Supercharging & stuff - losses, improvements, good vs. bad. vs. excellent etc

[tomo pauk]

In order not to clog another thread.

Look at the Merlin VIII engine or Merlin X engine in low gear to see the effect. Same supercharger turning slower is allowed to use higher boost and/or heats the intake air less.

Some people just lumped everything going on at low altitude into 'throttle loss' rather than go through a long explanation.

The decease of power due to the 'throttle losses' ought to be smaller for the engines with the low-speed gear (MS in British speke) than on the engines that are without that gear. It was not just what the 1-speed FS Merlins (like the Mk. II, III, 45, 47 or 50) experienced, but also the V-1710s.
One of the appeals of the swirl throttle was that it was supposed to cool the air a little bit, that helped with the power generated.
link
End result on the AM-35A engine (that was with power comparable to the Merlin 45/50 on +9 psi) was the gain of 100-150 HP at S/L.

Similar was the thing with the HS-12Y-45 vs. the -31 - the controllable vanes before the impeller allowed for the much lower loss of the power under the FTH, despite the -45 having a much more capable supercharger.

American test
Graph, comparing the boost (full lines) and power (dashed lines) of the run-on-he mill HS 12Y engine and the -45:

 

[Daggerr]

In order not to clog another thread.

It's a pity that my two efforts in that other thread (If German have access to high ocatane fuel, how does that impact the performance of their fighters? and If German have access to high ocatane fuel, how does that impact the performance of their fighters?) to explain why the difference in Merlin BHP between sea level and FTH has nothing to do with some mysterious "throttle loss" but are only the result of change in ambient air temperature between sea level and FTH, seem to have been a waste of my time.

 

[tomo pauk]

It's a pity that my two efforts in that other thread (If German have access to high ocatane fuel, how does that impact the performance of their fighters? and If German have access to high ocatane fuel, how does that impact the performance of their fighters?) to explain why the difference in Merlin BHP between sea level and FTH has nothing to do with some mysterious "throttle loss" but are only the result of change in ambient air temperature between sea level and FTH, seem to have been a waste of my time.

2-3 comments are warranted:
I value you input. On a public forum, people often disagree, however.
Term of the 'throttle losses' is not my invention.
Better/more efficient/less 'harmful' ways of throttling (that can be called by other names,) were invented and put into use back in ww2, with the net effect of allowing for the better power under the FTH than it will be otherwise the case.

 

[Shortround6]

Something was going on. And it was more than the difference in ambient temperature.

 

[Daggerr]

Something was going on. And it was more than the difference in ambient temperature.

No it was not.
If ambient temperature were constant with altitude then there would be no difference in BHP with altitude upt o FTH despite that throttle valve. Can we at least agree on that?

The Merlin supercharger power at sea level was not higher than at FTH.
The throttle valve was not consuming any power at the expensive of engine power.

Say I have my money in a savings account with a low interest rate, while my brother has his savings invested in stocks and ETF's, do I then loose money because my brother's capital grows faster?
No, I don't loose money, but by copying my brother I could make more money.
I would be increasing my profit, but not recuperating a loss that does not really exist.

Similar story with throttling: a simple throttle valve does not really cost energy, but replacing it by a swirl throttle, or a VSD, would of course result in a BHP gain.
That potential gain can however not be read from a Merlin power graph because the BHP difference between SL and FTH has absolutely nothing to do with a presumed 'throttle loss' due to that simple throttle valve.

2-3 comments are warranted:
I value you input. On a public forum, people often disagree, however.
Term of the 'throttle losses' is not my invention.
Better/more efficient/less 'harmful' ways of throttling (that can be called by other names,) were invented and put into use back in ww2, with the net effect of allowing for the better power under the FTH than it will be otherwise the case.

Heat engines (like an aircraft engine) follow the laws of physics/thermodynamics/chemistry. There is not really much room for opinion. Everything can be accurately calculated nowadays.

