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Thread: Straight eight and 7 cylinder engines?

  1. #16
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    I think he meant a radial engine, as in having a number of pistons arranged in a circle and driving a single crank throw.
    Not sure how the balancing would work out on those either, it wouldn't be like a boxer. A true boxer (in my way of thinking) has one piston per crank throw, so pairs of pistons are moving in opposite directions, hence cancelling the motion out (nearly, being offset from each other you get a couple generated).

    I think the bigger radials have two sets of pistons back to back. I imagine they are 180 degrees out of phase which would cancel out the vibrations. In fact, I understand some early aircraft radials had the crankshaft fixed to the plane and the cylinder block rotating with the propellor! Zero vibration

  2. #17
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    Quote Originally Posted by BjD
    I think he meant a radial engine, as in having a number of pistons arranged in a circle and driving a single crank throw.
    Not sure how the balancing would work out on those either, it wouldn't be like a boxer. A true boxer (in my way of thinking) has one piston per crank throw, so pairs of pistons are moving in opposite directions, hence cancelling the motion out (nearly, being offset from each other you get a couple generated).

    I think the bigger radials have two sets of pistons back to back. I imagine they are 180 degrees out of phase which would cancel out the vibrations. In fact, I understand some early aircraft radials had the crankshaft fixed to the plane and the cylinder block rotating with the propellor! Zero vibration
    when I saw this I think Hmm another misguided soul those airplane engines you are talking about with the motor block rotating... yeah they vibrate like there is no tomorrow!!! basically because when each piston fired it it put tremendous force on the block and shoved it around and if they had a number of pistons under 18 then they had to be on a heavily reinforced mount (one of the big reasons this engine got fased out) however they felt pretty good when you got them off the ground and had the revs up...
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  3. #18
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    Quote Originally Posted by hightower99
    when I saw this I think Hmm another misguided soul those airplane engines you are talking about with the motor block rotating... yeah they vibrate like there is no tomorrow!!! basically because when each piston fired it it put tremendous force on the block and shoved it around and if they had a number of pistons under 18 then they had to be on a heavily reinforced mount (one of the big reasons this engine got fased out) however they felt pretty good when you got them off the ground and had the revs up...
    Ahh, never thought about that aspect of it

  4. #19
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    Sorry, wrote rotary instead of radial in the hurry

    Flew in a couple of planes with 9 cylinder radials and they weren`t so bad vibrationwise.

    Btw. my "so called" analysis on the inline 8 correct?

  5. #20
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    Straight eight engine

    A modern straight eight engine is fully feasible, technically. Crankshaft vibration issues can be addressed by taking power from the center of the crankshaft, possibly using a gear on one crank web. Center gear power output drive was used on the famous Alfa Romeo Tipo B 2.l7 liter straight eight. Uneven air and fuel induction, the bane of classic straight eights, is no longer an issue with port or cylinder injection of fuel. In general, crankshaft rigidity is an outcome of the designer's preference, since he can alter journal diameters and web stiffness to order, on a clean-sheet engine design.

    I see no technical reason to use a straight eight: the weight, cost, and length all argue against such a choice. Yet, uniqueness sometimes has great market value, and I would probably be among those wishing to drive a straight eight powered car.

  6. #21
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    The reason 8 cylinder inline engines are not built now is because of their length, it has a great deal of torsional twisting for its weight. If your at full revs and throttle, the front rear cylinders end up running behind the front cylinders. This causes loss in performance and the engine becomes unbalanced.

    "V" engines have a shorter crankshaft and don't suffer from this problem as much. Also "V" engines are more ridged in construction. Keeping a straight eight engine block ridged over the increased length adds much more weight.

    Another issue is compactness. Early cars had a very long bonnet to accommodate these engines. The longer the body the more weight the vehicle is. Car designers are always seeking a very compact engine to streamline the car designs. The smaller the engine bay the larger the cabin and boot as well.

  7. #22
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    "Ridged" ---> "Rigid"

  8. #23
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    It's also the torsional rigidity of the crankshaft and that it is not that simple to align too many bearings properly.

  9. #24
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    Again, the only application I was envisioning was two motorcycle engines bolted together, which means a very short I8. Definitely shorter than most of today's I6s, as the displacement would be so much smaller.

  10. #25
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    Al ong time the limit set for Marine Diesel engines was 12 cylinders (inline) but now developements have gone further to 13 and 14 cylinder engines. The limit was set by the crankshaft, but obviously they have managed to overcome some of those difficulties. (Marine diesels pour out up 110000 BHP)
    "I find the whole business of religion profoundly interesting, but it does mystify me that otherwise intelligent people take it seriously." Douglas Adams

  11. #26
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    Yes but the weight is huge. Not as much as an issue in a ferry or large boat.

  12. #27
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    Quote Originally Posted by revetec View Post
    Yes but the weight is huge.
    but so is the torque...
    "I find the whole business of religion profoundly interesting, but it does mystify me that otherwise intelligent people take it seriously." Douglas Adams

  13. #28
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    SisuDiesel produces "small" straigh-7 diesel:
    7-Cylinder - AGCO Sisu Power

  14. #29
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    Quote Originally Posted by henk4 View Post
    ...A second disavantage is that it becomes impossible to put a straight eight in a transverse position, it will simply become woo wide. (The Miura V12 was the best they could )...
    I guess you forgot the Cizeta Moroder and its V16. I know it was quite some time ago, still...
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  15. #30
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    dont V6 have the worst inherant vibration because of the firing order necessary ?

    is not the V10 the most balanced design possible ?

    or have i been told wrong . . . . . . . ?

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