Mechanical Superchargers

[PhilPie]

I have a question about 2 stage mechanical superchargers - such as used in the Merlin P&W R-2800.

In a two stage - two speed supercharger, the first stage has two speeds. But I have never been able to determine if the second stage had only one speed or two speeds.

Any help?

 

[P-STICKNEY]

I have a question about 2 stage mechanical superchargers - such as used in the Merlin P&W R-2800.

In a two stage - two speed supercharger, the first stage has two speeds. But I have never been able to determine if the second stage had only one speed or two speeds.

Any help?

THe answer, as with pretty much anything else in aviation, is "That Depends."
Some 2-stage engines, like the Rolls Merlin 60 series and up, and the Rolls Griffon 60 series and higher, had the 2 supercharger iimpellers on the same shaft. This is simpler mechanically, and both impellers turn at whatever gear ratio is selected. But - it's much more complicated in terms of performance - the output (Pressure Rise and Mass Flow) of the 2 supercharger impellers needs to be carefully matched, and in this case, matched throughout their range of operating speeds (Both in terms of the gear ratio and the RPM range of the engine.)

It's an enormous tribute to Stanley Hooker of Rolls-Royce that he was able to not only make these systems work, but work at a high efficiency (SInce the overall efficiency is the product of the efficiencies of the two individual impeller/diffuser pairs.

A similar setup was used on the Daimer Benz and Junkers 2-stage engines - the DB605L, DB603L, and Jumo 213E. With their transverse superchargers, there wasn't room for 2 drive systems. Unfortunately, there's not a lot of good information on the internals of these superchargers - blower diameters and drive ratios and such - so a detailed analysis isn't possible.
(Other than the general statement that German superchargers lagged far behind everybody else in efficiency and output - which was balanced by the large displacement of their engines)

The approach used by Pratt Whitney on both the 2-stage R2800s and 2-stage R1830s was to have an independant drive for the initial (Auxiliary) stage, and a fixed single-speed drive for the second (Main or Engine) stage. The drive to the Auxiliary stage had 3 ratios, Neutral (No drive), Low and High) giving 3 critical altitudes for each engine at a particular power setting. In Neutral drive, the induction systems bypassed the Aux stage and its intercoolers, and fed the main stage directly. Diagrams of this can be found on the "Intallations" page of the Aircraft Engine Historical Society (AEHS) web site:
Installations

Allison used a somewhat similar setup for its 2-stage engines (G series, mostly)
A drive shaft from the back end of teh engine ran the input end of a hydraulic variable speed drive that ran the auxiliary stage. This drive was controlled to feed a certain delivery pressure to the engine stage., so the speed of the Auxiliary Stage would continuously vary from a low speed at low altitudes to its max speed at whatever altitude it could deliver the manifold pressure called for by the pilot's throttle setting.

By the way, that's also how turbosupercharged engines work - All large engines
(> 600 HP) and most of the smaller ones had their own integral mechanical superchargers. (If they weren't developing much boost, they were there to ensure that the fuel/air mixture was as evenly spread through the induction system as possible)
The turbosupercharger fed air at some pressure slightly over sea level (So that the turbo was turning at all times) to the engine's mechanical supercharger.

A drawback of the infinitely variable speed hydraulic coupling and turbosuperchargers was that the impeller/diffuser combination (A supercharger works by using the impeller to speed up the air that's fed to it (Increasing Kinetic Energy), and the diffuser outside the rim to slow down the air, converting the speed into pressure (Potential Energy)
One can't work without the other, and both perform about half the job each.
This means that the Aux Stage has a somewhat lower efficiency, since it can't be opimized for a small set of speeds like a directly connected gear driven blower.

Hope this helps

--
Pete Stickney

 

[pasoleati]

An excellent post, Pete! Good enough that I don´t need to add much except that the statement on German superchargers is perhaps a little over the top. After all, the Jumo 213 equalled the wartime performance of the Griffon with slightly lesser displacement...