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Thread: Engineering benefits of 6 cylinders?

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    Registered User lionheart33's Avatar
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    Engineering benefits of 6 cylinders?

    I can appreciate doing something because it can be done, but I am wondering what engineering problem does a 6 cylinder engine solve in a motorcycle compared to 4 cylinders?
    In motorcycle technology are engineers reaching some limitation in a 4 cylinder engine that can not be overcome?
    In the K1600 engine has BMW set a high standard from the start for efficiency and output or have they given themselves a starting point with years or decades of progressive improvements that can be rolled out over the years?

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    John. jstrube's Avatar
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    Smoothness. A straight 6 is balanced naturally with no harmonic vibrations. 4 cyl, 2 cyl, etc all have pulses in odd placed that make them vibrate to some extent.
    John.

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    Mars needs women! 35634's Avatar
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    For a big buck bike, marketing and bragging rights enter into the equation. Yeah, there are advantages to a 6, (and disadvantages-width, cost, complexity, expense) but much like having a V12 in you Ferrari/Mercedes/760i those extra cylinders will lure the deep pockets into the showrooms.
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    In theory, more cylinders for a given displacement can extract more power. Obviously, packaging, internal friction, and overhead reduce this to a finite number. Thus, technically 6 > 4, but the real reason in this case is balance, which promotes vibrational smoothness and firing sequence which promotes power delivery smoothness.

    The easy one first, firing sequence. It takes two revolutions (aka 4 strokes) to complete a cycle, thus each cylinder fires once every 720 degrees (really just said the same thing). Assuming even spacing (which isn't true in some cases), a twin cylinder engine will have a power pulse every 360 degrees (720 / 2). A four cylinder is 180 and a six is 120. For a given displacement, six 274.83cc (1649 / 6) spaced 120 degrees apart will provide a smoother power delivery than four 412.25cc firings spaced 180 degrees apart.

    You can see this in car engines in the extreme. A V-12 is basically two I-6s bolted together, but the firing sequence is now only 60 degrees apart. Basically as good as it gets for cars for smoothness.


    The second reason is balance. People get confused as there are several kinds of balance. The first order balance is reciprocating mass and is fairly simple. For any given mass that is moving you want another mass moving in the exact opposite direction. The boxer is a good example of this type of primary (first order) balance. Both pistons extend together, but since they are physically 180 degrees apart they cancel each other 100% on primary balance. I.e. both pistons move towards the crank at the same time; same for the valves. Any first order imbalance will have a frequency = RPM (technically revs per second, but who cares).

    While you can technically provide a first order balance for any engine configuration with enough crankshaft weight, it should be obvious that doing so it not ideal. Thus, an engine which provides a natural first order balance is generally superior. Interestingly enough this is what generally gives American V-8s their low reving and lumpy character (large crankshaft weights and a straight crank) vs. the high reving and slightly viby nature of European V-8s (cross-plane cranks).

    The second order balance actually has many parts to it and can't be balanced simply by adding or removing weight. Despite what BMW marketing may claim about the S1000RR, it's not possible to balance that I-4 just by paying attention to masses. A second order imbalance causes a vibration that is twice the frequency of the engine speed. This is what causes the boxer vibs and the I-4, etc. However, not all second order imbalance problems are created equal. I.e. Some can be ignored for small displacements or are easier to mitigate (less side effects), etc.

    Example: The largest vibration from a boxer-2 is due to the fact that the engine crank actually slows down and speeds up through every revolution. Ideally I'd always want a piston on the firing stroke of it's 4 part cycle so that there is always a driving force and in such cases I'd have a "constant" crank speed. The boxer doesn't have this so it vibrates a tiny bit as the crank speed is constantly changing even for a average constant RPM. The solution for this is to put a large flywheel so that the system has greater momentum and will slow less through each revolution. In the grand scheme of things this is minor, but the price you pay is an increased torque about the crank axis and that extra flywheel mass works against you when you want to increase/decrease revs quickly. There's never a free lunch.

    A I-6 isn't perfect on all accounts, but it does provide one of the best all around balances possible. However, it generally commits two cardinal sins in mass production vehicles: cost and packaging. It costs more because you really can't share parts of it with other configurations (e.g. V-6s and V-8s are very popular because only a few parts have to be swapped to create each configuration...one for the entry level and another for the high end cuts costs). The configuration is so long that it makes it hard to package (which drives costs up even more if your engineers have to get creative). Same for the boxers, technically superior in many ways, but doomed to obscurity.

