A host of questions not deserving of their own individual posts.

[don4331]

Are we sure that it is 'pressure_ratio x pressure_ratio', and not pressure_'ratio + pressure_ratio' for the superchargers with more than one stage?

Yes, it might be easier to think of it this way: The 1st stage increases pressure from 7 psi to 14 psi. 2:1 increase. The second stage "sees" 14 psi air, so when it does the 2:1 increase it goes to 28 psi.

Its quite easy to see on Diesels where you are getting 100+ psig.

 

[tomo pauk]

Yes, it might be easier to think of it this way: The 1st stage increases pressure from 7 psi to 14 psi. 2:1 increase. The second stage "sees" 14 psi air, so when it does the 2:1 increase it goes to 28 psi.

Does that math hold the water when in Hg or ata replaces psi?

Its quite easy to see on Diesels where you are getting 100+ psig.

Air pressure or fuel pressure?

 

[Shortround6]

Does that math hold the water when in Hg or ata replaces psi?


Air pressure or fuel pressure?

Pressure is pressure, doesn't matter how you measure it (what units).

It is claimed that the Merlin 61 was compressing the air at a 5 to 1 ratio at 23,500ft. I don't know it that includes RAM or not.
I also don't know if each compressor doing the exact same amount of work. They don't have to as long as they are designed that way.

 

[don4331]

Continuing:
4. The ideal location and method of injection is direct injection to the cylinder i.e. diesel. Now, as the technology to do that with gasoline in automobile engines didn't exist until about a decade ago, for WWII, it gets very grey. Is it better to cool the incoming air by injecting before the supercharger? Just after the supercharger/before the intercooler? Just after the intercooler?
If you have a small engine (relatively), that you want as much boost as possible to make it competitive, you want the gasoline injected as early as possible.
If you have an air cooled engine, that you need as much cooling as possible in the cylinders, do as late as possible so it is still evaporating

5. became separate list

6. Already answered.

7. You're describing exactly what Allison build with the V-1710-127: Almost 3k hp and BSFC of 0.405 lb/hp.

You just need a plane like the P-63 to be able to install it into.


8. Double edged sword: You don't necessarily want your airframe to be locked into a specific engine in case it fails. Whirlwind being point in case: Without the Peregrine, Westland didn't have an engine to power its fighter.

Conversely, you don't want your engine locked into a specific airframe for the same reason. If the Fw190A had failed, BMW might not have had a substantial market for its radial.

But overall, there was a lot of cooperation between airframe and engine manufacturers. And a lot of angst when thing changed between the mockup and the initial delivery.

9. Rolls-Royce is actually a pretty good example. The Griffon Mk. 1 was buried when it became apparent that fuel had moved on to 100 octane and beyond. So, a whole new engine was designed in late '39 and it finally began to overshadow the Merlin in '44.
6 more months and Vulture might have succeeded; but RAF didn't have 6 months. (Would have help if Manchester wasn't overweight and really needed 2,500 hp engines, not 1,750 hp ones.)
P&W R-2800 is about the same timeline.

10. Triptane

Not fuel additives, but Goring Ha-Ha gas (Nitrous Oxide), Water Injection and Nitromethane are all good, short duration, power boosters.​

 

[don4331]

Does that math hold the water when in Hg or ata replaces psi?


Air pressure or fuel pressure?

Buddy's Ram truck makes 104 psi air pressure! He has to over fuel it so it does over heat...so it rolls coal. But it makes stupid power (just very short term)

 

[don4331]

Charge cooling works well because several things are going on.
As the 'water' cools the intake charge/mix and the temperature goes down the charge gets denser (more lbs per cu ft ) and we are not burning pressure, we are burning lbs of fuel with air. At high altitude we have trouble supplying enough air to go with the fuel we can inject into the airstream. Pressure was just the simplest way to measure the air flow but it requires a lot of assertions.
Pressure works pretty good for measuring water since water is not compressible. But it is not as good as flow meter. A kink/bend in an output line will give you a high pressure reading even if you have restricted flow.
With air in a supercharged engine and the air at 300-400 degrees the density of air at 44in is lot lower than air at 150 degrees and 44in as an off the cuff example.
The cooler charge also heats the engine less for less wear on the engine and/or less problems with cooling system/s. It can allow for more boost than an engine that is not using a charge cooling system (inter cooler, after cooler or water injection) and keep the temperature limits in line and using extra boost on the cooler/denser air is a multiplication.

