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Thread: Straight eight and 7 cylinder engines?

  1. #31
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    Quote Originally Posted by Badsight View Post
    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 . . . . . . . ?
    Did you mean V10 or V12? V10s are pretty bad, where V12's are balanced because they areessentially two I6s, which are inherently balanced.
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  2. #32
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    I believe I4s are worse than V6s. My recollection may be in error but I believe 60 degree V6s are OK for balance. Most are made without balance shafts. 90 degree V6s are generally not good but can be very effectively balanced using a single balance shaft located in the valley between the cylinders. This is why the Mercedes V6s and Buick 3800 were quite smooth.

    I6s are inherently balanced. Cross plane V8s become inherently balanced by simply adding some weights to the crank shaft. V12s are inherently balanced by virtue of being a pair of I6s.

    Do keep in mind that inherent balance is only part of what makes a motor smooth. The number of firing pulses per revolution is also very important as are the engine mounts. Many people associate engine smoothness with cylinder layout but in fact it's a series of things working together.

  3. #33
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    Quote Originally Posted by culver View Post
    I believe I4s are worse than V6s. My recollection may be in error but I believe 60 degree V6s are OK for balance. Most are made without balance shafts. 90 degree V6s are generally not good but can be very effectively balanced using a single balance shaft located in the valley between the cylinders. This is why the Mercedes V6s and Buick 3800 were quite smooth.

    I6s are inherently balanced. Cross plane V8s become inherently balanced by simply adding some weights to the crank shaft. V12s are inherently balanced by virtue of being a pair of I6s.

    Do keep in mind that inherent balance is only part of what makes a motor smooth. The number of firing pulses per revolution is also very important as are the engine mounts. Many people associate engine smoothness with cylinder layout but in fact it's a series of things working together.
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  4. #34
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    Quote Originally Posted by KM2 View Post
    Firing order probably does have something to do with there not being 7 or 9 cylinder engines out but as there are some for marine applications, it shouldn't be impossible.
    Radial engines have odd numbers of cylinders in order to make the firing order work out.

    The Corsair fighter plane used during WW2 had an 18 cylinder Pratt&Whitney radial engine. This was pretty typical for the time. The 18 cylinders are arranged in two rows of 9 cylinders each.

    The firing order for one row of cylinders would be: 1,3,5,7,9,2,4,6,8 so that you have gone through 720 degrees of crank rotation by the time all 9 cylinders have fired. The cylinders are evenly spaced at 40 degree angles from each other and pairs of cylinders fire 80 degrees apart until you get back to where you started.

    Any odd number of cylinders works for a single row radial engine subject to limitations on how closely you can pack the cylinders together.

    There is only one crank journal per row of cylinders with a connecting rod assembly using 9 connecting rods which all pivot around one big end connecting rod bearing.

    Radial engines pose a bit of a packaging / form fit problem for automotive applications. They fit really nicely in the round nose of a fighter plane.

    This is from some of the interesting trivia I learned from my father who was a Marine aircraft mechanic during WW2 and the Korean War. Corsairs were his specialty.
    Last edited by Aloser; 02-07-2012 at 04:35 PM.

  5. #35
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    You've been doing you homework. Thanks for sharing it with us, if you right ( i don't know anything abouy this kind of thing, i couldn't tell if your right or wrong)
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  6. #36
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    As I understand it, straight 8's can be timed for perfect primary and secondary balance, but are not necessarily naturally capable of doing so. As mentioned, the long crankshaft is prone to flexing, and this produces enough vibration to require a harmonic damper. It also puts down more power strokes per revolution so under load, it can be more smooth than a six under heavier loads. Any of these shortcomings were things which could be overcome during the inline 8's heyday, and combined with the advantages they enjoyed popularity in mid-priced and larger cars.