    In order to create the awesome engine I-6 K1600 engine they had to get creative. Three major things were done: cylinder spacing, cylinder bore, and parasitic load location. They have special process and a difficult design to reduce the cylinder spacing to something like 5mm. That puts stress on your manufacturing processes and cooling design. The bores are small, but with long strokes to still provide that capacity. That typically means you end up with a low reving, high torque design. The other loads (e.g. alternator) are piggybacked on the engine rather than being on the end. That incurred some loss as that is chain driven (IIRC) and cooling of those parts can become problematic.

    Of course the devils in the details, but that is the generally reason why they wanted a I-6 and what they had to do to make it happen. I'm by no means an expert, but it is fun learning how things work.

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    God? What god? roborider's Avatar
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    And this is a good book to pick up and read:

    http://www.amazon.com/Modern-Motorcy...pr_product_top

    It is pretty technical in nature, but a fine read for anyone wanting to learn a lot about motorcycle engines and systems.
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    Registered User mneblett's Avatar
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    Quote Originally Posted by gunderwood View Post
    The second order balance actually has many parts to it and can't be balanced simply by adding or removing weight.
    (snippage)
    Example: The largest vibration from a boxer-2 is due to the fact that the engine crank actually slows down and speeds up through every revolution. Ideally I'd always want a piston on the firing stroke of it's 4 part cycle so that there is always a driving force and in such cases I'd have a "constant" crank speed. The boxer doesn't have this so it vibrates a tiny bit as the crank speed is constantly changing even for a average constant RPM. The solution for this is to put a large flywheel so that the system has greater momentum and will slow less through each revolution. In the grand scheme of things this is minor, but the price you pay is an increased torque about the crank axis and that extra flywheel mass works against you when you want to increase/decrease revs quickly. There's never a free lunch.
    Another issue with a boxer is the rocking couple created by the offset crank throws -- something which again a straight 6 doesn't have as much to deal with.

    As to the crank masses, everyone knows how to balance primary order masses. The real "mystical magic" is in deciding what the relationship between crank throw mass, piston weight, connecting rod weight and configuration should be -- i.e., balancing ideal piston/crank counterweight matching against con rod-counterweight matching when the crank pins and the counterweights are at the 90 degree positions (i.e., when the piston is 1/2 way up or down the bore and the crank pin and counterweights are on a line perpendicular to the bore). That relationship can significantly alter the "feel" of the engine, and for many manufacturers, their approaches to determining the "ideal" percentages/ratios/etc. for the feel they want are closely-held proprietary wizardry.
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    Registered User lionheart33's Avatar
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    Quote Originally Posted by gunderwood View Post
    In theory, more cylinders for a given displacement can extract more power. Obviously, packaging, internal friction, and overhead reduce this to a finite number. Thus, technically 6 > 4, but the real reason in this case is balance, which promotes vibrational smoothness and firing sequence which promotes power delivery smoothness.
    From a practical point of view what should we expect from the improved vibrational smoothness? On the surface it appears that the 1600 engine compared to the 1300 engine gives more torque, more HP at lower RPM - are there other benefits? Should we expect longer engine life, lower maintenance costs, ability to ride longer with less fatigue? Should we expect material improvements in both torque and HP over time from this initial engine?

    My '07 RT is my first BMW motorcycle - one of the primary reasons I chose it over the GT was to get an opportunity to experience the boxer engine given the history - I wonder if the 1600 engine will become a motorcycle engine with similar historic significance in time?

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    Registered User amiles's Avatar
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    My '07 RT is my first BMW motorcycle - one of the primary reasons I chose it over the GT was to get an opportunity to experience the boxer engine given the history - I wonder if the 1600 engine will become a motorcycle engine with similar historic significance in time?[/QUOTE]

    "Historical significance" is not necessarily synonymous with a "Successful Design"

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    Registered User mneblett's Avatar
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    Quote Originally Posted by amiles View Post
    "Historical significance" is not necessarily synonymous with a "Successful Design"
    True that -- Titanic, Olympic and Gigantic (ne: Brittanic) were "historically significant." Successful -- well ...
    Last edited by mneblett; 10-18-2011 at 09:05 PM.
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    Registered User lionheart33's Avatar
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    Quote Originally Posted by amiles View Post
    "Historical significance" is not necessarily synonymous with a "Successful Design"
    Yes, that is the reason I am trying to understand what problem(s) are being addressed from an engineering point of view. If I ever do buy a bike with a 1600 motor I want it to be for reasons other than hype. BMW needs to realize that its core market is made up of people who want more information, not less.

