how much power needs a supercharger?

[wells]

Niklas,

The 605A vs 605 AS would be a better comparison.  In that case, the difference was 1475 hp vs 1435 hp, no?  The 605D has a higher compression ratio that increases the thermal efficiency, raising power output to 1550 hp.

[niklas]

Hi wells

This is exactly my problem.

The first D engine, developed in ī42, had a higher compression and 1550hp. And a critical altitude of 6500m - not much more compared to an 605A.

The later models of the D engine had a critical altitude of 8500m and 1435hp, like the AS. BUT afaik they still had the higher compression.

I have the impression that the first D engines did not have the larger supercharger at the beginning, but just an improved A-charger.

So what caused the later D engines to drop from 1550hp to 1435hp? It can only be the installation of the larger supercharger. But the difference of over 100hp is very much imo.

Thatīs why i started this threat, basically. Does a supercharger of a 603 engine needs 115hp more power compared to the smaller one of the 605??

niklas

[chisel1]

At sea level I would not think a larger Blower would account totally for the Max HP differences, since they were'nt direct drive (slip clutch). There would be some friction losses tho.

Ignition timing could have been changed, maybe running a slightly different camshaft timing?

I dont know tho  

[HoHun]

Hi Niklas,

you could try to estimate the power requiry of the superchargers by the following approach:

A piston engine requires a certain air flow for a certain power output. A figure quoted by "Motorcycle Turbocharging, Supercharging & Nitrous Oxide" is 160 CFM (ft^3/min) per 100 HP - about 0.56 L/s/kW. (You could verify this by stochiometric calculations.)

The work spent compressing the intake air is described by

W = (p1 * V1 - p2 * V2) / (1 - y)

p1 is static pressure, V1 the intake volume. p2 is boost pressure, V2 the resulting volume of the compressed air. y is the thermal capacity of an ideal gas (1.401 for N2, 1.400 for O2).

V2 has to be calculated, too:

V2 = (1 + 1/c) * D * T * f * 0.5

D is engine displacement, c is compression ratio, f the crankshaft frequency ("RPM" expressed in Hz), T the examined time interval. 0.5 is a factor taking into account that a four stroke engine's piston draws air only every second revolution.

You'd arrive at the effective work (power could be calculated easily) required to compress the intake air. However, it's the supercharger efficiency that determines how much power would be subtracted from the propeller shaft output, with centrifugal supercharger efficiency being about 60% - 70% (according to "Motorcycle Supercharging").

(You're going to have to use the power including the power loss for the above calculation, by the way, not just the crankshaft power.)

One word of caution: These are just a few formulae from books which I never tried to combine before. I might be applying them totally wrong :-)

(Don't plug in numbers in imperial units either unless you know what you're doing ;-)

Regards,

Henning (HoHun)

[F4UDOA]

Heya Gents,

There is a great artical in this months issue of AirPower/Wings magazine on Mechanical Superchargers (F4U,F6F and FW190A) vrs Turbo Superchargers (P-51, P-47 and P-38). It really lays out the differences and theory very nicely. A must have for the serious aviation crowd. Airpoweronline

It's on the News stands now!

[HoHun]

Hi Hitech,

>Turpo Chargers are much better for avaition. There limitation is much more do to reliabality then there performance.

Another limitation I just read about is the incompatibility of turbo superchargers with nitrous oxide injection. N2O as an oxydizer increases exhaust pressure without requiring the greater airflow which the turbo would inevitably produce as a result of the increase.

Since in aviation, the main benefit both of N2O injection and turbo superchargeing is improved altitude performance, this is not much of a concern though.

It seems to be more relevant for motor racers, however.

Regards,

Henning (HoHun)

[gripen]

Niklas,
You can't compare critical altitudes and outputs of the DB605 series without MAP and RPM values. Generally the DB605A and AS with B4 fuel were running (start and emergency power) 1,42ata@2800rpm and critical altitudes (rammed?) were 5700m for the A and 6500m for the AS. But sometimes ratings are claimed with combat power 1,3ata@2600rpm and then critical altitude is probably higher.

gripen

[niklas]

gripen - they all use the same MAP and RPM. The critical altitude for a AS engine is 7500-7800m.

reducing MAP increases the critical altitude. Reducing RPM decreases it. Thatīs why german engines often have similar critical altitudes with combat power compared to emergency power.

The question still remains for me: What caused the later D engine to drop from 1550hp down to 1435 with same MAP and RPM, when the AS engine lost only 40hp with the larger supercharger?

[gripen]

Niklas,
Well, decreasing MAP certainly raises critical altitude but there is couple examples (V-1710 and Merlin) where decreasing rpm raises critical altitude, but I don't know if the DB605 acted same way.

AFAIK the DB605D required the C3 fuel for the higher compression ratio and it used different MAP than the A or AS, something like 1,5-1,6ata. But there appear to be many different D versions. Anyway, those above 7000m critical altitudes are pretty certainly rammed.

gripen

[-ammo-]

Originally posted by wells:
In the case of the R-2800, the difference in manifold pressures for the same power is 52" vs 54" (P-47 vs Corsair), so you're looking at about 75 hp there.  

In the P-39 with mechanical supercharger, 51" gives 1325 hp @ 3000 rpm and with the P-38, the same power is attained with 47", so a difference there of about 110 hp.

The P-38 and P-47 use exhaust driven turbo-superchargers, so there's no load on the engine.

actually not quite true. While the engine is not working to turn the blower, the blower is causing considerable back pressure, and robs it of some of its aspiration. However the benefits great outweigh that in horsepower.

[niklas]

Originally posted by gripen:
Niklas,
Well, decreasing MAP certainly raises critical altitude but there is couple examples (V-1710 and Merlin) where decreasing rpm raises critical altitude

can you give me an example please?

 

AFAIK the DB605D required the C3 fuel for the higher compression ratio and it used different MAP than the A or AS, something like 1,5-1,6ata. But there appear to be many different D versions. Anyway, those above 7000m critical altitudes are pretty certainly rammed.

gripen

I never heard about the boost increase. Do you have some documents which prove that?
Those altitudes are not rammed (7500m AS f.e). Messerschmittīs team used rammed altitudes for performance calculations, see http://buerger.metropolis.de/luftwaffe/109g-k/109G-K_1.html

niklas

[gripen]

Niklas,
You can find a example for the V-1710 from the "Vee's for Victory" by Whitney. Open page 157 and look how 38,3"@2600rpm could be maintained up to about 25k while at 3000rpm the F20R could do just about 35" at that altitude. I've seen claims about similar phenomena with the Merlin on the Boscombe Down reports.

My information about the DB605AS and the D is from a article published in the "Suomen Ilmailuhistoriallinen lehti" and written by Hannu Valtonen. The Unrammed critical altitude for the AS should be 6900m? There seems to be also some errors in your web page for example combat power MAP (30min) for the DB605A is 1,3ata (not 1,35ata, this can be verified from the original manual).

gripen

[EDO43]

Jeez, all you engineering types worried about altitude performance....Look at it from a mechanics point of view.... Look at the impeller and diffuser and intake manifolds; if there are no cracks, metal to metal contact or other damage, and it works the way it's supposed to;  get in and fly.... If ya gotta  worry about gear ratios,back pressure and the like, join the test pilots in the aircraft test arena. (soon to be up and running.LOL)

Relax and have fun...no engine I've ever seen will produce sea level horsepower at 25k or better without a low tension ignition system.  What about that eh?  You pencil pushers think of that?  How many of the airplanes we have in the MA have low tension ignition systems?  I haven't experienced an incidence of flashover yet while at 25k or better...They must all have them...go figure.