I had a quick look at the NACA translation of the German report on the Russian AM35 and AM38 that you posted earlier.
It looks rather disappointing. The authors did not do an effort to really analyse their test data and draw useful conclusions.

 

[GregP]

I don't find it to be true, Dagger, that everything can be calculated.

The theoretical can be calculated but, in execution, some engine have better-matched intake and exhaust openings, better finishes on the metal, more optimum angles used to turn a flow, and many other things that make one engine perform better than another one. Even using the same design, some engine just run better as a result of the combination of fit, finish, and better assembly or better polish and matching of joints, bearings, etc.

I have several friends who fly sporty aircraft with IO-360 engines in them, a couple with identical propellers, engine, and airplanes. Yes, I know the two airframes are not exactly identical, but that should not matter much in climb, and it does. One consistently outclimbs the other one by as much as 3% and cruises faster, too, at identical power settings.

The differences aren't major, but one engine puts out about 5% more on a dyno than the other one. Said another way, when they cruise side by side, one is using about 1.5" less manifold pressure to stay side by side. In the case of these Van's RV airplanes, one engine performs slightly better.

There is not way that Merlin or Kilmov engines were all identical. Some were a few percent better and some a few percent worse. It's the nature of manufacturing "identical" things. Yes, you can calculate the expected ideal performance and, if your fit and finish and torque value are all identical, you will average very similar performance. But, there will still be "good" and "bad" engines that start and run just fine in service, but run slightly differently from one another.

If you are talking average performance, with good design you can get decently close to ideal calculation. But you can also get significantly worse than ideal with a few less than optimum choices in design.

For my money, the Hispano-Suiza of the 12Y family were well-made engines that ran well but never quite came up to the performance of a Merlin. Then again, they didn't have Sir Stanley Hooker working on them, either, coming up with a very nice 2-stage supercharger unit, complete with intercooler. The HS engines I know virtually all had single-stage superchargers, and most were single speed. They had displacement over a Merlin (365L vs 27L), but not performance. So, they must not have come as lose to ideal calculations as the Merlin did.

 

[Bretoal2]

The difference in BHP between sea level and FTH for Merlin, or any other engine running at a constant S/C speed, is for 100 % the result of the change in ambient temperature with altitude.

This statement is not entirely accurate. Not 100 %, but almost.

Another element plays (a little) in the increase in power between ground and rated altitude : the progressive reduction of ambient pressure lowers the back pressure in exhaust.

And don't forget another factor : the variation of power absorbed by the supercharger, driven directly by the engine in the cases cited (HS 12Y, Merlin, Klimov) and many others. This factor can be positive or negative, depending on the evolution of the compressor efficiency according to the mass of air absorbed....

But in the end, yes, the main factor in the evolution of power at altitude is the increase in the mass of air absorbed by the engine, at identical intake pressure and rpm.

 

[Bretoal2]

For my money, the Hispano-Suiza of the 12Y family were well-made engines that ran well but never quite came up to the performance of a Merlin. Then again, they didn't have Sir Stanley Hooker working on them, either, coming up with a very nice 2-stage supercharger unit, complete with intercooler. The HS engines I know virtually all had single-stage superchargers,

The Hispano superchargers were not that good...

Let's read F.R. Banks, "I kept no diary" :

"When it came to supercharger design, Marc Birkigt's good engineering sense seemed to desert him [Nice introduction, right?]. His supercharger looked like an overgrown water pump, with the casing little larger than the rotor. It had no diffusers and the temperature rise was phenomenal, it even burned the paint of the outside casing at very low blower pressure ratio. British supercharger experience had shown that, without diffusers, the inside diameter ot th supercharger casing should not be less than twice that of the rotor, giving the mixture a chance to straighten itself from the turbulent condition before passing the induction pipes proper".

Graph, comparing the boost (full lines) and power (dashed lines) of the run-on-he mill HS 12Y engine and the -45:

Sorry, this is not the 12Y 45/49 curves... It's only the results of a test in Chalais-Meudon official installations.

