Hitech, Fuel consumption questions

[VO101_Isegrim]

Very good discussion ! Congrats, Gentlemen !


I`d like to ask a question on supercharger charactertics... it can be seen on all fixed-gear superchargers, that engine power output is less below critical altitude. Taking the 1st (fixed) speed of the DB 605A-1 for example, one can see that the engine power output is 1475 PS at SL, and steadily increasing to 1550 PS at around 2100m. Yet the boost pressure, RPM is constant.

This is because of "the extra work the SC has to do", as I have read....

Now, what is this extra work?

I have been thinking on this and the only explanation I could arrive at is that the SC, being fixed to a ratio towards the engine RPM, is trying to run at constant speed, compressing air in the process. Now, when it operates at lower altitudes, whereas it`s compressive capability has surplus, it overcompresses the air too much... since compressing already dense air is harder than thin air, it needs more power to keep supercharger RPM constant. It needs more torque. So it drains more power from engine.

Now, my question is, is my theory correct, or there is/are other reason(s)?

[Tilt]

Originally posted by joeblogs
There is a boost control lever in the Lavochkin......but again I always assumed that the two cables coming from it enabled either one super charger speed stage selection or another....


Blogs: I'm not sure.  Is that the propeller governor?

 

Thank you and ho hun for pointing out that throttle setting can adjust manifold pressure....... obviously I missed that flow rate via throttle is a function of pressure differential across the variable choke point.

Re La boost control its a big lever with two apparant positions attached to a wheel which push/pulls a boden cable...........actually the La9 now flying should be the same control system.

There was an additional "airscrew control" but the prop was a constant speed unit....... I assume this would be used to set engine speed as using the prop as variable ratio gear box.

To summarise

I can set manifold pressure as a function of rpm, stage of boost and throttle.

I can set rpm as a function of prop speed

the r7 govenor is a mystery .. could it be used to limit over rev?

would control via throttle or by prop pitch? how is this normally done on other aero engines?

The link between throttle and fuel mixture was directly copied from  an FW190

[joeblogs]

I don't believe the high output numbers on the fuel consumption of the DB605.  But I need to look at the document before I can say anything.

-Blogs

Originally posted by GScholz
That means that the R2800-8 consumed more than twice the amount of fuel at mil power while producing less than 200 hp more. I know German engineering was superb, but this sound incredible. You sure about this?

EDIT: Confused the alts. Still at 290 gph producing 2000 hp is horribly inefficient compared to your numbers on the DB605.

[joeblogs]

My guess is that if the supercharger is engaged, but the throttle is not fully advanced, you are paying a penalty for diverting power to the supercharger and not getting the most out of it.  In the absence of the supercharger, that power would go to the propeller.

This also appears in the German engines where the supercharger is connected to the engine with a continuously variable clutch.  At medium altittudes the supercharger is spinning, but not all that fast.  The result is diverted power and not much increase in manifold pressure.

If you didn't hold back the throttle at low altitudes, you would get too much compressed air.  In other words manifold pressure would be so high you would either produce detonation or simply blow somthing off the induction system.  

On the U.S. engines, you can take off with the supercharger disengaged, so there is no penalty.  At a higher altitude you would engage it at the lower of two gear ratios, immediatley pulling back on the throttle to avoid excessive manifold pressure.  This would be repeated again above the critical altitude of the supercharger in the low blower ratio. In a two stage engine, there would be one more of these steps at a quite high altitude.

In the example you use, the one difference across the altitudes is that the throttle is further advanced at higher altitudes, so that the supercharger is adding more pressure than it does at a lower throttle setting at a lower altitude.

-Blogs

Originally posted by VO101_Isegrim
Very good discussion ! Congrats, Gentlemen !


I`d like to ask a question on supercharger charactertics... it can be seen on all fixed-gear superchargers, that engine power output is less below critical altitude. Taking the 1st (fixed) speed of the DB 605A-1 for example, one can see that the engine power output is 1475 PS at SL, and steadily increasing to 1550 PS at around 2100m. Yet the boost pressure, RPM is constant.

This is because of "the extra work the SC has to do", as I have read....

Now, what is this extra work?

I have been thinking on this and the only explanation I could arrive at is that the SC, being fixed to a ratio towards the engine RPM, is trying to run at constant speed, compressing air in the process. Now, when it operates at lower altitudes, whereas it`s compressive capability has surplus, it overcompresses the air too much... since compressing already dense air is harder than thin air, it needs more power to keep supercharger RPM constant. It needs more torque. So it drains more power from engine.

Now, my question is, is my theory correct, or there is/are other reason(s)?

[joeblogs]

The airscrew control should tell the constant speed unit how fast the propeller, and therefore the engine should turn (it's not 1:1 as there is typically a reduction gear somewhere in between).

