Cams Replace Crank in new Engine Design

By Christopher A. Sawyer, Executive Editor

Crankshafts are inefficient devices for efficiently transferring power from the pistons to the driveline, with losses that can approach 36%. At the top of the piston stroke, where gas pressure is highest, force transfer efficiency is at its lowest, though it rises as the piston descends and the connecting rod’s leverage increases. Peak efficiency happens about 40% through the piston stroke, then drops at an exponential rate that mirrors its rise. In addition, the piston doesn’t travel a path that is strictly parallel to the bore, so an angular force equal to the pressure on the top of the piston is transferred to the cylinder wall. This increases friction, wear, and fuel consumption.

Bradley Howell-Smith, managing director of Revetec Limited (Sydney, Australia) and inventor of the Revetec engine, arrived at a solution to the problem in 1996, when he replaced the crank with counter-rotating cams driving an output shaft, and made the connecting rods rigid. (See: “How It Works”) “Within six months,” he says, “I had a twin-cylinder, 500 cc two-stroke demonstration engine no bigger than a loaf of bread running in my garage.” It put out more power and torque than an equivalent conventional design, and used less fuel. Convinced he was on to something, Howell-Smith suspended development, and spent the next few years raising capital and filing international patents (29 so far, including the U.S.). Development resumed in 1999, and included research into four-stroke, diesel, and alternate-fuel variants.
The latest version of the Revetec engine is the horizontally opposed 1800 SV; its swept volume is 1.8-liters (hence the name). It measures 280-mm long, 650-mm wide, 500-mm high, weighs 45 kg, and is scheduled to enter production in China later this year. Asked about his growing list of joint ventures with Chinese firms, Howell-Smith answers: “This is such a new and different concept that we thought it best to prove the technology in an emerging market.” In other words, he’d rather show established automakers a proven product than a proof-of-concept. “Life is too short to waste time battling the Not-Invented-Here Syndrome,” he notes wryly.

Inner cams attached to sleeve

One reason for the interest from China is the superior emissions performance Revetec claims for its design. Though the head design is totally conventional, the engine’s greater mechanical efficiency means 60% less fuel is needed to produce an equivalent output at wide-open throttle. That number rises to 75% in idle/no-load conditions, a big plus in crowded cities like Beijing. And less fuel means lower emissions.
Airplane manufacturers have expressed interest in the Revetec’s ability to drive counter-rotating propellers in a high-torque/low-rpm situation without the need for a separate drive unit or gear-reduction transmission. And ship builders are interested in the engine’s potential to maintain a near-constant piston speed. This increases control over propeller speed, and reduces the shocks conventional engine designs place on a ship’s drive system.