    As to the radial engines, they were made AFAIK in up to 4-row configurations to power the biggies. 4-row radial engine

    Both straight 8's and radials are dear to my heart, so I have read up on them a little. One of the best sounding cars I ever heard was a Ford powered by a supercharged Packard inline 8. Beautiful. And radial engines? Those are engine porn
    Last edited by jcp123; 02-03-2012 at 06:59 PM.
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  7. #37
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    As Culver says, there are two sides to smoothness. There is the vibration of the engine, and the smoothness of the power delivery. More cylinders allows a smoother power delivery, as more power strokes happen per revolution. A straight eight would deliver power very smoothly. However, based on an educated guess, I can only imagine that a straight eight engine would be subject to the same second-order vibrations as an inline four, perhaps at a higher frequency, depending upon the crankshaft design. The eight may also suffer, if the afore-mentioned flex in components affected the phasing of vibrations. If a crank shaft flexed, vibrations at some points along its length may not synchronise with the vibrations in a balancing shaft, for example.

    Radial engines have been built with as many as eight rows, as far as I know, but they were used for stationary purposes, such as power generation. As I understand it, a radial is inherently unbalanced, because the centres of mass of the crank throw, conrods, and most of the pistons, are moving in the same direction at the same time, as they orbit the centre of the engine. Vibration-wise, it would be analagous to having every piston in an inline engine moving the same direction at the same time. However, this is easily enough solved by including a single large counterweight for each row of cylinders, to completely balance the first-order vibration. I think the massive vibrations associated with radial-engined aeroplanes were probably the result of 40 litres of second order vibrations, and probably some manufacturing imprecisions.

    Rotary engines (the radial type, not the Wankel type), on the other hand, are almost perfectly balanced, because they have no large oscillating parts. The only vibrations in these engines come from manufacturing imprecisions, making the rotating mass of the engine slightly unbalanced, and with a rotating mass as large as the whole engine, that could cause considerable vibration. The forces generated by each ignition are internally resisted, in the same way as they are in any other engine, and do not cause vibration unless something is wrong.

    Ultimately, due to the performance and packaging advantages of other configurations of engine, straight eights became less good in pretty much every way, compared with modern inline sixes, V-anything else, and boxer engines. Likewise, an inline seven cylinder would have only slightly better performance than a six, slightly lower cost than an eight, and probably indifferent marketability. The advantages would not justify the design costs, under most circumstances.

  8. #38
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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 too wide...That rules out any option to use it for a FWD car if so desired.
    But why would you want to? GM's transverse V8s make me cringe, never mind a straight-8.

    FWD is for shopping hatchbacks / practical family boxes. Anything D-segment sized or above, if it's to be a driver's car, would be better with RWD / AWD IMHO.*Excluding cars like the Citroen C6 - that's a law unto itself and is awesome in it's own way!*
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  9. #39
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    Quote Originally Posted by Clivey View Post
    But why would you want to? GM's transverse V8s make me cringe, never mind a straight-8.

    FWD is for shopping hatchbacks / practical family boxes. Anything D-segment sized or above, if it's to be a driver's car, would be better with RWD / AWD IMHO.*Excluding cars like the Citroen C6 - that's a law unto itself and is awesome in it's own way!*
    I was more thinking about applications like the Bugatti F1 car from 1956....(there is a reason why few people remember this one)
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  10. #40
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    Quote Originally Posted by henk4 View Post
    I was more thinking about applications like the Bugatti F1 car from 1956....(there is a reason why few people remember this one)
    I remember reading about that, though I can't remember what it was called. Which model was it?
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  11. #41
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  12. #42
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    Quote Originally Posted by Wouter Melissen View Post
    Bugatti Type 251
    Thanks and yes: Strange design that.

    However, I'd love to see a straight-8 in something like a Morgan. - With an engine that long up front it'd feel like you were piloting a Spitfire (yes I realise that used a Merlin V12 - I should know since I live a couple of miles from the RR aero engine plant).
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  13. #43
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    I think I would prefer the Merlin to a straight-eight, if only for the engine sound. Based on the new Morgan three-wheeler, Morgan seem to have decided that the future looks like a classic fighter. I am not complaining.

    Do Rolls-Royce conduct ground-testing there? That must make it a noisy place to live.

  14. #44
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    Quote Originally Posted by Clivey View Post
    Thanks and yes: Strange design that.

    However, I'd love to see a straight-8 in something like a Morgan. - With an engine that long up front it'd feel like you were piloting a Spitfire (yes I realise that used a Merlin V12 - I should know since I live a couple of miles from the RR aero engine plant).
    A RR engine plant. I used to work at another one... though it wasn't always a RR plant.

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