Questions about compression ratio in WWII

Discussion in 'Engines' started by donkeyking, Oct 30, 2010.

  1. donkeyking

    donkeyking Member

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    In the following, I list compression ratio specification for engines of WWII
    From this list, I have some questions
    1. The English and American engines don’t show higher compression ratio than Axis’ engines. What are advantages in English and American engines to compare with Axis’ engines? For example Merlin 66 vs DB605?
    2. It seems that English and American engineers didn’t eager to improve their compression ratio in WWII. For example Merlin and Griffon have same compression ratio. British and USA have more and better avation gas in WWII. So they would more like to improve compression ratio

    British

    Rolls-Royce Merlin 6:1
    Rolls-Royce Griffon 6:1
    Bristol Perseus 6.75:1
    Bristol Hercules 7.0:1
    Bristol Centaurus 7.2:1
    Napier Sabre 7:1

    USA
    V1710 6.65:1
    Wright R1820 6.45:1
    Wright R2600 6.9:1
    Wright R3350 6.85:1
    PW R1830 6.7:1
    PW R2800 6.75:1

    German
    DB601 6.9:1
    DB605 7.5/7.3:1 with 87-octane fuel; 8.5/8.3:1 with 100-octane fuel
    DB603 7.5:1 left block, 7.3:1 right block
    BMW801 6.5:1
    Jumo 211 6.5:1
    Jumo 213 6.5:1

    Japan
    Mitsubishi Kasei 6.6:1
    Mitsubishi Kinsei 6.6:1
    Nakajima Sakae 7:1
     
  2. tail end charlie

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    The compression ratio is just a mathematical calculation of maximum and minimum volume. The actual compression is a function of the boost of the supercharger and other things like valve timing and gas flow and of course what the fuel used will allow. Conventionally aspirated high performance engines have much higher ratios.
     
  3. donkeyking

    donkeyking Member

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    Thanks Charlie. It is very clear
     
  4. engguy

    engguy Member

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    #4 engguy, Oct 30, 2010
    Last edited: Oct 30, 2010
    One of the main reasons for such low compression ratios in aircraft engines in WWII is since most all of them had some sort of super charger especially radial engines, they needed the lower compression ratio to compensate for the boost, there was no way to disengage the supercharger and de boost them, unlike a turbocharger, only way is throttle position and propeller pitch. The superchargers on some where able to be sped up or slowed down, but never dissengaged, and actually they were very important to fuel distribution, especially in the radial engines. With higher compression ratios they would constantly be in the detonation range, especially when high out put was required, and I suppose that was a constant in war time.
    To add to Tail ends post, its the actual pressure otherwise known as BMEP that is affected by the items he mentioned.
     
  5. Shortround6

    Shortround6 Well-Known Member

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    As has been said, it is a combination of boost and compression ratio together that define the limit.

    As an example take the Merlin and the V-1710. Both about the same size and both turning 3000rpm. The higher compression of the V-1710, while getting better fuel efficiency (or better mileage) also limited the boost that could be used to a lower amount than the the Merlin on the same fuel. It was estimated that this cost the V-1710 about 10% in power compared to the Merlin if everything else was equal (which it probably wasn't).

    Very last Allison V-1710s made dropped the compression ratio to 6.0:1 in order to use the very high boost levels that allowed them to exceed 2000hp. That along with a number of other tricks. :)
     
  6. donkeyking

    donkeyking Member

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    #6 donkeyking, Nov 1, 2010
    Last edited: Nov 1, 2010
    Thank your guys reply.

    From your replies, I summarize some information:
    1. The compression ratio specification is just for a nature aspirated engine which is non-supercharged in theory.
    2. Every plane had a supercharger in WWII.
    3. The pressure in combustion chamber is related not only nature aspirated air pressure, but also pumped air pressure by supercharger.
    4. If an engine’s compression ratio is set too high and it also has an excellent supercharger, the engine would get knock by too high pressure.
    If I’m wrong, can you please correct me?

    I'm not familiar with supercharger. Who can recommend a web site to introduce supercharger knowledge in WWII?
     
