SaparotRob
Unter Gemeine Geschwader Murmeltier XIII
"It only works for spherical chickens in a vacuum."
Dr. Leonard Hofstadter
Dr. Leonard Hofstadter
Supercharging & stuff - losses, improvements, good vs. bad. vs. excellent etc
- Thread starter tomo pauk
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GregP
Major
There are many worthwhile tidbits in The Big Band Theory, huh?
I'd ditch them all for Penny ...
I'd ditch them all for Penny ...
SaparotRob
Unter Gemeine Geschwader Murmeltier XIII
Penny for your thoughts.
Let's go back to post 1 and its link "American Test", illustration "Exhibit A etc." :
What is this altitude test bench? Just a ground test bench where the primary intake is "throttled" in order to simulate the lower atmospheric pressure existing at some heights. Nothing more, and no way to regulate the temperature.
Consequently, all tests are carried out at constant temperature ! And then the results are interpreted and corrected by calculation to take into account the famous decrease in temperature with altitude.
This test mode was that of the vast majority of engines in the 1930s, would they be American, English or French !
See also the results of these tests on the Planiol-Szydlowski compressor in Châlais-Meudon (illustration of the same post): the A° curve is the one obtained directly on the bench with original Hispano supercharger , before any correction of the results by calculation: we see that the power is rigorously constant from 0m to 3,200 m "simulated". Beyond that, the compressor can no longer raise the intake pressure sufficiently to maintain the power, which decreases.
After calculations, this curve is corrected and becomes Az, the "theoretical" curve of power at altitude of the original 12Ygrs engine.
It should also be noted that the constant power at constant temperature rule seems to don't work for the Planiol-Szydlowski supercharger (curve B°, interpolated after calculations in Bz). Yes, but no in fact.....Obviously, its regulation system works in a different way because its multiplier coefficient varies with the upstream pressure (curve B° and Bz corrected by calculations).
This multiplier variation (in French : circulation variable) is the whole point of this device!
It's a "there's a neat mathematical relationship when you test a spherical chicken in a vacuum chamber" type of case.
It doesn't change the fact that power loss below critical altitude is all downstream of the energy wasted by the butterfly valve. I don't know why
Daggerr
is trying to prove the valve is not a problem, but this kind of thing is usually a projection, i.e. it's not about butterfly valves, engines or aircraft.I know the test chambers used another throttle upstream. What's the relevance to the point I made?
To break it down another way: take a test engine with a two speed supercharger. Critical altitudes are sea level for low blower and 15,000 ft for high blower. We have 3 test cases at the same MAP:
A. High blower at 15,000 ft.
B. High blower at sea level, throttled.
C. Low blower at sea level.
Power in case B is lower than A because the temperature is higher due to the typical atmospheric temperature gradient.
Power in case C is higher than A because the supercharger is using less power, and the mixture temperature in the manifold is lower, because the air at 15,000 ft is not cold enough to cancel out the effect of a higher pressure ratio.
But the temperature in case B would be lower if the pressure were reduced efficiently. Doing so generates power, and the only practical use for it is to feed it into the supercharger. The swirl throttle accomplishes all of this. Not perfectly, so B with a swirl throttle is still less powerful than C.
The swirl throttle is more than just a trick to change the AoA at the impeller's inducer vanes. It takes substantial kinetic energy to do that, and the swirl throttle generates that from the expansion of air as the pressure drops. Energy which the butterfly valve wastes.
To break it down another way: take a test engine with a two speed supercharger. Critical altitudes are sea level for low blower and 15,000 ft for high blower. We have 3 test cases at the same MAP:
A. High blower at 15,000 ft.
B. High blower at sea level, throttled.
C. Low blower at sea level.
Power in case B is lower than A because the temperature is higher due to the typical atmospheric temperature gradient.
Power in case C is higher than A because the supercharger is using less power, and the mixture temperature in the manifold is lower, because the air at 15,000 ft is not cold enough to cancel out the effect of a higher pressure ratio.
But the temperature in case B would be lower if the pressure were reduced efficiently. Doing so generates power, and the only practical use for it is to feed it into the supercharger. The swirl throttle accomplishes all of this. Not perfectly, so B with a swirl throttle is still less powerful than C.
The swirl throttle is more than just a trick to change the AoA at the impeller's inducer vanes. It takes substantial kinetic energy to do that, and the swirl throttle generates that from the expansion of air as the pressure drops. Energy which the butterfly valve wastes.
Shortround6
Major General
I will note that the H-S engines and superchargers didn't operate like most British and American superchargers did. The supercharger, which had a flap/air limiter at the entrance blew air with no fuel through the intake system to the 6 carburetors where the air was blown through the venturi/s and then past a butterfly valve in each passage that lead to a pair of cylinders.
I will also note that there may have been differences between some of the 12Y engines aside from just unbolting one supercharger and bolting on a different one.
You would really have to work at making a worse intake manifold than this one from a point of view of flow. But H-S engines didn't use a lot of boost anyway.
12Y-45
Just viewing angles or are there bit gentler angles/transitions?
Swiss 12Y-51
Bend from leading into horizonal tube looks very good. The right angle bend coming out of the supercharger and turning forward looks about as bad as it gets without welding two pipes together with a totally square corner.
Photos may be mis-identified. Or there were different versions of the S-P supercharger? Granted the photo identified as a -45 engine is missing parts.
MIflyer
1st Lieutenant
Here is chart that shows you a big reason why turbos are so desirable.
