Speed vs. height curves and the nature of supercharging.

pathology_doc

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Me110G-level-speed.jpg




I note, particularly, the Me110G curve at the far left - zigzag pattern up to what we might as well call MS and FS gear full-throttle heights, and the gradual die-off thereafter - compared with the more or less unbroken and much smoother curve labelled "Fw190 n=2500" at the right of the graph.


I've also seen Messerschmitt 109 curves with similar patterns, e.g.:


Me-109E3-Russian.jpg







From what little I've read of German aero engines, I gather this is due to the nature of the supercharger drive - hydraulic versus direct belt or gear. I have also seen, elsewhere (print book, no link available) a similarly smooth-ish curve for the P-47D, which of course is turbo-exhaust driven. The latter is obvious - as atmospheric pressure drops off, the difference between exhaust manifold pressure and ambient will be greater, which will provide more oomph to the turbine drive. I'm gathering there's something about the construction of the hydraulic "clutch" in the German engine which provides a similar effect. Am I on the right track?


The curve for the Mig-3 above (AM-35A engine) shows a similar unusual appearance - either a single-stage supercharger with a very high rated altitude or something exotic going on.


Can anyone provide further details?
 

iverson

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pathology_doc said:
I note, particularly, the Me110G curve at the far left - zigzag pattern up to what we might as well call MS and FS gear full-throttle heights, and the gradual die-off thereafter - compared with the more or less unbroken and much smoother curve labelled "Fw190 n=2500" at the right of the graph.

I've also seen Messerschmitt 109 curves with similar patterns, e.g.:

From what little I've read of German aero engines, I gather this is due to the nature of the supercharger drive - hydraulic versus direct belt or gear. I have also seen, elsewhere (print book, no link available) a similarly smooth-ish curve for the P-47D, which of course is turbo-exhaust driven. The latter is obvious - as atmospheric pressure drops off, the difference between exhaust manifold pressure and ambient will be greater, which will provide more oomph to the turbine drive. I'm gathering there's something about the construction of the hydraulic "clutch" in the German engine which provides a similar effect. Am I on the right track?


The curve for the Mig-3 above (AM-35A engine) shows a similar unusual appearance - either a single-stage supercharger with a very high rated altitude or something exotic going on.


Can anyone provide further details?

WW2 superchargers were all gear-, hydraulic, or exhaust-turbine-driven. There there no belts strong enough.

The compressors were of the centrifugal type, rather than the Roots or vane types used in trucks and racing cars. This means that their output increases rapidly as the rotational speed of the compressor increases. They are inefficient at low speeds and very efficient at high speeds. So the rotational speed and size of the compressor means that the engine produces maximum output at one and only one "rated altitude".

To overcome this limitation, conventional superchargers could be given two or three different gear ratios, each suited to a different critical altitude. This is essentially the same principle as a car or bicycle transmission: you use gearing to turn the available crankshaft speed into the optimum compressor speed for two or three critical altitudes. This worked wonderfully in the Rolls-Royce Merlin. But it was still a relatively inflexible way to manage power in an aircraft that had to operate from sea level on up. For this reason, special low-level engines with single-speed drives and reduced-diamter ("cropped") impellors had to be developed for close-support, the Navy, and chasing V-1s.

The turbocharger automatically adjusted its output for altitude for the reasons you describe: at higher altitude the the turbine spun faster and compressed the lower density air to a greater degree than it would at lower altitude. This seemed like the ideal--hence the USAAF preference for it. But developing materials capable of withstanding the exhaust heat of a gasoline engine proved difficult and charge heating--with consequent low volumetric efficiency--was a problem until well after the war.

The hydraulically driven Daimler-Benz superchargers coupled a barometric controller with an infinitely variable hydraulic supercharger drive that performed, ins ome ways, like a turbocharger. The speed of the compressor was not limited by fixed gear ratios--it had the equivalent of automatic transmission. At low level the compressor turned relatively slowly and delivered low boost. At high altitude, the compressor sun rapidly and delivered much more boost. The downside of this approach would be less efficient conversion of engine power to boost (due to heating of the hydraulic fluid). The variable drive was probably made the DBs less sensitive to fuel quality than the Merlins were--an important feature for Germany.

