Hitech, Fuel consumption questions

[F4UDOA]

I have found some German PDF docs that show the SFC of the DB605 at 5.1 at a mil power of 1475HP.

This is 127GPH and a duration of exactly 50minutes meaning 25 minutes in the MA.

Chalk one up for HTC, they hit that nail on the head.

If compression in the secret to greater efficiency then explain this.

I just found the ASH-82FN compression is at 6.9 to 1.

This is not very high, how would you explain the duration of that engine??

[Tilt]

FYI compression ratios on Ash82 FNV

1st stage boost 7.14:1

2nd stage boost 10:1

I can see how higher compression ratio's would increase engine efficiency re conversion of fuel Kcal/hour into Kw's or HP.

Mechanical losses would be much the same at the same rpm and so efficiency would be increased even after the additional boster losses are subtracted.

Having said all this.......IMO it would be the application of a minor modifier when compared to the descrepencies F4UDOA describes.....

A direct corellation between HP and fuel consumption would be more accurate generally than we  have now IMO. Having achieved this it would then seem time to add modifiers re differing engine efficiencies.

All this is to nought if the corresponding (lower) consumptions at lower levels of rpm and boost can not be modelled in some way to allow the player to reach target albeit some what slower but with the same ratio of endurance and range enjoyed (or suffered) by those ac his ride represents.

Equally  should be considered is the attritional fuel model............

Fuel attrition did not limit short range AC (to shorter ranges) over long range AC as our % attrition model does now.

It limited range ...ie the number of gallons or litres available per mission.

If 100% = 600 litres then the 300litre capacity AC should still be granted a full tank at 50% attrition.

Whilst variable consumption with engine settings and the % attrition model remain as they are then I would argue that correcting actual consumption models would produce as many errors as they cure.

Having said that this does not reflect upon some glaring range issues with some AC (mossie and Ju88 come to mind)

[joeblogs]

F4u1doa is right on about fuel consumption at cruise and max HP settings.  Good engines could cruise with an SFC (in U.S. units) on the order of 0.41-0.45.  That's aviation gas only, not diesel.  Those numbers are based on my analysis of data on several hundred high output engines produced by all major combatants before and during WWII.

At military settings, SFC rises to 0.8 or much higher depending on the engine and altitude (power consumed by the supercharger).  Using WEP will lower fuel consumption as water is used to cool the charge, so the fuel mixture can be leaned out.  

Rich fuel mixtures in these engines is almost twice that of the lean (cruise) mixtures which explains the near doubling of SFC.  

In earlier posts I reported the best SFC of the ASH82FN was in the low 0.4x range, about the same as its western contemporaries.  There is no way then, using the same technology, this engine would have an SFC much below 0.8 at altitude and at military power.  Yet the endurance of the La-7 at military power in Aces High implies an SFC of an engine operating on a cruise setting.

It is true that higher compression is typically associated with better fuel economy.  But this only changes SFC by about ten percent and in some engines even less.  

An engine can have a better SFC at a lower compression ratio if either (1) it breathes better, giving a more even fuel mixture, or (2) it is cooled better.  So it is not immediately obvious to me that a Merlin would be all that less fuel efficient than a DB601.  

In my data the fuel economy of the Merlin varies considerably by its model number.  In addition, the Merlin was generally run on higher octane fuels, which is often associated with better fuel economy because the mixture can be leaned out more than when using a lower octane fuel.  An offsetting variable is the use of fuel injection in german engines versus a carburetor on most other engines.

-Blogs

[joeblogs]

Tilt:

You are comparing the blower ratios on the supercharger, which is not the compression ratio that Isegrim refers to.

The reason why compression ratio can influence fuel economy is that the engine generates higher pressure over the entire cycle of compression and decompression, which translates into more power generated per stroke.  So long as the fuel does not detonate due to the higher heat, you get more fuel economy.

Given my experiments with the FW190 in Aces, it does appear that cruise settings are reasonably well modeled for at least some planes.  The trick is to regulate both RPM and manifold pressure.  Why this does not seem to be properly modeled in all the planes is beyond me.

Originally posted by Tilt
FYI compression ratios on Ash82 FNV

1st stage boost 7.14:1

2nd stage boost 10:1

....

Having said all this.......IMO it would be the application of a minor modifier when compared to the descrepencies F4UDOA describes.....