What your describing is density ratio: We can't directly measure density (easily), but we can easily measure pressure and temperature and knowing those we can calculate density.

The one caveat: temperature needs to be measured in *R or *K, not *F or *C respectively to get the difference in density.​

(460+300)= 760* R is ~25% denser the (460+150) = 610* R assuming same pressure. (not the double that it might appear at 1st glance).​

 

[PStickney]

Continuing:
4. The ideal location and method of injection is direct injection to the cylinder i.e. diesel. Now, as the technology to do that with gasoline in automobile engines didn't exist until about a decade ago, for WWII, it gets very grey. Is it better to cool the incoming air by injecting before the supercharger? Just after the supercharger/before the intercooler? Just after the intercooler?
If you have a small engine (relatively), that you want as much boost as possible to make it competitive, you want the gasoline injected as early as possible.
If you have an air cooled engine, that you need as much cooling as possible in the cylinders, do as late as possible so it is still evaporating

5. became separate list

6. Already answered.

7. You're describing exactly what Allison build with the V-1710-127: Almost 3k hp and BSFC of 0.405 lb/hp.

View attachment 879778​

You just need a plane like the P-63 to be able to install it into.


8. Double edged sword: You don't necessarily want your airframe to be locked into a specific engine in case it fails. Whirlwind being point in case: Without the Peregrine, Westland didn't have an engine to power its fighter.

Conversely, you don't want your engine locked into a specific airframe for the same reason. If the Fw190A had failed, BMW might not have had a substantial market for its radial.

But overall, there was a lot of cooperation between airframe and engine manufacturers. And a lot of angst when thing changed between the mockup and the initial delivery.

9. Rolls-Royce is actually a pretty good example. The Griffon Mk. 1 was buried when it became apparent that fuel had moved on to 100 octane and beyond. So, a whole new engine was designed in late '39 and it finally began to overshadow the Merlin in '44.
6 more months and Vulture might have succeeded; but RAF didn't have 6 months. (Would have help if Manchester wasn't overweight and really needed 2,500 hp engines, not 1,750 hp ones.)
P&W R-2800 is about the same timeline.

10. Triptane

Not fuel additives, but Goring Ha-Ha gas (Nitrous Oxide), Water Injection and Nitromethane are all good, short duration, power boosters.​

About Point #4 - Direct fuel injection is very good, but not necessarily ideal. Rolls went with injecting fuel into the eye of the supercharger since it reduced the exit temperature of the fuel-air mixture, delaying detonation. (Like Water Injection) - This was enhanced by the introduction of Bendix-Stromberg Injection Carburetors, which provided fuel under pressure, providing consistent fuel flow under all conditions.
Some later engines - in particular some versions of the R3350 (Wright Cyclone 18) injected the fuel through the supercharger impeller, this provided not only a measure of charge cooling, but a more even distribution of the fuel-air mixture in the intake manifolds.
I don't know where you're getting the Direct Injection wasn't feasible in automotive engines until 10 years ago comes from - Spica mechanical fuel injection used in Alfa Romeo vehicles since the early 1930s. When Chevy introduced the 327 V-8 in the Corvette in 1962, it had Rochester Mechanical Fuel Injection as an option.
The P-26B and C Pursuits used mechanical direct injection R1340s. It wasn't new. It was, however, a P.I.T.A. to keep maintained for good performance.

 

[tomo pauk]

Pressure is pressure, doesn't matter how you measure it (what units).

How does multiplication work with psig, where the starting/sea level pressure is zero, and goes into negatives as we climb?

 

[tomo pauk]

As for the high-ethanol fuels.
it would've made a lot of sense for the Germans and other countries where the high-octane fuel in not readily available. Eg. on the Fw 190, where the E85-fueled DB 601E might've possibly run at 1.5-1.6 ata, and give comparable power to what the BMW 801C was making (as well as being lighter and more streamlined), and still not be a fuel hog the 801 was.

