'Magic' Superchargers

[MatzeH84 81]

 

2 hours ago, scandinavian13 said:

All the same, if we're stuffing random assertions into my mouth and running with it:

Well, obviously you do, because I never said you did an official statement. I was asking the question if this actually was an official statement.

Let's state the obvious: Switching the superchargers from high to low will decrease the manifold pressure. Why is that? Because the impeller turns less fast of course, thus producing less MP. I guess everybody is fine with that. Now what happens if you reduce the engine RPM- of course you will reduce the manifold pressure as well, because the impeller will also turn slower.

What makes it really difficult is the fact that charts and diagrams for this engine are hard to get on the internet for the regular user. As a partner of Boeing, I could imagine that PMDG has better access to the according charts.

This chart here is taken from the official maintenance manual, which goes the other way around, but in my eyes can be used to show the principle.

In the upper right corner you would see that a decrease of 100RPM would cause a drop of 2 inches MP. In this chart it's the other way around, because the intend of this manual is to calibrate the engine to fixed values. However, it shows the relationship.

This topic was also discussed at A2A's forum, as both their competing propliners show the exactly same behaviour as the DC-6 regarding supercharger performance and MP reaction. The developers admitted they used the supercharger modeling implemented by Microsoft, which exactly reproduces the current MP reaction. Their custom coded warbirds circumnavigate this issue and feature the correct MP reaction, where MP is influenced by RPM; for example the P-47, which in turn is equipped with earlier versions of the R-2800.

Due to all these informations, it seems to the experienced and technically interested customer, that the current supercharger modeling uses the standard model, which neglects the dependence of MP from RPM.

No offense with anything intended, english isn't my mother tongue.

[downscc 1,327]

I like that graph but I have a question regarding CARB UPPER DECK PRESSURE.  Upper deck pressure is normally associated with turbocharging and is the pressure output of the compressor stage at the throttle plate. In the supercharged engine, the carburetor is on the suction side of the compressor so I assume the carb upper deck pressure is at the intake to the carburator, the rise from 25 inHg to 56 inHg MP is the 31 inHg of boost provided by the compressor and includes the loss through the carburetor. Correct me if I've made the wrong assumption.  I understand the variance in MAP due to RPM is essentially a variance in compressor boost.  This implies that the upper deck pressure is held constant and the throttle plates are wide open (unobstructed).  This is where my engineer thinking gets in the way.  In your example, using a load generated by the props to reduce RPM, I don't think its likely that the carb upper deck pressure remains the same because of the decrease air flow.  So we are back to the basic problem in that we are lacking data for a sufficient analysis.  The curve you submit as evidence is valid evidence but only  for a specific set of conditions, which are not maintained in your decreasing RPM scenario.

Very interesting topic, I sincerely appreciate the effort you are displaying in presenting your thesis.

[randomTOTEN 13]

11 hours ago, scandinavian13 said:

I get that you finished with "in my opinion," but please provide references, data, and background if you're going to make statements like this.

Hi,

Please see the post above the one you just quoted. Specifically, "The only way BMEP would increase is if I decreased RPM while maintaining the same power output. My argument is that power output decreases with a reduction of engine RPM due to the design nature of a supercharger as opposed to turbosupercharging." And,

On 7/30/2017 at 6:35 PM, randomTOTEN said:

Quote

but you are ignoring that when the engine turns slower it requires less air by weight.

So then why is an engine with an automatic mixture adjustment producing more (or equal) power when being provided with less air.. by weight?

 Another user has already proved a chart for the R-2800 (including CB-16) Which demonstrates a 2" correction which must be applied for a variation of as little as 100RPM.

I submit the following references for this information:

"The actual power output of any reciprocating engine is a direct function of the combination of engine torque and rotative speed... If all other factors are constant, the engine power output is directly related to the engine airflow [because fuel meters with air, this must be true]... The pressure received by the supercharger is magnified by the supercharger in some proportion depending on impeller speed....

Of course, the engine airflow is a function of the RPM for two reasons. A higher engine speed increases the pumping rate and the volume flow through the engine. Also, with an engine driven supercharger or impeller, and increase in engine speed increases the supercharger pressure ratio. With the exception of near closed throttle position, an increase in engine speed will produce an increase in manifold pressure." (pg. 137-138)

"As altitude is increased with the supercharger or "blower" at low speed, the constant MAP is maintained by opening the throttle and the BHP increases above the sea level value because of the reduced exhaust back pressure. Opening the throttle allows the supercharger inlet to receive the same inlet pressure and produce the same MAP. Finally, the increase of altitude will require full throttle to produce the constant MAP with low blower and this point is termed the "critical altitude" or "full throttle height." If the altitude is increased beyond the critical altitude, the engine MAP, airflow, and BHP decrease.

