OT: Which Engine Configuration has the Most Power?, Curious Minds Want to Know Options

[Britain Smith]

I was discussing the engine layout of a Porsche motor in a horizontially opposed configuration and the question came up on which engine configuration produced the most HP/liter and why? I know that an advantage of a Porsche motor is that the center of gravity is really low, but does its configuration have any power advantages when compared to say a 90 degree V6 or an straight 6? For simplicity, lets keep the discuss focused on normally aspirated motors. What are you thoughts and why?

-Britain

[lapuwali]

Torque is more or less fixed by engine size and breathing ability, and horsepower is torque x rpm, so if engine size is fixed, you get maximum power by spinning it as fast as possible, modifying as you go to get it to breathe best at engine speeds the rest of the parts will allow without breaking apart. Since you lose mass far faster than strength as you make parts smaller, this argues for many small cylinders rather than a few big ones. So, a short-stroke V12 should make a lot more HP at peak than, say, a six of equal capacity, simply before the lighter pistons and rods will allow higher revs because you run up against parts breaking, and higher revs with equal torque means more power.

Eventually, you hit a limit where things like the increased bearing area of a 16-cylinder engine generates so much additional friction that you simply don't see much gain over a 12.

The layout of the parts (inline, vee, boxer, W, H, whatever) are usually dictated by other factors, like packaging. Vee engines tend to be fairly compact overall, so they work well in engines with many cylinders. The VW W engines take the same idea another step farther. As long as the layout doesn't cause harm, esp. for high revs, then it's not really all that important to power production. Renault recently tried to make a wide-angle V10 work, as a flat-10 has aerodynamic negatives (engine and exhaust hang into the underbody airstream), but a 90d V10 has a higher CG. The 115-120 degree engine, however, had serious vibration problems, and problems with block rigidity, so it ultimately didn't work.

The friction/speed balance is going to depend a lot on the strength of available materials. 10 years ago, Ferrari were alone with a V12 in a field of V10s in F1. The V10s could spin fast enough that the additional size, weight, and internal friction of the V12 pretty much cancelled the rpm advantage (at that time, about 16K rpm). Next year, rules dictate V8s, so they're planning on running that at 20-21K rpm using better (more expensive) materials than they used in 1995, in order to reduce the power loss from dropping the size from 3.0 to 2.4 liters. So much for reducing costs (which the rule was intended to do).

[redshift]

A six is better than a four, troll.

(IMG:http://www.914world.com/bbs2/html/emoticons/laugh.gif)

M

[Rotary'14]

I believe the current Rx-8 makes about 238 HP @ 8500 rpm from just 1.3L I think that little bugger has this contest beat. It has a redline of 9000 rpm! (IMG:http://www.914world.com/bbs2/html/emoticons/happy11.gif)
But not too much torque though,, (IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif)

Just gotta keep the car light. (IMG:http://www.914world.com/bbs2/html/emoticons/smilie_pokal.gif)

[flesburg]

I am not an engineer and cannot add much to what has been said, except that I have read somewhere about vibration effecting the need for mass to counter-act the vibration, such as heavier components or balance shafts etc. I think I also read that in theory, the rotary, such as used to be used in aircraft was theoretically the design with the least inherent (spelling) vibration, and had a very short crankshaft, and all said and told offered the most power with the least weight.

However rotaries were only used in a few automobiles because of frontal area requirements.

I think that horizontally opposed, or boxer engines, were the second best layout again because of lack of inherent (sp) vibration. Therefore they have also been widly used in aircraft.

Dr. Porsche selected the horizontally opposed layout for those reasons. According to Ferry Porsches biography, both he and his father were advocates of air cooling because of the power per pound advantages of air cooled vs liquid cooled engines, and the ability to tune them more accruately for maximum power output, because of their tendency to operate at a more consistent head temperatue. They even designed air cooled engines for armored tanks.

It has someting to do with the angle of the crankshaft in relationship to connecting rods at the moment of the power stroke of each piston.

I would like to hear other imput, from those of you with an engineering background.

Long live air cooled horizontally opposed engines. Someday we may have to use aircraft engines in our 914s to maintain the "faith".

[lapuwali]

QUOTE (Rotary'14 @ Aug 25 2005, 04:06 PM)

I believe the current Rx-8 makes about 238 HP @ 8500 rpm from just 1.3L I think that little bugger has this contest beat. It has a redline of 9000 rpm! (IMG:http://www.914world.com/bbs2/html/emoticons/happy11.gif) But not too much torque though,, (IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif) Just gotta keep the car light. (IMG:http://www.914world.com/bbs2/html/emoticons/smilie_pokal.gif)

Assuming you accept Mazda's definition of a rotary's size. It can be argued the actual useful capacity is 3x that, since all three chambers are in use at any given time, but the 1.3L figure only uses the swept volume of one chamber per rotor. Where rotaries are allowed to compete against piston engines, there's always an equivalency formula to make up for the fact that a "1.3" rotary can't really be compared to a "1.3" piston engine. If you want to start talking non-four-stroke engines, the peak years of the two-stroke GP bikes got about 200hp from 500cc (normally aspirated) for 400hp/liter. This pretty completely trounces the current F1 engine of 300hp/liter, which itself pretty easily crushes the best rotary figures I've seen for non-turbo engines, even w/o the equivalency figures.

