how does....

[Asa]

dunno if this should be in the lounge or here, feels like it should be here though
today's idle pondering:
how does the amount of cylinders affect the power of an engine?
say you have 300 cubic inches to work with, each will flow the same amount of air, each will have rods and rotating assemblies of the same strength, the engines will be as close to each other as is mechanically possible
the only variable is the basic engine set up, 4-cyl, I-6, V-6, I-8, V-8, etc. etc. all the way to V-12 if you want to
what engine will produce the most power and why? i know one of the big things about six cylinders is that they have a balanced engine, and i also know that (most) inlines have the bottom end strength, but what else? which would be the best?

 

[SuperMag]

All else being equal, an engine with many small cylinders will have the potential to make more horsepower than one with a minimum number of large cylinders.

(1) The reciprocating assembly weighs significantly less, thus the potential to spin a lot faster (good for horsepower, more power strokes per minute).
(2) Flame fronts are more easily managed; they don't have as far to travel. Less liklihood of detonation (Good for higher compression and more advanced timing).

Case in point: Check out the technical specs of any multi-cylinder motorcycle engine and compare it to a motorcyle engine of the same size, but of just one cylinder....

FWIW, the Cox engine on my model airplane spins at somewhere between 35,000 and 40,000 RPM with a flywheel (no prop) and 20,000 RPM with a prop. It displaces .049 cubic inches, and IIRC, power is rated at 0.4 HP, or 8.2 HP/CI !

 

[Asa]

number 1 was about what i was thinking, but for slightly different reasons...
i seem to do that, pick the right answer for something, but pick the wrong reason why... got any insight to that one?

 

[SuperMag]

Either... You have good intuition, but not much experience. (Once you get older you can pick the right answer to stuff based on not just intuition, but other semi-related things you know as well. )

Or... You're just lucky. (Makes up for some poor bastard somewhere who always gets it wrong. You know, the ol' cosmic balance thing...)

 

[jwhoss76]

It's gotta be the second part! Just kidding evan!

 

[Asa]

i think that given time i could have come up with the answer on my own, that one seems to fit because of the fact that i never actually sat down and really thought about the question...

as for lucky....
if i'm lucky i'd rather be lucky in areas other than mental speculation... maybe in areas like the lottery, or finding a perfect condition shelby in the back of a junkyard for $100 (well, maybe not perfect, new paint at least, and don't ask what kind of shelby because any Ford make will work), besides, if i get it right on luck that leads to the one bad situation, the one time that i could think "ok, it's this one..." and nope, it wasn't, now i don't have brakes (or some other situation)

 

[SuperMag]

as for lucky.... if i'm lucky i'd rather be lucky in areas other than mental speculation...

I hear ya. Some days I wish I was good looking...

 

[twentyover]

SuperMag-

Think you're right but I think there is a better reason than weight of the rotating and reciprocating assemblies.

A general understanging among engineering types is that the product of
B(rake) M(ean) E(ffective) P(ressure), cylinder face area, and RPM yield horespower. The best tuned NA four storke engines in the late eighties were producing, IIRC correctly, was about 180 psi. To get more horespowerm you needed to increase face area (Bore) or RPM.

Since we have a fixed displacement here, we can increase the face area of the piston and shorten the stroke. This will increase both face area and rpm potential, (and engine speed of maximum horespoer assuming all other events remain the same), but open up the flame propagation issues.

By increasing the # of cylinders, you can have high face area and high rpm, with the resulting higher horsepower.

My understanding is that the maximum torque occurs at a higher rpm, and will be lower in the multi cylinder engines due to problems with cylinder filling at high rpm.

I think the examples of this can best be seen in the relatively unregulated motorcycle racing scene on the early '60's. To get adequate horespower to compete with the two stroke twins, Honda built a 5 cylinder 125cc engine. No typo, 5 cylinder 125cc. Also built a 6 cylinder 250 cc mount. The didn't do it because it was cheaper or lighter that way, they did it so the could get rpm, and rpm is where the hoserpwer lives.

In the late '70's, a much more regulated motorcycle F1 deal, limited to 4 cylinders, Honda built the NR-500, with oval pistons and 2 con rods per piston, becuase they were not permitted to build a v-8 motor for the class. Melt the pistons together so they are oval, stick 8 valves per cylinder in place, and you effectively get a V-8 with 4 cylinders.

Wasn't particularly sucessfull in F1, but the concept was later proven in their endurabce racing bikes.

 

[SuperMag]

Twenty,

You are entirely correct, but that phenomenon speaks more to the issue of "what makes more horsepower, bore or stroke?" I'm reminded of Smokey Yunick's efforts at building a stock-block Chevy V8 in the 1960s. IIRC, the bore was a typical SBC dimension of 3.5", but the crank had a throw of only 2". Anyone infatuated with SBCs will instantly recognize that stroke figure as being, well, ludicrous. But the advantages are two fold- it makes more horsepower for the reasons you cite, and it lowers the accelleration/deceleration stresses in the reciprocating assembly, thus providing for a higher redline.

