Variable Stroke Engine

Discussion in 'Off-Topic' started by BR-549, Aug 16, 2016.

  1. BR-549

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  2. jpanhalt

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    Why not use a supercharger/turbocharge and vary the boost?

    john
     
  3. BR-549

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    john, the way I understand it, is this was invented so that one can efficiently use a turbo and/or super charger at different load ranges.

    At light loads, with a charger, you want high compression.

    But at high loads, with a charger, you want low compression.
     
  4. BR-549

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  5. crutschow

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    The goal is a achieve a constant maximum peak compression pressure, independent of the throttle setting and any turbo boost.
    That is achieved, at least to some degree, with a variable compression ratio.

    I liked the mechanical simplicity of the Saab approach of a few years ago where they just pivoted the crankcase slightly with respect to the block/head to vary the compression ratio but, for whatever reason, they never put it into production.
    Wikipedia says it was shelved by GM when they took over due to "cost".
     
    Last edited: Aug 16, 2016
  6. dannyf

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    Seems fairly complicated. Variable displacement has been around in production cars for a few years now.
     
  7. crutschow

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    Which ones??
     
  8. jpanhalt

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    I believe the Corvette is one. Some Fords too. Maybe others. Some cylinders are deactivated using electronic valves.

    BR-549 is talking about variable compression, not displacement.

    John
     
  9. shortbus

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    At first glance that, the attachment in the first post, looks similar to an Atkinson cycle engine.
     
  10. crutschow

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    Sort of.

    But the Atkinson cycle engine has a longer effective expansion stroke as compared to the compression stroke. This gives a compression ratio different from the expansion ratio
    This difference can be generated mechanically or, as some modern engines do such as the Prius, by control of the valve timing.
    This can improve engine efficiency but at the expense of less maximum power available (it takes in less air/fuel on the intake stroke).
    That's how the Prius is able to get such good highway efficiency, where the electric motor is not being significantly used, except to add to the peak power when needed.

    The variable compression engine has the same stroke length for both, but varies the position of the stroke to vary the compression ratio. The compression ratio and expansion ratio are the same.
    This must be done by some mechanical means.
    This improves efficiency without reducing the maximum power available.
     
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