I'm not sure what the boost control does, but I've seen similar switches on some British inlines.  It may be an easy way to change manifold pressure when you are engaging different gears on the supercharger, but I am not sure on that. This is especially important for engines running at very high manifold pressures at low altitudes.

I suspect the R7 is an overspeed control for dives.

A good source for information on actually running one of these high output engines is Bent McKinley's Aircraft Powerplants. Also a number of people have posted links to a series of excellent articles written for a pilot's magazine.  If I can find the links, I'll post them here.

-blogs

Originally posted by Tilt
Thank you and ho hun for pointing out that throttle setting can adjust manifold pressure....... obviously I missed that flow rate via throttle is a function of pressure differential across the variable choke point.

Re La boost control its a big lever with two apparant positions attached to a wheel which push/pulls a boden cable...........actually the La9 now flying should be the same control system.

There was an additional "airscrew control" but the prop was a constant speed unit....... I assume this would be used to set engine speed as using the prop as variable ratio gear box.

To summarise

I can set manifold pressure as a function of rpm, stage of boost and throttle.

I can set rpm as a function of prop speed

the r7 govenor is a mystery .. could it be used to limit over rev?

would control via throttle or by prop pitch? how is this normally done on other aero engines?

The link between throttle and fuel mixture was directly copied from  an FW190

[joeblogs]

the column is Pelican's Perch from Avweb:

http://www.avweb.com/news/columns/182146-1.html

[vorticon]

always good how people use there own charts as reference...


get at least 1 more reference on that (preferably in book form) then whine

[joeblogs]

The first chart F4u1doa posted is based on data from a specific engine chart for the corsair fighter.  I made that chart from a scan of the original US Navy document. He's also just found some new data on the ASH-82 FN engine.  

Tilt is citing a Russian flight test report for La-7 using that engine.  

Batz posted a chart for the 109e4.  Isegrim is posting engineering data for the DB605 engine.  

All of this information is from primary sources, although translations are sometimes a problem.  

My data is collected from a few hundred engine specifications published in the 1940s and a number of engineering books published during that period.

So what's the problem?

-blogs

Originally posted by vorticon
always good how people use there own charts as reference...


get at least 1 more reference on that (preferably in book form) then whine

[HoHun]

Hi Isegrim,

>it overcompresses the air too much... since compressing already dense air is harder than thin air, it needs more power to keep supercharger RPM constant. It needs more torque. So it drains more power from engine.

>Now, my question is, is my theory correct, or there is/are other reason(s)?

That's how I understand it, too.

I think Tilt's comment shows where the surplus power vanishes to:

"flow rate via throttle is a function of pressure differential across the variable choke point"

Here's the title of an engine book I found very enlightening even though it doesn't mention aviation at all:

"Motorcycle Turbocharging, Supercharging & Nitrous Oxide" by Joe Haile.

Except for computerized fuel injection, pretty much all of the technology he describes was employed on WW2 fighters, too :-)

The great thing about the book is that (on merely 200 pages) it has in-depth explanations that are easy to read, with a good measure of drawings and photographs (and motorcycle examples of course).

The author is not an engineer, but a journalist - which may explain the great readability ;-)

Regards,

Henning (HoHun)

[HoHun]

Hi Tilt,

>ASh82FNV engine speed was controlled by an R7 govenor. I assume this works by effecting the throttle in some way to limit the system at maximum speeds?

Could it be that the R7 unit actually controls the throttle (and perhaps mixture etc.) to make sure any throttle lever position corresponds to a constant boost?

Without any automated system, the pilot would have to keep an eye on the boost indicator continuously and manually change the throttle setting constantly to adapt it to changing altitude, outside air temperature, ram pressure etc.

Regards,

Henning (HoHun)

[joeblogs]

Any idea what PS means?

Am I right that most continental calculations of specific fuel consumption are in grams/ps/hour?

-Blogs

Originally posted by 214thCavalier
This may be useful for comparison.

One PS is approx 98.6% of a HP

To convert from BHP to PS multiply by 1.01387

[GScholz]

PS is metric HP. I guess different averaging in the calculations result in the slight power difference.

[HoHun]

Hi Joe,

>Any idea what PS means?

Pferdestärke = horse power.

>Am I right that most continental calculations of specific fuel consumption are in grams/ps/hour?

Yes.

Regards,

Henning (HoHun)

[F4UDOA]

Vorticon,

Are you serious? That chart in the beggining is right out of the F4U and F6F manuals. JoeB just added the SFC calculations to make it easy.

If you thing any of this is wrong please point it out old boy.

Gsholz,

I have a German doc that says that is the right consumption. I find it hard to believe myself.

What I really don't get it how the consumption doesn't worse with increased power. It just seems to stay the same.

Of course the La-5/7 is the greatest mystery so far.

[F4UDOA]

Anybody know how much MW50 the 109G10 carried?