The critical altitude with a particular supercharger installation is specific to a given combination of MAP and RPM. Obviously, a lower MAP could be maintained to some higher altitude or a lower engine speed would produce less supercharging and a given MAP would require a greater throttle opening." (p. 143)

Hurt Jr., H. H. (1960). Aerodynamics for naval aviators (Report no. NAVWEPS 00-80T-80). Direction of Commander, Naval Air Systems Command United States Navy.

 

Quote

 

There are three cockpit controls that affect supercharger operations:

The throttle lever.

The propeller control lever.

The supercharger gear-selection lever.

1. The throttle lever position determines the boost pressure that is delivered by the supercharger and maintained constant up to the rated height by the boost control unit. It is a boost selection lever and in conjunction with the propeller control lever determines the power output of the engine. In addition its position largely controls the amount of fuel entering the engine and both power and enrichment jets are interconnected with it.
2. On propellers fitted to most engines the blade angle can, within limits, be adjusted by the pilot to increase or decrease speed, and the selected speed will be maintained by the action of the constant speed governor unit. The propeller control lever hence becomes in effect an engine speed control lever.
3. By means of the supercharger gear lever, the pilot can select high or low speed and it is very important to understand the effect of the two speeds on the power output of the engine. In either gear ratio, the boost pressure recorded on the gauge is identical and depends on the throttle lever position. But the power output of the engine is very different.

...

The propeller control lever has a marked effect on supercharger efficiency, for it will be remembered that the supercharger is geared to the engine crankshaft, and consequently changes in engine speed are reproduced in the supercharger.

 

 

Bristol Superchargers (quoted for theory) http://enginehistory.org/Turbochargers/Superchargers/BristolSuperchargers.shtml

Quote

Centrifugal superchargers add additional boost as the speed of the motor increases. This means the centrifugal supercharger will provide more horsepower at high engine speeds and less boost at lower engine speeds.

https://en.wikipedia.org/wiki/Centrifugal-type_supercharger#Power_output

(quote not directly cited)

Quote

The pilot controls the output of the supercharger with the throttle and indirectly via the propeller governor control

https://en.wikipedia.org/wiki/Supercharger#Altitude_effects

Again,

As MatzeH84 pointed out (before I was able to), if the propeller speed governor has no effect on supercharger speed... THEN the entire purpose of having a dual speed supercharger is redundant. What's the point of changing the gearing ratio on the supercharger, if the speed of the supercharger has no effect on it's boost output?

Just what is happening when an engine is switched to "high blower?" Why is MAP increasing?

Thanks,

Robert Toten

 

[randomTOTEN 13]

2 hours ago, downscc said:

This is where my engineer thinking gets in the way.  In your example, using a load generated by the props to reduce RPM, I don't think its likely that the carb upper deck pressure remains the same because of the decrease air flow.

By what ratio? Assuming ISA, at 25" the throttle plate is only restricting airflow to reduce pressure 5" (17%) below ambient. Yet so little as a 100 RPM variance results in 2" change of boost pressure? How drastic could that 5" be reduced by a 4% change in RPM? That 4% change results in 2" lost.. or near half of the total restriction caused by the throttle plate at the carburetor...

Remember, this test is still at sea level. Does it apply to an engine operating in ambient pressure of less than 18", but still putting out ~35" to the cylinders?

Robert Toten

[MatzeH84 81]

7 hours ago, downscc said:

I like that graph but I have a question regarding CARB UPPER DECK PRESSURE.  Upper deck pressure is normally associated with turbocharging and is the pressure output of the compressor stage at the throttle plate. In the supercharged engine, the carburetor is on the suction side of the compressor so I assume the carb upper deck pressure is at the intake to the carburator, the rise from 25 inHg to 56 inHg MP is the 31 inHg of boost provided by the compressor and includes the loss through the carburetor. Correct me if I've made the wrong assumption.  I understand the variance in MAP due to RPM is essentially a variance in compressor boost.  This implies that the upper deck pressure is held constant and the throttle plates are wide open (unobstructed).  This is where my engineer thinking gets in the way.  In your example, using a load generated by the props to reduce RPM, I don't think its likely that the carb upper deck pressure remains the same because of the decrease air flow.  So we are back to the basic problem in that we are lacking data for a sufficient analysis.  The curve you submit as evidence is valid evidence but only  for a specific set of conditions, which are not maintained in your decreasing RPM scenario.