A direct corellation between HP and fuel consumption would be more accurate generally than we  have now IMO. Having achieved this it would then seem time to add modifiers re differing engine efficiencies.

All this is to nought if the corresponding (lower) consumptions at lower levels of rpm and boost can not be modelled in some way to allow the player to reach target albeit some what slower but with the same ratio of endurance and range enjoyed (or suffered) by those ac his ride represents.

...

[HoHun]

Hi F4UDOA,

>I have found some German PDF docs that show the SFC of the DB605 at 5.1 at a mil power of 1475HP.

I'm not sure that this is equivalent to MIL power - it really depends on the definition.

Usually, WEP is a 5 min rating while MIL is 15 - 30 min. In German terms, the former would be "Notleistung" while the latter would be "Steig- und Kampfleistung" (and last 30 min).

Methanol-water injection would be "Sondernotleistung" for which there doesn't seem to have a direct equivalent in American WW2 terminology (though the technology was employed). In post-war American technical slang, it would be "wet" WEP.

So if you're using MIL-equivalent power for a DB605 capable of 1500+ HP, it should be in the region of 1300 HP at a fuel consumption of around 106 GPH.

Regards,

Henning (HoHun)

[Tilt]

Originally posted by joeblogs

Using WEP will lower fuel consumption as water is used to cool the charge, so the fuel mixture can be leaned out.  

 

This is AC dependent isn't it?.......not all AC injected water.

You can help me understand something re supercharging........

The Ash82FNV had "two stages of boost"  Stage 1 used below 4500metres  Stage 2 used above 4500.

I always took this to be either a two speed supercharger or indeed the ability to super charge the super charger.

A look at the engine in Kbely showed just one super charger and since then I have assumed it could run at two speeds.

Or at least two speeds obtained as a result of gearing from the main crank.

Its clear that manifold pressure (at a given stage of boost) is a direct correlation of engine rpm and that manifold pressure relates also (in this engines case) to power.

So as engine rpm changes so does boost and (real) compression ratio with a compound effect upon manifold pressure.

To me this explains the significant increase in power from 1650hp to 1800hp when the Ash 82 FNV merely increases revs from 2400 to 2500 (Man pres. from 1000mm to 1200mm) when at its first stage of boost at sea level. (btw at 2600 rpm power was rated 2000hp).

At the 2nd stage of boost the effective compression ratio is raised significantly and it seems that this then gave a less than linear relation ship of Power to engine rpm. Its clear that the 1st stage was backing off its linear power/alt curve at 3500 metres but the 2nd stage did not kick in until nearly 4500 metres.

At 4500metres (2nd stage) we see again manifold pressures of 1000mm so power was in effect still only 1650hp and fuel consumption still that of a reduced compression ratio at lower altitude.

This leads me to three conclusions

a) that fuel consumptions at less than mil power could vary widely between AC with differing boost systems and differing fuel mixture control systems.

b) that fuel consumptions at more than mil power could vary widely between AC with differing WEP systems.

c)that use of compression ratio as a guide to engine efficiency is misleading if not qualified with its boost component  at various altitudes.


I must admit to some lack of confidence in my logic and so would welcome any correction.

[F4UDOA]

HoHun,

What I think is the mil power setting shows the use of fuel only and reflects with a PS rating of 1475 at 480 ltrs per hour or 127 gph. The next level above that uses both C-3 fuel (560ltrs hour) as well as MW50 (150ltrs hour) at a PS rating of 1800.

JoeB,

Where you been man?? I have found a German doc that says the La-5 with the ASH-82FN had a rated power duration of 40 minutes. I think that is pretty much the deal closer as far as I am concerned. That mean Mil power could not have lasted longer than 30 minutes.

I need to post that DOC asap.

BTW, Tilt, I know I said 30min. I just reread it. Sorry.

[Tilt]

Originally posted by joeblogs
Tilt:

You are comparing the blower ratios on the supercharger, which is not the compression ratio that Isegrim refers to.

 

My appologies


if ******* is compression then its 7.0  (+/-0.1) :1


edit

I type the correct cyrillic in the text  and this BB changes it to gobble di gook...........I have changed it to stars.

Please refer to line 7 on this doc

http://www.tilt.clara.net/pics/page122.gif

[Staga]

http://www.kolumbus.fi/staga/wwiiol/scans/db601_enginechart.gif

[joeblogs]

Great post tilt

>>This is AC dependent isn't it?.......not all AC injected water.

