High Altitude Heavy Bomber for RAF

[Shortround6]

The blowers on Merlin and Allison engines at the very least ran well into supersonic air-speeds.

They sometimes ran on the edge. In the pressures and temperatures inside the supercharger there was a higher limit as to what was supersonic vs in outside air. But once the impeller tips went supersonic in the 'atmosphere' of the supercharger things went to hell in short order. Hundreds if not thousands of mini-sonic booms per minute (second?) really disrupts airflow in the supercharger and the airflow through the supercharger and into the cylinders drops very quickly, which can lead to surging and other problems.

 

[Shortround6]

It really is an issue.
The original DB 605 AS had a critical alt of 26,000' with the single stage supercharger.
BTW, the old Rootes type blowers are simple but very inefficient.

Eng

The DB 605 AS was about as good as it got for a centrifugal compressor in WW II and it took several years after the war for any centrifugal compressor jet to do any better (I am not going to argue about 4.1 vs 4.2

for the Mercedes race car in the 1938 season they used two roots blowers operating parallel. Once they sorted out the 2 stage system (unequal sized blowers) they got 7% more power using 10% less boost. Power to drive the supercharger was reduced and net power was increased and power that achieved in the older engine at 8000rpm was now obtained at 7500rpm.
1939 boost was 2.31 Ata using the special racing fuel.

 

[Engineman]

This may be true according to certain research or conditions. But it was not what was being taught in some of the text books of the time or in industrial practice(air compressors for ventilation or industrial processes). Mercedes had used a 2 stage compressor in their 1939 Grand Prix car (both stages were roots superchargers).
Superchargers were not well understood in the 1930s as the fuel would not support high boost. In the US the R-2600 is often said to be the first Wright engine to use a Wright designed supercharger. Others claim it was one of later R-1820 engines. Wright was building superchargers (very low pressure ones) from the late 20s on, but they were GE designs and in fact GE was often supplying at least some of the components which were often built by Allison under subcontract.
Obviously what a lot of designers 'thought' could change very quickly. Allison, P&W and Wright all figured out that GE superchargers weren't very good at about the same time (1-2 years?)
Also superchargers have sort of a sweet spot as far as efficiency goes. On a compressor map
View attachment 841951
We can see that peak efficiency of a modern, well designed compressor can vary from 78% down to the high 50s depending on flow with the worst being at the extremes.
At least one text book from 1943 (a few years out of date so as not to give away any secrets) was using 65% efficiency in most of their examples on inter coolers and temperature rise in aux superchargers with altitude. Being American there was little about 2 stage mechanical superchargers and it was assumed there would be an intercooler to reduce the intake air temperature before it entered the 2nd stage.

Turbo for a car is rather simple. Supercharger for an airplane is harder in that the mass airflow (pounds per minute?) is also affected by the density of the air and efficiency at cruise may not be peak efficiency at max power.
A Merlin III supercharger was maxing out at about a 3 to 1 pressure ratio and it was the best in the world in the late 30s for a variety of reasons (bad intake for one) so even after Hooker worked on the supercharger for the Merlin XX/45 when he wanted a 5-6 to 1 pressure ratio for the Merlin 60 he had to go to two stages. But the small Merlin (and Allison) need a lot of supercharging to move the air needed to make high power.

The problem with an inefficient supercharger is that not only does it take more power to drive the supercharger (compress the air to degree you want) but that the excess power is converted to heat over and above the heat created by simply compressing the air.
And the heat goes all the way through the engine. If you increase the heat by 100 degrees in the intake manifold you increase the peak temperature in the cylinder by 100 degrees and the exhaust temperature goes up 100 degrees.

Yes, happy with all that.
The thought that two stages of easy working supercharge can be more "efficient" than a single stage providing the same flow and pressure may be due to the poor efficiency of the single stage in the example. Certainly, inefficient compressors are made.
Prof Dr-Ing. Kollmann was designing high tech compressors of the time.
Yes, the inefficiency will result in extra heating of the charge, over and above that incurred in compression without losses.
For info, the early DB600 supercharger was working at about 61% efficiency at Take-off power, by the time of the DB605A (1941) they had improved that to 71% at Rated altitude and 76% at Take-off.

Eng

 

[Engineman]

BTW, the compressor map is fine but, you only get the efficiency that coincides with the mass flow/pressure that the engine can demand. If the running line of engine demand falls outside the area of high efficiency, you get the low efficiency wherever the demand line falls. This is part of matching the compressor to the engine.

Eng

 

[Engineman]

The DB 605 AS was about as good as it got for a centrifugal compressor in WW II and it took several years after the war for any centrifugal compressor jet to do any better (I am not going to argue about 4.1 vs 4.2

for the Mercedes race car in the 1938 season they used two roots blowers operating parallel. Once they sorted out the 2 stage system (unequal sized blowers) they got 7% more power using 10% less boost. Power to drive the supercharger was reduced and net power was increased and power that achieved in the older engine at 8000rpm was now obtained at 7500rpm.
1939 boost was 2.31 Ata using the special racing fuel.

Here are compressor maps from Daimler-Benz for representative DB600 and DB 605A compressors. They include running lines for the engines and you can see that the DB 600 one
is badly matched on a fairly poor compressor, but the DB 605 A one is well matched on a much better performing compressor.

DB original, from the book TSCT by Calum Douglas

Eng

 

[Engineman]

For anyone wishing to expand their knowledge of WW2 Aero-engine tech, there are two indispensable books by Calum Douglas.
The first is, The Secret Horsepower Race (TSHR).

The second book is, Turbo/Supercharger Compressors and Turbines for Aircraft Propulsion in WWII (TSCT).

Both books are unsurpassed for original detail and depth of content. TSCT is based on the technical papers of Prof Dr-Ing. Karl Kollmann,
co authored by Calum Douglas and S Can GULEN.

Eng