After riding the “mystery Yamaha” last week, the Supertrax crew has been trying to guesstimate what’s under the hood. Our imaginations have been running wild and one thing we can’t overlook is the uncanny smoothness of what we think is a new 4-cylinder snowmobile engine in that particular sled.
Our guessing goes over to the GP road race Yamaha ridden by world champion Valentino Rossi. His GP bike uses a unique engine built by Yamaha called the “Cross-Plane”. Huh? Never heard of it? Kevin Ash of ashonbikes.com explains it this way:
“A conventional four-cylinder engine has its crankpins all in the same plane – a flat-plane crank – with the two inner ones 180 degrees from the two outer ones. The inner two pistons move up and down together, and so do the two outer ones, and it’s this particular configuration that generates inertial torque. This is independent of the main torque output generated by the combustion and cylinder pressure and happens entirely because of the crank layout.
On a flat-plane engine (like the Apex Genesis), the pistons are traveling at high speed when they’re half way along their cylinders, and at this point they have a lot of kinetic energy. Yet 90 degrees of crankshaft rotation later, all four pistons are stationary, two at the top, two at the bottom. Their kinetic energy has been transferred to the crankshaft, which was responsible for slowing the pistons down. Another 90 degrees on and the pistons are back up to maximum speed, accelerated by the crank which has returned some energy to them and in turn, it’s slowed down again.
On Yamaha’s cross-plane crankshaft, these fluctuations are all but eliminated. In this layout the crankpins are distributed at 90 degrees to each other around the crankshaft (in two planes which form a cross when viewed on end). So as one piston is slowing down and losing energy to the crank, another is speeding up and taking the same amount back. At no point do all the pistons stop together, as they do on a flat-plane crank. Instead the energy flow is evened out and the rotation of the crank is much smoother.”