Overdrive Magazine

April 2019

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42 | Overdrive | April 2019 T he rapid industrywide adop- tion of the automated man- ual transmission has been accompanied by the related trend of downspeeding and lower axle ratios. The slower engine speeds have brought about significant savings in fuel costs and other efficiencies. "Downspeeding is about reduc- ing the amount of trips the piston takes up and down the engine, which reduces friction," says John Moore, powertrain product market- ing manager for Volvo Trucks North America. "This is actually where we're saving the fuel." Volvo's XE13 package that debuted in 2011 reduced rpm at cruise from about 1,370 to about 1,150. Newer iterations of downsped drivelines routinely cruise near 1,050 rpm. Every reduction of 100 rpm is worth about 1.5 percent in reduced fuel consumption. So being able to maintain highway speeds at 1,150 rpm versus 1,370 constitutes a fuel savings of more than 3 percent. A downsped driveline lets the engine operate at its most efficient rpm while also generating the mini- mal horsepower required to maintain a 65-mph cruise. "There is a combination of fac- tors for the engine fuel eciency improvements," says Carlos Pinotti, Meritor's senior director of rear drivetrain engineering. He cites reduced engine friction due to lower piston speed and reduced heat trans- fer and flow losses due to lower gas velocities and improved thermody- namic eciency. "The slower that engine is turn- ing, you're not running the water pumps, accessory drives and all those things nearly as fast, not to mention fewer cycles per minute," says Mike Garrison, Eaton application engineer. Reduced heat and friction also lead to less component wear, potentially extending engine life. "An AMT is capable of making shifts at the appropriate time to keep the rpm low," Garrison says. It takes into account real-time driving factors to optimize the shift strategy and bal- ance performance with fuel economy. Behind the AMT, a taller rear gear ratio lessens engine speed and improves fuel eciency. Axle ratios have been getting faster with a lower numerical value, Garrison says. "For overdrive transmissions, com- mon axle ratios were in the mid and low 3s," he says. "Today, with a lot of these linehaul downspeeding specs, it starts with a 2." Nearly 25 percent of the rear ends Meritor sold in 2011 came with a 3.55 axle ratio. By 2016, that share had dropped to about 5 percent. Across 40,000-pound axle, linehaul and 6x4 applications, a 2.64 ratio has climbed from about a 7 percent order share to about 13 percent in that same timeframe. It's now the leading ratio among Meritor products. "The axle ratio's range shied toward faster ratios due to downspeeding," says Pinotti. "By the end of 2016, 2.64 and 2.47 ratios rep- resented 25 percent-plus direct-drive and overdrive transmissions." A 2.47 ratio was the biggest gainer over those five years, climbing from less than 3 percent to a nearly 10 percent order share. The engine's output torque has Downspeeding uses high-speed rear axles combined with high-torque lower-rpm engines. With this powertrain combination, the engine revs less at any given highway speed and receives a corresponding fuel economy boost. With downspeeding comes new efficiencies BY JASON CANNON E Q U I P M E N T

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