Another point on layout, some lend themselves to low levels of vibration, which can raise the rev ceiling, and the boxer-6 and boxer-12s are generally the layouts that produce the least vibration. The boxer-12 is very bulky, and not a convenient shape for many cars, however.

[Tobra]

QUOTE (lapuwali @ Aug 25 2005, 03:40 PM)

this argues for many small cylinders rather than a few big ones. So, a short-stroke V12 should make a lot more HP at peak than, say, a six of equal capacity, simply before the lighter pistons and rods will allow higher revs because you run up against parts breaking, and higher revs with equal torque means more power.

Your post reminded me of a V-12 XKE that my father used to have. 5 liter twelve, I can'recall of the top of my head, but it had dinky little pistons and a short stroke. Would go 70 in first though, sounded just like an airplane.

I have always thought that a horizontally opposed engine made the most sense. For example, ride a Harely V twin, then get on a BMW. You will be astounded by how little vibration one has relative to the other, and how smooth it sounds. This is because of the long period of time between the second piston power stroke and the first going again. As the previous post said, you get less vibration from a flat motor, less vibration means higher revs, and up to a point, higher revs are a very good thing for making power, as an F-! motor designer whose name escapes me at the moment once said, revs are free.

[porsha916]

I believe that a radial engine is the most powerful air cooled engine, if you take into consideration RPM, single row has only one connecting master rod, smooth operation, low rotating weight per displacement! (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)
Not very practical for a car! (IMG:http://www.914world.com/bbs2/html/emoticons/sad.gif)
My two cents!
Bill

[J P Stein]

QUOTE (Rotary'14 @ Aug 25 2005, 04:06 PM)

I believe the current Rx-8 makes about 238 HP @ 8500 rpm from just 1.3L I think that little bugger has this contest beat. It has a redline of 9000 rpm! (IMG:http://www.914world.com/bbs2/html/emoticons/happy11.gif) But not too much torque though,, (IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif) Just gotta keep the car light. (IMG:http://www.914world.com/bbs2/html/emoticons/smilie_pokal.gif)

The only people that call one 1.3 L work at Mazda.
The FIA call it 2.6L....still, gud hp, no torque.

If one is totally unfettered......the most hp per liter I ever heard of was BMW's FI turbo engine.... an estimated 1500 hp from 1.5L in qualifing trim. 80 ish lbs of boost....a 4 banger.
It got down to the least amount of frictional losses.

[flesburg]

Another 2 cents worth.

V configuration engines have to be designed with a certain number of degrees between cylinder banks, or vibration increases dramatically, and therefore the need to have balance shafts or even a brace the engine to the car itself.

For example, a 6 should have 0 (ie inline like BMW), or 60, or 120 or 180 (horizontially opposed) between cylinder banks. That is why my buick v6 which had cyl banks at 90 degrees (GM just cut two cylinders off of a v8 to save tooling costs) had a big brace between the engine and the chassis (to keep the engine from shaking itself apart). Likewise my ford tarus (yea a ford tarus, shhh!!!) (no caps on purpose)

A 2 cylinder shoud be 0 or 90 or 180 degrees. The reason a Harley sounds like it does and ha rumps - ha rummps along, is because its cylinders are at some odd and incorrect angle, but if they fixed it, it would not be a Harley, and even Honda (who really knows how to build engines) copied the wrong angle in their Harley clone. The other reason they sound like they do is because they have pistons like 2 pound coffee cans. Anyone grow up on a farm and remember the 1 cylinder John Deere tractors with I think like 6 or 8 inch bore. Remember the comment above about piston mass!!!!

An 8 cylinder should have 0 or 90 or 180 degrees between cylinder banks.

A 10 is something else, and a 12 is still different, BUT they will ALL balance better at 0 or 180 degrees.

As I understand it, the ONLY advantage of a wankel is that is vibration free. Everybody other that Mazda who tried to build one (including Merc.) gave it up as no good. Even Mazda gave up except for the "sports car", (it rook rike spots ca)

Horray for horizontally opposed (180 degree) AIR COOLED engines.

by the way, electric power generators are motors. Internal combustion power generators are engines!

[J P Stein]

QUOTE (flesburg @ Aug 25 2005, 06:35 PM)

by the way, electric power generators are motors. Internal combustion power generators are engines!

Well ....duh.

When's the last time you heard " I put the engine on em'"?
Folks would think you picked it up and sat it on em'.

