Engine power vs altitude

Discussion in 'Aviation' started by Rapecq, Sep 8, 2009.

  1. Rapecq

    Rapecq New Member

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    I would like to ask about differences in the way that engine power decrease with altitude, but first some performance charts:

    BWM 801D2
    [​IMG]

    or Merlin 66:
    http://www.spitfireperformance.com/merlin66hpchart.jpg

    Both engines have 2-speed supercharger. We can see in both cases that the power of engine rise till FTH of the particular gear is achieved, then there is a power drop untill the second gear of supercharger is engaged.

    However, on Jumo 213 E-1:
    [​IMG]

    we can notice that there is a constant power decrease with altitude?

    Where does these differences come from? Could we use all those charts for direct comparison of engine performance?

    Regards

    PS. Sorry for my English - isn't as good as I want it to be.
     
  2. drgondog

    drgondog Well-Known Member

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    Decrease in air density (and temp) with altitude is the reason.

    Getting sufficient mixture of oxygen to fuel is the reasone for superchargers (and all mechanical methods) to bring air into the engine/carburation system
     
  3. Shortround6

    Shortround6 Well-Known Member

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    It was accepted that most piston engines would loose power with altitude until at about 55,000-56,000ft the engine would only make enough power to overcome internal friction and not be able to provide any power for propulsion.
    If you continue the lines on the graphs they will all end at about that altitude.

    It has to do with the air density. The chart I have doesn't go to 55,000ft but only 50,000ft. At 50,000 ft the air is about 15% as dense as sea level so an engine, taking in the same cubic feet of air, could be expected to make 15% of the power at sea level.
    This is gross power and not propeller shaft or 'net' power. Say, as a hypothetical, you have an engine that gives you 1100hp at sea level to the propeller. Such an engine might actually be making an extra 150hp that is used up in friction of pistons and bearings and being used to drive oil and fuel pumps, etc. this gives us a total of 1250hp gross. At 50,000 ft the engine would have 15% of this power or 187.5HP but since the friction and pump dirves still need the same amount of power (150) this leaves only 37.5 hp left for the propeller.
    TO go further let us assume that this engine has no supercharger. In this case it's 1100 hp sea level power would have fallen to about 788hp at just 10,000feet ( 74% air density X 1250 gross - 150 friction/pumps).
    Now we fit a supercharger that can give us sea level density at 10,000ft for a cost of 100hp to drive it. This can give us 1000hp at 10,000ft. a very worthwhile improvement.
    But the higher we go the closer the power of the supercharged engine gets to the power of the unsupercharged engine. If I have done the math correctly the unsupercharged engine would give 317.5hp at 30,000ft to the propeller (37.4% air density X 1250 gross - 150 friction/pumps) compared to the supercharged engines 382.5 HP ( 50.6% difference between 10,000ft and 30,000ft X 1250 gross-250hp for friction, pumps and supercharger drive) Still better but both are going to zero out at about 55-56,000ft.
    Adding a second gear to the supercharger or a second stage is going to have a similar effect. At some point the extra boost, which has to be paid for with power from the crankshaft, is not going to be enough to compress the air enough to cover the power needed by the supercharger.
    THe Germans may have made slightly different asumptions about just where the zero point is, and/or used a different density chart. Just how many research flights had been made to 50,000+ feet to measure air density in the late 30s and early 40's:)

    I am not sure if this helps with you question but I hope it does.
     
  4. Rapecq

    Rapecq New Member

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    Hello

    Thanks for answers, but I still don't know what may cause the difference in power drop beetwen for example BMW 801D/Merlin 66 and Jumo 213 E-1. As I've written before on BWM 801 chart the power goes up from SL till reach FTH of particular gear, then power drops untill 2nd gear is engaged.

    On the other side we have Jumo 213 E-1 chart when at each gear the power of engine is pernamenlty dropping. What may be the reason of this?

    Regards
     
  5. red admiral

    red admiral Member

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    I think the difference is because of how the boost is regulated on the two engines. For the Merlin, the available boost is greater below the full throttle height but the engine is limited in how much boost it can make. As a result the throttle isn't fully open below the full throttle height and the engine makes less boost and less power. I imagine the BMW works in a similar way.

    I'm not entirely sure about what method the Jumo 213 uses but it seems to keep the boost constant between full throttle heights, maybe some sort of dump valve. Or it might be something to do with the MW-50 boost system, I'm not sure.
     
  6. fusbeta

    fusbeta New Member

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  7. Aurum

    Aurum Member

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    Don't forget that Jumo hydraulic driven second stage that allowed change supercharging smoothly. So line of power between 1st 2nd stage max. power points is not so wavy as other have.
     