One very important thing is that, well beyond the question of throttle position , the maximum inlet pressure allowed for the "stock" 12Y is 920 mm Hg at takeoff, giving 790 hp. With the same boost of 920 mm Hg, the Planiol-Szydlowski supercharger increases the power to 890 hp. Why? Because the intake flow is less hot, thanks to the much better efficiency of the supercharger. Less temperature means greater mixture mass at identical pressure, therefore more power....

 

[tomo pauk]

Heat engines (like an aircraft engine) follow the laws of physics/thermodynamics/chemistry. There is not really much room for opinion. Everything can be accurately calculated nowadays.

I had a quick look at the NACA translation of the German report on the Russian AM35 and AM38 that you posted earlier.
It looks rather disappointing. The authors did not do an effort to really analyse their test data and draw useful conclusions.

Do you have a take on the test data, analysis and useful conclusions?

 

[Shortround6]

Another element plays (a little) in the increase in power between ground and rated altitude : the progressive reduction of ambient pressure lowers the back pressure in exhaust.

One booklet (published by GM) claims that a 1000hp at sea level engine would gain 80hp at 20,000ft due to reduced back pressure provided other things remain the same.
Italics are in the original. There maybe a little bias in the book (GM being the parent of Allison) but they try to give a simple explanation (2-3pages including illustrations) of 7 different types of supercharger set up for a theoretical 1000hp engine.

This may be for a 'free' exhaust?
But very few exhausts were wide open. There were trade-offs between exhaust thrust (keeping the exhaust velocity high) and lowest possible back pressure. Allison exhausts were cut back, at times with hacksaws (using templates), when WEP was approved and the need to getter better exhaust flow at low altitudes while flowing 30-40% more air trumped better exhaust thrust at higher than FTH. They changed the square inches of nozzle area for more flow at low altitude.

 

[Bretoal2]

One booklet (published by GM) claims that a 1000hp at sea level engine would gain 80hp at 20,000ft due to reduced back pressure provided other things remain the same.
Italics are in the original. There maybe a little bias in the book (GM being the parent of Allison) but they try to give a simple explanation (2-3pages including illustrations) of 7 different types of supercharger set up for a theoretical 1000hp engine.

This may be for a 'free' exhaust?
But very few exhausts were wide open. There were trade-offs between exhaust thrust (keeping the exhaust velocity high) and lowest possible back pressure. Allison exhausts were cut back, at times with hacksaws (using templates), when WEP was approved and the need to getter better exhaust flow at low altitudes while flowing 30-40% more air trumped better exhaust thrust at higher than FTH. They changed the square inches of nozzle area for more flow at low altitude.

The expression " provided other things remain the same" opens the door for many interpretations! And most certainly depends on exhaust system architecture (true free, jet exhaust, turbo...).

To return to the question of increase in the mixture mass due to the drop in temperature between ground and FTH at constant intake pressure: this increase in mass also increases the real compression ratio. It also increases the lamination of gases in the intake ducts and the valve faces and seats.

It therefore seems to me that it is very adventurous to "guess" by calculation what the behavior of the engine will be... even if Hooker tried!

 

[Daggerr]

This statement is not entirely accurate. Not 100 %, but almost.

Another element plays (a little) in the increase in power between ground and rated altitude : the progressive reduction of ambient pressure lowers the back pressure in exhaust.

Yes, I know. Follows from Hooker's 0.422 ....... formula.
I left that small effect out so as not to unnecessarily confuse the discussion.

My point was: if ambient temperature does not change with altitude then the inlet temperature and pressure of the S/C and the temperature and pressure of the piston engine will be exactly the same at SL and at FTH and any altitude in between. Therefore the air mass flowrate, and consequently BHP, would not be affected by the amount of throttle valve opening. It's not the throttle valve that causes the actual BHP difference between SL and FTH, but the actual ambient temperature profile. And as you pointed out, the decrease in exhaust backpressure with increasing altitude also has a small effect.

