High vs Low RPM motors

So a former team member and I were having a discussion on why it is that high rpm engines seem to be ideal for racing applications. I understand the fact that the longer you can maintain your torque into the rev range the better. But with all else being equal it seems to be that short stroke, lower torque, high rpm engines seem to be the ideal choice. I've always understood the fact that for towing applications you would want a long stroke so that each power cycle produces a higher amount of torque and your engine doesn't have to be screaming to get moving. However I've never been able to wrap my head around why it goes the other way.

Does it come down to gearing restrictions where if the high torque/low rev motor is geared high enough then power delivery becomes very rough and throttle response is decreased?

Just interested to hear other members thoughts on this point.

A high rpm engine will of course be smoother, but I think in the range of engines you find in FSAE that you wouldn't really get to the point where the driver feels a choppy power delivery. However, singles and (low speed engines?) can be harder to start and idle because there is a much greater speed and power variation throughout one cycle. Which I've heard can become significant to combustion dynamics at low speeds.

It might be because at low rpm you lose a much larger percentage of combustion heat to your engine block. Your heat rate is roughly constant and so the longer a stroke takes the more heat will be lost. If your engine goes fast enough, this affect will be reduced but you will begin to lose more energy to friction loses. Peak thermal efficiency is somewhere between these two extremes.

Longer stroke motors also see higher piston accelerations and higher forces. Which means they need to be build heavier and are likely to accelerate more slowly than a lighter high rpm engine. Although, I don't know what happens when you apply a suitable gear reduction and the fact that your longer stroke engine is operating at lower speeds/accelerations.

Actually, another idea,for a given engine volume because Power=Torque*RPM,you make more power at higher rpm because RPM increases faster than torque drops off from having to shorten the stroke. But for towing you just need lot of torque to get it moving so you go for the low rpm long stroke. For FSAE we want to convert lots of power into kinetic energy, so high RPM is good. Of course then there is the restrictor problem at high rpm....

I'm not an engine guy so could be wrong on stuff, but it was an interesting question and got me thinking. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Horsepower is what accelerates the vehicle.

For the same displacement and torque, you get more horsepower with increasing engine speed. Therefore generally, an engine with high power-to-mass ratio will run at a higher engine speed.

It really is that simple generally, although lots of semantics can be discussed.

Quote:

---

long stroke so that each power cycle produces a higher amount of torque

---

Diforesi,
Explain to me please how a long stroke engine produces more torque?

Pat

Like others have said, it is primarily because you will get more usable power at higher revs(P = T*omega).

I believe the phenomenon of high RPM racing engines is mostly because of head geometries. A high RPM engine like found in F1 is finely crafted to operate perfectly in a very small range. A lower RPM engine has a much broader powerband, but is not extracting the peak potential of the engine cycle.

In higher levels of motorsports, you can afford to have a peaky engine because you can also afford a driver capable of using it. For towing a trailer, you do not want to be flitting between gears every 3 seconds. Also, like you said, they have to have some get up and go in the lower range just to get rolling (no double clutch launch http://fsae.com/groupee_common/emoticons/icon_wink.gif).

Just my take on it, there's tons of finer details beyond my own understanding.

Quote:

---

Originally posted by PatClarke:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">long stroke so that each power cycle produces a higher amount of torque

---

Diforesi,
Explain to me please how a long stroke engine produces more torque?

Pat </div></BLOCKQUOTE>

I think he's talking about a stroker. You know same bore, more stroke, more displacement.


As for the OP's question. For the same displacement the motor that revs higher (if properly tuned) will make more power. More power means if you gear it right you can put torque to the wheels for longer and have more force to accelerate. This is because power is a function of RPM and torque. If a racing series is displacement limited without restrictors you will almost always see motors being pushed to the highest RPM possible to get as much power out as they can.

Quote:

---

Originally posted by PatClarke:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">long stroke so that each power cycle produces a higher amount of torque

---

Diforesi,
Explain to me please how a long stroke engine produces more torque?