 

[don4331]

How does multiplication work with psig, where the starting/sea level pressure is zero, and goes into negatives as we climb?

You use psia (absolute pressure).

 

[tomo pauk]

You use psia (absolute pressure).

IOW, the measuring unit matters?

 

[AspiringHowardHughes]

Of course someone thought of the combination turbocharger and supercharger already.
Nothing new under the sun....
On the plus side, it is nice to see solid evidence that my idea had merit!

Thanks again for the answers, all of you.

 

[AspiringHowardHughes]

Going back to question number one, I have come into some evidence that pressure is actually not the characteristic of the exhaust gas that is being harnessed.
This video by Greg's Airplanes and Automobiles on turbo-compounding (power recovery turbines) is suggesting that there was actually no meaningful back-pressure generated by the turbines (which resemble those of the world war two turbochargers I mentioned earlier), that they operated using the velocity of the exhaust instead.
This claim is made around the twenty-minute mark.

View: https://www.youtube.com/watch?v=gbMgwDIdScY

This is understandable as a design choice (it makes installing a turbine onto an engine not originally designed for one easier) but would sacrifice what I know to be a serious benefit of turbochargers: The compressor producing an amount of boost associated with the exhaust pressure, making the whole system somewhat self-consistent.

Is this a reason making engine control systems work with turbochargers was so difficult?
Was the convenience of (allegedly) not creating much back-pressure important enough alone to necessitate this turbine design?
I hear claims that this design was chosen by General Electric partly because the developments would be more applicable to their steam turbine field. What truth is there to this?

 

[Simon Thomas]

It is always worth cross checking anything Greg says with actual facts and data. Typically most of his claims will generate a response of "sort of, but that isn't the whole picture."
In this case, it isn't an either/or situation. I am sure you know the kinetic energy formula.

 

[don4331]

Going back to question number one, I have come into some evidence that pressure is actually not the characteristic of the exhaust gas that is being harnessed.
This video by Greg's Airplanes and Automobiles on turbo-compounding (power recovery turbines) is suggesting that there was actually no meaningful back-pressure generated by the turbines (which resemble those of the world war two turbochargers I mentioned earlier), that they operated using the velocity of the exhaust instead.
This claim is made around the twenty-minute mark.

View: https://www.youtube.com/watch?v=gbMgwDIdScY

This is understandable as a design choice (it makes installing a turbine onto an engine not originally designed for one easier) but would sacrifice what I know to be a serious benefit of turbochargers: The compressor producing an amount of boost associated with the exhaust pressure, making the whole system somewhat self-consistent.

Is this a reason making engine control systems work with turbochargers was so difficult?
Was the convenience of (allegedly) not creating much back-pressure important enough alone to necessitate this turbine design?
I hear claims that this design was chosen by General Electric partly because the developments would be more applicable to their steam turbine field. What truth is there to this?

I would suggest you have that bass ackwards (and I attempted to make the same point in my response above).

GE's expertise was with steam turbines; so they were applying that knowledge to the exhaust turbine. And while the primary means of power recovery is velocity, there is a pressure reduction as well.
​

Recognize that the exhaust gas probably has more energy than the propeller is absorbing. And the compressor only needs a fraction of that power - even at 25k', the compressor for an Allison only needs about 20% of the energy. Why add back-pressure when you can't take advantage of it.

The distance between the turbocharger and the cold at altitude are the primary reasons that making engine control system work were so difficult, IMHO.

 

[AspiringHowardHughes]

Yeah, I had heard some concerns about how accurate that information is - particularly about the supposed "bomber mafia".

As I understand it, a maximally efficient power recovery system (or turbocharger, for that matter) would extract all pressure and velocity from the exhaust, leaving it "stationary" in the air as the plane continues onward. This same idea is what determines the most efficient rocket burns while in an elliptical orbit (Oberth effect, if my memory serves)
Obviously this is never the case for a real installation, but am interested in the methods designers used to approach this.

The comparison between impulse (velocity-driven) and reaction (pressure-driven) is well-presented in this image.

