Torque Target

John Baxter | August 07, 2011

Meritor also offers 6×2 setups, says Charlie Allen, global service director. He says a special suspension design allows the operator to lift the rear tandem and place all the weight on the forward drive axle while driving on slippery roads.

Blake says another fuel-saving feature is that Detroit Diesel engines can wait till the coolant reaches 225 degrees F. to turn on the fan because of the reduced heat loads of SCR engines, thus significantly reducing fan on time, a big source of fuel consumption.

The value of multi-torque

Torque enhancement doesn’t stop with the higher ratings and the more potent torque curves of today’s engines. Every manufacturer offers a multi-torque system that cranks up torque beyond its base level under certain conditions. These serve the same purpose as recommending a torque curve with torque that rises a lot as rpm falls — an incentive to keep an operator from downshifting too early.

Such arrangements are ideal for company drivers who may not drive with the same attention to fuel economy as an owner-operator. Multi-torque is also a practical way to increase performance while retaining fuel economy.

The systems normally confine an ECM-controlled torque boost of 100-200 lb.-ft. to certain operating conditions, such as being in the top two gears. They provide maximum cruise performance at minimum cost because the torque to which driveshafts and axle parts are subjected increases by the ratio in the transmission. For the top two gears, the torque increase is only a fraction of what it would be down in the lowest gears.

Eaton’s Shane Groner says the company designs a number of its transmissions for multi-torque applications, including 13- and 18-speed models. Driveshafts and axles are relatively insensitive to the torque increase, but the input side of the transmission needs to be designed for the extra twisting force. The added torque shows up in the top two gears with a 10-speed and in the top two gear splits, both of which are used only in the top main gearbox position, in the 13- and 18-speed models.

The other advantage multi-torque provides is a more powerful incentive for the operator to shift properly. An engine with a good torque curve tends not to lose as much speed on hills, thereby encouraging the driver not to downshift. A multi-torque arrangement may actually provide a slight power loss when downshifting, or at least no more than a negligible increase. When the driver realizes this, he just leaves the transmission in the higher gear.

“With multi-torque,” says Volvo’s Ed Saxman, “you can get up to 1,750 lb.-ft. of torque even on a 425 [hp engine]. It will pull with a 475. Heading down toward 1,300 rpm, the torque comes on. It will lug down on a 3 percent grade and suddenly the truck stops slowing down.”

Mack’s Dave McKenna says the company’s Econodyne engines have a special characteristic called EconoBoost. If the driver is climbing a hill at full throttle and keeps the engine below 1,300 rpm for at least four seconds in the top two gears, he gets an extra 200 lb.-ft. of torque. The MaxiCruise engines have extra torque rise in all the gears, but the EconoBoost arrangement means an even stronger incentive not to downshift.

“Our SmartTorque system gives an extra 100-200 lb.-ft. in the top two gears,” Zack Ellison says of Cummins technology. “The benefit is that you don’t need as strong a transmission and other drivetrain parts. You can climb a lot more hills without a downshift. The longer you keep it in top gear, the more efficient the whole system is. The percentage of time spent in top gear indicates what kind of fuel economy you are going to get.”

In Detroit Diesel’s system, says Chuck Blake, several ratings with 1,550 lb.-ft. of torque increase to 1,750 lb. ft. when in the top two gears. “The multi-torque systems protect the driveshafts.”