Pat </div></BLOCKQUOTE>

Quote:

---

long stroke so that each power cycle produces a higher amount of torque and your engine doesn't have to be screaming to get moving

---

I think he is talking about more torque in the lower rpm range. Which can be true:

Longer stroke --> smaller bore --> smaller valves and ports --> higher speed of air/fuel mixture at a given speed --> more tumble/swirl --> more turbulence --> (together with more compact shape of the combustion chamber; and less wall heat transfer) faster/more efficient combustion --> ignition timing can be adjusted accordingly --> more torque http://fsae.com/groupee_common/emoticons/icon_smile.gif

Quote:

---

Longer stroke --> smaller bore --> smaller valves and ports --> higher speed of air/fuel mixture at a given speed --> more tumble/swirl --> more turbulence --> (together with more compact shape of the combustion chamber; and less wall heat transfer) faster/more efficient combustion --> ignition timing can be adjusted accordingly --> more torque

---

Well put Mr. Dressler. You can also see how these characteristics would limit torque at higher rpms, affecting peak power.

It's probably worth mentioning too that a fundamental limitation for IC engines is piston speed. This is why you can't get the cylinder filling benefits of a long stroke and still have high rpm/power.

Quote:

---

Originally posted by Owen Thomas:
Well put Mr. Dressler. You can also see how these characteristics would limit torque at higher rpms, affecting peak power.

---

Thanks. And yes, it certainly affects peak power http://fsae.com/groupee_common/emoticons/icon_smile.gif

Quote:

---

Originally posted by Owen Thomas:
It's probably worth mentioning too that a fundamental limitation for IC engines is piston speed. This is why you can't get the cylinder filling benefits of a long stroke and still have high rpm/power.

---

Well, you wouldn't get very high performance out of a long stroke engine at high rpm anyway, even if it could do that high rpm, as you mentioned.

The other way round, you will get into problems performance-wise if you design your bore too large - At a bore-stroke ratio of 2:1 and plus, the tumble flow field of your ordinary 4-valve pentroof combustion chamber will have increasing problems to develop properly. That's where valve designs other than the conventional poppet valve might come into play...

Additionally it has to be mentioned that a rather large valve area and ports for a given swept capacity (and therefore larger bore/stroke ratio) not only comes with higher engine speed, but also with turbo-/supercharging.

Quote:

---

Originally posted by PatClarke:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">long stroke so that each power cycle produces a higher amount of torque

---

Diforesi,
Explain to me please how a long stroke engine produces more torque?

Pat </div></BLOCKQUOTE>
My understanding (possibly lacking and/or wrong) is that for one, the combustion pushes the piston for a longer amount of time. But, what I think is more significant is that by having a longer stroke, the journals are further from the center of the crankshaft, giving the piston/connecting rod more leverage. Because torque is rotational force, by having more leverage on the crankshaft it is easier to rotate, so there is more torque from a longer stroke.
Remember, these are the musings of a high school student, so some or all of it may be dead wrong.

Danny,

Just because a long stroke engine has a larger crank radius doesn't mean it's going to produce more torque. The torque it produces is a result of the combustion process, which can have vastly different levels of effectiveness (efficiency) in different engines. Also, a longer stroke means a longer time spent on the compression stroke where you are losing piston energy.

Or, for a given capacity;

A long stroke engine has a smaller bore, = smaller piston area, = smaller force acting on its longer lever arm.

A short stroke engine has larger bore, = larger piston area, = larger force acting on its shorter lever arm.

Bottom line is that for a given capacity the torque is (approximately) the same regardless of bore-stroke ratio (ie. ~100Nm/litre, (+/-20% from pulse tuning)).

But, as pointed out above by Owen (and in more detail on another thread recently) all engines have a workable maximum mean piston speed of about 20-25 m/s, so a short stroke engine can rev higher, and so produce more power.

Z

(PS. F1 engines for the last ~20 years have B:S ratios >2 (B = ~95mm, S= ~45mm), yet produce similar torque to any other engine of the same capacity, and have similar "racing engine" maximum mean piston speed of 25 m/s. The decades long movement to shorter strokes and higher revs is a direct consequence of seeking more power from a given, rules-mandated, maximum engine capacity. It is not the most efficient way of making power (frictional losses), but in motorsport expensive + inefficient = good !)

Just adding to what has been said and demonstrate this with some simple equations:
Torque = Force X Distance
Torque = Pressure X Area X Distance
Torque = Pressure X Area X Stroke/2 (Crank pin length will be half stroke length due to crank slider mechanism)
Note here that Area X Stroke = Volume
Torque = 1/2 * Pressure * Volume

So unless you increase MEP, you cannot increase torque for a given volume. MEP will be affected by several things such as volumetric effiency, ignition timing, compression ratio, fuel type, A/F ratio etc. These are all tunable parameters or affected by the design of intake/exhaust (cams/ports/valves too).

