EV camping tech talk
A data driven insight into EV tow vehicle camping, its potential and limitations, with currently available options.
By Michael Zutech, Houston Electric Auto Association
The goal of this piece is to provide data driven insight into EV tow vehicle camping, its potential and limitations, with currently available options. The tow vehicle is a 2018 Tesla X100D, and the camper is a 16’ Casita fiberglass shell camper, chosen for it light 2,165 lb dry weight and aerodynamically favorable rounded external shape. The X is the big SUV Tesla model, but its low 0.24 drag coefficient means its wake is minimized, and only the lower part of the camper gets a wake benefit - the whole top is exposed to full speed airflow, and adds a lot of aerodynamic drag.
In selecting the Casita, we considered pop- up and A-frame camper designs, which would lie more fully within the Tesla wake, giving greater range. For us, it was more important to have minimum set-up time, which is really nice when arriving at a site late, or in bad weather. We wanted full features, including internal head/shower, frig, stove, sink, A/C, table, bed and so on, so accepted higher energy use to get it. Note however that the design does include a curved flush door that lessens drag, and avoids the large roof top heat pump that is so common, but precludes good airflow on
the camper top, so it’s a pretty good design as fixed shell campers go.
Before purchasing the Casita, I made a spreadsheet model estimating the rolling and air resistance of the Tesla and various campers to help the selection process. Good data for the Tesla was available via EPA filings, but I had to make estimates for the campers. Fortunately my last four decades have been spent as a designer/researcher in wind turbine rotors, so the low speed aerodynamics background needed for that task was already well established. Measured energy use data over a two month road trip with 3,805 miles of towing have now been used to calibrate the initial estimates closer to reality. The rig as shown, with visible wheel well skirts and hard to see aft vortex generators (VGs) (for more info see http:// airtab.com) is good for about 1.8 mi/kWh at 60 mph with no wind, based on those best values from 3,805 miles of towing. Calculated values of drag and range when combined with the Tesla X, and for the Casita alone, follow on the next page. If anyone has access to better values for any of the data, I’d sure like to get it.
One frequently asked question was whether they weren’t on backward –— shouldn’t the pointy end face into the wind? To understand why not requires an explanation of how they work. Their function is to scoop away airflow slowed by trailer surface disturbance, so it gets replaced by faster moving air from outside what is called the “boundary layer”. Of course it creates some drag to do this, so that raises a further question, of how that lowers drag? The answer is that the faster moving flow sticks to the curve of the afterbody more strongly, follows the curve further, and results in a smaller vehicle wake. Making the wake smaller is like fooling the air into thinking the trailer was smaller, and that reduces drag a lot more than the VGs add, so you can come out a few percent ahead overall, though with a curved afterbody like the Casita it can be more.
VGs in a line also create what’s known as a “vortex sheet” that better separates the free stream airflow from the vehicle wake, allowing it to extend a little further aft, which reduces drag from turbulent mixing across the boundary —net effect is like a small afterbody extension, which also reduces drag. This is why VGs can work even on squared of trailer backs.
The photo right, shows flow indicating streamers, that were used before departure to see how well the VGs were working. The photo below right was taken from a trailing EV, and shows that they are working well across the camper top, but poorly along the sides.
Across the top, the streamers are stuck to the curved surfaces, showing excellent wake size reduction, while along the sides, many of them are disturbed. The ones behind the side windows are shown to be a particular problem, not surprising since the airflow must come up over the molding, then pass across a depression, since the aft part of the window is inset to slide open inside the front part. I’ve now added an acrylic pane to bridge the depression, and mounted VGs on it so they protrude into the flow to see it that can fix the difficulty.
The bad news is some aspects of drag reduction are hard, but the good news is that more reduction is possible, including things like underbody and wheel fairings, and aft sunshade that’s also a fairing and solar panel mounting area, and if the sides are otherwise intractable, side mounted deflector vanes. Such things may be topics for another time, after testing.