  8. Yo-Yo

    Yo-Yo New Member

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    Jumo has vanes before the blower so it can control the boost level preventing from the excessive boosting at low altitude.
     
  9. msxyz

    msxyz Member

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    #9 msxyz, Nov 9, 2012
    Last edited: Nov 9, 2012
    Each engine aircraft is built for a critical altitude, that is the altitude at which it can develop the maximum power.

    At sea level, manifold pressure must keep below a certain level even if the supercharger could, in theory, pump more fuel air mixture, the limit being the ocurrence of self ignition before the piston reach the top dead center. What's interesting is that power at sea level is usually slightly lower than power at critical altitude becuase air is thinner and thus the expanding hot gases encounter less resistance during their trip down the bore, pushing the piston, and out of the exhaust (this factor is called 'back pressure').

    Power curves at a certain altitude are also affected by engine control design: on a car engine you would see power dropping at the same rate of atmospheric pressure. Single speed supercharged aircraft engines start with a certain power, increase it slightly till the critical altitude and, from there on, power will decrease in accordance with barometric pressure. Two speed superchargers follows the same principle of one speed designs except that, above a certain altitude, the engines equipped with them benefit from a higher boost setting.

    Daimler Benz designed a hydrostatic, continuos variable coupling controlled by a barometer to use with their engines. The power output of the DB601/5 remains basically flat till about 4500 meters thanks to this automatic compensation system. A simpler, albeit less efficient arrangement, involves variable geometry stators to modify the pressure ratio of a centrifugal compressor by altering the airflow.

    Every technical solution to tackle the loss of power at altitude has its pro and cons and influences the engine behavior in a different way.
     
  10. Yo-Yo

    Yo-Yo New Member

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    Exhaust backpressure is not the only factor reducing the power under critical altitude. The main factor is air (mixture) density. As the manifold pressure regulator keeps MP constant air temperature after blower is higher at low altitude so the density that is proprtional to pressure/abs_temperature ratio drops.
     
  11. tomo pauk

    tomo pauk Creator of Interesting Threads

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    I've inserted some commas - hopefully that's okay?
     
  12. Shortround6

    Shortround6 Well-Known Member

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    General Motors ( Owner of Allison) published a booklet in 1943(?) describing a number of different intake/supercharger systems and their effects on power at altitude for a theoretical 1000hp engine. Back pressure at 20,000ft was enough lower that the engine would gain 80hp (8%) IF the engine could be supplied sea level AT NO COST to the engine ( more power to drive supercharger). Please note that this is NOT exhuast thrust and please note that exhaust thrust is not totally free either as the size nozzles needed for exhaust thrust do raise the back pressure some. It does mean that the power to run a Turbo is not totally free however. Balance this against the power needed to compress air at 20,000ft to a sea level air pressure.
     
  13. davparlr

    davparlr Well-Known Member

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    #13 davparlr, Nov 12, 2012
    Last edited: Nov 12, 2012
    This is a good opportunity to compare the strengths and weaknesses of supercharging vs turbo-supercharging. If we compare the turbo-supercharged R-2800-57 used in the P-47N to the supercharged Jumo 213E (used in the Ta-152),

    http://www.wwiiaircraftperformance.org/p-47/p-47n-88406-speed.jpg

    we can see that the hp of the R-2800 is basically flat rated up to 33k ft. with none of the saw toothed power levels (note that this graph shows max continuous power. For WEP max power is 2800 hp). Brake Horsepower (BHP) is next to the last graph, right before True Speed. At 33k ft, the Jumo is only generating about 1300 hp, or less than half the power of the R-2800. As pointed out by Shortround6, this is not free. Turbo-superchargers are bulky and heavy, the P-47 engine system is about 400 lbs more on the P-47 than the engine system of the F6F. In addition it does not use exhaust thrust, which other people can discuss.

    This does show why turbo-supercharging was such a tempting technology. This also shows that, while the P-47 was never known as a great low level dog fighter, it is known as a formidable fighter at high altitudes.
     
  14. tomo pauk

    tomo pauk Creator of Interesting Threads

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    #14 tomo pauk, Nov 12, 2012
    Last edited: Nov 12, 2012
    The production engines, mid 1943 to mid 1944 (not the V-1710-117, at least not used in production fighters; here is for comparison). The V-1710-93 and -117 were used in P-63A and -C. V-1710-89/91 are on P-38J.

