Detroit Diesel is too early in its test process “to tell what effects this type of set-up will have on deposits, etc.,” says Williamson. “But we can say our engines do perform well in a set-up of this nature and continue to meet/exceed emissions standards, maintain good combustion and deliver leading fuel economy.
“Simply lowering the engine cruise rpm on existing products does bring an efficiency advantage, but to perform correctly and get the most benefit a system approach is needed. Only when the system is optimized are other tradeoffs like driving performance, durability, and reliability avoided. We currently do not see any issues when the system and its shift points are managed correctly.”
Ed Saxman says Volvo’s engine, which already has low soot, should eliminate any potentially adverse effect of downspeeding. “With ultra-high pressure fuel injection, we can burn all the fuel, and it is difficult to measure the soot in engine exhaust,” even before the diesel particulate filter, he adds. “There should be no soot issues relating to cruising at a low rpm.”
‘Downspeeding’ speeds up
Will drivetrains incorporating “engine downspeeding” become a trend? For Eaton Corp., the answer is a resounding, “Yes.”
The company’s automated transmissions already depart from the traditional single cruising gear concept, says Shane Groner, Eaton’s manager of development and product planning. Its Ultrashift and Ultrashift Plus II products have included a “kickdown” feature for acceleration since 2005. They respond to driver demand by dropping down from top gear at highway speeds, as acceleration becomes more important to the operator than efficient cruising. Groner refers to this as “modulating capability,” meaning the transmission can shift up and down in an orderly manner in response to changing throttle demand.
“We’ll be seeing new products incorporating engine downspeeding technology within a year,” Groner says. “They will provide new, small gear steps in the range of 17 percent, giving a change in rpm after a shift in the range of 250-300 rpm or even less. We have a number of ratios in our arsenal that we can use to accomplish this and already have the required modulating capability in our boxes. The idea is to squeeze more miles out of the fuel.
“We can provide several designs to meet different applications, including both overdrives and direct drive units, and both 10-speeds and multi-speed, small-step units.” Groner notes, “There’s no putting one design into every application.”
Multi-speed units are the 13-and 18-speed boxes with splitters. Such units are intelligent enough to split the gears to keep the engine in the sweet spot during subtle changes in highway cruise conditions, yet shift full gears for faster acceleration when the truck is getting up to highway speeds, Groner says. Such units are in field testing with Paccar and Detroit Diesel.
Fuel economy and torque output improve when running at 1,150 rpm rather than 1,375, says Volvo’s Ed Saxman. “This means the engine puts out more power from each ounce of fuel, so customers gain about a 1.5 percent fuel efficiency improvement for every 100 rpm of ‘downspeeding.’ So fleets that spec XE13 can expect up to a 3 percent improvement when compared to another overdrive transmission in a similar operation.”
About Allison’s TC10, Todd Dygert notes that it’s “been shown to offer better fuel economy than automated manual transmissions in customer trials using real-world duty cycles.” Fuel economy improvements in fleet tests ranged from 3.3 percent to 11 percent.
Allison contends the powershift concept — the avoidance of de-throttling the engine during shifting — saves fuel by eliminating the dead period during shifts when the engine and drivetrain turn over but produce no power. The lower cruise rpm is a large contributor to these savings, however.
Fuel economy gains at lower speeds are a “result of reduction in parasitic losses,” notes Bill Kendrick, Cummins’ assistant chief engineer for vehicle performance. This refers to the reduced energy required to turn the engine over and operate the oil pump, water pump and other accessories.