The flow restriction provided before the impulse waterwheel is perhaps needless for the engine comparison, as there is already high pressure available.
There would obviously be some additional obstruction of flow (and thus pressure increase) caused by the impulse waterwheel, but not a significant amount.
Of course, these examples represent two extremes. It is rare that turbomachinery selects either one of them over some degree of a hybrid solution, as far as I am aware.
Modern turbochargers, for example, use a hybrid design.
Evidently there was not enough incentive to use anything but an impulse turbocharger back in the day.

I would also note that while GE of course had plenty of previous experience with turbomachinery, war-related projects were a great source of government funding and thus were probably surging ahead of their domestic industrial design work as far as level of technological advancement goes. Just like every engine manufacturer during the war, really.

 

[Shortround6]

Without numbers it is very hard to say if the turbines were impulse or reaction.
Or even it maters much.
I don't have numbers for the Wright engines, the only ones to use turbo compounding in commercial service (?).
I do have numbers for the RR XX engine for the exhaust velocity, the charge mass (air+fuel) and calculate HP From the Rolls-Royce Heritage Trust book "the Performance of a Supercharged Aero Engine" Technical Series No 3. This are for simple jet thrust exhaust nozzles.
What is interesting to me is figures for the numbers above at different altitudes, I will list 3 out of the 6 listed in the book.

Altitude.......................................15,000ft..........................20,000ft....................30,000ft
Back pressure in hg...................23.2.................................22.3..............................16.3
Atmospheric pressure..............16.88..............................13.74............................8.88
Charge flow lb/min...................140.5..............................144.0...........................107.2
Exhaust Vel, Ft/sec.....................1395...............................1695.............................1901
Exhaust HP.....................................86.5................................113.0............................89.0

Notice that the exhaust HP is nearly the same at 15,000ft and 30,000ft but different flow weight/mass and different velocity of exhaust.
The change in back pressure to atmospheric pressure accounts for much of the difference in Exhaust velocity.
The turbines in the turbo charged engines or in the power recovery turbines have to operate at altitudes that are from around 5,000ft to 35,000ft and at different aircraft speeds and throttle settings full power or 60% cruise or lower? large changes in charge flow and the turbine system as to operate in these different conditions. Not a peak efficiency but it has to at least pay each own way (offset weight and bulk)

As I understand it, a maximally efficient power recovery system (or turbocharger, for that matter) would extract all pressure and velocity from the exhaust, leaving it "stationary" in the air as the plane continues onward..................................
Obviously this is never the case for a real installation, but am interested in the methods designers used to approach this.

Obviously such a solution is nonsense in the real world. Object of the Turbo charger is to spin an impeller at very high speeds. Pressure generated by the turbo impeller goes up the square of speed. Slowing down the turbo to make the turbine part more efficient makes the compressor section much less efficient............Unless you introduce a gear system to run the compressor impeller at higher speed than the turbine is running and you can design a highly efficient gear system that won't fail under the loads and conditions (the turbine section is running at hundreds of degrees and even on WW II turbos required special alloys for the housing and heat shields and cooling air forced though/around the turbine section.
A lot of trouble for a theoretical advantage.
Getting the exhaust gas pressure down even close to zero pressure and velocity means the exhaust gases are not moving in the pipes or even leaving the cylinders.
Look at the numbers again, At 20,000ft even the smallish Merlin XX engine is moving 144lbs of burned fuel and air per minute and moving it at 1695 fps (1155mph).
That 144lbs of mass is at around 1700 degrees F. slowing it down or stopping it is going to lead to a major meltdown real quick. If you are lucking the engine just shuts down real quick. If you can't get the exhaust out of the engine you can't the new air and fuel into the engine.

 

[don4331]

Getting the exhaust gas pressure down even close to zero pressure and velocity means the exhaust gases are not moving in the pipes or even leaving the cylinders.
Look at the numbers again, At 20,000ft even the smallish Merlin XX engine is moving 144lbs of burned fuel and air per minute and moving it at 1695 fps (1155mph).
That 144lbs of mass is at around 1700 degrees F. slowing it down or stopping it is going to lead to a major meltdown real quick. If you are lucking the engine just shuts down real quick. If you can't get the exhaust out of the engine you can't the new air and fuel into the engine.

