Cams Replace Crank in new Engine Design
Leave it to the Aussies to design a piston engine that transfers power from the pistons to the output shaft via counter-rotating, three-lobed cams instead of a crankshaft.
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.
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.
Howell-Smith sees automotive as the biggest prize, and touts the engine’s modularity and simplicity alongside its mechanical efficiency to emphasize his case. “Development has shown us that we can have a generic block covering certain capacities, and tailor it to the client’s needs through the cam design,” he says. One example he uses shows how a car, tractor and truck would use the same basic engine–“everything would look the same on the outside,” he says–but different cam designs to produce horsepower and torque tailored to the application. “We can change the cam to apply far more leverage to the output shaft for the truck, or go to a five-lobe cam and low rev limit to get the most torque for the tractor.” Couple this with his claim that the Revetec bottom end carries a 20% savings compared to a conventional engine, and you can understand why he thinks his company has a realistic chance getting its foot in the door at cost-conscious automakers.
How It Works
In the Revetec engine, pistons are attached to their horizontal opposites via a rigid connecting rod. Just below the lower edge of the skirtless pistons sit two roller bearings mounted on a common shaft, one on each side of the connecting rod. The pre-loaded bearings ride along the outer edge of the cams (also found on either side of the connecting rod), and drive them in opposite directions. The outer cams are splined to the output shaft, while the inner set ride on a sleeve fitted over the output shaft. The inner cams transfer their force to a gear situated in the sump that drives a counter-rotating balance shaft. This planetary gear arrangement reverses the direction of the force so it can be transferred to a drive gear splined to the end of the output shaft. (An animation of the engine is available at:www.Revetec.com.)
Each cam rotates three times per revolution, making the swept volume of the Revetec three times its static volume. Accordingly, an 1,800-cc swept volume four-cylinder engine measures 600 cc, or just 150 cc per cylinder. Maximum force is applied to each cam within 10% of top-dead center and throughout the majority of the piston stroke. Which means the free-revving engine reaches its torque and horsepower peaks quickly, a potential problem in terms of drivability.
“Electronic controls and variable valve timing will help,” says Howell-Smith, “but there’s still a lot we can do with the cam shape, including profile changes, matching the dwell to the expansion rate of the fuel, etc.” The current engine idles at just 300 rpm, and Revetec’s analysis suggests this can be lowered to 120 rpm with a switch to variable valve timing. Howell-Smith expects the rev limit to remain at 6,000 rpm.
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