SAE J2971, “Recommended Practice Truck and Bus Aerodynamic Device Terminology” was issued in April 2013. It provides a standard naming convention for aerodynamic devices and technologies used to control aerodynamic forces on trucks, including trailers.
Finally, other promising methods include CFD and the constant-speed torque test. It is generally accepted by experts that constant-speed torque tests require standardization. CFD is proving very useful for product development, but there are practical limitations to its use as a compliance tool for whole vehicle evaluation; however, this is likely to change with time as CFD continues to improve. The recommended practice providing guidance for the use of CFD in evaluating commercial vehicle aerodynamic performance is SAE J2966, “Guidelines for Aerodynamic Assessment of Medium and Heavy Commercial Ground Vehicles Using Computational Fluid Dynamics.”
New tractor specifications throughout the industry have been substantially influenced both by the SmartWay program and the CARB regulation. The committee prepared a questionnaire for several of the largest tractor manufacturers in an effort to quantify this result. The tractor manufacturers that were contacted accounted for about two-thirds of industry sales in the period noted in Table 6-4.
Among those manufacturers surveyed by the committee, nearly 60 percent of tractors sold (first two rows under Share of Sales) are fully equipped sleepers whose fuel consumption already benefits from SmartWay specification. (The manufacturers not contacted, which account for roughly one-third of sales, could have been either more or less likely to produce SmartWay-certified equipment.) This percentage will increase beginning in 2014, as will the performance of other new tractors, since the federal regulations affecting tractors will have become effective.
TABLE 6-4 Industry Sales Penetration of SmartWay Tractors and Components, circa mid-2012 to mid-2013
|Tractor Type (Classes 7 and 8)||Share of Sales (%)||Share of S/W
|S/W Duals||S/W WBSTs|
|SmartWay fully compliant
|Day cab w/roof fairing and/or other aero||26||49||5|
|Day cab, no added fairings||8||49||6|
|Classic and vocational||4||23||1|
NOTE: Percentages are manufacturer sales-weighted.
a S/W, SmartWay.
b May apply a S/W label.
c Short one or more S/W components.
SOURCE: Responses to committee’s questionnaire for tractor manufacturers in October 2013.
It is significant that another 26 percent of tractors sold, day cabs, are also aero equipped; half of those use low-rolling-resistance (LRR) tires. Most tractor manufacturers supply well aeroengineered fairings for day cab tractors. It may be possible to separate the use of these tractor types into higher and lower speed applications. In the case of the higher average speed applications, requiring “full” aero-treatment could further reduce fuel consumption. There are certain trailers that are best served by tractors without aeroroof fairings (often called low-roof tractors). Among these trailers are flatbeds and many tankers. Finally, some day cab tractors and some sleeper cab tractors are not single purposed but are used in mixed “utility” haulage. This means pulling a van trailer at times and non-vans at other times; as well, either of those activities leads to performance at a mix of average speeds as duty cycle differences and congestion dictate. Regulators and even carriers need to consider if all these applications can benefit from high aero content.
Further, smart speed recorders might serve as a tool for setting a more stringent aero requirement. (Here the committee imagines a speed recorder computing the cumulative product of tractor moving time and speed-cubed that could be periodically evaluated. Such a time-averaged speed-cubed level would represent the added value of a high-aero configuration.)
A final observation from Table 6-4 is that a full 77 percent of all tractors were equipped with LRR tires,17 reflecting the carrier’s perception of their good value. Probably some of the non-LRRs legitimately require special tire operational features not achievable within an LRR specification.
The committee oversaw the collection of information on the use of aerodynamic devices on trailers by observing nearly 5,000 tractor/trailers operating on interstate highways in seven parts of the country. Persons with knowledge of trucking were provided photographs of the trucks, trailers, and aerodynamic devices of interest and instruction on how to conduct the informal surveys. Observations were made from the side of a highway where traffic could be clearly observed in one or both directions. The results were recorded on the individual forms. The observations were made at the following locations:
17 Found by multiplying the values in the column Share of Sales by the percentages thereof having duals and WBSTs and summing.