Blogs: Yes, but the general principal of all these systems was either to (1) reduce the temperature of the mixture going into the engine; or (2) increase the mass of air going into the engine when the plane was at higher than critical altitude for the supercharger.  The Germans did (2) a lot because they tended to rely more on single stage superchargers. Effects in terms of induced horsepower, fuel economy, and duration depended on the installation...


>>You can help me understand something re supercharging........

The Ash82FNV had "two stages of boost"  Stage 1 used below 4500metres  Stage 2 used above 4500.

I always took this to be either a two speed supercharger or indeed the ability to super charge the super charger.

A look at the engine in Kbely showed just one super charger and since then I have assumed it could run at two speeds.

Or at least two speeds obtained as a result of gearing from the main crank.

Blogs: Sounds like a single stage two speed supercharger.  The way to tell is that a two stage supercharger (without a variable clutch) will have two critical altitudes - one for the first stage and the other for the second stage.  For US and British planes, you will see charts of the climb rate with two or more notches in them.

>>Its clear that manifold pressure (at a given stage of boost) is a direct correlation of engine rpm and that manifold pressure relates also (in this engines case) to power.

Blogs: The compression ratio that isegrim referred to is a constant in WWII era engines.  It's simply the difference in the volume of a cylinder with the piston at the bottom dead center versus when it is at top dead center.

Holding everything Else constant, increasing engine RPM has a tendency to Lower manifold pressure.  Think of an engine as a giant air pump.  As RPM increases, the engine is pushing more air out of the exhaust and sucking more air into the intake manifold, where pressure is measured on these engines.  The only way to maintain the manifold pressure then is to push the throttle forward a bit.  I can't remember if this is modeled in Aces.

In your example, you get a 10 percent increase in horsepower by raising rpm by 4 percent and manifold pressure by 20 percent.  So the engine is pushing an additional 150/4 times its displacement (I can't remember exactly how big this engine is but it's around 2600 cu in) in heated air (divide by 4 as there is one intake stroke in 4 cycle engines) plus the previous 1650/4 of  cycles now have contain 20 percent more hot air.

>>At the 2nd stage of boost the effective compression ratio is raised significantly and it seems that this then gave a less than linear relation ship of Power to engine rpm. Its clear that the 1st stage was backing off its linear power/alt curve at 3500 metres but the 2nd stage did not kick in until nearly 4500 metres.

At 4500metres (2nd stage) we see again manifold pressures of 1000mm so power was in effect still only 1650hp and fuel consumption still that of a reduced compression ratio at lower altitude.

Blogs: What you are observing is two things.  First running the supercharger at a higher speed requires even more power from the engine (a deduction from horsepower).  Second you may be observing lower manifold pressure  if your data has the plane above the critical altitude for the supercharger.  With a two stage supercharger, you can go even higher, but you have to deduct additional horsepower to run the second blower and the larger blower is typically less efficient too.


>>a) that fuel consumptions at less than mil power could vary widely between AC with differing boost systems and differing fuel mixture control systems.

Blogs: Yes definitely.  Engines varied in fuel economy and even across planes using the same engine as induction systems also vary in their efficiency.  Different carburetors or fuel injection systems matter.  Boosting systems matter because they consume different amounts of power and differences in gear ratios mean that different planes have sweet spots at different altitudes.

>>b) that fuel consumptions at more than mil power could vary widely between AC with differing WEP systems.

Blogs: Certainly true.  I am trying to locate as much info as I can on fuel consumption of German engines at high power settings.  Some of the information in this thread helps me.

>> c)that use of compression ratio as a guide to engine efficiency is misleading if not qualified with its boost component  at various altitudes.

Blogs:  I'd put it another way, compression ratio just doesn't matter enough to argue about relative to other variables.  Maximum boost, for example, is typically a far more important indicator.

[joeblogs]

Just back from a long work trip... Your post induced me to pull out some of my notes...

-Blogs

Originally posted by F4UDOA
HoHun,

What I think is the mil power setting shows the use of fuel only and reflects with a PS rating of 1475 at 480 ltrs per hour or 127 gph. The next level above that uses both C-3 fuel (560ltrs hour) as well as MW50 (150ltrs hour) at a PS rating of 1800.