Very interesting topic, I sincerely appreciate the effort you are displaying in presenting your thesis.

I searched the net several hours yesterday in an attempt to find any graphs showing the relationship between RPM and MP with throttle untouched, however I couldn't find such data. I guess there may be such data available for similar engines, however I think it would be refused as not referring to the R2800-CB16. Thus, the best curve I found was the one I presented.

From my knowledge, I'd say you are fully correct when you state the carb upper deck pressure will drop when the RPM decreases. The chart suggests this as well,assuming MP drops with decreasing RPM (relationship mp-carb upper deck pressure in the chart, assuming constant RPM, but in my eyes the relationship can't change). So assuming we fly at a pressure altitude where we achieve a good MP within engine limits with throttle fully open, reducing the RPM would first cause a torque peak due to engine momentum, then the torque will settle in with the new parameters. At the same time, the impeller speed drops together with engine RPM, producing less MP, and therefore a lower carb upper deck pressure. This is resulting in less fuel being introduced by the carburetor due to lower air demand, thus the output power decreases (less air, less fuel, less RPM). If you would do this with a partial throttle setting, you would then be able to slightly increase the MP again to obtain the desired power setting with the RPM set.

However, as you already said, without proper curves it is not possible to provide hard numbers.

[scandinavian13 5,192]

There it is...

See...now we have a discussion based on some sort of supporting facts. Thank you.

World of difference, no?

[downscc 1,327]

I cannot follow that Matthais, I have a hard time coming up with a scenario where I decrease RPM without first decreasing MP so I don't know what to expect.  Always decrease RPM before MP and always increase RPM before MP.  That is how the model is designed too I suspect. The transient behavior of how MP lags the throttle for example is realistic and displays some of the supercharger RPM vs MP pressure dependence that this is all about.

[MatzeH84 81]

Dan, the principle stays the same. Of course you would follow throttle->prop->mixture when reducing power. Reduce from climb MP to cruise MP, then reduce to cruise RPM => MP will drop below the cruise setting previously set and has to be reset once more to obtain the cruise rating.

Right now it is, reduce MP, reduce RPM, and you're set.

[downscc 1,327]

7 hours ago, MatzeH84 said:

Dan, the principle stays the same. Of course you would follow throttle->prop->mixture when reducing power. Reduce from climb MP to cruise MP, then reduce to cruise RPM => MP will drop below the cruise setting previously set and has to be reset once more to obtain the cruise rating.

Right now it is, reduce MP, reduce RPM, and you're set.

Now you're making sense to me.  I agree, I wonder how much the drop in MP is in the typical scenario where I reduce MP from 40 to 30 inHg then RPM from 2400 to 2200.  i wonder if the decrease in MP with the 200 RPM drop would be more than 1 inHg?  Your chart indicates a 2 in drop for 100 RPM but at a constant upper deck pressure and in this case upper deck pressure will decrease along with RPM (and the increased throttle plate restriction).  Given that it's a 8.3% drop in RPM will the upper deck pressure drop more or less than 8.3%... I don't know...I'm the wrong kind of engineer (BSEE). Then with a given change in  upper deck pressure how does that add to a 200 RPM change.  I expect a change but is it more or less than 1 inHg (which is 3.3% of 30 inHg).  My guess is that the MP drop would be less than 8.3% but how much less, don't know.  I suspect the drop is minimal because none of the DC6 pilots called it out.  We have a DC3 pilot who says that setting the MP is always a  process, you set it and then set it again and maybe again.  We are not seeing that.

EDIT:  Also just realized upper deck pressure would be less of a vacuum when RPM decreases such that it trends towards ambient pressure.  Those upper deck pressure numbers on your chart probably represent something the A&P would see in a static test at sea level.  I wonder what the upper deck pressure to MP curve looks like at upper deck pressures in the teens? Is it steeper or flatter?  I suspect flatter as we will be on the mid to upper part of the compressor curve.

Good topic.

[randomTOTEN 13]

I found another source which is relevant to this discussion.

Quote

Gear-Driven Centrifugal Blowers

A centrifugal compressor consists of an impeller and a diffuser housed in a helical casing, or scroll. The diffuser, sometimes called the stator, occupies the annular space between the impeller and scroll. Passages created by the diffuser vanes open wider as they approach the discharge throat. Vanes on the impeller wheel are arranged radially and may be straight or curved. The use of curved vanes came relatively late in the period and improved efficiency.