Pic 1 is a normal HS engine (12 Y 31 ?) with original supercharger : one vertical lower scoop and a flame trap into each manifold (that's the hump between S/C and the first carb.)
Pic 2 is a 12Y 45 or 49 with Planiol-Szydlowski S/C : two side entry scoops, 3 flame traps on each side, one under each carb.
Pic 3 shows a Saurer engine with modified HS S/C and latest manifolds with individuals flame traps.
I think the awful right-angle tube incorporated inner guide vanes. See :
Shortround6
Major General
could be.
Retired firefighter and right angle bends just scream "kink" when you are working with water. They give you the heebie-jeebies
you may not need much bend to get a lot better.
But since firefighters are in hurry they sometimes dumb things down. Like they give you a 'standard' friction loss through the "gun" but what it takes to push 500gpm through the gun is NOT what you need to push 1000gpm through the gun. You take a few tips off and open the bore size but you have friction loss in the hoses leading to the gun and friction loss in the gun itself. Our dept once set up some experiments where we put pressure gauges in the hose junction right at the gun intake so we had pressure at the truck (pump), pressure at the inlet to gun and with a pressure gauge (hand held pitot gauge) in the water stream we could figure out what was going on. It has been a lot of years but the friction loss in the gun was closer to 4 times when we doubled the flow. But that few feet of twisted pipe was worth a lot of feet of hose and pipe was supposed to have less loss per ft if it was straight.
Granted water is incompressible (for all practical purposes) so some things don't transfer well.
I will say that when I started we had two stage pumps in service that could be switched from parallel to series (volume to pressure) but those went away as engine power increased. Sheer power was better than sophistication
We used ball valves for most of the inlets/outlets but I can tell you, getting something jammed against the intake grate/screen can make a huge difference (rock, gasket, even a fish).
MIflyer
1st Lieutenant
Here is an article from GE on turbosuperchargers.
Daggerr
Airman 1st Class
The "atmosphere at constant temperature" concept was brought up by me just to show that if the ambient temperature at SL would be the same as the ambient temperature at FTH then the Merlin BHP at SL would be about the same as at FTH and nobody would talk about "throttle loss".
In reality the temperature at SL is higher than at FTH and while the BHP at SL is lower than at FTH.
So same ambient temperature --> similar BHP, and different ambient temperature --> different BHP, while boost is the same.
Therefore ambient temperature is to blame for this difference in BHP, not the difference in % opening of the butterfly throttle.
It is not so that because at SL the butterfly throttle is partly closed the S/C "has to work harder" at SL than at FTH and therefore SL BHP is lower than at FTH, as some members seem to think.
The S/C does not work harder at SL and its power consumption is not higher than at FTH.
However the piston engine shaft power at SL is lower than at FTH due to the lower manifold density (which is due to higher manifold temperature, which is due to the higher ambient temperature) and that causes the lower BHP at SL.
BHP = SHP minus S/C power minus other power users.
I trust that satisfies your curiosity.
MIflyer
1st Lieutenant
I have had people tell me on-line that if you supercharge at low altitudes then the aircraft goes slower because of the energy absorbed by it and by the fact that if it is linked to the throttle you cannot use full throttle because you hit the so-called limit of 60 inches max manifold pressure.
I suppose that could be true if you had an extreme example, like the high speed of a two stage supercharged Merlin being engaged at sea level. But that is absurd.
The Mustang pilots on Iwo Jima that escorted the B-29's on raid to Japan found that the Japanese fighters stayed at 15,000 ft or below, at around their maximum performance point. P-51 high speed superchargers engaged at around 18,000 ft, automatically, if the switch was set to Automatic. That meant that at 15,000 ft they would be in automatic Low Speed and reaching the upper limit on performance for that setting. So one Squadron Cmder on Iwo, frustrated at being unable to go down and get the Japanese fighters, directed that the spring loaded momentary contact for High Speed Supercharger be replaced with one that would stay in that position. The Packard tech rep warmed against that ("Canna do that, Captain! You'll blow up my engines!) but they did it and were pleased with their ability to fight better at the altitude the Japanese were willing to use. One idiot tried to tell me that the P-51 would go SLOWER if you enaged the high speed at 15,000 ft. If that were the case we would have no superchargers on automobiles.
With just this "small" detail that automobiles only have one area of evolution, the one where the atmospheric pressure is 100% of the nominal, while airplanes evolve close to the ground but also in places where the pressure is 50 or 30% of normal, and the temperature 50 to 75 ° C less...
NeilStirling
Airman
- 27
- Oct 6, 2006
wuzak
Captain
With the same boost and at the full throttle height (FTH) in MS (low) gear, the Merlin has more power in MS (low) gear than it does in FS (high) gear.
If the intake air temperature was the difference in altitud eperformance, why would this be.
In high gear mixture is more heated by the supercharger rotating faster ( delta temp is proportionnal to the square of the impeller tip speed).
So higher temperature of mixture at same manifold pressure (=boost) gives lower quantities in the cylinders, and lower power.
Basic !
Shortround6
Major General
You are also closer to the detonation limit. Hotter mixture at the same pressure is more likely to detonate.
One source claims the S-P supercharger heated the intake air 60 degrees C less than the H-S supercharger. This opens up a lot of possibilities.
However even the compression ratio in the cylinder affects detonation so trying to compare one engine to another gets really tricky.
See post #8.