IMHO there is more hype than deserved regarding high rpm vs. low rpm engines. People often speak of a high-rpm engine having absolutely no power at lower rpms, while if you actually overlay a couple dyno charts it will only be down by maybe 20%. While 20% is a significant drop, it certainly isn't "absolutely no power." However, when driving the high rpm engine it may seem that way, as it drives just as lame as the low rpm variant until it "hits the cam" and things get wild.

A great example of this is comparing the Buell XB-12 with the Buell 1125R. They were both made at the same time, and had similar displacements (1200 vs. 1125cc). The XB-12 had essentially a hopped-up Harley 45º air-cooled v-twin that they managed to get to survive reliably to over 6,000 rpm. The 1125R had a modern Rotax DOHC water-cooled, fully balanced 72º v-twin that happily spun to almost 11,000 rpm. Both were tuned to Erik Buell's specifications, so you really can't get a better comparison of a high-rpm vs. low-rpm motor. Here are the two dyno curves:

http://i167.photobucket.com/albums/u...vsXB12dyno.gif

Another misconception is that high rpm requires power over a narrow range of rpms. There is a lot that goes into cam design that determines the breadth of the power curve; a high rpm motor does not necessarily have to be peaky. You will get more output from a peaky engine, but it won't be as usable. Regardless, that is a function of cam design and is largely independent of rpm (although high-rpm engines gain more by being peaky...). What I find even more interesting about comparing these two is that the high-rpm motor actually flattens out more on top than the low-rpm motor. It actually has a broader powerband!

My personal preference is a highly linear output like the two engines shown above. Power comes on very gradually and predictably as a function of rpm. You can short-shift it and hold a higher gear in sweepers or on slaloms for smoother torque management, and just lean into it on exit, and if you're really good you can mesh the progression into the power band with corner exit. It just makes it easy to be smooth IMO.

Something you also have to take into consideration is that the engine is just a part of the whole car. So the weight, the center of gravity and the dimensions are also very important.
As already said by the others, an engine that revs higher, will need a shorter stroke. With a shorter stroke you will have a smaller crankshaft. So it can be positioned lower, which also lowers all the other internals. Additionally you can make your connecting rod shorter, so your cylinder head moves down, too.
And although your cylinder head will have to be a bit wider, I would say that this engine would also be lighter.

In F1 (until 2005), they did put a lot of effort into the engine development. Not just because of the power, but also because of the cog, the weight and the dimensions.

Quote:

---

Originally posted by Paul Gerisch:
With a shorter stroke you will have a smaller crankshaft.

---

Smaller in diameter, but wider (in case of a multi-cylinder engine), as you mentioned below.

Quote:

---

Originally posted by Paul Gerisch:
Additionally you can make your connecting rod shorter, so your cylinder head moves down, too.

---

Keep in mind that conrod length affects the engine's secondary balance. So, with shorter stroke --> higher rev range, your conrod should not be TOO short...

P.S.: Welcome to this forum, Paule http://fsae.com/groupee_common/emoticons/icon_wink.gif

I love adambombs graph. This shows the sacrifice to a higher rpm engine quite well. Because friction goes up with RPM, you shouldn't expect to get the same torque at higher rpm, but ultimately torque is most closely linked to displacement. Stroke and Bore have an impact on where the peak torque rpm is, but very little influence on the peak amplitude.

There are certainly engines out there that do not live up to their displacement potential. For instance many stationary, and high duty cycle engines. These are engines the designers worked to increase reliability and lifespan on over outright output and shouldn't be used for comparison.

But if you look at racing engines from AMA superbike through drag racing pro-stock engines and everywhere in between, the torque vs displacement points make a pretty straight line.

It would be rather interesting to take those two engines as tested, fit restrictors to them, retune them, and test them again.

It could well be, that the old Harley banger with it's higher Ve at lower rpm out torques and maybe even out powers the Rotax.

The Rotax probably has much larger ports and valves, and much more radical valve timing. Strangled right down with a restrictor, it is never going to be able to reach anything like 11,000 rpm.

And at the lower max airflows limited by the restrictor, it could quite likely be operating at a severe disadvantage.

Quote:

---

Originally posted by Slim:
A high rpm engine will of course be smoother, but I think in the range of engines you find in FSAE that you wouldn't really get to the point where the driver feels a choppy power delivery. However, singles and (low speed engines?) can be harder to start and idle because there is a much greater speed and power variation throughout one cycle. Which I've heard can become significant to combustion dynamics at low speeds.