  7. engguy

    engguy Member

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    Number 1 is NO. All recip engines have a compression ratio specification. It just needs to be a bit or more lower for a boosted one. And especially one that is going to work at such high power setting that an aircraft engine works at. Its not just putttting down the road at part throttle like your car does. Its like always trying to climb a hill, less your in a dive, cause as soon as to start backing off the power in an airplane it starts its decent, without pushing forward on the controls. And is the reason you can read stories how an old heavily loaded plane, can barely maintain a low altitude with 2 of its 4 engines stopped, and that is with those 2 engines running at max takeoff power.

    And a supercharger is any PUMP or COMPRESSOR that is used to pressurize the induction system of a Piston engine. In theory the highest pressure that the intake manifold of a NATURALLY ASPIRATED engine
    can see is what ever the atmospheric pressure is at the time. I say in theory becase in real life it can be boosted by tuned ram effect( but that is a different topic), but nothing as high as a pump boost will be. So can you see that in an airplane as you climb higher the air gets thinner ie less pressure, so it is handy to have a pump to help pump in more air and thus raise the pressure in the intake system of the engine.


    Supercharger - Wikipedia, the free encyclopedia
     
  8. donkeyking

    donkeyking Member

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    Thanks for your reply. I shall read it firstly,and talk to you later
     
  9. donkeyking

    donkeyking Member

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    #9 donkeyking, Nov 5, 2010
    Last edited: Nov 5, 2010
    I can't understand this example.

    If I were an engineer to choose a combination between higher compression ratio engine + lower boost supercharger and lower compression ratio engine + higher boost supercharger, I would choose compression ratio engine + lower boost supercharger. Of course they would have the same output power

    The reason is higher boost supercharger needs more power to drive. So my choosed option has more power(useful power) to drive prop. Am I wrong?
     
  10. Shortround6

    Shortround6 Well-Known Member

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    They don't have the same power output.
    While they may have the same "effective" compression ratio and while the engine using higher boost may use a bit more power to drive the supercharger the engine using higher boost has actual crammed more fuel and air into the cylinder to begin with.

    Say, for arguments sake, the the lower compression engine uses 10% more "boost" and 10% lower compression in the cylinder. Lowering the compression ration 10% does not lower the power 10%. But the 10% higher boost means 10% more fuel and air is going into the cylinder and burning.

    As an example say we have an engine that delivers 1000hp to the prop. say also that it uses 100hp in internal friction and pump drives, say also that it uses 100hp to drive the supercharger. 1200 gross for 1000hp net. Now change the supercharger to use 10% more boost. 1320 hp gross-100hp for friction/pumps=1220hp-110-120hp for supercharger= 1100hp to prop. yes lowering the cylinder compression will drop the power a bit but not the whole 100hp.
    The higher compression engine will get better fuel efficiency and better range.
     
  11. Ivan1GFP

    Ivan1GFP Member

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    #11 Ivan1GFP, Nov 6, 2010
    Last edited: Nov 6, 2010
    I believe the Nakajima Sakae had a 7.2:1 compression. Sources are somewhat contraditory but I believe Smithsonian docs I requested said 7.2.

    The BMW 801D series engine I believe had a compression slightly over 7:1.

    BraMo 323 Fafnir - 6.4:1 compression.

    I don't believe anyone has mentioned that in general the German engines had a greater displacement and less supercharging than allied engines for pretty much the same output at medium altitudes.

    - Ivan.
     
  12. donkeyking

    donkeyking Member

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    #12 donkeyking, Nov 6, 2010
    Last edited: Nov 8, 2010
    Thanks Ivan and shortround6

    I learned a lot of about compression ratio and supercharger from your replies
     
  13. tail end charlie

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    DK

    consider the logic of your post. If a lower boost and higher compression was the way to improve this would logically end up with no boost and a very high compression ratio...that is a conventionally aspirated engine.

    A supercharcher/turbocharger will always produce a higher power output even at sea level which is why (for example) formula 1 bans them, they produced too much power for the sport even with 1.5 litre engines.

    For an airplane at altitude some sort of boost is essential to compensate fore the rarefied air. at altitude the forced inlet compensates for the drop in air pressure rather than actually increasing performance
     
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