Tweaking Specifications on Truck Engines Can Raise Fuel Economy, Study Finds

By Frederick Kiel, Staff Reporter

This story appears in the Sept. 3 print edition of Transport Topics. Click here to subscribe today.

Fleets can save fuel — possibly by as much as 5% — by “overspec’ing” engines, according to PHH FirstFleet, an asset, financial and technology management company for fleets.

“By slightly overspec’ing engines to run more consistently in the ‘sweet spot’ — the middle 41% to 60% of the engine’s torque rating — we’ve found that you can maintain a truck’s optimal rpm range of around 1200 to 1450,” Michael Lewis, president and general manager of PHH FirstFleet, told Transport Topics.

“This reduces over-revving, maintains a truck’s life cycle and decreases fuel consumption, without overly affecting driver satisfaction or shifting patterns.”

PHH FirstFleet said it demonstrated the fuel savings in an 18-month study on more than 200 trucks from 11 private carrier fleets. About 95% of the trucks in the study were Class 8s, both sleepers and day cabs, Lewis said.

“In our system, you basically lower the [numeric] gear ratio of the rear axle by one notch and increase the torque up one notch,” Ezel Baltali, applications engineer for PHH FirstFleet, told TT.

“We conducted tests on more than 200 trucks in 11 fleets, using telematics to measure their miles-per-

gallon fuel use, both before and after we changed the torque and gear ratio,” Lewis said. “The trucks that we worked with were not applying any of our other recommended measures for improving fuel efficiency, so that they were averaging 6.0 miles per gallon. After we adjusted the engine, their consistent fuel use rose to 6.3 miles per gallon, or a 5% increase.”

Lewis said that the procedure consistently raised fuel efficiency by 0.3 miles per gallon on trucks with different miles-per-gallon rates.

“That means that if a fleet has instituted other fuel economy measures and their trucks were getting 7.0 miles per gallon, our adjusting would increase fuel economy to 7.3 miles per gallon, which is less than 5%,” Lewis said.

“To give an example, we modified one Class 8 day cab by moving the gear ratio of the rear axle from 3.9 down to 3.73, and increasing the torque from 1550 to 1650 [foot-pounds]; that is, one notch in each direction,” Baltali said.

The engine adjustment can “equate to a savings of approximately $2,000 of fuel per tractor per year, assuming 100,000 miles are driven a year,” Lewis said.

“Adjusting the engine will work on trucks with both 2007 engines and older ones,” Lewis said. “But ’07 engines have increased gear and torque options that make it even easier.”

Spokesmen from engine companies agreed that the principle behind the study was sound, though one did not think it would increase fuel savings by a full 5%.

“A fleet can improve fuel efficiency of the engine by overspec’ing it,” David Siler, director of marketing for Detroit Diesel, told TT. “This means that, in today’s diesel engines, there is a direct relationship between engine rpm and fuel consumption: The lower the rpm of the engine, the better the fuel economy.”

Siler said that “in order to pull a load, an engine has to produce torque and horsepower; yet to run efficiently, it has to keep the engine rpm down as much as possible. You accomplish this by using a numerically lower gear ratio and a higher-torque engine to make that happen. If managed properly, you can actually save fuel by increasing the torque output of the engine and numerically reducing the rear axle ratio.”

Lewis said the study can be downloaded from the company’s Web site,  www.phfirstleet.com.

“We’re extremely concerned with environmental issues and, since this adjustment cuts fuel use, it also cuts emissions, so we want everyone to know about it,” Lewis said.

“The one-time cost of increasing the engine’s ratings costs roughly $1,000 on a new truck, compared with $2,000 per year per truck in fuel savings,” Lewis said.

Louis Wenzler, director of on-highway communications for Cummins Inc., was skeptical.

“For the most part, I agree with the idea, but I don’t think adjusting the engine itself will produce a 5% increase in fuel economy,” he told TT.

“A fleet can cut fuel use with this system by PHH FirstFleet, that would mean less performance,” Wenzler said.

Lewis said no loss of performance takes place.

“Lowering the gear ratio does decrease performance,” he said, “but raising the torque under our guidelines offsets that loss.”

Lewis emphasized that each engine has to be adjusted to deal with how the truck will be used.