Concerns stemming from oil prices and greenhouse gas emissions have re-intensified efforts to design acceptable aerodynamic designs for long-haul Class 8 trucks that optimize fuel economy. In this effort, wind tunnels and models designed for tractor-trailer combinations play an important role.
Auto Research Center, Inc. (ARC), an independent aerodynamics research team that is based in Indianapolis and used by vehicle manufacturers, NASCAR and other racing teams, has developed a rolling road wind tunnel and truck model for Class 8 trucks that it says provides better insight into the on-road aerodynamic impact of design decisions.
Aerodynamics as a competitive differentiator. The Freightliner Cascadia (earlier post) was recently assessed as more aerodynamic than other Class 8 commercial vehicles, according to Freightliner in consultation with ARC.
ARC researchers compared the Cascadia with four other similarly spec’d Class 8 vehicles—the International ProStar, Kenworth T660, Peterbilt 386 and Volvo 780—using Freightliner’s wind tunnel, the only aerodynamic testing facility built specifically for Class 8 vehicles.
Testing took place in June and July. Researchers measured wind drag on the front, sides and back of the tractor, as well as the front of the trailer. There was a tractor-trailer gap of 48 inches. The amount of drag for the Cascadia was consistently lower and better than competitors. The ProStar recorded 7.8% more drag than the Cascadia; the 780 showed 9.6% more drag; the T660 showed 18.8% more drag; and the 386 showed 22.9% more drag.
My overall conclusion from this test is that based upon the results, the Freightliner Cascadia is the most aerodynamic of the five tractors tested.—Mike Camosy, ARC operations manager
Using anticipated fuel costs and differing applications, the Cascadia’s efficiency could save customers as much as $950 to $2,750 a year per truck, according to Freightliner. Fuel savings were calculated assuming each truck was driven 144,000 miles per year, with fuel at $3 per gallon, driving 60 mph.
Rolling Road. ARC, which operates one of only 3 moving ground wind tunnels in the US, has built the US’ first rolling road semi truck model for aerodynamic development (wheels rolling on a moving ground plane in the wind tunnel).
The model is 1/8th scale, with instrumentation to record areas of high/low drag under different configurations.
At the recent Aerodynamics of Heavy Vehicles II Conference, ARC presented a technical paper—“Advanced Experimental Methods for the Analysis and Aerodynamic Design of Heavy Vehicles”—describing its experimental findings in comparing rolling road with non rolling road results, using two different models based on the NASA Generic Conventional Model (GCM): one with rolling wheels, the other with non-rolling wheels.
For both model configurations, NRW [non rolling wheels] and RW [rolling wheels], the road off conditions gave closely matching results. However, the road on condition revealed quantitative differences between the road off conditions with respect towards each model configuration as well as differences between these configurations themselves while tested in the road on condition.
The ARC detailed model tested in the tunnel represented a more “real world” class 8 truck. Multiple pieces to this model were systematically fitted to build it to a final more accurate specification as individual test runs. While fitting these basic parts, several of them caused a reversal of force trends between the road off and road on conditions. A combination of three of these basic parts, (landing gear, air tank, spare wheel), actually recorded a 0.42% drag increase in the road off condition, while recording a 0.95% drag decrease in the road on condition representing a 1.4% variance.
Three development items were chosen to represent underbody changes that would be influenced by rotating wheels as well as a rotating ground plane. The test items were a tractor rear wheel deflector, a rear trailer diffuser and a set of wheel covers. Two of these three development items showed a trend reversal of the forces.
The tractor rear wheel deflector gave a 1.04% drag increase in the road off condition, while giving a 1.52% drag decrease in the road on condition representing a total variance of 2.56%. The rear trailer diffuser gave a 0.82% drag decrease in the road off condition, while giving a 0.93% drag increase for the road on condition representing a variance of 1.75%. Although the wheel covers gave the same force trend for both the road on/off conditions, this change was much larger in the road on condition (1.33% greater change in road on).
ARC concludes that without rolling road testing, current development efforts to increase truck aerodynamics may be misleading.