://www.raafwarbirds.org.au/targetvraaf/p40_archive/pdfs/P40N RAAF A29-412 418 Test.pdf


Charts are good for comparison reasons but also note that they depend on conditions such as the atmosphere and ram air intake of the engine.
Its possible the same plane can provide a different FTH at different speeds on different days.

 

[tomo pauk]

The P-63 example follows what you've wrote - WER beats MIL, but one needs to fly lower. The ADI increases WER power, again UNDER full throttle height.
Both engines from OP are mech supercharged, so turbo is here out of equation.

The R-2800 for Naval birds, at lest that's stated in America's 100K, allows WER (with ADI injection) at ALL heights (up to 30kft +) , beating them, power-wise, some 10-15% in same height. That stands in contrary with what I've thought and what you've just wrote