It might be because at low rpm you lose a much larger percentage of combustion heat to your engine block. Your heat rate is roughly constant and so the longer a stroke takes the more heat will be lost. If your engine goes fast enough, this affect will be reduced but you will begin to lose more energy to friction loses. Peak thermal efficiency is somewhere between these two extremes.

Longer stroke motors also see higher piston accelerations and higher forces. Which means they need to be build heavier and are likely to accelerate more slowly than a lighter high rpm engine. Although, I don't know what happens when you apply a suitable gear reduction and the fact that your longer stroke engine is operating at lower speeds/accelerations.

---

High RPM engines doesn't have anything to do with how smooth the operation of the engine actually is. The "smoothness" of an engine is determined by the number of cylinders, firing order, balancing factor of the crankshaft and any additional balancers added to the engine (be it a balance shaft or balancing gears). These items create torsional vibration and torsional bending of the engine block as the corresponding cylinders are fired. This is where firing order plays a role as the arrangement of "bangs" setup torsional waves along the crankshaft and into the block.

What do you mean by choppy power delivery? Do you mean how the power delivery feels between a single cylinder and a 4 cylinder? Or do you mean from a tuning aspect.

Heat transfer between the combustion process and the cylinder block have nothing to do with RPM (per say), but to do with ignition timing (that is why I said per say, spark timing is dependent on RPM and your mixture. So I guess you are right. However -&gthttp://fsae.com/groupee_common/emoticons/icon_wink.gif. Generally at lower speeds your engine has a low spark advance value as there is more time available for the peak pressure to reach an optimal 15-16 degrees ATDC. As you increase your operating speed of the engine more advance of spark timing is required to allow for peak in cylinder pressure to remain close to the previous value of 15-16 degrees ATDC. Now since you've added more advance onto your spark timing you expose the cylinder walls to the flame front for a longer period of time increasing the heat transfer between the flame front and the cylinder walls. Therefore you are loosing energy from the combustion process to raising the heat of the coolant/cylinder walls/block. Is it possible to retard the timing at higher RPM's to generate a faster combustion rate? I'll let you think about it.

Jan_Dressler is correct with what he mentioned. Just one thing, this probably isn't the case in FSAE as a restrictor is already found upstream of the ports, but if your ports are to small you have the potential of reaching coke at the throat of the port. Port design is also extremely important aspect of engine design, however more often so the majority of the engine geometry is already predetermined from the manufacture leaving little room for additional improvements in gaining fluid energy through the ports. Also try to eliminate swirl in port injected engines, the centrifugal forces of the air-fuel mixture swirling into the combustion chamber will wet the cylinder walls removing oil from the wall = trouble http://fsae.com/groupee_common/emoticons/icon_wink.gif.

I am also surprised that no one mentioned anything about the momentum of the air/intake pressure waves and its correlation to RPM? Or maybe I missed it.

Quote:

---

Originally posted by Warpspeed:
It would be rather interesting to take those two engines as tested, fit restrictors to them, retune them, and test them again.

It could well be, that the old Harley banger with it's higher Ve at lower rpm out torques and maybe even out powers the Rotax.

The Rotax probably has much larger ports and valves, and much more radical valve timing. Strangled right down with a restrictor, it is never going to be able to reach anything like 11,000 rpm.

And at the lower max airflows limited by the restrictor, it could quite likely be operating at a severe disadvantage.

---

Tony, I'm not so sure I agree with these points.

Things are different when you get into low cylinder counts and odd-fire engines in restricted scenarios. We run such large plenum volumes on singles because the restrictor chokes the induction event on a pulse basis, not at steady flow. We must time-average the flow through the restrictor to negate its effect. Since the restrictor is limiting the volume of a single induction event, it limits peak torque and not peak power. If the two engines were the same, but one had more peak torque, you might find that the torque peak of one was truncated while the other seemed unaffected in comparison.

But the two engines have different firing orders and the 72 degree firing order plays in favor for the 1125 in a restricted scenario. With only a 45 degree separation between induction events, the xb-12 might as well be treated as a thumper.

Finally, with the added valve periphery of 4 valves instead of 2, the Rotax undoubtedly runs less duration than the XB-12. I found these specs for the XB-12. Taking ramps for pushrods into account, duration at 1 mm is likely over 265/265. There is no way the 1125 would need this much cam--with great thanks due to 4-valve breathing.

