enginehistory.org

[rinkol]

I've come across a graph that plots the maximum power for selected DB 600 series engines. This graph is plotted with the altitude on what looks like a logarithmic scale. One thing that is curious is that the power curves of the DB 600G and DB 601A differ significantly from those of the later engines that also have single stage superchargers. For the earlier engines, the power curves show a gradual linear increase up to the rated altitude and then fall off linearly with a larger slope (I am excluding the short duration takeoff power which appears as an additional step near zero altitude for the earlier engines). The DB 601E and most later engines with single stage superchargers show a rather more complex behavior. The downward sloping line has a breakpoint so that the initial slope is low and then abruptly increases to a value that more closely corresponds to the behavior of the earlier engines above their rated altitude.

It is not obvious why the later engines behave differently. The DB 601E introduced an improved supercharger, which presumably accounts for the difference. However, I have not seen any details of the changes and they would appear to be somewhat subtle as the rated altitude did not change significantly in comparison with the earlier engines. Can anyone explain what is accounting for this behavior?

Robert

[szielinski]

Are you able to post those graphs?
(interested even if I can't tell you the answer...)
_________________
“Make everything as simple as possible - but not simpler” – Einstein.

[rinkol]

I could scan it, but there doesn't appear to be any way of embedding it in a post. the simplest might be to send it via email.

Robert

[jwells]

If it's any help, the graphs being refered to can be viewed in "Flugmotoren und Strahltriebwerke" by Gersdorff & Grasmann, 1995 p107.

Raymond Young (Wright Aero), 1941 says, "In the case of the DB 601A, there is no very definite critical altitude point and the power output fluctuates considerably in the altitude range where the automatic controls probably disturb each other with resultant hunting of the engine."

It is worth remembering that the fluid couplings in the DB 600 series engines were controlled by TWO pumps, so by jigging the speed or valving of the second pump, in particular, different performance curves could be obtained.

Jerry W.

[rinkol]

jwells

That is interesting. Possibly one other consideration is the interconnection of the supercharger control with the throttle. I suspect that, below the rated altitude, the throttle is used to fine tune the supercharger boost and not left in the fully open position.

One thing i was wondering was whether any of these superchargers had a feature similar to the "lock-up" mode of some automotive automatic transmissions where, under appropriate conditions, the torque converter will be bypassed via a clutch providing a direct drive arrangement. Above the rated altitude, this would avoid the power losses in the torque converter.

Robert

[kmccutcheon]

Links to the graphs Robert described:
http://www.enginehistory.org/Misc/Jumo211/jumo211_manual_cover_page.pdf
http://www.enginehistory.org/Misc/Jumo211/DB_60X.pdf
http://www.enginehistory.org/Misc/Jumo211/jumo211f.pdf
http://www.enginehistory.org/Misc/Jumo211/jumo211j.pdf
_________________
Kimble D. McCutcheon

[jwells]

Just responding to Robert's last comments, we have to get the terminology right here. To the best of my knowledge, TORQUE CONVERTERS were never used in piston aero-engine supercharger control mechanisms. The torque converter is a TYPE of fluid coupling which has vanes or STATORS between the impeller and the turbine. These are only used in automotive applications.

Regarding "lock-up" devices, again these are only found in cars and were only introduced in the 1970s when fuel shortages forced manufacturers to produce better fuel economies from their vehicles. Also, because of the presence of the stators in a torque converter, it can never achieve a better minimum slip value than a straight fluid coupling.

However, thanks to Robert for raising this matter and promoting this discussion - we really do need some of the German researchers to get stuck in and dig out all this info on the German engines that we lack (and, of course, publish their findings in English!)

Jerry W.

[rinkol]

jwells, thanks for the correction. I've seen the fluid drive described on the internet as being similar to the Dynaflow transmission from the 1950s, but this is not strictly correct.

Does anyone have any idea of the losses involved when the slippage is reduced to the minimum possible? DB seems to have stayed with the fluid drive, the engines which were exceptions (such as the DB 614 which had a three speed drive) seem to have all been abandoned.

Robert

[jwells]

The slippage losses for the DB fluid couplings are usually given as 2-3% when the coupling is full and operating at its maximum capacity.

Regarding DB engines which didnot have fluid drives, if you have a look at the top line on the graph you originally refered to, which is for the DB603N, you will note that the low-speed part of the curve is of the mechanical drive type (which it was) and the high-speed part is of the fluid drive kind. This would have been done to try and avoid the large slippage (and, therefore high heat production) produced by the coupling when it was trying to deliver a low supercharger ratio.

You really should get hold of a copy of the SAE paper on the DB601 engine listed in my post elsewhere on the "General Discussion" forum. This has lots of info on the fluid coupling.

[jjuutinen]

In short about the DB 605A:
-there were indeed two pumps supplying oil to the clutch
-the other pumped all oil to the coupling and while the other was barometrically controlled
-at Notleistung of 1.42 ata the throttle was fully open but below 2100 m an additional barometrically controlled throttle acted as an ABC
-from 2100 m to 5700 m at 1.42 ata the second pump oil flow increased gradually till maximum
-5700 m was the crit. alt.

[rinkol]

Following Jerry's advice, I've ordered the SAE paper on the DB 601A (and, for good measure, the Jumo 211B).

Robert

[cdouglas]

Hi,
The different graphs are due to the method of driving the supercharger.

The DB600 had a gear driven supercharger like the Merlin. Hence a straight line up, then a falling off.

The later engines all had fluid coupling driven SC. Which gives you a "two pointed" characteristic.

The initial rise, as the compressor is kept at its lowest level (about 0.7x crank rpm). Then the coupling is filled up slowly by oil via an air pressure bellows connected to an oil pump, and the impeller RPM is slowly increased, which gives you the "trough between the mountain peaks", then once the impeller is at 0.98x crank speed ( 2% hydraulic slip) power cannot be maintained and it falls away.

I suspect this graph (http://www.enginehistory.org/Misc/Jumo211/DB_60X.pdf) is somewhat "idealised", and should not be taken as being the exact curves for each engine.

I do not know with certainty, but I suspect that its possible that very ealy DB601a may have been made with gear-driven SC. However by 1939 the DB601a curve will look in shape like that shown for the 601E, 603 and 605 in that graph series.

The Jumo211F has a two speed gearbox SC drive (in working effect just like a Merlin XX), hence the shape of its curves with harsher peaks, as it has no way to "blend" the speed change, just 1...then 2.

Personally I hate log graphs, but when all you have is a pencil and a ruler, all graphs magically turn into log graphs !

Regards