Perhaps my original response was oversimplified. When I said "all else being equal" I especially meant that to include bore/stroke ratios. A V12 with a square bore/stroke ratio will far surpass a similarly stroked 6 cylinder in its potential to produce power, if for no other reason than the pistons weigh less and only have to cover half as much 'ground'. Not to mention the reduced mass and stress in the valvetrain...

 

[twentyover]

Well, the position that I take is that what makes horsepower is RPM. Stroke controls maximum rpm as rings start to flutter between 4000 & 4500 average fpm piston speed- varies a little with rod length.

I didn't specifically address bore/stroke ratio because I consider it ireelevent on this point. If we fix it's value, you still end up with a greater percentage of pistion area as you increase the number cylinders.

IIRC Cox motor is 2 stroke, and I believe that the FIA uses a 1.7 correction for strokers. Still, the key is it revs.

I had the rare opportunity in May to visit Cosworth's CART engine rebuilding facility in Los Angeles. Obviously, they didn't let us in with micrometers, but I can relate some impressions. Connecting rod center to center distance looked to be about 4-4.25 inches. Blocks were aluminum casting w/ wet sleves. Outside sleeve diameter looked to be about 4.00 inches. Assume a 3.5" bore and 2" stroke and piston speed is 4000 average fpm at 12,000 rpm. They spin higher so I'm assuming the bore is larger and the stroke is shorter.

Back to the theoretical root question- what makes mo' power- can be reduced from the engine configurations list to a simpler more cylinders or less cylinders. There should be no theoretical advantage (with the exception of balance) between an I6 and a V6 for instance if you assume the crank doesn't deflect to alter timing. the cylinders all receive the same portion of air, etc.

When you get into the real world, I think that crank whip, bunches of main bearings, and mixture distribution in real life throw advantage to short crank engines like V-type or even VW's W8 configuration (assuming it get's good muxture feed- I've not seen the port layout, but I can't believe the top engine option in a German car isn't pretty well scienced out.) over an inline type.

Next aspect is 'what would be the best'- and here I will willingly wuss out. I want a max horsepower 300 cid motor- what configuration will I build it? Maye a W24 or radial 18 or 27 cylinder. Minimum bearing loss. Want maxmum low end grunt? Seriuously undersquare single cylinder. Want to put those in a car? Uh-oh- didn't ask that question.

Okay, thinking I may be answering the original question- More cylinders develop more high rpm horespower. Short cranks assist. Fewer cylinders develop less horsepower at lower rpm.

Race car or tractor, what do you want? That determines what is "best".

All this being said, it's just my opinion, and is worth exactly what you're paying for it. If I've offended, I apologize- it was unintentional.

Good open discussion like this is really cool as none of us Albert Einstein- we all can learn.

 

[SuperMag]

Not offended in the least

I'm not disputing what you say at all; we're both in agreement that increasing the number of cylinders increases the absolute maximum power potential. My assertion to you is simply that miniaturizing the rotating and reciprocating assemblies results in the greater potential for the generating of power than simply increasing the total bore area. I just lack the math skills to prove it . But in essence, I think we're arguing the same thing...

FWIW: Yes, the Cox engine is a twostroke. Divide its power output by 2 for argument's sake and you're still left with 4.1 HP/CI. Simply amazing for an engine with no piston rings and only four moving parts...

 

[Anonymous]

Opps...inadvertently posted. Disregard for now...

 

[SuperMag]

Awww, Stan. You know you want to jump in here. Show us some real math!

 

[BIGREDRASA]

This also harkens back to the days of the Stovebolt C*** six vs. the Ford Flathead V-8. The flathead would outrun the six on top end, but the six would eat the 8's lunch anywhere there was a need for more torque.
The question can also be restated in terms of the "area" under the horsepower curve, within the useable RPM range. The engine with more "area" under the curve will win under most competition circumstances, except one in which the speed remains relatively constant. Such a "peaky" engine would be useless on the street.
Back in the early '60s, there was a movie entitled "RPM 7000," based on the things Ford did to raise the redline on the 427. When they were trying to compete with the Hemis, Ford found that they were not reaching their peak HP until after the redline was exceeded. Ford went to sodium-filled valves and other tricks, to get the 427 to live past 7000. They had the horsepower, but were still being beaten on tracks that required a broader/flatter horsepower curve.
Nowadays, they wind the 358 CID motors to 9700 or so. I know I'm rambling, but I guess the point I'm trying to make is: there is a difference between "maximum" horsepower and useable horsepower. I'll take the latter.

 

[Anonymous]

Fascinating discussion!

A couple of points, though...

asa67_stang wrote: i know one of the big things about six cylinders is that they have a balanced engine

This only applies to inline 6's. All 90-degree V-6's are inherently unbalanced. The odd-fires are worse than even-fire V-6's, but still shake, rattle and roll. :roll:

FWIW, inline 8's and 12's are also inherently balanced (though not 4-bangers), and are silky smooth.

twentyover wrote: I didn't specifically address bore/stroke ratio because I consider it ireelevent on this point.

