Piston Speed Versus Piston Acceleration

Admin · June 15, 2023

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/)

In response to my stories on the Vance & Hines Suzuki-inspired Pro Stock Motorcycle drag bike, Rod Callaghan quite rightly reminds me that much has changed since the days when people imagined a “limiting piston speed,” and speculated that engine pioneer Harry Ricardo (1885–1974) would find much to interest him were he to look in upon today’s scene. The piston speed of the V&H Suzuki-inspired drag engine reaches 6,740 feet per minute!

Piston engines reached a piston speed of 4,000 ft./min. as long ago as 1907 when a single-cylinder engine having a 250mm (roughly 10 inches!) stroke was designed by Frenchman Maurice Sizaire for the “voiturette” class (minicars). This engine stood 41 inches tall and its valves were operated by a 3D cam that not only opened and closed the valves but varied their lift as well.

Related: Vance & Hines’ Dominant Suzuki Hayabusa Pro Stock Motorcycle

The idea of limiting piston speed is nonsense. (Giovanni De Sandre/)

There is nothing but friction to limit sliding velocity, so the idea of a “limiting piston speed” is nonsense. The rocket sleds used for testing by the US Air Force at Holloman AFB are guided by sliding on rails at supersonic speeds.

In the early days of steam railways it was speculated that people might not be able to breathe at such terrific speeds as 40 mph. Soon thousands of people read the morning papers at that speed as commuter trains carried them to their city jobs.

What does set a limit to piston life is acceleration—the cyclic stress caused by the super-quick starting and stopping of an engine’s pistons. This stress is directly proportional to stroke but is also proportional to the square of rpm. Double the stroke and get double the acceleration. But double the rpm and instead you get four times the acceleration. What this tells us is that engines with fairly large strokes such as a big British single (Manx Norton or Matchless G-50) but turning quite moderate rpm such as 7,000 will have moderate peak piston accelerations (maximum occurs at TDC). But smaller cylinders with shorter strokes, but turning much higher revs, will have very high piston accelerations.

The stress produced in a piston by such accelerations sets a limit on how long the part can be run before fatigue failure becomes a likely outcome. Because higher temperature accelerates the fatigue process, it must also be taken into consideration. When it was decided that MotoGP engines must be capable of lasting through three races, the manufacturers developed mathematical piston temperature models and validated them with data collected in preseason circuit testing. This corrected model was then combined with piston stress distribution from finite element analysis to allow redesign to minimize peak temperatures and stresses. The resulting pistons are quite beautiful as the design process gives them graceful organic shapes.

Working through the numbers for the Vance & Hines drag engine at its peak of 14,000 rpm and stroke of 73.4mm gives us peak piston acceleration equal to that formerly present in Formula 1 back in the days of highly oversquare 20,000 rpm V-10s and V-8s (oversquare in this context means bore greater than stroke).

The 81 x 48.5 mm bore and stroke engines of MotoGP are now said to peak at about 18,000 revs, which gives a similar (if slightly higher) peak piston acceleration.

Even higher accelerations could be survivable if technical rules permitted use of dispersed-phase-strengthened aluminum alloys. Disappointing? Ultimates are very attractive to us humans, but they often turn out to be beyond our financial reach.

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