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    Registered User mneblett's Avatar
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    Quote Originally Posted by lionheart33 View Post
    Yes, that is the reason I am trying to understand what problem(s) are being addressed from an engineering point of view. If I ever do buy a bike with a 1600 motor I want it to be for reasons other than hype. BMW needs to realize that its core market is made up of people who want more information, not less.
    Not sure I understand the reason for the last statement-- what information do you believe BMW hasn't given us?

    Are you suggesting that BMW doesn't have a clue about what its "core market" is/wants? Given their pretty rigorous product development processes, that seems a bit, um, unthoughtful.

    As far as problems being addressed from an engineering point of view, I beleive you've received several answers, including primarily: superb smoothness which cannot be matched by most motors, particularly inline fours.

    What else are you looking for? The fact that the marketers get to have a field day with a "new" design that is very different from any current competitor? That aspect may be a significant factor in the selection of the straight 6 for the new bike, but does that somehow make the existence of the 6 any less attractive?

    Sorry -- can't figure out where you are going with this.
    Last edited by mneblett; 10-18-2011 at 09:49 PM.
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    Registered User lionheart33's Avatar
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    Quote Originally Posted by mneblett View Post
    Not sure I understand the reason for the last statement-- what information do you believe BMW hasn't given us?
    All I have seen is superficial marketing materials. At the most fundamental level I am not sure I understand why there is even a need for a 6 cyl. motorcycle engine. I am not being negative toward the brand or any individual.

    Are you suggesting that BMW doesn't have a clue about what its "core market" is/wants?
    On the automotive side BMW has changed its emphasis away from its traditional core market and has focused on a broader appeal. There is no doubt they make a great product, but in the past, at least from my point of view, it seemed that a BMW product was an engineered product more than a product driven by marketing and accounting. I have owned three M3 automobiles and each succeeding generation had its improvements but each generation lost some core elements in character. I have no interest in the latest 8 cyl. M3. That is just my personal choice and my personal point of view.

    Given their pretty rigorous product development processes, that seems a bit, um, unthoughtful.
    Product development is often a function of trade-offs - I would love to better understand BMW's long-term strategy with their motorcycles. I clearly don't understand what happened with the K1200GT to the K1300GT, seems like the cycle was cut short for some reason. Was that thoughtful on their part?

    As far as problems being addressed from an engineering point of view, I beleive you've received several answers, including primarily: superb smoothness which cannot be matched by most motors, particularly inline fours.
    "Smoothness" is a vague concept. If that is all there is, so be it.

    What else are you looking for?



    Sorry -- can't figure out where you are going with this.
    My questions are pretty simple. There is no hidden meaning or agenda.

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    Registered User lkchris's Avatar
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    There are few "engineering" reasons for the 6 and VERY MANY marketing reasons.

    BMW needs to be unique, exclusive, and expensive. 6 fits this quite nicely.

    BMW owns the luxury car market by marketing "sporting." Seems they like that approach as regards the bikes, too.

    Note that R1200C is gone. Note that the K1200LT is gone. Lets hope BMW continues to ignore USA dealers' opinions on the bikes they should offer and that they never venture into the "chopper" and "bago" markets again.

    6 is a pretty cool replacement for the LT, don't you think? It's not a "me too" anything.

    NEVER EVER think that engineering EVER trumps marketing because it NEVER does.
    Last edited by lkchris; 10-19-2011 at 03:19 AM.
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    Registered User lionheart33's Avatar
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    Quote Originally Posted by lkchris View Post
    6 is a pretty cool replacement for the LT, don't you think? It's not a "me too" anything.
    I can appreciate the innovation in the engine regardless, even if there are no tangible functional benefits. However I do suspect that the engine design may be the ground work for an engine that will stand the test of time.

    NEVER EVER think that engineering EVER trumps marketing because it NEVER does.
    The way to sell me is to sell me on innovative functional design. I think this has been a cornerstone of BMW's approach to its products and sales success. A perfect situation is having a great product and understanding what makes it a great product - marketing has its role. In the long run marketing a poorly engineered product leads to failure.

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