Timing @ .053” Lift Open / Close:
INT. 25 / 44
EXT. 59 / 10

Duration @ .053” Lift:
249 / 249

Max Lift:
.551” / .551”

Quote:

---

Originally posted by Mbirt:

Things are different when you get into low cylinder counts and odd-fire engines in restricted scenarios.

---

Very true.
Being forced to run a restrictor is a real game changer, and the more uneven the pulsing of the induction airflow, the worse it all gets.

Very difficult to predict what might happen, which is why I find this particular engine comparison a particularly interesting one.

But the thought still remains. As a restrictor effectively causes power to plateau at some relatively lowish rpm, an engine with a higher Ve at, and especially below that lower rpm may be working at less of a disadvantage with a restrictor.

Getting the hottest available high rpm race engine, and then strangling it right down with a restrictor may not always be the best approach to the problem.

Quote:

---

Originally posted by Mbirt:
But the two engines have different firing orders and the 72 degree firing order plays in favor for the 1125 in a restricted scenario. With only a 45 degree separation between induction events, the xb-12 might as well be treated as a thumper.

---

Mbirt,

Because I am a nit-picking PITA I should point out that the above quote is back-to-front. Assuming the two V-twins are of conventional design the 45 degree Harley will fire every 315-405 degrees (360+/-45), and the 72 degree Rotax will fire every 288-432 degrees (360+/-72). So the induction events on the Harley are more evenly spaced than on the Rotax, so giving potentially better breathing through the restrictor.

The very old Harley design was probably a case of the designers trying to figure out the best way to get more power from their single cylinder engine.
"Maybe increase bore and stroke?"
"But then we have to change all the tooling..."
"How about we use two of our existing engines side-by-side, parallel-twin style, for twice the power?"
"Errrr..., but then it will weigh twice as much..."
"Ok, how about we just stick an extra cylinder onto the crankcase, here, just behind the existing cylinder, and use pretty much the same crank?"
"Yeah, saves the weight of that extra crank and crankcase, and keeps it all nice and narrow. And if we push the two cylinders as close together as possible we can get pretty close to 360-360 firing..."

Z

I agree with Z's possible Harley scenario. Even more plausible when you consider the forked connecting rod design where the two cylinders are directly in plane with each other. The crank pin is not wide enough for two full size rods.

Thanks, Z. I stand corrected. I had assumed the potato-potato sound and seemingly poor VE were due to a 45 degree firing order to match the 45 degree vee angle. Should've done my homework...

I thought the Harley v-twins weren't "conventional design", though? Ye faithful Wikipedia tells me that Harley engines have simultaneous power strokes, which means on that engine the firing interval is actually 45 degrees apart. This is what gives them the "potato-potato" noise (still chuckling about that). I can't speak for the Rotax, but I would assume it is more of a conventional (288/432 degrees) setup, yielding smoother power delivery.

Anyways no one seems to be wrong, but in this specific scenario I think Mbirt was correct with his original statement. Regardless, looking at the pressure pulses definitely adds some complexity.

Back to the topic at hand, it seems like if an engine did have an odd firing sequence like a v-twin, the variation in manifold pressure would have a smaller effect at higher RPMs.

Quote:

---

Originally posted by Owen Thomas:
I thought the Harley v-twins weren't "conventional design", though? Ye faithful Wikipedia tells me that Harley engines have simultaneous power strokes, which means on that engine the firing interval is actually 45 degrees apart. This is what gives them the "potato-potato" noise (still chuckling about that).

---

I also assumed that too, that the Harley firing interval was 45 degrees, but apparently it is not.
The Harley ignition system is very crude, with one set of points and no distributor cap, it is essentially a wasted spark system where both plugs fire simultaneously.
This obviously cannot work if it had only a 45 degree firing separation, but it does work in a fashion with the 315/405 firing interval that these monsters apparently have.

Hmm, I was also under the impression that "conventional" for a v-twin meant that both cylinders fired on the same rotation, ie 45º bank angle = 45º firing interval. I was also pretty sure that the majority of cruiser engines used the "quasi thumper" layout to get the potato® Harley sound. It gets really obvious with the large displacement ones that idle at like 300 rpm; they just sort of go "badup....badup....badup...."

As for the 1125, will have to consult the service manual for my 1125CR tonight! http://fsae.com/groupee_common/emoti...on_biggrin.gif

OK, I had to research this more myself, and sure enough it does appear that the Harley uses the "more even" firing order of 0-315-720. There is also some interesting discussion of Harley's trademark on the potato:

"The mark consists of the exhaust sound of applicant's motorcycles, produced by V-twin, common crankpin motorcycle engines when the goods are in use"

Harley's unique sound goes back to the beginning

What gives a Harley-Davidson motorcycle its distinctive sound?