And don't forget another factor : the variation of power absorbed by the supercharger, driven directly by the engine in the cases cited (HS 12Y, Merlin, Klimov) and many others. This factor can be positive or negative, depending on the evolution of the compressor efficiency according to the mass of air absorbed....

Yes, at constant ambient temperature from SL up to space, the small change in S/C flowrate with increasing altitude (due to change in exhaust backpressure) could have a small effect on S/C efficiency due to a slight move of it's operating point in the compressor map, but such shift in efficiency would then be minor and could go either slightly up or slightly down, depending on the exact location of the operating point in the map.

 

[Daggerr]

I don't find it to be true, Dagger, that everything can be calculated.

Please note that I was referring to the physics/thermodynamics/chemistry heat engines, not to aerodynamics or propellors or .....

When it comes to "identical engines" I agree that they need not be entirely identical due to tolerances, but if one would know the exact dimensions of each engine one could do a detailed calculation for each of them, resulting in slightly different powers or fuel consumption or ....

Nowadays much more accurate calculations and simulations (such as CFD) are possible that was not possible in WW2.

 

[Daggerr]

Do you have a take on the test data, analysis and useful conclusions?

In the mean time I noticed that that NACA translation is not a translation of the whole German study report but merely some excerpts.
If only somebody would have the whole German report.

Now it is not clear exactly how the tests were done. Were they all done including the piston engine? All done in a test facility, or some also in a flying aircraft? Are some tests only done with S/C and swirl throttle?
Not clear what kind of gasoline was used: German B4 or C3 or captured Russian gasoline or .......
No mention of S/C impeller diameter.

Correct me if I'm wrong but afaik both the AM38 and AM35 had carburators. So like in a Merlin gasoline was sprayed into the air upstream the swirl nozzle and S/C. Depending on the air temperature, pressure, and type of gasoline, part of the gasoline would vaporise upstream and part inside the S/C thereby lowering gas temperature. That has an impact on both the inlet and outlet temperatures of the S/C. The translated report does not talk about that at all. The authors seem to assume that S/C inlet temperature is ambient temperature.

Some numbers in the calculations seem to be pulled out of a hat, or maybe they were calculated in parts that were not translated.
For the AM38 there is simply mention of 4920 kg/h air with swirl throttle, which is then used to calculate 4650 kg/h air without swirl throttle. Then it is somehow calculated that these flowrates correspond with 1780 and 1680 hp at SL respectively, so a 100 hp gain at SL due to the swirl throttle.
There is also reference to Figure 30. However in Figure 30 the 100 hp gain at SL is between 1300 and 1400 hp, not between 1680 and 1780 hp.

And so on.

The few calculations in the translated part are all done rather sloppy.

Strangely enough there is no mention of measured swirl throttle pressure drops under different vane angles as function of flowrate et cetera.
All it says on page 4 is: "The pressure loss in supercharger duct and carburetor with fully opened swirl throttle amounts to 100 millimeters of mercury."
Not clear at what air flowrate, temperature and pressure that was measured but that seems very high to me. Just imagine how high it would be with a partly closed swirl throttle.
As a consequence it is difficult/impossible to know what the actual S/C inlet pressure was under different circumstances. Several graphs show S/C outlet pressure but without also knowing the S/C inlet pressure one can't really calculate the actual S/C performance and power consumption under different vane positions.

My preliminary conclusion:
- not enough data in the tranlated excerpts to do a proper analysis.
- a power gain of only 100 hp at SL, and less at higher altitude, sounds disappointing.

 

[tomo pauk]

Now it is not clear exactly how the tests were done. Were they all done including the piston engine? All done in a test facility, or some also in a flying aircraft? Are some tests only done with S/C and swirl throttle?
Not clear what kind of gasoline was used: German B4 or C3 or captured Russian gasoline or .......
No mention of S/C impeller diameter.