Air enters at the impeller hub, rotates with the impeller and, under the influence of centrifugal force, moves outward in a path defined by the impeller vanes. Upon contact with the diffuser, the air expands and slows, converting much of its kinetic energy into static pressure.

Because the impeller cannot be allowed to make physical contact with the shroud, there is always some leakage between the vane tips and the scroll. The seal consists of air, an elastic medium. At low rotational speeds the impeller merely flays about delivering little or no output. As tip velocity increases, the air seal becomes more positive and the compressor begins to pump. Unlike Roots blowers that move the same volume of air per revolution, centrifugal compressors are dynamic machines, whose output increases as the square of speed—double the speed and the output theoretically quadruples.

http://enginehistory.org/Piston/InterWarSCdev/InterWarSCdev.shtml

Robert Toten

[CrashTronic 3]

I submit John Deakin's excellent engine management articles

https://www.avweb.com/news/pelican/Pelicans-Perch-15-Manifold-Pressure-Sucks-182081-1.html

https://www.avweb.com/news/pelican/182544-1.html

The first article on, "Those fire breathing turbos" covers supercharging

I see some concept obfuscation going on. Let's get everyone on the same page. I remember this discussion from "that other also amazing propliner"(tm)

some background: Manifold Pressure is the absolute barometric pressure the engine is operating at. Case in point, what is the difference between a blocked intake and a closed throttle....none. To determine power production in ANY piston engine you need 3 pieces of information, 1) how much air (MAP), How much fuel (PPH), and how fast the combustion occurs (RPM). they are all proportional, increase one and you increase total power. 

Quote

Assume you're cruising at some low altitude (say 4,000 feet), throttled well back to about 20 inches MP and 2,000 RPM. (Remember, this means the throttle plate is somewhat cocked, restricting induction airflow.) Now reduce the RPM to 1,200 without changing anything else, and you'll see the MP rise sharply. Why? Simple: The ambient pressure hasn't changed; the throttle plate hasn't changed; the only thing that has changed is the speed at which the pistons are pumping the air. Since they are moving much more slowly at the lower RPM, they are not sucking nearly as hard — not creating as much of a vacuum — so the MP goes up, towards ambient pressure. The natural extension of this experiment is to reduce the RPM to zero, when the MP will rise all the way to outside ambient pressure (about 25 inches at 4,000 feet).

When you slow down a super charged engine, the supercharger will spin more slowly but the effect of slowing down the pistons has a larger effect on the MAP than the slowing of the supercharger.

 

Go read those articles they are amazing and very informative

[downscc 1,327]

Robert:  I found a generic gas centrifugal performance curve in a Wiki article that is representative of actual curves I've seen for other applications:

The N lines are constant speed lines and flow rate is related to pressure ratio as a function of the rotor speed. Quantitative values for this family of curves can be easily found for auto racing compressors and turbocharger, I have not found one yet for the two speed R-2800 supercharger.  I can play with the above typical by presuming P&W is maintaining something along the peak efficiency line, then our decrease RPM scenario is accomplished by moving from one N line to another while also moving to the left as a result of less flow.  I can imagine a small decrease in speed and a realitively significant change in flow (as in the change from 2400/40 to 2200/30) maintaining a relatively uniform pressure ratio (supercharger boost directly related to MP).

9 minutes ago, CrashTronic said:

1) how much air (MAP), How much fuel (PPH), and how fast the combustion occurs (RPM). they are all proportional, increase one and you increase total power. 

Not so fast Mike, I will read your references but how much air might better be measured by SCUF (standard cubic feet, a standard unit of air mass) than pressure.  I can pack a lot more air by weight at the same pressure if it is cooler.  That's why RAM puts intercoolers on our converted TSIO-520s in the Chancellor.

 

[randomTOTEN 13]

downscc,

Quote

I can imagine a small decrease in speed and a realitively significant change in flow (as in the change from 2400/40 to 2200/30) maintaining a relatively uniform pressure ratio (supercharger boost directly related to MP).

What do you mean by "uniform pressure ratio"?

The chart you posted depicts pressure ratio on the vertical axis.

