# Maximum Allowable Piston Speed (Jennings)

Discussion in 'Fabrication, Welding, Machining' started by MACE, Sep 24, 2001.

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I have been reading Gordon Jennings' Two Stroke Tuners' Handbook. It's quite a piece of work. The stuff he covers in the first chapter (which he considers "basics") is all new to me. I guess my HiTorque two stroke education was not quite complete...

Here's the question:
Jennings states that maximum engine speed is determined by piston speed. He states that (if I remember correctly) the max (average) speed is 4000 ft/sec. He gives a formula to find average piston speed.

---What is the factor that determines this limit? Lubrication (which improves with technology), inertia (which is actually accelleration - not speed), some physical material property of the piston or cylinder, or something else entirely????

---Why would this be a factor of average speed and not maximum speed?

I would have guessed that the maximum engine speed would be determined by inertia. In other words, the weight of the parts that reciprocate would want to be low to minimize the forces from accellerations but would want to be heavy to be strong enough to withstand the accellerations, but that would increase the accelleration loads - and so on and so on...... The optimal solution is never obvious.

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2. ### dirt bike dave Sponsoring Member

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IMO, inertia and maximum piston speed would be limiting factors. Perhaps Jennings uses average piston speed as a general rule because it is much easier to calculate in a basic 'how to' book.

One factor related to average piston speed would be air flow. Maybe there is a physical limit on how fast you can fill and empty the cylinder, and Jennings calculates that is at or near 4,000 fps average piston speed.

My \$0.02

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3. ### spanky250 Mod Ban

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4000 ft/min used to be an astronomical speed, achieved only by the most exotic of racing engines. Now there are production cars that reach and even exceed this piston speed. I think modern advances in metalurgy are the reason this has become possible, and better design and production techniques are certainly a contributing factor. Modern engines use piston designs that are both stronger and lighter than even state-of-the-art racing engines used just a few years ago. Inertia has decreased, and piston, rod, and crank strength has increased to allow properly designed and built engines to run these speeds with excellent reliability and durability.

I also think friction has a lot to do with this limit. I have read articles that talk about engines having a dramatic and steeply curved increase in friction once piston speeds begin to climb much past 4000 ft/min, resulting in a point beyond which the gains of performance reach an equlibrium with the increased power loss created by the friction. Modern lubricants have extended this ceiling somewhat, but only to a degree.

jm\$.02

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Last edited: Sep 25, 2001
4. ### Rich Rohrich BioHazard Staff Member

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There was a time when japanese race engines following strict maintenance schedules saw 4000 fpm as the upper limit mean with customer engines usually running lower. As you know Mace, materials science has changed the rules in a lot of engineering disciplines and it's now common practice for the japanese to sell customers engines that see rpm peaks reaching a mean of 4500 fpm or higher, and power peaks in the 3800-4000 fpm mean range. Just take a look at the Yamaha R6 for an example of how much the peaks have risen on engines that can go thousands of miles with mimimal maintenance.

Engines built for drag racing will often times see mean fpm in the 5500-5800, and depending on who you believe the F1 guys aren't too far behind in some cases.

In the end I think plotting time at or near the mean piston velocity versus the actual mean gives an interesting insight into the potential longevity of components.

It's a tough design decision. If you make the parts light enough to achieve higher peak speeds you reduce the service life of the parts and force the user to adhere to strict maintenance schedules. Make the parts tougher/heavier to lengthen service life and you are forced to lower peak speeds which will ultimately cost you in terms of peak power. A tricky juggling act at best and that's BEFORE we even start thinking about bill of material costs. YIKES!!!

Luckily there are some new forging techniques in the pipe that should provide us with lighter yet tougher forgings for pistons so the balancing act will get even more interesting as time goes on :)

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5. ### dirt bike dave Sponsoring Member

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Rich,
Are there any 2-strokes that greatly exceed the 4,000 fps figure? Can supercharged or turbocharged 4 strokes acheive much higher piston speeds than normally aspirated engines?

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6. ### Rich Rohrich BioHazard Staff Member

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For a given displacement two-strokes tend to have smaller bores, larger strokes and subsequently lower piston speeds than comparable four-strokes. Piston ring seal and ring flutter caused by the huge ports they have to contend adds additional variables for the designers of two-strokes. In spite of all that some of the specialty two-stroke kart engines reach some pretty amazing speeds.

As for turbo engines, all else being equal you'll tend to have heavier pistons in these engines to deal with the increased heat and cylinder pressure , so the advantage would be with the normally aspirated designs.

One thing is for sure, for every hard and fast rule we think we have in this area there are 5 successful exceptions to it :)

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7. ### David Trustrum Subscriber

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Quite right, ring flutter is a problem at elevated rpm especially with engines designed for lower RPM & thicker rings which are more susceptible. Some primitive designs will spit their tips (by the ring peg) off in disagreement lowering compression somewhat. Serious ring flutter will cause overheating problems due to decreased contact with the bore. Small bores can get away with pushing the figures a bit.

Large ports & high revs can cause the rings to nibble at the pegs pushing them in or dislocating them meaning the rings are free to have a bit of a look around the front of the engine where they tend to stick a leg out into the exhaust at an inopportune moment.

Also the pistons tend to smear over the rings sticking them in place. Road race engines are keen to provide this service if not feed pistons often enough.

Don’t you hate when someone is riding your bike & you can hear them stuffing it down the gears forcing the engine to spin far higher than it would normally of it’s own accord??

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8. ### EricGorr Super Power AssClown

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F1 Piston Speeds - When I worked for a couple of F1 vendors that made pistons, liners, and coatings, they were dealing with 6,800 f/min
The liners were honed with the hot-tensioned method to insure roundness at operating temperature. The pistons were the slipper types and only about 35mm in length. One time I snagged a sample piston to weigh it, a 97mm F1 piston weighed a punny 110 grams, thats about the same as a CR125 piston!
How do they do it? A new piston manufacturing process called ISO-Therm. The tooling is heated along with the alloy and pressed to shape. This enables distinctly difference shapes for strength than possible with traditional casting or forging techniques.
I've heard of all sorts of interesting ring packs being tested including aluminum and albamet with Keronite coatings.

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9. ### dirt bike dave Sponsoring Member

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Eric & Rich,
Do you think it is physically possible to get that kind of piston speed (6,800 fps) on a two stroke?

My thinking is that on engines of similar bore & stroke, the two cycle must move twice as much air as the four cycle per foot of piston movement. At a certain point, the friction of the air passing through the intake and crankcase would be a physical limitation.

For me, this is easier to conceptualize if you think of the engine as a pump and replace the 'air' with a liquid, say maple syrup. At some point you just can't suck in the intake charge fast enough, especially if you have to fill the cylinder every revolution instead of every other revolution.

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