Thanks for the overview.
Impeller diameter probably remained the same as it was before. Both AM35A and AM38 were rated for the Soviet high octane fuel, so the B4 will not cut it.
Making engine tests on the captured aircraft is probably not going to happen well until the tests on the ground facilities are made.

Correct me if I'm wrong but afaik both the AM38 and AM35 had carburators. So like in a Merlin gasoline was sprayed into the air upstream the swirl nozzle and S/C. Depending on the air temperature, pressure, and type of gasoline, part of the gasoline would vaporise upstream and part inside the S/C thereby lowering gas temperature. That has an impact on both the inlet and outlet temperatures of the S/C. The translated report does not talk about that at all. The authors seem to assume that S/C inlet temperature is ambient temperature.

Mikulin engines were with carbs (that were also with throttle bodies) downstream vs. the S/C, with multiple carbs per engine. See here for example.

My preliminary conclusion:
- not enough data in the tranlated excerpts to do a proper analysis.

Okay.

- a power gain of only 100 hp at SL, and less at higher altitude, sounds disappointing.

I'm not sure that the swirl throttle was ever advertised as a device that brings improvement at higher altitudes; the 1st implementation was on the higher altitude engines so it can help with power down low.
Power gain down low due to the swirl throttle was judged as big enough to warrant the implementation by Mikulin, Jumo, DB and Klimov.

 

[Shortround6]

My point was: if ambient temperature does not change with altitude then the inlet temperature and pressure of the S/C and the temperature and pressure of the piston engine will be exactly the same at SL and at FTH and any altitude in between. Therefore the air mass flowrate, and consequently BHP, would not be affected by the amount of throttle valve opening. It's not the throttle valve that causes the actual BHP difference between SL and FTH, but the actual ambient temperature profile. And as you pointed out, the decrease in exhaust backpressure with increasing altitude also has a small effect.

We seem to ignoring the fact/s that we are trying to trick the supercharger into operating like it is at high altitude and forcing it to operated at a high pressure ratio by restricting the mass flow with the throttle plates (note that this only applies to certain superchargers.)
Otherwise we are restricting the mass airflow (total amount of air) to desired amount and increasing the pressure of this air only a little. Or some combination of the two.
Unlike a turbo or even a DB supercharger, in British or American (or some others) we cannot change the speed of the impeller to suit the supercharger to desired airflow.
Is the pressure in the Merlin supercharger actually dropping to 16.2in Hg in the inlet at take-off with the gated throttle and the supercharger boosting it back up to 42.2in Hg pressure in the manifolds or are we getting air that is somewhere between the two pressures but just less of it? Or is the air in the supercharger being heated even more than the air is heated at 16,000ft despite entering colder?

We can use a multispeed drive (almost always two), Just about everybody used a two speed supercharger drive to significantly increase low altitude power. They often got little more high altitude power out them but that was because single speed was a real compromise the really sacrificed low altitude power for altitude.

Merlin X with two speed supercharger was good for 1025/1065hp for take off at 2850rpm using 87 octane fuel. The 6.389 supercharger gear heated the air a lot less. Engine was rated at 1145hp at 5250ft at 3000rpm.
The supercharger on the Merlin III (8.588 gear) was using about 80% more power to turn the impeller and was heating the air more.
The slightly higher hi supercharger gear on the X (8.75) was good for 1025hp (?) at 17,750ft but was heating the air even more while dealing with slightly lower air temperature, about 3 D C.
Another thing going on is that air at 18,000ft has got 49.93% of the sea level air pressure but it has 56.97% of the density (56.97% of the weight of air per cubic ft.) This may very well be due to the colder air being denser but different superchargers don't act the same way as they get lower in altitude. They are going to have different temperature rises in the supercharger to get the same pressure

 

[Daggerr]

Is the pressure in the Merlin supercharger actually dropping to 16.2in Hg in the inlet at take-off with the gated throttle and the supercharger boosting it back up to 42.2in Hg pressure in the manifolds .......

If the supercharger runs at a tip speed that delivers a pressure ratio of 2.6 then yes it is.
There may however be a slight difference between the pressure ratio delivered at SL and that at FTH due to temperature difference, see below.

