Piston Pace Versus Piston Acceleration

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

In response to my tales on the Vance & Hines Suzuki-inspired Professional Inventory Bike drag bike, Rod Callaghan fairly rightly jogs my memory that a lot has modified because the days when individuals imagined a “limiting piston pace,” and speculated that engine pioneer Harry Ricardo (1885–1974) would discover a lot to curiosity him have been he to look in upon right now’s scene. The piston pace of the V&H Suzuki-inspired drag engine reaches 6,740 ft per minute!

Piston engines reached a piston pace of 4,000 ft./min. as way back 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 have been operated by a 3D cam that not solely opened and closed the valves however various their elevate as properly.

Associated: Vance & Hines’ Dominant Suzuki Hayabusa Professional Inventory Bike

The thought of limiting piston pace is nonsense. (Giovanni De Sandre/)

There may be nothing however friction to restrict sliding velocity, so the concept of a “limiting piston pace” is nonsense. The rocket sleds used for testing by the US Air Power at Holloman AFB are guided by sliding on rails at supersonic speeds.

Within the early days of steam railways it was speculated that folks won’t have the ability to breathe at such terrific speeds as 40 mph. Quickly hundreds of individuals learn the morning papers at that pace as commuter trains carried them to their metropolis jobs.

What does set a restrict to piston life is acceleration—the cyclic stress brought on by the super-quick beginning and stopping of an engine’s pistons. This stress is instantly proportional to stroke however can also be proportional to the sq. of rpm. Double the stroke and get double the acceleration. However double the rpm and as a substitute you get 4 instances the acceleration. What this tells us is that engines with pretty giant strokes corresponding to a giant British single (Manx Norton or Matchless G-50) however turning fairly average rpm corresponding to 7,000 could have average peak piston accelerations (most happens at TDC). However smaller cylinders with shorter strokes, however turning a lot increased revs, could have very excessive piston accelerations.

The stress produced in a piston by such accelerations units a restrict on how lengthy the half will be run earlier than fatigue failure turns into a probable final result. As a result of increased temperature accelerates the fatigue course of, it should even be considered. When it was determined that MotoGP engines have to be able to lasting by means of three races, the producers developed mathematical piston temperature fashions and validated them with knowledge collected in preseason circuit testing. This corrected mannequin was then mixed with piston stress distribution from finite component evaluation to permit redesign to attenuate peak temperatures and stresses. The ensuing pistons are fairly stunning because the design course of offers them swish natural shapes.

Working by means of the numbers for the Vance & Hines drag engine at its peak of 14,000 rpm and stroke of 73.4mm offers us peak piston acceleration equal to that previously current in Components 1 again within the days of extremely oversquare 20,000 rpm V-10s and V-8s (oversquare on this context means bore better than stroke).

The 81 x 48.5 mm bore and stroke engines of MotoGP are actually mentioned to peak at about 18,000 revs, which supplies an analogous (if barely increased) peak piston acceleration.

Even increased accelerations may very well be survivable if technical guidelines permitted use of dispersed-phase-strengthened aluminum alloys. Disappointing? Ultimates are very engaging to us people, however they usually develop into past our monetary attain.