Innovative approaches to drivetrain design promise more efficient highway cruising.
Volvo recently shook up the standard view of how a drivetrain should behave.
The truck maker introduced the XE13 option, a system combining a Volvo I-shift automated 12-speed overdrive transmission with axle ratios in the range of 2.64-2.69:1. This gives a 65-mph cruise rpm of 1,150 rpm, a little more than 200 rpm lower than most trucks built today, even when they have conservative gearing.
Volvo calls the concept “engine downspeeding.” It’s among new developments from drivetrain manufacturers aimed at using less fuel to maintain desired highway speeds.
The XE13, equipped with a 425-hp engine producing 1,750 lb.-ft. of torque, incorporates “modified software.” What this means is the XE13 drivetrain produces good performance by downshifting at a higher road speed when climbing a grade, says Ed Saxman, marketing manager for Volvo Drivetrain. It downshifts at a speed that is adapted to fast gearing – typically, he says, at about 58 mph when geared for the most fuel-efficient 65 mph cruise.
Long-haul drivers tend to prefer manual drivetrains that will pull hills without frequent downshifts. Give the engine enough torque, the right torque curve and the right rear axle, and the truck will climb most hills even when staying in top gear. Thus, trucks are normally geared for a cruise rpm that represents a compromise between efficient steady-state cruising and the ability to climb hills without a shift.
A truck geared for 65 mph cruise at 1,370 rpm can normally drop down at least to 1,100 rpm before the driver needs to shift. With a typical 10-speed transmission geared for 1,370 rpm at 65 mph, the driver would not have to downshift until reaching about 52 mph, or 1,096 rpm.
Part of the recipe for making the standard drivetrain a pleasure to drive is the significant difference between the cruise speed and the first downshift point. Another part of it is the relatively high horsepower produced by compromise gearing, typically a rear axle around 3.40:1. This results from the additional 220 rpm versus the cruise rpm of Volvo’s XE13, for instance.
Allison Transmissions this year will introduce an exotic 10-speed, twin-countershaft automatic that incorporates a similar drivetrain concept, with a couple of interesting tweaks. Allison’s TC10 TS will get its 10 speeds by combining a five-speed main box with a two-speed range box, just as standard 10-speed manual transmissions do. It will, however, use a torque converter in place of a clutch, and shift between gears with the hydraulically actuated clutches of other Allison automatics. The torque converter will lock up at about 4 mph and then behave like a standard engaged clutch to save fuel.
The TC10’s torque converter will multiply the engine’s output torque by 1.76, allowing a powerful start while the transmission’s gearing provides a 7.4:1 first gear, significantly faster than the equivalent gear in standard 10-speeds. They normally have a first gear in the range of 11:1 if top gear is an overdrive. (The first gear in the overdrive versions of Volvo’s I-shift is 11.73:1.) This means the gearbox can have closer steps than other 10-speeds without having more gears.
Also, the 8-9 and 9-10 shifts in the TC10 will have narrower steps than the lower gears. The Volvo 12-speed I-shift has consistent steps of about 28 percent, while the Allison’s 9-10 step is 17 percent and its 8-9 step is 24 percent.
This means that the Allison will cruise in 10th gear at the chosen cruise speed, say 65 mph, but can downshift to 9th without a drop in vehicle speed if the driver demands more power for acceleration or hill climbing with a significant increase in throttle. With such narrow steps, this occurs without revving the engine enough to get the rpm above the sweet spot. The result is that while steady-state, level road cruise will occur at an unusually low rpm, the drivetrain remains quite capable of delivering normal, or even superior, responsiveness.