The Fw190 had a fairly conventional geared supercharger, as near as I know. They were designed for medium-altitude operations and lacked power at high altitude. For high-altitude aircraft like the the Ju388, BMW developed a turbocharged BMW 801 TJ late in the war.

The MiG's Mikulin engine was indeed a conventional "supercharger with a very high rated altitude". The superharger was geared, but the ratios were chosen to deliver maximum boost at maximum altitude and the compression ratio was lowered to accommodate high boost at rated height. I have read that this made the MiG-3 something of a dog at lower levels, where the supercharger was less efficent and consumed more power. This lack of flexibility made the MiG less than useful, because, on the Eastern Front, air battles were almost always fought at low levels.

The early Allison Mustangs, on the other hand, made their name as tactical reconnaisance because the Allison was designed without a decent gear-driven supercharger. The USAAC thought that the turbocharger was the only way to get power at altitude. As result, the Allison was a strictly low-altitude engine that was no good in foghters. But its breathing was unrestricted and no power was lost to supercharging, so nothing could catch it on the deck.

Eventually, Allison developed its own hydraulically driven centrifugal supercharger as an alternative to the problematic turbochargers.
 

Granit

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pathology_doc said:
The curve for the Mig-3 above (AM-35A engine) shows a similar unusual appearance - either a single-stage supercharger with a very high rated altitude or something exotic going on.

Can anyone provide further details?

AM-35A had a supercharger with inlet guide vanes with rotary blades designed by Polikovsky. It worked like a hydraulic drive on German engines.
 

pathology_doc

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iverson said:
There were no belts strong enough.


Noted. I was mostly trying to contrast direct engine drive with other forms.

The compressors were of the centrifugal type, rather than the Roots or vane types used in trucks and racing cars. This means that their output increases rapidly as the rotational speed of the compressor increases. They are inefficient at low speeds and very efficient at high speeds. So the rotational speed and size of the compressor means that the engine produces maximum output at one and only one "rated altitude".


I believe much the same problem was found with early marine steam turbines in destroyers - the turbines spun too fast, and the shafts needed multiple props on them to absorb all the power (couldn't use larger props due to hull clearance and draught issues).

To overcome this limitation, conventional superchargers could be given two or three different gear ratios, each suited to a different critical altitude. This is essentially the same principle as a car or bicycle transmission: you use gearing to turn the available crankshaft speed into the optimum compressor speed for two or three critical altitudes. This worked wonderfully in the Rolls-Royce Merlin. But it was still a relatively inflexible way to manage power in an aircraft that had to operate from sea level on up. For this reason, special low-level engines with single-speed drives and reduced-diameter ("cropped") impellors had to be developed for close-support, the Navy, and chasing V-1s.


IIRC all single-stage Merlins fitted to production Spitfires (the ASR.IIc was a later conversion and the III never got beyond prototype) were single-speed and vice versa; the only two-speed, single-stage engines the Spitfires got were the Griffons fitted to the MkXII (and derivative Seafires), while the speeds required for chasing V1s were gained by allowing a more open throttle and consequent higher boost pressures rather than altering the gearing per se.

The turbocharger automatically adjusted its output for altitude for the reasons you describe: at higher altitude the the turbine spun faster and compressed the lower density air to a greater degree than it would at lower altitude. This seemed like the ideal--hence the USAAF preference for it. But developing materials capable of withstanding the exhaust heat of a gasoline engine proved difficult and charge heating--with consequent low volumetric efficiency--was a problem until well after the war.


I read somewhere that Jacky Fisher asked one of his designers (it may even have been Sir William White) for a gas turbine engine and the designer told him no - metallurgically impossible. Of course the designers of the early jets had similar problems, but there the Allies had the edge.

