Revs, how high do they go?

[xRIEx]

Tango13 said:

The online calculator I used was for average speeds, the formula for peak speeds is a lot more complex and needs the con rod length which I didn't have.

I was convinced in my mind that conrod length should be immaterial, but my quick sketches and Pythagoras say I'm wrong. Even though I'm convinced my quick maths is right, I still can't picture why the piston isn't at midpoint of the stroke when the crank is at 90deg to TDC, irrespective of conrod length.

[anonymous-user]

Con Rod length changes the acceleration profile, but obviously not the average velocity over the stroke

[Skodaku]

xRIEx said:

Probably a four stroke, Honda have a deep dislike of 2-smokes, which is why they came up with all the fantastic multi-cylinder four strokes, the ones already mentioned and things like the NR500 and NR750.

I have a memory of a long-past Kriedler 50cc race 'bike engine that was a 12 cylinder. Quite possibly utterly false memory. Anyone ?

[intrepid44]

What you want is an old 100cc air cooled kart, 22,000 rpm!

https://www.youtube.com/watch?v=BpehdSmhsLE

[conanius]

Honda S2000 in F20C guise (so, all UK cars) has nice high rev and piston speed combo.

"The F20C was designed with high maximum rpm capability in mind, for increased power output; redline is at 8900 rpm, with VTEC engagement at 6000 rpm. Its relatively long stroke of 84mm results in a mean piston speed of 4965 ft/m, or 25 m/s, second highest than any other production car to date.[3] It is only beaten by the B7 Audi RS4 (2006-2008). "

Lifted straight from Wiki, but I knew it was high.

I wonder what the Spoon Sports S2000 that revs to 11k manages then !

[xRIEx]

conanius said:

I wonder what the Spoon Sports S2000 that revs to 11k manages then !

I make it 30.8m/s

[cheddar]

I wonder what F1 engines would rev to now if the 18,000rpm ceiling was lifted?

IIRC they were close to 20,000rpm pre-limit.

Would we be near 25,000rpm and 1200hp?

[zebra]

HaloGen8 said:

Ask any Corsa / Saxo boy down the local Maccy D's.

Think they will offer a definitive answer

Why bother going down to Maccy D's; loads of them on here unfortunately.

[MC Bodge]

Skodaku said:

I have a memory of a long-past Kriedler 50cc race 'bike engine that was a 12 cylinder. Quite possibly utterly false memory. Anyone ?

14 gears, non?

[eldar]

Skodaku said:

I have a memory of a long-past Kriedler 50cc race 'bike engine that was a 12 cylinder. Quite possibly utterly false memory. Anyone ?

I suspect a 4cc cylinder is possible using model aircraft engine technology. I'm not sure it would translate into anything useable, though.

[MC Bodge]

Ki3r said:

My XJ6 redlines at 13,000, so would think an R6 would be higher.

You must have modified it a fair bit

[AJB]

xRIEx said:

I was convinced in my mind that conrod length should be immaterial, but my quick sketches and Pythagoras say I'm wrong. Even though I'm convinced my quick maths is right, I still can't picture why the piston isn't at midpoint of the stroke when the crank is at 90deg to TDC, irrespective of conrod length.

Because at the top of the stroke and the bottom of the stroke the conrod is vertical. The midpoint of the stroke would therefore be crank at 90 deg with a vertical conrod. However, when the crank is at 90 deg, the con rod is at an angle (with the angle depending on con rod length vs crank length), and that angled con rod will bring the piston a bit lower. So, at 90 deg crank the piston is slightly more than half way in its travel from top to bottom of its stroke.

As an aside, it's this piston movement due to con rod angle which introduces double engine speed vibrations, and that's why balancer shafts in an inline 4 rotate at double engine speed. Given enough time I might be able to remember some of the maths of that, but not at this time of night!

[fourwheelsteer]

AJB said:

xRIEx said:

I was convinced in my mind that conrod length should be immaterial, but my quick sketches and Pythagoras say I'm wrong. Even though I'm convinced my quick maths is right, I still can't picture why the piston isn't at midpoint of the stroke when the crank is at 90deg to TDC, irrespective of conrod length.

Because at the top of the stroke and the bottom of the stroke the conrod is vertical. The midpoint of the stroke would therefore be crank at 90 deg with a vertical conrod. However, when the crank is at 90 deg, the con rod is at an angle (with the angle depending on con rod length vs crank length), and that angled con rod will bring the piston a bit lower. So, at 90 deg crank the piston is slightly more than half way in its travel from top to bottom of its stroke.

As an aside, it's this piston movement due to con rod angle which introduces double engine speed vibrations, and that's why balancer shafts in an inline 4 rotate at double engine speed. Given enough time I might be able to remember some of the maths of that, but not at this time of night!

Something to do with the fact that the acceleration of the piston on the downward stroke is not the same as that of a corresponding piston going up. I didn't believe it until I did some maths but the numbers can't lie. If it were possible to eliminate connecting rod angularity the pistons would follow simple harmonic motion and an inline four would be perfectly balanced.

[xRIEx]

fourwheelsteer said:

If it were possible to eliminate connecting rod angularity the pistons would follow simple harmonic motion and an inline four would be perfectly balanced.

Now that's an interesting (but probably financial pointless, if it's even possible) idea. Some sort of cam-like crank to allow the conrod to remain vertical, then convert it to simple angular motion like a wankel rotor around its output shaft.

[fourwheelsteer]

I think the solution is something called a scotch yoke. The problem may be that it consumes more energy in friction and/or is harder to lubricate than a conventional crank. There's an animated demonstration of the principle on the Wikipedia page.

http://en.wikipedia.org/wiki/Scotch_yoke

[Limpet]

The R6 is an interesting case as Yamaha gave the 2006 model a 17,500 RPM redline which the marketing team made quite a big deal out of. A few months later when bikes started hitting dynos with calibrated tachometers connected to them, it was discovered that the engines were actually limited at 16,200 RPM. The rev counter was very optimistic. Quite an embarrassing position to put yourself in as a manufacturer, all things considered.

My early 90s Kawasaki ZZR600 would pull a genuine 14,000 RPM. Bike engines have been capable of these kind of revs for a quarter of a century now, and with excellent reliability and longevity.

[Shadow R1]

1000cc.
Revs to 13,750
Bore and stroke 77 mm × 53.6 mm.

[Chewykneeslider]

fourwheelsteer said:

Something to do with the fact that the acceleration of the piston on the downward stroke is not the same as that of a corresponding piston going up. I didn't believe it until I did some maths but the numbers can't lie. If it were possible to eliminate connecting rod angularity the pistons would follow simple harmonic motion and an inline four would be perfectly balanced.

Interesting stuff there.

I always think of the % balance factor you need for a single cylinder engine.

If you balance the crank in such a way that it counterbalances 100% of the combined mass of the big end, conrod and piston, then the engine will vibrate, but the direction of the vibrations is at 90° to the motion of the piston.

If you ever see a Manx Norton ticking over at a standstill, you can see this in the way that the front wheel shakes in a fore and aft manner.

[CBR JGWRR]

MC Bodge said:

You must have modified it a fair bit

Yamaha XJ6 not Jag XJ6.

[Captain Muppet]

mozza42 said:

Kawasaki zxr250 was over 20k rpm IIRC

22,000rpm according to a magazine thing I read on them back when they were on sale.

There was also a Yamaha 4 cylinder 250 that'd do about the same revs.

Both were standard production engines on sale to the public 20 years ago.