Musings on the Internal Combustion Engine

I recently started a thread on a message board comparing the benefits of Wankel Rotary vs. Inline-6 cylinder combustion engines (I still haven't worked out which one is smoother, so please offer me an opinion if you have one).

This discussion prompted me with a number of questions I'd like to explore here. Pardon me if the tone of these musings tread into the incomprehensible due to heavy use of technical jargon that I don't bother to explain in the body of this essay. Specifically, I am going to go over what (I believe) causes engineers to select certain configurations over others in designing engines for specific applications. Of course, expect that in addition to this my own opinions will be liberally injected and open to debate.




















Perhaps I should start with the first question I mentioned. Which is better, and Inline-6 or a Rotary? This is difficult to address without a given displacement or manufacturer, but for this question I can at least set up some likely boundaries: An Inline-6 cylinder engine possesses both perfect primary and secondary balance. Additionally, any configuration involving inline 6s will have this same balance advantage (i.e. V-12s, V24s, etc). Due to this balance advantage, an inline 6 engine can also be scaled up to monstrous proportions with few ill-effects. The Wankel Rotary by comparison looks as if its going into battle with one of its limbs already shot off. Unfortunately, this engine has only been made in significant production numbers by Mazda. It's most common with a displacement of 654cc per rotor, with the 1.3 litre 13B being the most common engine. So not only is much rarer, but its displacement is pretty much set. However the 13B does have some advantages. It also has perfect primary balance with the use of appropriate counterweights, and secondary imbalance isn't an issue really as it has no reciprocating pistons to produce any non-sinusoidal movements. In addition, a select few engine building gurus have managed a few times over the years to craft specialty 4, or 6 rotor engines; far exceeding Mazdas biggest factory effort with the 20B. As these are unique one-off creations however, and they cannot be compared to a mainstay of automotive and industrial engineering production such as the Inline-6. Indeed, only the 13B has had high enough production numbers to be compared with an Inline-6, and in that there are a number of shortcomings. First of all is the need of counterweights to balance the engine. This makes the 13B slightly slower revving despite a typically higher redline. In addition, while the power delivery and motion of the engine is silky smooth, its combustion frequency is less so. With a 2-stroke-like power output the twin-rotor 13B is identical in combustion frequency to an Inline-4, even if its overlapping power strokes eliminates the odd transition of the Inline-4, it still isn't as frequent as the Inline-6. Then again, any Inline-6 is likely to weigh significantly more than a rotary... And so i feel as if I am left at an impasse on this issue. It may be conspicuously obvious that I have left reliability and longevity out of this equation. This because it relates to the operating conditions and treatment of the individual engine.

Well, this is turning out to be quite the wall of text and I haven't really gotten to the meat of things, but what can be expected when there are few concrete conclusions to be drawn? My current economic reality doesn't allow for me to play around with vehicles that have the previously mentioned engines. Motorcycles make more sense in my current situation, and they present a number of interesting engine configurations that deserve mention as well. Rotary bikes like the Norton Commander or Suzuki RE5 are cult bikes in their own right, but I always thought a Mazda rotary powered bike would be awesome for a number of practical reasons; primarily that the 13B had a much longer, and refined production. I'd thought it might make more minimalistic sense to try mounting it transversely with one of these, even though I don't know how the transmission would work. This point is kinda tangent to my earlier objective, but I just figured I'd mention it as it helps segue into the engine configurations of actual production motorcycles...

Pretty much everything has been tried in motorcycles before. That said there are a few popular configurations that have proved to be more effective than others; namely Single, V-twin, Inline twin, Triple V-four, and Inline four. While 2-stroke engines can make things a bit more complicated here, basically every motorcycle engine currently in production is one of these designs, (Ok, BMW has a Boxer-twin, but that's just the exception to the rule). 

This may get ridiculous
Single pistoned motorcycles, or "thumpers" are so called for the heavy rhythmic vibration that come with them. In small displacements they can actually rev quite high, to make the most of their diminutive size (Not uncommon to see 10,000 RPM out of a 125cc single). On the other hand if scaled up the vibration increases significantly, and as such the redline is generally much lower (The 650cc Suzuki Savage comes to mind with its 6,000 RPM redline). Despite how some ladies may enjoy the vibration of these bikes, they are often difficult to live with despite their simplicity. Seriously, most singles (2-stroke dirtbikes aside) will find themselves challenged by a steep enough hill. If we move on up to twin cylinder bikes we find a number of interesting arrangements, and starting here is where the angle of crank throws needs to be addressed. An Inline twin can have its two crank throws at 360° (both cylinders moving in the same direction at the same time),  180°, (both cylinders moving in opposite directions at the same time) or 270° (emulates the feel of a 90° V-twin, and mixes the imbalances of the previous two crank configurations). None of these yield perfect primary or secondary balance (without using counterweights, commonly called "balance shafts") but their advantage over other configurations is their simplicity with less complicated valvetrains, easier packaging in the frame, and reduced weight. The V-twin department has different advantages, and disadvantages. A V-twin can reduce some weight and complexity by using a single pin crankshaft, and if the cylinders are banked at 90° Perfect primary balance is also achieved. Anything less than a 90° bank however will induce some vibration no matter how cleverly it is reduced. As usual with engine imbalance scaling up the displacement increases the vibration. Hence how a 1200cc Harley with its 45° bank will rattle you all over the road until your hands are numb. Inline triples may just take the crown however when it comes to balancing smoothness with practicality for a motorcycle. Secondary imbalance is less than on an Inline 4, and if we ignore its inherent rocking motion it has good primary balance as well, that is on top of a powerplant that balances the weight of the motorcycle better than an Inline 4. Sure enough, these days all but Ducati have moved to the use of Inline 4 engines for their racing applications, So there has to be something great about this engine configuration; and their is. It has perfect primary balance, and despite its relatively strong second order imbalance, it can achieve extremely high RPM (I remember seeing a few with 17,000 redlines). This is possible despite the secondary imbalance problem because with 4 cylinders the individual cylinders' displacement is quite low. It is essentially scaled down compared to what would be common in a car with a lower redline. For these reasons Inline 4s in motorcycles tend to be of low displacement and low torque with peaky high RPM power, or they tend to be of larger displacement with a lot more torque and less top end.

Ok, so basically my finalists for the best motorcycle engine configuration go like this: The 90° V-twin for its perfect primary balance, and its low transverse cross section. The Inline 4 for its similarly perfect primary balance, and its variation in availability and application. And lastly the Inline Triple for balancing cross sectional girth with operational smoothness and vibration... In my mind the all round winner here is the Inline Triple. That said there is a specific reason why I would not own one:  Noise. Due to the Otto cycle there is always one stroke every two rotations that is "dead" in which there is no combustion at all happening. This brings up umm... let me call it an auditory imbalance. These things sound disgustingly horrible. It is for that reason that I personally would always prefer a V-twin or an Inline 4. The 4 is perfectly rhythmic, with no combustion overlap, and at the appropriate RPM these things sound glorious; practically like mini-F1 cars. The rhythm of the V-twin is unbalanced, but somehow still sounds beautiful most of the time. I attribute this to it's auditory signature being similar to the human heart beat. In this way perhaps it resonates with me on a more visceral level than most other engines. In any case that's about it for this article. If you came here for some talk about South Korea but were mad enough to read through all of this, you have my sincerest apologies.

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