Cat C13
As engine makers transition to lower emissions, they are adopting varying approaches on horsepower and displacement. One trend, however, is better torque curves that often help produce more peak torque. This means that choosing the correct axle ratio for the tire size, transmission top gear ratio and engine torque curve is more important than ever. “Simply put, torque does all the work, and horsepower determines at what speed that amount of work will be done,” says David McKenna, Mack’s powertrain marketing manager.
“Some people with long experience in the industry still have not fully embraced ‘gear fast, run slow,’ ” says Mike Powers, product development manager at Caterpillar. “Because of the fuel economy penalty related to meeting emission standards, the sweet spot is smaller than it was.”
Powers says nothing has changed for 2007: Caterpillar owners should follow the same ACERT gearing recommendations that have been in place since 2004. In that year, for example, Cat’s 475 hp rating went from 1,650 lb.-ft. of torque to 1,850, typical of the change that “allows a lower rpm at cruise,” he says. Match carefully, and be prepared to use a faster axle ratio than what you’re used to with many engines. C15 engines with less than 1,750 lb.-ft. of torque should cruise at 65 mph at 1,400 rpm, while those with 1,750 lb.-ft. and above should cruise at 65 mph at 1,325 rpm, Powers advises.
Owners of Cummins ISX engines, meanwhile, should cruise at 1,450 rpm at 65 mph, whether it’s a pre-2007 or a 2007 engine, says Lou Wenzler, on-highway market communications director at Cummins. “The 2007 ISX has a family of ratings that are similar, if not identical, to pre-’07 ratings. All ratings provide a robust power curve, which enables great lugback performance. The ISX ratings are fully compatible with a full range of automated and fully automated transmissions.”
In the Volvo D13, the sweet spot has moved downward, says Ed Saxman, Volvo’s product manager for drivetrains. On the D13, peak horsepower has increased from 465 to 485 while torque remains at 1,650 lb.-ft. But the sweet spot is now 1,300 to 1,500 rpm, down from 1,400 to 1,600. “You get the best fuel economy by staying within that rpm band,” Saxman says. “The torque is so good, a 1,450 rpm cruise will feel good even at 69 mph.” Gear accordingly.
The D13 is a 12.8-liter engine rather than a 12.1-liter machine with the same bore but a slightly longer stroke, intended to allow the engine to meet the 2007 emissions standards with “the same durability, and staying within the same design parameters, such as peak cylinder pressure, by lowering the density of horsepower a bit,” Saxman says.
Similarly, Detroit Diesel has standardized its Series 60 by using the 14-liter design at all horsepower ratings for 2007, while also improving the piston design and cylinder liner finish to minimize oil consumption. “We have not modified the torque characteristic significantly for 2007 other than to extend the peak torque speed down to 1,100 rpm from 1,200 rpm,” says Tim Tindall, director of Detroit Diesel component sales. “This has been done to allow a slightly broader operating range for the engine.”
Detroit Diesel isn’t changing its driveline recommendations for 2007 engines over what it recommended for 2006, Tindall says. “We do offer engine configurations which are matched for fully automated transmissions as well as for manual transmissions which are automatically shifted.”
International’s new MaxxForce 11 and MaxxForce 13 engines – to be introduced around yearend – will provide their peak torque all the way down to 1,000 rpm, allowing “for earlier upshifts when accelerating and fewer downshifts when climbing hills,” says Jacob Thomas, vice president of International’s Big Bore Business Unit. For the 13-liter design, that means up to 1,700 lb.-ft. with a peak horsepower rating of 475. The high-pressure common rail injection system can produce its maximum 26,000 psi even at peak torque rpm, where other heavy-duty injection systems cannot deliver maximum injection pressure, Thomas says.
The view at Mack is that the match of available horsepower and torque on the new Mack MP engines is better than ever. “We have managed to lower the cruise speed RPM settings with the MP series engines, with much improved horsepower ramps,” says Mack’s McKenna. “All Mack MP engines are right at home with a 10-speed transmission.” McKenna recommends multi-speed gearboxes as optimal for mountainous interstate or heavy haul, however. In Mack’s case, however, the new lineup offers better horsepower density than the earlier ASET engines, he says.
Tighter links
Another significant trend is linking transmission and engine electronically so torque levels can be varied depending upon the gear ratio. Volvo’s I-Torque allows use of “commonly available components,” which are less costly, Saxman says. I-Torque limits torque coming from the top-rated Volvo 16-liter engine to 1,650 in first and second gears, to 1,850 in third and fourth, and then supplies the maximum of 2,050 above that because what really matters is the “torque input to the driveshaft and the rear of the transmission.”
Cummins has introduced two new SmartTorque ratings, the ISC 425 ST and 485 ST, Wenzler says. The 425 gives 1,750 lb.-ft. in the top two gears, while the 485 gives 1,850 in the same ratios in order to induce the driver to stay in the higher gears and optimize the balance between performance and fuel economy.
Caterpillar’s Powers cites as an example the Eaton Fuller Convertible Top 2, a 10-speed that can be converted to a 13-speed at resale. The unit drives like a normal 9-speed, and then makes automated shifts between ninth and 10th, which are actually splitter ratios. RPM after an upshift is as low as 1,100 to 1,150, keeping the engine in the sweet spot, yet maximizing the speed range available from the top gear position.
Engine makers and their partners must deal with ever-tightening emissions, of course. But they are fighting to keep your fuel costs under control.
Help from the maker
Don’t overlook useful engine spec’ing tools
Our discussion of engine makers’ advice on sweet spots assumes a typical on-highway operation. But is there really a typical operation? Due to your type of haul or geographical range, your needs aren’t the same as all others, so be sure to tap into engine manufacturers’ spec’ing assistance to optimize the balance between torque and fuel economy.
Caterpillar’s program, DesignPro, is available from the company through the truck dealer and offers comparisons in miles per gallon, Powers says. Even though you often won’t get exactly that mileage in operation, you can change axle ratios or tire sizes and see the relative impact on fuel economy. DesignPro goes further, however, by showing how truck design, such as aerodynamics, affects fuel economy.
Volvo’s spec’ing assistance program offers a graphical representation of a tachometer and speedometer on a computer screen, Saxman notes. You can input the tire size and transmission top gear ratio and the program will then help you to choose the axle ratio that will keep the engine in the sweet spot. Horsepower at the wheels is also available, which helps determine gradeability.
Cummins’ PowerSpec program, which can be downloaded at this site, offers a gearing/spec’ing calculator that aids customers in properly gearing their vehicle with Cummins power, Wenzler says.
Mack’s Predictor program “guides our salesman and customer to the best specifications desired, whether for best fuel economy, best power, low speed job site operations, transmission matching, and so on,” says McKenna.
Detroit Diesel’s offers Specification Manager, a simulation tool designed to evaluate various driveline options across a multitude of operational routes. “This tool allows us to evaluate gradeability and fuel consumption differences in comparing one specification to another, as well as optimizing the number of gears required,” Tindall says.
Engine manufacturers know their products better than anyone. Let them help.
