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Thinking About Piston Speed


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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>.
Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

When I was a boy enthusiast reading Ken Purdy’s The Kings of the Road, I learned that the average piston speed in racing engines must not exceed 4,000 feet per minute. (You compute this by taking the stroke in feet, times two, multiplied by rpm.) There was no explanation, which suggests that the authors of such wisdom didn’t have one.

I kept that in mind until I later learned that, oops, the “limiting piston speed” had somehow increased to 4,500 feet per minute, in the case of early Superbike engines. And then to 5,000. And more; current MotoGP engines, making power at 18,500 on a 48.5mm stroke, are close to 6,000 feet per minute.

And then, splash! We fall into the cold water of reality. The present watercraft speed record of 317 mph (465 feet per second) pencils out to nearly 28,000 feet per minute. Consider that the boat is a slider lubricated by a liquid, and is therefore entirely comparable with an IC engine’s pistons sliding on oil. There is nothing whatsoever to stop pistons from sliding much faster than they do; there is no “piston speed barrier.”

Related: The Plight Of The Modern Piston

While there is no limit to the speed a piston can move through a cylinder, fatigue limits how long a piston will last when subjected to massive forces.
While there is no limit to the speed a piston can move through a cylinder, fatigue limits how long a piston will last when subjected to massive forces. (Ducati/)

Or what about the rocket sleds, supported and guided not by wheels but by sliders gripping rails? Their speed record? The folks at Holloman Air Force Base will admit to Mach 8.5, which is about 9,500 feet per second, or 574,000 feet per minute—very close to 100 times the peak-revs piston speed of MotoGP engines.

I reckon piston engines still have some leeway for future improvement.

So what does limit rpm in piston engines? In a word, fatigue. More specifically, the level at which a piston’s fatigue cracking becomes unacceptably likely within the lifetime required of the engine. The cause of stress leading to such fatigue cracking is the high acceleration/deceleration that pistons experience twice per revolution. The hottest 600cc sportbike engines were at around 7,000 peak Gs when their market let go, and F1 and MotoGP engines have reached for 10,000 G.

Other failures can obviously limit rpm as well; connecting rods break, valves float, cranks break in two from torsional vibration. But piston cracking is a primary concern.

Sick, Tired, and Houred Out

Top Fuel dragster engines are rebuilt every four seconds (they run only 1,000 feet now), but MotoGP engines must survive roughly three Grands Prix plus practice and qualifying, about 10 hours of operation. I’ve used the following example before, but will do so again here because it is so telling: One year at Laguna Seca the US Suzuki crew were obviously celebrating after the close of practice. I asked them what the occasion was. “The factory just put our pistons back on the 100K list!” one slightly tipsy mechanic said.

Every part in a race engine has a lifetime determined by the factory: so many hours for the crank, so many for the con-rods, etc. When Moto2 used Honda 600 engines, their reciprocating parts were replaced every three races. Just before MotoGP bikes adopted pneumatic valve springs, teams were changing the metal springs every day. Racing is full of stories of heroic all-nighters, but every night? And so the Suzuki crew celebrated; their pistons were now good for 100 kilometers of use, a whole 60 miles.

Steel parts have what is called a “fatigue limit,” a stress level below which their life is essentially infinite. Immortality! For most aluminum alloys, however, there is no such limit. That means when you apply cyclic reversing stress to an aluminum part such as a piston, its “meter is running.” Every stress cycle causes some damage within the metal’s structure, and the higher the operating temperature, the greater the rate of damage accumulation. The eventual result is a crack, typically around the wrist-pin bosses, that progresses to failure. To win at this game you test, redesign, and test again until you know with high statistical accuracy that your parts have a very low chance of failing in their required term of service. The pistons being replaced are, in a real sense, “sick.” They are old metal, close to being timed out, riddled with microdefects.

Better, stronger, more durable materials exist, materials whose extraordinary strength comes from so-called dispersed-phase hardening, where fine ceramic dust is uniformly distributed within the aluminum by the large shear forces of extrusion. But it’s expensive, so the F1 managers decided to ban it as a composite. The fine particles of the dispersed phase act as “keys” to slow or stop the movement of dislocations, irregularities in crystals which make it easier for atoms to slip across each other under stress. The result is greater resistance to deformation.

That leaves manufacturers with conventional alloys and methods such as refining the piston’s shape based upon dynamic stress analysis (eliminating stress concentrations) and reducing operating temperature with features such as piston-cooling oil jets.

The Rule Book as Design Redline

Before the current limits on bore and stroke ratio, another approach was commonly used. The formula for calculating peak piston acceleration shows it is proportional to rpm (squared) and directly to stroke. So for years designers chose shorter strokes and bigger bores; fortuitously, the bigger bores also provided room for the bigger valves needed to fill cylinders in the shorter times allowed by the higher revs. But eventually this led to distorted monstrosities whose bores were close to 2.5 times their strokes, resulting in wide and very thin combustion chambers that burned slowly and suffered correspondingly larger heat loss due to all that piston-crown and combustion-chamber surface area, exposed to combustion over that longer time.

This is why Dorna decided to set the arbitrary upper limit for MotoGP’s four-cylinder engines at 81mm, corresponding to a stroke of 48.5mm (in the King’s measure that’s 3.189 x 1.909 inches), for a bore/stroke ratio of 1.67. While that’s rad for a production engine, it’s not even close to the extremes of F1 back in its heady days of 20,000 revs.

We feel the attraction of extremes, but realistically, we can’t afford them.

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5 minutes ago, Bender said:

Well done, now keep it up 😂 

 

I have done loads you have missed you cheeky cow! :D 

  • Haha 1
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