Creating the optimum downsped engine/axle/drivetrain mix

The perfect combination: The Zen of creating the optimum downsped engine/axle/drivetrain mix

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“Most of the downsped drivetrain combinations recommended by Navistar to our line-haul customers utilize an overdrive transmission and a rear axle ratio in the 2.64 to 2.80 range. This covers top gear cruising speeds of 62 to 65 MPH and low profile 22.5 tires,” says Jodi Presswood, vice president and general manager of the heavy-duty product line for Navistar. “We are preparing to release a downsped direct drive transmission combination that will require axle ratios in the 2.26 to 2.39 range. These direct drive combinations will be focused on fleets that have a typical cruising speed of 62 MPH or less.”

Presswood points out that Navistar’s Performance Engineering team works directly with dealer sales personnel and customers to fully characterize how a vehicle will be used.

“We have developed a customer questionnaire that provides sufficient information to confidently recommend the correct gearing for each unique customer,” Presswood says. He adds that the ProStar ES is its recommended spec for maximizing fuel economy with a down sped drivetrain, noting, “The ES has been developed for line-haul operation at 65 MPH. Customers that fall outside of that duty-cycle should contact Performance Engineering for guidance in selection of the optimal drivetrain combination.”

Downsped drivetrains are gaining popularity with a broad group of fleets and customers not only due to the fuel economy gains but also to the additional benefits of automated transmissions and the integration of other systems in the vehicle.

“Do not think of them strictly as a line-haul solution as the technology fits in a growing number of applications,” Presswood says.

Do the math

When recommending axle ratios for downsped engines, John Moore, Volvo Trucks’ product marketing manager-powertrain, says, “You have to do the math. Rear axle ratios are dependent on desired cruise speed, transmission type—direct drive or overdrive—and tire revolutions per mile. The first question to ask is what do you consider downspeeding? Some consider it to be a direct drive transmission with a 2.47 rear axle ratio that would yield 1,370 engine RPM at 65 MPH with 512 revolutions per mile tires. Volvo considers it to be an overdrive I-Shift with a 2.64 rear axle ratio and with 512 revolutions per mile low pro tires that would yield 1,142 RPM at cruise. This is commonly known as the Volvo XE package.

“The second question is what’s your minimum vehicle speed? For speeds under 62 MPH, the direct drive 2.47 ratio is best. For speeds at 62 MPH or over, the XE package with a 2.64 rear axle ratio is super-efficient. The XE package uses peak engine torque at lower engine rpm to maintain performance, while downspeeding and delivering superior fuel efficiency,” he says.

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