2007 Engine Scorecard

Engine makers have done a better job with fuel economy and reliability.

Steve Sturgess
Executive Editor

Reducing fuel consumption also cuts air pollution. That's common sense, but does the supposition also add up to scientific fact? Yes, said researchers at the federal Environmental Protection Agency, after testing a tractor-trailer set up to more easily roll down the road and slip through the air.

They compared the modified rig against a standard vehicle by the amount of fuel burned and oxides of nitrogen produced, and found that both were lower with the more fuel-efficient rig.

The EPA's prime mission is cleansing the environment, so its test in 2005 focused on measuring pollutants and how they correlated to fuel use. Researchers used a portable emissions measuring system, or PEMS, which metered fuel flow to the engines and exhaust from their stacks. PEMS is easier and more meaningful than dynamometer testing, because not many dynos can accommodate an entire tractor-trailer, said the researchers. A PEMS will show the effects of vehicle modifications acting on a moving vehicle, not just of the diesel itself.

Researchers figured that NOx, an ingredient in smog and a target of the agency's increasingly strict diesel exhaust emissions limits, is a direct result of engine power, and is reduced when less power is needed to move a truck down a highway. (Particulate matter, another exhaust pollutant that's been cut by EPA decree, is caused by additional factors like fuel composition, variations in revs and motor oil slipping past piston rings, so it was not counted in the test.)

The test was run on a dogbone-shaped track at Aberdeen Proving Ground, a U.S. Army facility in Maryland, using Society of Automotive Engineers' J1321 Type II procedures (which were developed some years ago with the Technology and Maintenance Council of ATA). The track has a 3-mile straightaway with a 35-mph turnaround loop at each end. The 2004-model tractors with their 427-horsepower diesels were serviced and tuned to factory specifications. Drivers were trained on the procedures and instructed to do the runs the same way each time.

"At a steady speed of 65 mph on a flat road, aerodynamic drag and rolling resistance account for 21 percent and 13 percent, respectively, of the total energy used by a Class 8 heavy-duty tractor-trailer," the researchers stated in an SAE paper written to explain their findings. Cruise speeds like that were planned, so researchers outfitted both of the Mack Vision daycab tractors with factory-approved roof fairings, which is common practice among truck operators.

To achieve better fuel economy with one rig, researchers fitted it with wide-base single tires, which in other testing by manufacturers have reduced fuel consumption by 5 percent or more. One brand of tire went on the tractor and another went on the Fruehauf trailer, and all tires were mounted on aluminum rims.

Crews also installed several aero improvers on the trailer - something most owners don't do. The devices included a gap fairing mounted on the trailer's nose and a "belly skirt" on each side, plus an inflatable boat tail at the trailer's rear.

"The skirt fairings reduce crosswind and underside drag, the gap fairing reduces turbulent drag between the tractor and the trailer and reduces drag on the front of the trailer, and the boat tail reduces turbulence at the rear of the trailer, maintaining laminar flow over the trailer," researchers explained. The gap fairing and skirts are permanently mounted, while the boat tail deflates and folds against the trailer's rear doors to allow them to open for loading and unloading.

Testing compared a baseline rig, which had normal dual wheels and tires on its tandems and no aero devices on its trailer, with the modified test rig. To see what each type of improvement accomplished, the test rig was run with the wide-base single tires only, with the trailer aero devices only, and with both types of equipment. Rigs were run at 55 mph, 65 mph, and a simulated "suburban" cycle with slowdowns and speedups. Altogether, there were 36 test runs, but several were voided due to various malfunctions. The average results:

Wide-base single tires: Improved fuel economy and cut NOx emissions by an average of 6.04 percent and 36.9 percent, respectively, at 55 mph; by 12.6 and 30.5 percent at 65 mph; and by 10 and 13.9 percent on the suburban cycle.

Trailer fairings: Improved fuel economy and lowered NOx emissions by an average of 5 percent and 18.5 percent at 55 mph; by 12.6 and 33.5 percent at 65 mph; and by 3.21 and 10.3 percent on the suburban cycle.

Wide-base tires combined with trailer fairings: No data were recorded during the 55-mph test because the boat tail deflated during the runs. It also deflated during one of the suburban runs; but when it stayed inflated, it plus the other fairings and the wide-base single tires appeared to improve fuel economy by 41 percent and reduce NOx emissions by 25.1 percent. At 65 mph, the fuel economy improvement was 17.9 percent and the NOx reduction was 44.6 percent.

Statistical "confidence intervals" were less than desired in a number of the runs, and there were brief times when the sophisticated measuring devices recorded relatively poor economy and higher than normal emissions levels. However, the instruments also found that the test rig's engine had to make 16 percent less power than needed by the baseline rig during the runs, confirming that the improvements did indeed allow the test rig's engine to work less hard. This, of course, is what saved fuel and lowered the amount of NOx made by that engine. EPA researchers were also pleased that such testing can be done and meaningful data collected while running trucks on a track or road, not just while they are on dynos.

Overall, researchers found that the wide-base tires generally reduced fuel use and NOx emissions at all speeds. And as expected, the trailer fairings usually were more worthwhile at higher speeds, in NOx-reduction performance if not always in outright fuel economy, at least on these runs.

"These test results should be of particular interest to the freight industry, because most fleets and operators will be using existing heavy-duty trucks for many years or even decades to come," they concluded. "The simple, cost-effective components tested here not only have the potential to reduce fuel costs, they may also provide a method of NOx-control 'retrofit' that pays for itself."

The SAE paper, "Effects of Single-Wide Tires and Trailer Aerodynamics on Fuel Economy and NOx Emissions of Class 8 Linehaul Tractor-Trailers" (05CV-45), was authored by Anthony Erb, L. Joseph Bachman and Cheryl Bynum. It can be found on EPA's website or by doing a web search using that title.