Humped torque curves were the norm in North American heavy-duty on-road diesel engine power maps for decades. Today's torque curves are much flatter, but the height and width of the torque curve remain limited. Volvo Trucks has broken that mold in dramatic fashion with its new XE13 powertrain option.
Volvo's XE13 powertrain makes it possible to stay in top gear longer, which saves fuel and improves drivability.
Volvo's XE13 powertrain makes it possible to stay in top gear longer, which saves fuel and improves drivability.

I had the pleasure in mid-September of being the first driver outside of Volvo's development and testing program to take this new powertrain for a test drive. Without putting too fine a point on it, I was blown away.

We've been through a long dry spell in powertrain improvements between October 2002 and the post-2010 era. Truck and engine makers had their hands full meeting EPA's objectives. Fuel economy, while always a concern, took a necessary back seat to emissions reductions. But with this initiative, I believe Volvo has made the first dramatic improvement in powertrain technology in some time.

What's interesting is that Volvo's XE13 requires no bolt-on parts, no moving parts, no additives, no material upgrades and no extraneous gizmos. There's nothing more to the XE13 concept than some software programming and very, very tall rear ends.

The design objective was to create a powertrain that would operate at optimum efficiency on 90% of the interstate highway grade profiles in the country with an absolute minimum of gear changing. Volvo has engineered a powertrain that will stay in top gear much longer along most of America's Interstates. When it has to shift, it takes full advantage of the 1,750 pounds-feet of torque produced by the 425-horsepower D13 engine down to as low as 1,050 rpm.

It cranks out 1,750 pounds-feet between 1050 and 1,500 rpm. It'll make over 1,400 pounds-feet as far down as 900 rpm. That's right: 1,750 pounds-feet of torque across a 500-rpm band from a 13-liter engine rated at only 425 horsepower.

A D13 engine coupled to I-Shift overdrive transmission and run through a set of 2.04:1 rear ends on 22.5 tires produces a cruise speed of 65 mph at 1,156 rpm. At 1,150 there is still 250 rpm to the south where the engine produces more than 1,400 lb ft. Imagine pulling a hill at 900 rpm with more than 1,400 pounds-feet of torque?

I can tell you, it's astonishing. A driver watching the revs closely could pull such a hill without a downshift beside a driver with a less capable powertrain who would have downshifted and might be running 1,600-1,700 rpm. Who do you think is pulling the hill on less fuel? Who do you think would make it over the hill faster?

The guy with the Volvo XE13 powertrain.

If it seems like I'm impressed with Volvo's XE13 powertrain, I am, but I'm not saying anyone else has done a poorer job. Volvo's engineers have done something remarkable here and they deserve credit.

Volvo has opened up the torque on the D13 and widened the torque band to where you have massive pulling power from 900 rpm all the way out to 1,500 or better. Gone is the humped torque curve that limits peak output to just a very narrow range. XE13's wide, flat torque curve allows for two things: fewer fuel-consuming gear changes and far more time in top gear, and a much lower engine speed at highway cruise speed with no compromise in drivability. Both are big fuel savers, and big driver comfort enhancements.

Truthfully, much of what has made this possible was the close integration of the engine and the transmission. The rest of the powertrain has not had to be beefed up to protect against the 1,750 pounds-feet of torque because of how engine and transmission software apply the power. There's no additional risk of driveline damage due to low-rpm operation, Volvo assures me.

And best of all, from the driver's perspective, when the D13 is running as low as 900 rpm, it still sounds like and seems like it's running at 1,400 rpm. It doesn't sound like it's about to come apart, which is probably why drivers of other makes of engine are reluctant to take the revs down so low.

The XE13 powertrain is available on VNL models spec'd for 80,000-pound on-highway operation. It requires a D13 engine, an overdrive I-Shift, and 2.64:1 gears -- the overall ratio is 2.04:1.

I'll be writing a Drive Test report on the XE13 package, along with another very cool bit of programming just announced by Volvo called Eco-torque, in an upcoming issue of Heavy Duty Trucking. I'll get into more detail on how it all works then.

In the meantime, you can read our coverage of the XE13 here.