JoeB,

Where you been man?? I have found a German doc that says the La-5 with the ASH-82FN had a rated power duration of 40 minutes. I think that is pretty much the deal closer as far as I am concerned. That mean Mil power could not have lasted longer than 30 minutes.

I need to post that DOC asap.

BTW, Tilt, I know I said 30min. I just reread it. Sorry.

[Tilt]

Originally posted by joeblogs

Holding everything Else constant, increasing engine RPM has a tendency to Lower manifold pressure.  Think of an engine as a giant air pump.  As RPM increases, the engine is pushing more air out of the exhaust and sucking more air into the intake manifold, where pressure is measured on these engines.  The only way to maintain the manifold pressure then is to push the throttle forward a bit.  I can't remember if this is modeled in Aces.

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? I also always assumed that the supercharger (which I can now confirm was a two speed centrifugal unit) was geared directly of the engine so its max pressure (at each stage)was subject to engine rpm.


In your example, you get a 10 percent increase in horsepower by raising rpm by 4 percent and manifold pressure by 20 percent.  So the engine is pushing an additional 150/4 times its displacement (I can't remember exactly how big this engine is but it's around 2600 cu in) in heated air (divide by 4 as there is one intake stroke in 4 cycle engines) plus the previous 1650/4 of  cycles now have contain 20 percent more hot air.

1650 > 1800 is 9% increase (1800 is 109% of 1650)

I was under the impression that manifold pressure had gone up as a function of the gearing between engine and supercharger............ie as the engine goes faster the supercharger goes faster........... the massive increase of 20% manifold for 4% rpm suggests that the relation ship is not linear.

This is born out by the fact that a further increase to 2600 rpm (3.85%) brings about  a further increase in Hp to 2000hp (not used in flight) which is 11% increase.

I assume that the near exponential curve is a function of the centrifugal type supercharger . Its efficiency is highly speed dependent?

I note the following manifold presures taken at 1000m alt.

rpm2400   man 1040
rpm2200   man 875
rpm2000   man 745
rpm1800   man 665
rpm1600   man 610
rpm1500   man 580

When you look at level flight endurance for this range of settings you note that 365 litres of fuel would last 1hour 45 minutes at the lower setting but only 35 mins at the higher

swept volume is 41.2 litre

You seem to imply that manifold pressure can be raised independently of engine rpm........ I assume this would be the case where supercharger speed was independantly variable of engine speed. 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.

Having said all that i lost the precise point of your maths......

are you saying

Given the airpump had increased capacity by 10% and only 4% of that was due to increased rpm (ie at constant pressure the volume /hour had only gone upto 104%) then the remaining 6% was a function of increased manifold pressure?

This shows that a 20% increase in manifold pressure and therefore a 20% increase in weight of air/fuel only returns a 6% increase in power. In fact 6/104 is  5.8%.  I assume this is a function of the mixture running leaner plus some air flow efficiency factors.

FYI the la7 did not have pilot mixture adjustment.........it was synchronised off the the throttle.........this was a first.........in the La5 the pilot had to set throttle and mixture separately.

[joeblogs]

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? I also always assumed that the supercharger (which I can now confirm was a two speed centrifugal unit) was geared directly of the engine so its max pressure (at each stage)was subject to engine rpm.

...You seem to imply that manifold pressure can be raised independently of engine rpm........ I assume this would be the case where supercharger speed was independantly variable of engine speed. 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?

Typically RPM and manifold pressure (throttle) are controlled independently. I can't remember if that was true of the nifty german units on the BMW 801 etc.

The way this works is that the throttle controls a choke in the carburetor, or a metering valve in the fuel injection unit.  That choke limits the amount of air that can pass through the induction system at any point in time.  Thus, holding RPM constant, pushing the throttle forward opens the choke and allows more air to enter a cylinder on every fourth stroke.  

Conversley, holding the throttle constant but raising engine RPM increases the number of strokes per minute but does not change the amount of air sucked into the cyclinders on every fourth stroke.  The combination of more RPM and more throttle increases both the number of strokes per minute and the amount of air drawn into the cylinders on every fourth stroke.

Gear driven superchargers are essentially synchronized with the demands of the engine.  When RPM is higher, so more air is pumped through the engine every minute, the supercharger runs comensurately faster.  Manifold pressure should stay roughly constant, but it can drop off a bit if the induction system is not well designed.  This is a second order effect and is not worth emphasizing.

