This 2020 summer is great for aero mods. It seldom rains, climate is warm. Not too much work to do around the house, so lots of time to work on the car.
I picked the idea of a rear diffuser extension from a SAE publication about a movable extension that could increase fuel economy.
See:
Kang, S., Cho, J., Jun, S., Park, H. and Song, K., "A Study of an Active Rear Diffuser Device for Aerodynamics Drag Reduction of Automobiles", SAE International. 2012-01-0173, 2012, doi: 10.4271/2012-01-0173
This work is also cited with details in:
Umut Aktas Christian Abdallah, 2017, Aerodynamics Concept Study
of Electric Vehicles, Drag Reduction and Range Increase, Master thesis, Chalmers University of Technology, Gothenburg, Sweden, 2017
A movable diffuser extension relative to vehicle speed is probably not doable on a production car. Neither, is it practicable on an older car. So, I decided to try a fixed diffuser extension. What the heck, when I have to reach for something in the trunk, my legs never touch or come close to the bumper cover. So, there is room for some appendage at the back of the car that extends from the diffuser and under the bumper cover. Picking some stuff in the trunk, I measured that my knees were at least at 20 centimeters (8 inches) from the bumper cover. Kang et Al, 2012, obtained drag reductions of 5,8 % and 5,9 % at 80 and 100 Km/h respectively. Over than 1 or even 2 % drag reduction by a passive mean is worth an inquiry. The best diffuser extension was the whole rear bumper cover width and having rounded corners.
The extension I fabricated is made of 3/16 in (4 mm) choroplast. To make sure the extension would not bend down and sag, 6 little ¾ in (18 mm) plastic corner moldings 4 in. long (100 mm) were glued perpendicular to the material creases at 8 inches center apart (200 mm). The plastic corners are not angled but about perpendicular to the rear extension line. The extension is 48 in. long and centered on the car width. It also falls nicely between the rear wheels "boat tails" as I did not want the extension to interfere.
For good measure, the extension is painted black as the car is that colour. The extension was then affixed under the bumper cover with self taping screws on two rows where feasible to strengthen against a possible bend. See pics.
The results are presented in the pictured table. The average speed increase measured with the “throttle stop” method is 3 Km/h in the speed range of 105 to 115 Km/h. The fuel economy is almost in average 0,1 L/100 km (0.6 mpg). The drag reduction being proportional to the before-after speed squared is averaging at 5,34 % which is lower than the Kang et al, 2012, results but not by far for speeds that are superior to 100 Km/h (62 mph). For a speed of 120 Km/h (75 mph), Kang et al, 2012, got a 4,5 % drag reduction.
If this diffuser extension resists the wear and tear of weathering by wind and rain and other physical damages, it will remain on the car. Some damage is already done to it by the exhaust pipe hot gases. The extension opening facing the tail pipe will be enlarged hoping that the damage can be limited.