    V-1650-3 weights: Engine + eng. controls + cooling + lubricating: 1670+30+663+101 = 2464
    V-1710-93: 1620+105+347+135 = 2207 (no intercooling here); ADI system adds 50 lbs
    V-1710-117: 1710+114+346+135 = 2305
    V-1710-89/91 weights, same as above + turbo: 2730+321+81+1065+194+613 (turbocharging system) = 5004 for 2 engines; 2502 lbs for 1 engine

    TO power: 1380HP* (1490 for the -7) vs. 1325 (both 2 stage Allisons) vs 1425 (for turbo Allison)
    max WEP: 1600/1720 HP (low gear) vs. 1500 (1820 with ADI; bot at SL) vs 1600 (for turbo Allison)
    max WEP at altitude: 1330* (-3; with ram: at 29000 ft) or 1370 (-7, military power, no ram: at 21400 ft; 1505 HP at 19250 WER, no ram) vs 1150 (no ram: at 22400 or 25000 ft) vs. 1600 HP (no ram: at 25000 ft; with ram: 26500ft)
    all data from AHT book.
    Agreed that, while making high speed dash, the non-turboed engines have the exhaust thrust to help them out. They also (at least in P-38 vs. P-51 case) use the ram effect much better. Contrary to that, neither ram effect nor exhaust thrust are of much use when plane is climbing, ie. flying in low or moderate speed.

    Fuel consumption, max continous/military power/WER, at altitude: V-1650-3: 140/190/204 USG/hr/engine. In low blower: 115/165/189.
    V-1710-89/91: 113/161/187.

    * the table for the V-1650-3 gives 1400 for the take off, 1410 @ WER at 23750 ft, no ram.

    It indeed took much space weight to have the R-2800 make 2800 HP at high altitude. OTOH, seem like the turbo V-1710 was as good (for the needs of the USAAF) as the 2 stage Merlin.

    The single engined fighter with turbo V-1710: a missed USAAF's opportunity?

    added: fuel consumption for R-2800-21 (P-47), max cont/mil/WER (2300 HP): 210/275/315 USG/hr
    R-2800-8W (F4U), high blower: 250/280/245(???-from the engine table). Low blower: 240/275/245(???)
     
  15. gjs238

    gjs238 Well-Known Member

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    I think the challenge is where to fit the turbo, and preferably, the intercooler.
     
  16. tomo pauk

    tomo pauk Creator of Interesting Threads

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    The best location for non-late-war turbo (ie. the blades are not hollow, for cooling purposes) is where it was for P-47 - within the rear fuselage, down. The intercooler can be a part of the 'cooling group', as it was in the Merlin Mustang, the intercooler here being air-to-air type.
     
  17. Shortround6

    Shortround6 Well-Known Member

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    Unfortunately the few planes that were built using turbo V-1710s showed a lack of imagination or some rather poor assumptions on drag or perhaps the packaging problem is a bit more difficult than we think.

    Curtiss_XP-60A_061024-F-1234P-016.jpg

    061024-F-1234P-013.jpg
     
  18. tomo pauk

    tomo pauk Creator of Interesting Threads

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    The second page depicts a non-turbo plane; open exhausts?
    Ugly as it was*, the XP-60A was claimed to make 420+ mph. A good info, better/more thorough than one in the 'Vees for victory' would be most welcomed :)

    *P-47 was seldom, if ever, described as a pretty plane...
     
  19. Shortround6

    Shortround6 Well-Known Member

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    Curtiss also guaranteed the XP-46 would do 410mph at 15,000ft. The light weight prototype made it, the fully combat equipped 2nd prototype dropped to 355mph at 12,200ft.

    A fire in the XP-60A damaged the exhaust shrouds while ground running, the plane was "repaired" by fitting normal exhausts and the turbo removed with all ducting. First flight was Nov 1st 1942. One source claims a revised turbo installation was installed and tested (ground or flight?) Work stopped Nov 6th and the plane was dismantled with parts used in other aircraft. With only 6 days at best and work going on between flights it seems doubtful high speed runs were done. The XP-60B was supposed to use an Alison engine with a Wright instead of a GE turbo supercharger but was never completed with this engine, being converted to mount an R-2800.

    You are correct on the second picture. There seems to be some confusion as to which picture belongs to which version of the P-60. That picture is for the XP-60 with a Merlin 28. (I think?) This airframe was later modified to have a bigger tail and was supposed to have a Merlin 61 installed (becoming XP-60D) Plane is reported to have crashed after the tail was modified but before the engine swap was done.
     
  20. rinkol

    rinkol Member

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    I think you are referring to the scheme used on the HS 12Y-45 (Planiol supercharger) where throttling was performed using vanes whose incidence could be varied at the supercharger input - http://www.enginehistory.org/Accessories/S-Psc/S-PscTst.shtml. A similar arrangement was used by the Mikulin AM-35 and some other V12s developed in the Soviet Union. Unfortuantely, I don't know much about its applications in the German engines, though it is clear that the Germans would have been aware of the idea.
     
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