Are you another case of "not getting out much"? (IMG:http://www.914world.com/bbs2/html/emoticons/laugh.gif)

[messix]

easy answer.... the one that is engineered to run at the most efficient peak power level [do you want longevity, short term power like the 8,000hp top fuel]
but i think the best power config has to be gas turbine. 0 vibration, low weight, great fuel effiecency for the power output. and reliablity

[jonwatts]

Don't forget VW's VR6, with 35 degrees between staggered cylinders. It has nothing to do with this discussion, but just don't forget it.

[bd1308]

the worst engines in my experience balance wise is the Jeep inline series engines....

they are bullet-proof....OHV and strong engines (i had a 2.5L I4)
but the balance is horrific.....even with the harmonic balancer it was like chattering my teeth.....

i miss that jeep. until i fill up my 914 for $26...

[Crazyhippy]

Inline motors tend to make good tq, and are arguably the longest lived. They have a main bearing in between each and every rod bearing, where boxers and v's have 2 rods per main.

[bd1308]

i'll agree with that...i have friends that have over 300K on thier 2.5L engines without a problem at all...

[goose2]

QUOTE

I believe that a radial engine is the most powerful air cooled engine

Don't know about that, but here's an interesting example of a radial engined race car:

[ArtechnikA]

QUOTE (Crazyhippy @ Aug 26 2005, 01:12 PM)

... They have a main bearing in between each and every rod bearing, where boxers and v's have 2 rods per main.

Porsche 6 cylinder engines have 8 main bearings. one crankthrow per bearing pair, plus a nose bearing outboard of the intermediate shaft chain drive...

[lapuwali]

QUOTE (Crazyhippy @ Aug 26 2005, 09:12 AM)

Inline motors tend to make good tq, and are arguably the longest lived. They have a main bearing in between each and every rod bearing, where boxers and v's have 2 rods per main.

This isn't true. There have been PLENTY of inline engines w/o a main bearing between each throw. Early BMC B-series engines, for example, only used three main bearings on inline fours. Later engines had five main bearings. All A-series engines only had three main bearings (inline fours, again). Millions of these engines were produced, powered a wide variety of cars for 50 years of production.

Some vee engines have rods share one crank throw, others have all rods on separate throws, still others have a main bearing between each throw. Vee engines are primarly made to be compact for the number of cylinders they have, so dropping one set of main bearings is advantageous, as it shortens the crank. This also makes the crank stronger and stiffer, thus extending its life. Inline sixes, in particular, have very long crankshafts that need to be massive to handle torsional forces in the engine. If it weren't for the fact that inline sixes have excellent primary and secondary balance, they'd not be a good engine configuration.

There's also nothing inherent to the inline package that blesses them with more torque. Torque for a given displacement is primarily a function of piston area and stroke, so a 2.0 four will usually make more torque than a 2.0 six, simply because the four will have more area per piston, and will USUALLY have a longer stroke. It will also have less internal friction as there's less total ring area, unless the four is very oversquare (big bore, short stroke), in which case the four can end up with equal or greater ring area, and thus more friction, and possibly even less torque than an undersquare six.

There are enough parameters on an engine that you can juggle them around to produce almost any characteristic you want. If you only look a common mass-produced engines, you're looking at a set of compromises that usually have little to do with ultimate power, since there are a great many concerns facing an engine designer that have nothing to do with specific output. Even in full-on racing engines, there are aerodynamic concerns, size, weight, and often fuel consumption (in some series), and cost (in some series). Rules often require the designer to make sub-optimal choices. For street engines, cost, fashion, and emissions play far bigger parts than how much power it makes.

You can usually learn a lot by looking at the oddball engines, and figuring how why they were made. For example, Honda in the late 70s was fully behind the four-stroke idea, when GP bike racing was totally dominated by two-strokes. Honda felt that a 500cc four-stroke V8 would be competitive against the 500 two-stroke fours of the day, using exotic materials to keep the weight down. The rules limited you to four cylinders; but, the rules didn't say the cylinders had to be round, so Honda made a V8 with cylinders siamesed together so each piston was oval, and there were only four of them. Eight rods, 32 valves, and four throws on the crank with shared rods, but it was still technically a V4. But it had the piston and valve area of a V8. It could rev to a limit that was never fully disclosed, but was known to be above 20,000rpm. Unfortunately, they also chose to use a radical new chassis design at the same time, so it flopped, as there were simply too many teething problems to tackle all at once. The engine reappeared in the mid80s in 750cc endurance form using a conventional chassis, and was known to produce more than 200hp in that form. This was all sufficiently successful, and Honda were beginning a new F1 car engine program, that F1 engine regulations were quickly amended to state that cylinders and pistons must be round, before Honda could produce and oval-piston F1 engine. Honda experimented with this design for many years (known from patent filings unearthed later), but never found it promising enough to produce it on a mass scale. The main lesson learned here was the importance of valve area to breathing in a four-stroke, and how limiting round cylinders and round valves are to total valve area. It reinforced a similar lesson re-learned in the 1960s (by Honda) when they re-introduced the idea of four-valve heads, even after as eminent an expert as Harry Westlake had dismissed the idea that anything more than two valves were necessary for optimal flow.

[bd1308]

good insight james.