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Thread: variable compression ratio

  1. #1
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    variable compression ratio

    saab prototype claims that their variable cylinderhead can vary the compression ratio anywhere between 8:1 to 14:1 allowing supercharger to run at 2.8 bar
    ...

    i have no doubt that they can vary the compression ratio but once you tilt the cylinder head dont you affect the geometry of the piston/crankshaft movement?

    and 2.8 bar on an 8:1 compression ratio ... wow

  2. #2
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    Interesting stuff here, never heard of anything like this before. I just don't see how they would be able to vary compression. Do you have any information on how this system works?
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  3. #3
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    Saab have ben playing with this for years.
    Some crazy prototypes were talkd about yearsa go with multiple combusiont chambers and valves to introduce these 'swirl' chambers when needed.

    But the SVC works by moving the head up away from the crank, thus decreasing the compression ratio. Thhe cylinders are part of the head, so nothing too hard to engineer.
    See http://www.edmunds.com/news/innovati...7/article.html
    They're claiming 30% improvement.
    Interesing idea - but somethign else to breaka dn can't see that it woudl be viabel with a performance engine where the combustion pressures are significantly higher.

    The current engine can vary from 8:1 to 12:1 compression and has already completed 200,000 km durability tests.

    If you need pictures to grasp the concept, see Saab USA ( I'm saying nothing ) http://www.saabusa.com/main/GLOBAL/en/vepsilon/
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  4. #4
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    Very interesting thing. We need imaginative people to create that kind of invention (which works and reliable of course... )

  5. #5
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    i still think the 2.8 bar supercharging at 8:1 compression is bogus

    but the theory is sound, an engine which is capable of varying it's compression ratio would have the same impact of a good CVT but applications are hard, many current CVTs arnt much better than a standard automatic because of mechanical inefficiency (my info is kinda out of date but i hvnt heard anythign ground breaking in a while)

  6. #6
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    Quote Originally Posted by Ferrari Tifosi
    Interesting stuff here, never heard of anything like this before. I just don't see how they would be able to vary compression. Do you have any information on how this system works?
    http://autozine.kyul.net/technical_s...gine_4.htm#SVC

  7. #7
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    can we start this thread by a simple explanation of what exactly is "compression ratio". what does it represent and how significant are its values.

    just a quick explanation of the ratio, no need to make as complicated as that OTHER ratio !! (you all know what im talking about!)
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  8. #8
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    Compression ratio = combustion chamber volume at BDC / volume at TDC

    So piston at bottom of cylinder say 250cc
    Piston at top of cylinder say 25cc
    Compression ration = 250/25 = 10:1

    "A woman without curves is like a road without bends, you might get to your destination quicker but the ride is boring as hell'

  9. #9
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    Quote Originally Posted by KnifeEdge_2K1
    i still think the 2.8 bar supercharging at 8:1 compression is bogus
    Explain ?
    many current CVTs arnt much better than a standard automatic because of mechanical inefficiency (my info is kinda out of date but i hvnt heard anythign ground breaking in a while)

    The thing with CVT is that a good one enables the engine to be kept at maximum efficiencey so the overal improvement can be huge.
    The drawback is that people don't feel comfortabel driving fully variable CVT. So we end up with computer controlled CVVT which varies the engine revs to make it sound liek a "normal" gearbox and have stepped changes to make it feel as if it's actually changing gear - the Citroen one actually has paddles to switch gears on their CVT !! !!
    "A woman without curves is like a road without bends, you might get to your destination quicker but the ride is boring as hell'

  10. #10
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    Quote Originally Posted by Matra et Alpine
    Compression ratio = combustion chamber volume at BDC / volume at TDC

    So piston at bottom of cylinder say 250cc
    Piston at top of cylinder say 25cc
    Compression ration = 250/25 = 10:1

    ok need some educating

    you said 250/25 = 10:1 how do you get 10:1

    could someone explain please?
    and yes i am very dense

  11. #11
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    Quote Originally Posted by paul
    ok need some educating

    you said 250/25 = 10:1 how do you get 10:1

    could someone explain please?
    and yes i am very dense
    250 / 25 is 250 divide by 25. so that gives 10.
    So the CR is 10 to 1 commonly written 10:1
    "A woman without curves is like a road without bends, you might get to your destination quicker but the ride is boring as hell'

  12. #12
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    basically the higher the compression ratio the more you get out of your "bang", but the higher the ratio the easier the engine will misfire, knock ect
    thats why you use higher octane fuel in high compression ratio cars, higher octane means lower chance of detonation

    on turbo charged cars the turbocharger compresses the air before going into the piston which bassically accomplishes the same thing as a high compression ratio NA engine, but the drawback is since a turbo already compressed the air before hand you cant have as high a CR because it would again be too high causing knocking detonation misfires ect

    so to summerize it, a turbo is like using brute force, while NA is more precise, but since the power and torque gains you get from a turbo are so huge, and the potential is everything but limitless, the tiny drawbacks it has are inconsequential

    matra will prolly cover anything i've missed or mistaken

  13. #13
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    then of course there are the diesel engines which have ratios of 20 or more, without this misfiring etc.
    "I find the whole business of religion profoundly interesting, but it does mystify me that otherwise intelligent people take it seriously." Douglas Adams

  14. #14
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    but then you're talking about a different source of fuel :P

  15. #15
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    Quote Originally Posted by KnifeEdge_2K1
    but then you're talking about a different source of fuel :P

    diesel -- gasoline -- different source.
    I think I've lost the thread somewhere.
    Can you explain please.
    "A woman without curves is like a road without bends, you might get to your destination quicker but the ride is boring as hell'

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