The hydraulically driven Daimler-Benz superchargers coupled a barometric controller with an infinitely variable hydraulic supercharger drive that performed, in some ways, like a turbocharger. The speed of the compressor was not limited by fixed gear ratios--it had the equivalent of automatic transmission.


More like Continually Variable Transmission - automatic, after all, is generally a multi-stage stepwise thing.


The variable drive was probably made the DBs less sensitive to fuel quality than the Merlins were--an important feature for Germany.


*Nod* It's fascinating to read late-war accounts of "1.5 ata boost pressure must not be used" coming out of Reichlin, while Rolls-Royce etc. are happily running +25 without too many issues... (Incidentally, I'm unclear: is 1.5ata the total pressure going into the manifold or the pressure above atmospheric? The former would be equivalent to +7.5lb boost, the latter to about + 23lb).

The MiG's Mikulin engine was indeed a conventional "supercharger with a very high rated altitude". The superharger was geared, but the ratios were chosen to deliver maximum boost at maximum altitude and the compression ratio was lowered to accommodate high boost at rated height. I have read that this made the MiG-3 something of a dog at lower levels, where the supercharger was less efficent and consumed more power. This lack of flexibility made the MiG less than useful, because, on the Eastern Front, air battles were almost always fought at low levels.


Sort of what you'd get from a Spitfire VIII or IX without the MS gear.
 

iverson

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pathology_doc said:
iverson said:
There were no belts strong enough.


I believe much the same problem was found with early marine steam turbines in destroyers - the turbines spun too fast, and the shafts needed multiple props on them to absorb all the power (couldn't use larger props due to hull clearance and draught issues).

Not exactly. Propeller sizes are limited by tip speeds, particularly in an incompressible fluid like water. If the tip speed exceeds the local speed of sound, you get cavitation around the propeller. Cavitation causes huge losses in efficiency and destructive shock waves that rapidly erode the propeller. Given the same output shaft speed, multiple small propellers can transmit the needed power at a much lower tip speed than a single large propeller.

The critical-altitude issue is different. While cars are usually supercharged to make more power by operating above sea-level pressure, airplanes generally are not (except for Reno racers). In military aeroengines, supercharging is used to provide the equivalent of sea-level power at higher altitudes. Ideally, you'd like to maintain constant full-throttle power at all altitudes. Turbochargers can come close to this ideal, but, in practice, all aircraft engines had to be optimized for a narrow range of tactically important altitudes.

An engine that is designed to deliver maximum full-throttle power at sea level is set up to burn fuel in dense, sea-level air. Above sea level, it starts to lose power. An engine that is supercharged to deliver maximum full-throttle power at a critical altitude of 12000 or 30000 feet can't deliver enough air to maintain that power above the critical height. Nor, less obviously, can these altitude engines deliver full power below critical altitude.

Power is limited by the amount of fuel that you can burn, and the amount of fuel that you can burn is limited by the amount of air that you can mix with it. To burn sea-level amounts of fuel in an internal combustion spark-ignition engine at altitude, you thus have to pack more, thinner air into the same space. You have to raise the compression ratio. The highly successful "over-compressed" high-altitude engines that the Germans developed in WW1 achieved this by significantly raising the compression ratios of otherwise conventional engines. This made it possible to fly well above the altitudes that Allied fighters could reach. But it also introduced a problem: the over-compressed engine would not run and might even fail at lower levels. To reduce power to a safe level, the pilot had to throttle back or use a decompression fitting on the engine that released pressure from the head.

Why high-compression engines had problems at sea level was not fully understood until Harry Ricardo and Sam Heron discovered detonation post-war. Detonation occurs when compression heats the fuel/air mixture until ignites on its own, without a spark. The resulting shock waves can wreck engines.