Or is the air in the supercharger being heated even more than the air is heated at 16,000ft despite entering colder?

Note that at SL the air is not entering colder but entering warmer than at 16,000 ft

Merlin X with two speed supercharger was good for 1025/1065hp for take off at 2850rpm using 87 octane fuel. The 6.389 supercharger gear heated the air a lot less. Engine was rated at 1145hp at 5250ft at 3000rpm.The supercharger on the Merlin III (8.588 gear) was using about 80% more power to turn the impeller and was heating the air more.
The slightly higher hi supercharger gear on the X (8.75) was good for 1025hp (?) at 17,750ft but was heating the air even more while dealing with slightly lower air temperature, about 3 D C.
Another thing going on is that air at 18,000ft has got 49.93% of the sea level air pressure but it has 56.97% of the density (56.97% of the weight of air per cubic ft.) This may very well be due to the colder air being denser but different superchargers don't act the same way as they get lower in altitude. They are going to have different temperature rises in the supercharger to get the same pressure

Not sure what point you try to make here.

A centrifugal supercharger behaves like any other centrifugal compressor in the industry.
It delivers an 'isentropic head' (aka 'adiabatic head') that is proportional to its tipspeed squared, and also depends on the volumetric flowrate through the S/C. That's why in a compressor map the performance lines droop.
Isentropic means reversible adiabatic.
Reversible basically means: without friction.
Adiabatic means: without heat transfer to or from surroundings.

In the real world nothing operates isentropic. In a centrifugal compressor there is a lot of friction going on between gas and impeller and casing.
To account for that humans invented the term 'isentropic efficiency' (aka 'adiabatic efficiency'), which for WW2 superchargers is in the order of 70 %.
The 'adiabatic head' can be expressed in meters or in kJ/kg (or kcal/kg or Btu/lb or .......) and represents the power consumption per kg gas under isentropic operation. To get actual power per kg divide 'head' by efficiency.

From the 'adiabatic head' and the temperature and physical properties of the gas (in our case air/fuel mixture) one can calculate the pressure ratio that the S/C actually delivers.

If the ambient temperature from SL up to space were constant then the pressure ratio at SL and at FTH would hardly differ, say 2.6 in both cases, and at a controlled MAP of 42.2in Hg the S/C inlet pressure would be 16.2in Hg at any altitude between SL and FTH thanks to the variable opening of the throttle valve.
As S/C inlet temperature would be the same in both cases (due to same ambient temperature) so would S/C outlet temperature and S/C temperature rise. Manifold density is the same in both cases (same boost and same temperature) so air/fuel mass flow is almost the same in both cases (except for the small effect of lower exhaust backpressure at FTH), so BHP at SL and FTH would also be almost the same (as I have pointed out before more often than you probably wanted to hear).

In reality the ambient temperature drops with altitude, so for the same delivered 'head' the pressure ratio of the S/C is not exactly the same at SL as at FTH, but a little lower at SL due to the higher temperature. So in your example above: if presssure ratio is 2.6 at FTH then it could be 2.4 at SL, so S/C inlet pressure at SL could then be 17.6 in Hg versus 16.2 in Hg at FTH.

It is not a mystery what happens inside a S/C, no laws of physics/thermodynamics that still need to be discovered, no quantum effects.
It's fairly straight forward really, but unfortunately there are many lousy textbooks and websites that can cause confusion.

 

[overbeck]

It is true that power would be roughly unchanged between SL and full-throttle altitude if the T remained unchanged. That's an interesting observation. But power at SL is significantly higher than at critical alt if the supercharger RPM can be lowered to avoid throttling. It's not a question of temperature vs the inefficiency of the butterfly throttle. It's both.