Fun fact:

Almost all of the WW2 high performance V12's were fork and knife connecting rods in order to keep the engines as short as possible and to reduce the vibrations as a result of offset banks...

DB V12....

http://legendsintheirowntime.c...10_sk_DB601_p177.png

RR V12:

http://www.ww2aircraft.net/for...conrods-2-resize.jpg

Allison V12:

http://www.enginehistory.org/Misc/AllisonRods.jpg

Oneupmanship is everything...
This guy has the right idea.
http://thekneeslider.com/radial-engine-motorcycle-2/

Any chance any of those V12's are based on Straight 6's? Internet didn't turn up much, but there are other benefits to having no bank offset.

Eliminates offsets in intake runners to a common plenum.
Makes exhausts exact mirror images, no offsets to accommodate
Cam drive for both heads 'in plane'.
Block main webs can be thicker, and no piston travel clearance machining.
Faster and easier machining of block bores (remember no CNC in 20s and 30s when many of these were designed).

Interesting, looking things up, that the first Allison V-12 V-1710 had no crank counterweights. Of course with a bore offset, you could also get away with that, but it would take seriously strong bulkheads and main webs to distribute those forces.

...and while I am at it here is a link to something I found that currently holds my fascination...

http://hildstrom.com/projects/boxer/index.html

One thing to consider as bearing width narrows (split rod or even grooved bearings in modern engines) is the hydrodynamic wedge does not have an even load distribution. Near the edges of bearings the oil can leak out the side rather than all be drawn through radially. I've seen load distribution graphs and they look like an upside down flattened parabola. So when you split a bearing into two half width sections, you'll need to add total bearing area.

A benefit to the narrow sections is higher oil flow so they will run cooler than the inner bearing (peak bearing pressure being equal), but as you can imagine if you had old F1 tire like grooves in the bearing, it will run cool until the load capacity is reached and then things get hot real fast no matter how much oil you've got.

Another problem with forked anything is load distribution is never even to the two sides due to dimensional tolerances. So you'd have to protect for some amount more than 50% of total load dependent on tolerances and system compliance.

The enormous diameter main bearing idea is certainly not new. If the mains are made a larger diameter than the crank throws, the crank is installed axially into the end of the crankcase.

The Offenhauser four cylinder race engines were built that way, with splash fed roller main bearings that never wear out. Very strong, very stiff, and low friction.
Here is a picture of a bare Offy crankcase.
http://img.photobucket.com/alb...20Print/IMG_2571.jpg

Forked conrods have been used in quite a few successful engines, and as far as I know have never proved to be a design weakness.

The main rotational conrod bearing surface is always one piece and made full width, the knife and fork part just oscillates back and forth with changes in relative conrod angularity, so lubrication is less critical. You only require one oil hole in the crank, not three.

Yeah the guy who wrote that doesn't have the background to understand the tribology behind it but the the concept seemed sound. Balancing the system wedges would be an interesting problem to work out. Also after seeing the speeds that the Honda oval piston engine were running and the period that the engine was built, a twin rod piston has had some practical success even though a different configuration. The design of the engine would be very interesting if paired cylinders were brought in plane.

When it comes to perfect balance, the ninety degree Vee twin with cylinders in line, and with forked conrods is probably the best solution.
Boxer engines come close, but the crank throws become problematic.

60 degree V12's are smoother in mechanical balance as well as torsional excitation than a V8's. The history of airplane engines as well as race engines have always favored them for that,fashion,etc. Boxer 12's even more so but most flat 12's are actually 180 degree V12's and not a true boxer engine. Here is a neat graphic on it as well as some other great engine links...

http://www.epi-eng.com/piston_...m_piston_engines.htm

...if I had my druthers I would have do 5 liter flat 8 (103mmx75mm) with the Coxial Flat Twin setup so it would fit in my 95 Impreza project car I am building. I would prefer a 12 cylinder but an 8 is the largest that will fit and it would still sound pretty awesome. Here is a 3 liter version...

http://www.youtube.com/watch?v=2lNm_bQEbCM

http://www.youtube.com/watch?v=8Z0rjor-s1w

...and just for fun a very cammy 1.5 liter (180 degree flat 8 i think i.e. not boxer)...

http://www.youtube.com/watch?v=DItHwFZUzk0