Thanks,

Robert Toten

[downscc 1,327]

Correct, the vertical axis is the pressure ratio p₁/p₀  and  what I mean is the pressure ratio might change very little if the flow rate change is much more in comparison to the compressor speed change.  For example, start at N₃ at the peak efficiency line.  It doesn't matter where because this is a generic chart without dimensions but I pick N₃ because my example is climb power. From that start point reduce the flow rate so move to the left on the N₃ line any distance but not near the surge lines and the pressure ratio increases and this shows up as a lag in the MP gauge as you decrease throttles. Ok, now decrease compressor speed but since my example is only  a change from 2400 to 2200 (representing a change from climb power to cruise power setting of 2200/30), which is less than 10%, we are not going from the N₃ to the N₂ line which might be as much as a 20% change but midway between the N₃ and N₂ line.  Note we end up with about the same pressure ratio as we started out with.

I'm not trying to prove anything with this example.  If we could find this curve for the R-2800 two speed blower then there would not be much of a discussion (I got close finding HP and performance curves but not a compressor curve).  I am only trying to establish that it is not unreasonable to believe that the DC6 MP behavior when changing from climb power to cruise power is realistic.  I don't see why this wouldn't hold true that it is also reasonable for any other power change of the same magnitudes.

What we need is a DC6 pilot to chime in on this, that would be the easiest way to settle it.  Next time we PMDG gets the beta team together I will raise this issue.

[randomTOTEN 13]

downscc,

We need to remove the throttle plates from your engines You're seriously getting hung up on those!

You'll see in my OP I went to an altitude where the air was thin, and where I purposely left the throttles fully open, to remove any 'complications' arising from restricting inlet airflow via the throttle plate. There are no restrictions to airflow that are dependent on a suction action from the engine to reduce MAP. We're wide open baby

It does appear that you acknowledge there is a problem with the modelling, now we're just describing the finer points.

 

You said you're an engineer, and of that I have no doubt. You're adding as much complexity into this conversation as you can find, and that's okay.. it's another "future feature" which will make these engines even more of a challenge to the average FSX user. That's why we enjoy PMDG products (and why your type can be a very demanding student.)

I'm not a mathematical thinker, but I am a visual one. So.. I've plotted your scenario from my perspective. Lets go through your scenario in great detail, plotting the changes on the chart:

Quote

For example, start at N3 at the peak efficiency line... From that start point reduce the flow rate so move to the left on the N3 line any distance but not near the surge lines and the pressure ratio increases and this shows up as a lag in the MP gauge as you decrease throttles. Ok, now decrease compressor speed but since my example is only  a change from 2400 to 2200

Note we end up with about the same pressure ratio as we started out with

All of this is now plotted. A pilot is tasked to reduce from climb power (2400 RPM and 40" MAP) to cruise power (2200 RPM and 30" MAP). These are perfectly fine settings under normal conditions. This pilot is trained and knowledgeable, and knows that power reductions start with manifold pressure before RPM reduction. This is to respect internal pressures inside the cylinders.

Condition A: The supercharger is producing climb MAP from an engine turning at climb RPM.

Condition B: The pilot reduces the throttles to first set cruise manifold pressure. As you mentioned, the decrease flow will increase the pressure ratio, and the rate at which MAP falls will not be proportional to amount of throttle decrease (interesting lesson). HOWEVER, the pilot is not concerned with this. The pilot has no way of calculating pressure ratio, and no interest. This is engineering data, not aircraft operating data. The pilot is tasked with setting cruise MAP, and will continue pulling the mechanical throttle levers until the MAP gauge in front of his face indicates 30". At that point, we have arrived at B. The MAP is 30" and the RPM is still in climb RPM of 2400.

Condition C: This is the part that I think you're struggling to grasp, and the event which is not currently found in the PMDG DC-6. After safely reducing MAP, the pilot now pulls back the RPM to 2200. Our supercharger output curve moves in response to the change (between N2 and N3). Is the new pressure ratio the same as A? YES! Is the new output pressure the same as A (40")? NO! Is the output pressure at C going to be the same as B? NO! because our decrease in RPM shifted the curve. You can mention the decreased suction behind the throttle plate, but you're still tripping over that plate lol. I reflected that by following the efficiency lines which shows a slight decrease in flow rate. You might get some slight increase in inlet MAP, but look how substantial that pressure ratio changes.

Robert Toten

 

EDIT: Another great thing this graph shows is the prediction that a change in Carburetor Temperature (CAT) will have a effect on air density, thus the flow rate. There should be noticeable changes in MAP based on changes in CAT.

Also, condition A is off the peak efficiency line because it was easier to draw in MS Paint