Steam turbines get their exhaust gas pressure down to high vacuum pressures (0.1 atm or 1.5 psi) and the steam is still moving in the pipes/leaving the boilers.

We're getting into 4th year turbomachinery engineering...​

​

Also note: The turbine(s) have reduced temperature of the steam from 1,500 *F to <100 *F.

As I understand it, a maximally efficient power recovery system (or turbocharger, for that matter) would extract all pressure and velocity from the exhaust, leaving it "stationary" in the air as the plane continues onward.

The comparison between impulse (velocity-driven) and reaction (pressure-driven) is well-presented in this image.
View attachment 880034
The flow restriction provided before the impulse waterwheel is perhaps needless for the engine comparison, as there is already high pressure available.
There would obviously be some additional obstruction of flow (and thus pressure increase) caused by the impulse waterwheel, but not a significant amount.
Of course, these examples represent two extremes. It is rare that turbomachinery selects either one of them over some degree of a hybrid solution, as far as I am aware.
Modern turbochargers, for example, use a hybrid design.
Evidently there was not enough incentive to use anything but an impulse turbocharger back in the day.

I would also note that while GE of course had plenty of previous experience with turbomachinery, war-related projects were a great source of government funding and thus were probably surging ahead of their domestic industrial design work as far as level of technological advancement goes. Just like every engine manufacturer during the war, really.

I like the questions; hopefully my explanations help.

Yes, the maximal efficient power recovery system would extract all pressure, velocity and temperature from the exhaust, leaving it "stationary and at 'room temperature'" in the air. e.g. A high efficiency furnace allows the steam produced during combustion to condense into water, extracting all the energy from the condensation process. Unfortunately, a airplane can't afford the weight and volume of such a system and there is no ability to take advantage of the energy of condensation - which is why ICE engines can't get much over 50% efficiency (to get to 100% theoretical efficiency, you would have to cool the exhaust to absolute 0* to take advantage of condensation of the CO2 and air in the exhaust).

An exhaust turbine has an efficiency curve very similar to the compressor curve above. So, it is only very efficient at a narrow band of Pressure differential, mass/flow and speed. Also note, just like the compressor, as you reduce velocity/pressure (when allowing the exhaust to expand upon exiting), the result is the temperature is reduced.

If you compare the Allison with Power Recovery Turbine to the best of the 2nd stage Allison's - V-1710-143:

The V-1710-143 outputs 2,250hp to the propeller, and sends 490hp to the 1st and 2nd stage superchargers.​

The V-1710-127 turbine removes the supercharger load (the 490hp) for 2,730 hp at propeller + adds another 250 hp for the 2,980 hp at the propeller. All with just a single stage turbine!​

​

This is why the engine designers were getting all excited about jet engines. If you had the materials to develop multi stage turbines, you could make huge amounts of power - and put those ICE boys out of business.

Also note GE is getting cubic $$$ on the steam turbine side from the gov't for all the destroyers/cruisers/carriers/battleships in the navy. And the steam turbine improvements directly translated to domestic industry.

 

[Tom Fey]

Spark plug (cooling) blast tubes on the Allison V-1710's used in the P-38 Lightning and perhaps other installations as well.

 

[tomo pauk]

FWIW, a video where the E85 (ie. 85% of ethanol) is tested against a very high octane fuel, as well as against the 93 oct fuel, on a naturally-aspirated engine. 550 HP vs. 521 vs. 430. One comment under the video:

@deathscythehell7937

3 years ago (edited)
Yes, yes, yes finally a true same day test of E-85. I've been running E-85 in all my street/race cars for the last 5yrs. I own 4 supercharged Shelby's running E-85 all are 1000rwhp except for one making 925rwhp. Thinking about ditching the 427 & procharger and going to a Whipple 5.0 or 5.2 to crack 1000rwhp, a 2600lbs car with 1000rwhp, ooooh scary. I've known what E-85 is capable of for a while now, I wouldn't go back to 93 or 110 if you paid for the fuel. Yes you sacrifice some fuel mileage, but running 24lbs of boost on a street car can't be beat. Nice to see you spreading the info on to people who aren't able to get to a dyno and experiment for themselves. Keep up the good informative work.

VIDEO