Torque Target

John Baxter | August 07, 2011

New engine technology promises not only improved engine efficiency, but also the ability to spec your drivetrain to further enhance fuel economy.

Exhaust gas recirculation creates higher exhaust backpressure and slower combustion in the cylinders, which reduces fuel economy. How can the situation be remedied?

One answer is proving to be selective catalytic reduction. SCR technology in 2010-2011 engines has a beneficial side effect: It allows engineers to reduce EGR levels by retuning engine parameters. This yields an often-overlooked side benefit – an enhanced ability to produce power and torque. By smartly spec’ing your transmission and drive axles to give a lower cruise rpm, you can take advantage of engines that pull better, and can increase fuel economy even beyond the gains produced by the retuning.

Don’t underestimate the value of low rpm cruising. The most efficient diesels are the slow-turning, ocean-going ship engines. Some turn at only a tenth of the rpm of a modern truck diesel.

“We’re selling torque, not horsepower,” says Ed Saxman, product marketing manager for Volvo. He points to Volvo’s 13-liter engine, which produces 500 hp and 1,750 lb.-ft. of torque, as an example.

For Detroit Diesel’s DD15, “The torque lasts down to 1,000 rpm,” says Chuck Blake, Detroit Diesel’s senior technical sales support manager. The company’s amplified common rail system produces maximum injection pressure at these lower rpms, which smoothes out low rpm combustion and enhances torque. In addition, the camshaft no longer has the job of operating unit injectors, and is driven by the rear end of the crankshaft where the flywheel is. These changes mean much less vibration produced by the engine’s mechanical parts. As a result, “It won’t miss a beat at that rpm,” Blake says.

Manufacturers are pegging their engine sweet spots in a tight range. Saxman says Volvo’s is 1,375 rpm. Mack’s Dave McKenna, Mack powertrain products marketing manager, says 1,350-1,450 is satisfactory, but for best fuel economy he recommends 1,300-1,350.

Cummins’ Zack Ellison, manager of customer technical support, says for the 15-liter ISX, 1,350-1,360 is recommended for most fleet specs, while up to 1,400 is considered satisfactory for higher-powered versions, depending upon desired performance. Cummins’ online Power Spec tool pegs the ideal cruise rpm at 1,380 for an on-highway truck with a 500-hp engine hauling less than 80,000 pounds at 65 mph. Cummins’ higher-powered ISXs have their torque peak at 1,200 rpms, while the standard fleet engine peaks at 1,100.

Blake says the DD15 should be geared at between 1,350 and 1,450 rpm at cruise, and at the high end of the range when climbing hills.

“Torque is important,” says Steve Slesinski, director of global product planning for Dana Holding Co. “Three or four years ago, it was very different from today. We’re seeing a trend downward in rear axle ratios from 3.70:1 and 3.55:1 axles down to 3.42:1 and 3.36:1. The industry will continue to migrate to low rpm and faster axles as drivetrains improve.”

Slesinski says you should gear for your average cruise speed, not for an occasional condition. He says you’d normally spec’ a 3.55:1 axle with a 0.73:1 overdrive ratio for 65 mph, but could go as low as 3.25:1 for a 75-mph cruise.

“Cruise rpm will continue to drop,” says Shane Groner, Eaton North America product planning manager. Typically it will go “from 1,400 down toward 1,300 rpm, with torque on the average engine holding at well below 1,200 or even 1,100 rpm.” strives to maintain an open forum for reader opinions. Click here to read our comment policy.