Since supercharging is just a more efficient way of raising the compression ratio in the engine, you have to be careful not to overboost an engine so much that you raise the effective compression ratio to the point where detonation starts. A supercharger that will compress the air enough to give maximum, full-throttle, sea-level power in the thin air at 30000 feet will raise the compression ratio enormously if run at sea level. Sea-level air is something like three times as dense as that at 30000 ft. If you ran the high-altitude engine at full throttle under those conditions, detonation would destroy the engine in no time. So the high-altitude pilot had to throttle back lower down. No engine produces its best power throttled, so the power of the high-altitude engine drops significantly when run at low level. Worse still, the supercharger machinery is just dead weight at sea level, the impellers further throttle the airflow, and extra friction and moving parts consumes additional power.

For this reason, low-altitude fighters usually had single-speed, single-stage superchargers that delivered minimum boost.

IIRC all single-stage Merlins fitted to production Spitfires (the ASR.IIc was a later conversion and the III never got beyond prototype) were single-speed and vice versa; the only two-speed, single-stage engines the Spitfires got were the Griffons fitted to the MkXII (and derivative Seafires), while the speeds required for chasing V1s were gained by allowing a more open throttle and consequent higher boost pressures rather than altering the gearing per se.

True. Gearing was for multi-altitude flexibility. The low-level Merlins were single-speed because they were intended for use in airplanes that operated at a single level--low-level. The superchargers were "cropped"--given smaller impellers that delivered lower volumes of lower pressure air--so that the engines could run at full throttle in dense, low-altitude air.

For low-level operation, the Germans and Americans also used water injection to lower the charge temperature, delay the onset of detonation, and allow higher boost for brief periods--usually during take off and landing or emergencies. In the USAAF it was called ADI (Anti-Detonation Injection). In Germany it was called MW-50, for the methanol that was added to the water as antifreeze. The British do not seem to have used water injection to any great extent, perhaps because the design of the Merlin's supercharger provided a measure of detonation protection. In war-time Merlins , the supercharger was located between the carburetor and the engine's induction manifold. So the vaporization of fuel in the carburetor cooled the charge significantly.

For high altitudes, the Merlin used a two-stage supercharger with an intercooler between the stages. This lowered the total end-to-end temperature rise for a given level of supercharging.

American-made fuel was, however, the key to Allied high-altitude performance. Tetraethyl led and high-quality American crude oil produced gasolines with very high octane numbers. High octane fuel resists detonation and allows more boost and thus more power. German fuel never approached the detonation resistance of US fuel. So German engines tended to be of larger displacement for the same power. Direct fuel injection may also have helped to maximize the detonation resistance of engines running on lower quality fuel.
 

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Context -


Me110G-level-speed.jpg




I note, particularly, the Me110G curve at the far left - zigzag pattern up to what we might as well call MS and FS gear full-throttle heights, and the gradual die-off thereafter - compared with the more or less unbroken and much smoother curve labelled "Fw190 n=2500" at the right of the graph.


I've also seen Messerschmitt 109 curves with similar patterns, e.g.:


Me-109E3-Russian.jpg







From what little I've read of German aero engines, I gather this is due to the nature of the supercharger drive - hydraulic versus direct belt or gear. I have also seen, elsewhere (print book, no link available) a similarly smooth-ish curve for the P-47D, which of course is turbo-exhaust driven. The latter is obvious - as atmospheric pressure drops off, the difference between exhaust manifold pressure and ambient will be greater, which will provide more oomph to the turbine drive. I'm gathering there's something about the construction of the hydraulic "clutch" in the German engine which provides a similar effect. Am I on the right track?


The curve for the Mig-3 above (AM-35A engine) shows a similar unusual appearance - either a single-stage supercharger with a very high rated altitude or something exotic going on.


Can anyone provide further details?
You might find my recent post, "Slosh" or "Whine" Monseur? of interest.
 

Hobbes

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. I'm gathering there's something about the construction of the hydraulic "clutch" in the German engine which provides a similar effect. Am I on the right track?

The hydraulic drive could transfer a variable amount of power, this was controlled by the amount of fluid in the system. This level was controlled by a mechanical device that used various inputs (altitude, throttle setting). This was a primitive form of computing, similar in principle to the devices used in fire control systems.
 

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