Heavy Duty Trucking, June 2017

JUNE 2017 HDT 67 WWW TRUCKINGINFO COM that the location and shape of the leading edge of the skirt is a likely dominant contribution to the performance of side skirts This implies that as long as the flow is redirected along the sides of the trailer instead of underneath the drag reduction is significant It is likely that the leading edge contributes most to the flow split between what enters the underbody region and what is redirected around the trailer bogie Extending the skirts over the trailer wheels provides an additional reduction in drag as identified by the increased performance of the extended side skirts over the standard side skirts The success of side fairings has led to the introduction of a number of other devices to sculpt air around trailers Most of these target the underside of the trailer which is a low air pressure area to begin with a problem compounded by the drag inducing tandem axles toward the rear of the trailer To counter these problems fleets can now spec a variety of devices for trailer undersides ranging from removable landing gear to bogie fairings to various devices designed to smooth out the underside area of a trailer completely Bogie fairings are essentially a wind deflection structure that sit in front of the trailer axle and divert air around it SmartWay found these systems can save up to 184 gallons of fuel a year A belly box structure which can serve as a dual storage aerodynamic device by keeping air from hitting crossmembers and other drag inducing components underneath a trailer was found to save around 475 gallons of fuel per year Boat tails bring up the rear The low pressure area at the rear of a 53 foot trailer has long been recognized as a problem area in tractor trailer aerodynamics complicated by the fact that access doors are also located in this area Boat tail structures that taper the final air flow past a trailer to reduce the low pressure area in its wake are the most popular solution for fleets today These systems usually consist of a set of panels that extend aft of the trailer To allow access to the cargo door these panels have some sort of extension and retraction system that allow them to fold or flip out of the way as needed Of the boat tail systems it surveyed SmartWay found that long four panel boat tails provide the greatest increase in base pressure with the three panel and shorter four panel boat tails providing a smaller difference Despite the larger increase in base pressure for the long four panel boat tail pressurization inside the boat tail cavity that acts on the inside surfaces of the inward facing panels provides an added component of drag researchers note This is likely a contributing factor toward the small difference in drag reduction between the short and long boat tails Little difference is observed for the drag reduction and the pressure distributions between the long three panel and the tapered three panel boat tails indicating that the size of the side panels do not have a major influence on the boat tail performance Performance wise SmartWay Canada found little variation in boat tail devices regardless of which style or size of device was used Long four panel boat tails for example were found to save approximately 500 gallons of fuel a year while three panel tapered devices delivered the worst performance but still saved around 422 gallons per year A never ending puzzle Mike Roeth executive director of the North American Council for Freight Efficiency notes that although his organization gives aerodynamic devices consistently high confidence ratings in terms of their ability to deliver fuel savings for fleets he cautions that getting the formula right can be challenging As each device is added the performance of other devices will be impacted Roeth cautions The airflow over each device changes the operating conditions for the other devices The performance of a combination of devices will not simply be the additive total of each device operating alone That said however NACFE has found that the greatest aerodynamic drag reduction comes from using devices in three main areas gap underbody and rear Addressing the aerodynamics of all three points of drag should give the greatest fuel savings for the vast majority of fleets Using similar formulas to SmartWay Canada Roeth says NACFE was able to determine that ROI for the three most popular aerodynamic systems occurs surprisingly quickly around 55000 miles in many cases Making that easier Roeth says is that the cost of trailer side skirts have decreased substantially over the past three to five years due to far more market entrants driving cost competition and much higher deployment volumes reducing cost per unit Based on interviews with fleets NACFE estimates that costs for side skirts have dropped roughly 70 compared to cost estimates compiled as part of the 2010 National Academy of Sciences study that investigated fuel efficiency technologies for commercial vehicles Taken as a whole the findings seem clear Even limited use of tractor trailer aerodynamic devices yield significant fuel savings for fleets And fast ROI times mean even cash strapped fleets can start with one device then add others until a full fuel saving package is in place According to the SmartWay Canada study trailer nose fairings like these OptiFlow Front Fairings from Wabco are the most effective way to reduce low pressure in the fifth wheel area and can yield fuel savings of around 422 gallons a year PHOTO WABCO