I'll use an example of a Merlin compressing 22 in. Hg to 48 inches. At sea level the butterfly throttle lowers the pressure from roughly 30" (not accounting for carb venturi loss) to 22". The expansion of the air accompanying the pressure drop does not result in a temperature drop because of one problem: the expansion also releases a significant amount of energy, and the throttle dumps this right back into the air as heat. Initially it lowers P and T by causing the air to accelerate through a restriction, but this increased velocity is dissipated as heat by turbulence immediately downstream of the throttle, bringing the temperature back to where it started and causing the familiar hissing sound of gas flowing through a restriction.

It is possible to get a temperature drop by expanding the air through a turbine. The NACA did this when they tested their axial flow compressor circa 1940. IIRC they plumbed the turbine section of a GE turbocharger into the axial compressor's inlet duct, and the turbocharger's compressor section dissipated the resulting power.

After the Merlin's supercharger compresses the air at 288 K from 22" to 48", I calculate a T of about 384 K assuming 75% efficiency. At this T and 3000 rpm the Merlin will use about 1.45 kg air per second, and an imaginary inlet turbine at 100% efficiency would yield an inlet T of 264 K and 25.5 kW or 34 hp. If some madman were to gear that to the prop, combined with the T drop it would roughly cancel out the power lost by needing to compress by a ratio of 22/48 instead of 30/48 (not taking into account less than ideal supercharger efficiency).

The swirl throttle goes some way to performing the function of an inlet turbine using the impeller. The swirl vanes reduce the angle of attack at the impeller's inducer vanes (maybe even sending it negative), causing the impeller to do less work and therefore lowering its pressure ratio and power consumption. But swirling also raises air velocity, and being done passively it lowers P and T just like in a venturi or a butterfly throttle (turbines typically have "nozzle" vanes immediately upstream which do this too). But unlike in a butterfly throttle, the accelerated air does not become turbulent and dissipate its energy, so the swirl throttle lowers P and T upstream of the impeller.

 

[Daggerr]

It is true that power would be roughly unchanged between SL and full-throttle altitude if the T remained unchanged. That's an interesting observation.

Glad to hear that you agree with me on that.

But power at SL is significantly higher than at critical alt if the supercharger RPM can be lowered to avoid throttling.

???????

It's not a question of temperature vs the inefficiency of the butterfly throttle. It's both.

The Merlin BHP power difference between SL and FTH is caused by difference in ambient temperature, not by inefficiency of the butterfly throttle.
I don't want to explain that again but suggest you read all my posts again, also the ones in the other topic mentioned above.

Of course BHP at SL can be increased by VSD or Swirl Throttle but that is another story.

After the Merlin's supercharger compresses the air at 288 K from 22" to 48", I calculate a T of about 384 K assuming 75% efficiency. At this T and 3000 rpm the Merlin will use about 1.45 kg air per second,

You may want to check your calculations.

and an imaginary inlet turbine at 100% efficiency would yield an inlet T of 264 K and 25.5 kW or 34 hp. If some madman were to gear that to the prop, combined with the T drop it would roughly cancel out the power lost by needing to compress by a ratio of 22/48 instead of 30/48 (not taking into account less than ideal supercharger efficiency).

Better do such calculation with a more realistic 70 % efficiency instead of 100 %.

Some additional remarks:
- There will be a temperature drop over the throttle valve due to higher duct velocity downstream than upstream. Kinetic energy increase at expense of enthalpy.
- Due to vaporization of avgas introduced in the carb the S/C inlet temperature will drop further. Part of avgas is vaporized upstream, part inside the S/C. At SL most avgas is vaporized upstream, at FTH less upstream and more inside.

 

[GregP]

Question for you guys in the technical engine discussion. Not sure what you are talking about but, on this planet, we don't live in an atmosphere where temperature doesn't change with altitude.

So, why are you discussing an atmosphere at constant temperature?

Understand, I'm not saying your conclusion are incorrect but, since temperature DOES change with altitude, the only way to get to there is in a temperature-controlled altitude chamber. If you are in there, your engine isn't performing in the real world, so what is the point?

Just curious, not taking a swipe at anyone.