I'm surprised at your statement, and Cosworth obviously think otherwise, as well. Their 2.65L V-8 was turning 15,500 revs last year, and speculation has it that the engine's stoke was about 0.60 of the bore. This conforms to current racing engine practice of making the bore as large as possible while still maintaining proper inlet velocity, within the allowable number of cylinders and swept volume permitted by the rules. Furthermore, a shorter stroke for a given displacement permits greater valve area, lower peak and average piston speeds, and hence higher revs within the mechanical limits of the rings, pistons and rods -- all grist for the hp mill.

Not yet touched upon in this thread is the observation that a turbine engine is the logical extension of the "hp increases as the number of cylinders increases" argument, since it represents an "infinte" number of cylinders for a given displacement, without the internal friction and pumping losses associated with piston engines.

Gotta run!

 

[SuperMag]

Yup, here they come. "Area under the curve" and "infinite number of cylinders for a given displacement." That's calculus folks, so I'll just sit back and watch now, lest I get too close and get pencil-whipped...

 

[Lazy JW]

As an example of the other extreme, I own a 1941 John Deere Model A tractor. This is the famous two cylinder Poppin' Johnny with 5.5' bore and 6.75' stroke for about 321 cubic inches. Mine has been bored .090" over, so goes closer to 330 cubes. The crankshaft/flywheel assembly weighs well over 300 pounds, the pistons are cast IRON and each piston has five rings that are 1/4" wide. The compression ratio is a whopping 4.45-1 stock, this tractor is one of the "All Fuel" models with the heated intake manifold for burning low octane fuel such as kerosene, distillate, #1 diesel or gasoline if you wish. So what is the HP of this monster? A whopping 27 BHP at 975 rmp as tested at the Nebraska tractor test station! Even with the low compression ratio these engines deliver surprisingly good fuel economy. I have no idea what the torque rating of this engine is, but in Nebraska test #472 a John Deere Model 60 (which is an upgraded "A" with higher compression ratio for burning gasoline) developed 251 ft/lb of torque at 653 rpm and 38 hp at 975 rpm. Nowhere near what the 6 cylinder 250 cc Honda made for horsepower, but it will do a better job of plowing my field
Lazy JW

 

[Anonymous]

I promise that there won't be any calculus and there's no test...

So, the question is..."what engine will produce the most power and why?"

I think it's fair to say that based on the posts so far, there is concensus that more cylinders equals more power, all things being equal. Now let's look at why.

Let's start by considering two extremes. First a single cylinder, severely undersquare (the bore is smaller than the stroke) engine of 300 CID. The other engine also displaces 300 CID, but lets assume it's a V-12 with a similarly oversquare configuration (the bore is bigger than the stroke). The first engine might have a 6" bore and a 10.6" stroke, while the second is blessed with a 4" bore and a 2" stroke. To further grossly simplify, let's assume they share comparable quality parts and volumetric, thermal and mechanical efficiencies, and are each able to maintain a 4000' per minute piston speed. That means the single will have a redline of 2264 rpm, while the V-12 can rev to 12,000 rpm before reaching the same piston speed. In other words, we are comparing apples to apples -- two engines identical except for their number of cylinders and resultant bores and strokes.

Now, shaking off the old memory cobwebs, we dimly recall something from Shop 101 - an internal combustion engine is nothing but a pump. Air and fuel in equals power out (along with some waste heat and pollutants).

With that in mind, let's consider the power potential of the two engines. Each engine has an intake stroke every other rotation, so that figures out to 1132 for the single versus 6000 for the V-12. Since the two engines share the same displacement and overall efficiencies, the V-12 will inhale 6000/1132=5.3 times the same fuel/air mixture as the single at their respective redlines, and hence put out more than 5 times the power of the single.

Now you know why, all things being equal, more cylinders equals more power.

 

[Asa]

ok, all of the above answers my question pretty firmly...
basically, when i thought about it and tried to work it out, i was thinking along the lines of the fact that with more cylinders, you'd have more forces acting on the crank than you would with 1/2 as many cylinders
........ but now that i think about that, that isn't exactly correct either... because with the same displacement you'd have the same force, just spread out more evenly

 

[twentyover]

54 Ford-

The problem with language is that I know what I mean and you have to assume what I mean by what I say.

I did not mean to imply i think bore/stroke ration is irrelevent in the production of power, what I thought I was saying is that in the example if I fix bore/stroke ratio between all configurations (responding to Super Mags 11:19 post about using similiar bore/stroke ratio between configurations) it will not affect the outcome, since it doesn't matter what the ratio is, the greater the number of cylinders still yields the greater horsepower.

I do recognize there are certain features that racing engines seem to share, including large bore/stroke ratios (1.5-1.8), large rod/stroke ratios (>2:1).

Lazy JW-

Don't know that the Deere wouls plow better than the Honda. Just put a sand tire on the back and pop the clutch at 12,000 rpm- plow a field 1 furrow at a time