Tilt: I was under the impression that manifold pressure had gone up as a function of the gearing between engine and supercharger............ie as the engine goes faster the supercharger goes faster........... the massive increase of 20% manifold for 4% rpm suggests that the relation ship is not linear.

This is born out by the fact that a further increase to 2600 rpm (3.85%) brings about  a further increase in Hp to 2000hp (not used in flight) which is 11% increase.

I assume that the near exponential curve is a function of the centrifugal type supercharger . Its efficiency is highly speed dependent?

I note the following manifold presures taken at 1000m alt.

rpm2400   man 1040
rpm2200   man 875
rpm2000   man 745
rpm1800   man 665
rpm1600   man 610
rpm1500   man 580

When you look at level flight endurance for this range of settings you note that 365 litres of fuel would last 1hour 45 minutes at the lower setting but only 35 mins at the higher

Blogs: I don't have the test report in front of me but I think the complication is that we observe different combinations of RPM and throttle.  So lots of variables are changing.  You are correct that as engine rpm rises so does the rotation of the supercharger.  The ratios you mentioned earlier are constants.  So if the supercharger gear ratio is 6.7:1, andy increase in engine RPM is multiplied by 6.7 at the supercharger.  But in a well designed system you are increasing the airflow proportionally so the resulting increase in horsepower should be a linear function of RPM.  It might actually be a little less than proportional if there are friction losses, and the supercharger becomes less efficient when airflow approaches the speed of sound.

I think what is suggesting the geometric increase you observe is the combination of more RPM and more manifold pressure.  In that case you get the combination effect I described in earlier in this post.  

I'm working on some charts that show how airflow, horsepower and fuel consumption increase at different engine settings and I'll post those soon. Airflow and horsepower increase proportionately, as does fuel consumption with one exception--that point on the power curve where the pilot must shift from a lean fuel mixture to a rich mixture (to cool the charge).

Tilt: Having said all that i lost the precise point of your maths......

are you saying

Given the airpump had increased capacity by 10% and only 4% of that was due to increased rpm (ie at constant pressure the volume /hour had only gone upto 104%) then the remaining 6% was a function of increased manifold pressure?

This shows that a 20% increase in manifold pressure and therefore a 20% increase in weight of air/fuel only returns a 6% increase in power. In fact 6/104 is  5.8%.  I assume this is a function of the mixture running leaner plus some air flow efficiency factors.

Blogs: Well output went up by 10 percent.  I am not sure of the relative contribution of the increase in RPM vs. the increase in manifold pressure, but it should be easy to work out: compare the change in the number of strokes times the airmass contained in each stroke (the RPM effect) to the change in the airmass drawn times the initial engine RPM (the throttle effect).

Fuel air mixture shouldn't change dramatically except for the jump from lean to rich mixture at about 50 percent of rated horsepower in most engines and again when WEP is engaged.

Tilt: FYI the la7 did not have pilot mixture adjustment.........it was synchronised off the the throttle.........this was a first.........in the La5 the pilot had to set throttle and mixture separately. [/B][/QUOTE]

Blogs: Sounds like some of the german units.  I think those worked the following way.  If the measured manifold pressure reaches a certain level, the quadrant automatically enriches the mixture before detonation sets in.  When the throttle is pulled back and manifold pressure drops, the reverse is done.  This works because the temperature of the charge is an increasing function of manifold pressure.  

-Blogs

[GScholz]

Originally posted by F4UDOA
I have found some German PDF docs that show the SFC of the DB605 at 5.1 at a mil power of 1475HP.

This is 127GPH and a duration of exactly 50minutes meaning 25 minutes in the MA.

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.

[HoHun]

Hi Tilt,

>You seem to imply that manifold pressure can be raised independently of engine rpm........ I assume this would be the case where supercharger speed was independantly variable of engine speed.

Manifold pressure can be raised independly of engine rpm even with a fixed gear ratio as long as you're below the full throttle altitude for the respective rpm/manifold pressure combination.

Below this altitude, the supercharger produces more pressure than desired so that the engine has to be throttled back. Advancing the throttle then will give you greater manifold pressure even though you don't spin the supercharger any faster than before.

Above this altitude, the throttle is completely opened anyway, and manifold pressure and supercharger speed aren't independend any more.

Regards,

Henning (HoHun)