Testing a Smooth Engine Undertray

When I investigated the effects of a splitter on my Prius, I discovered something unusual: gauge pressures on the stock engine undertray were a lot higher than I expected. Julian Edgar’s Vehicle Aerodynamics: Testing, Modification & Development includes several examples of engine undertrays with measured pressures much less than atmospheric. My test showed that the Prius undertray was developing pressures at atmospheric or higher. What was going on?

Gauge pressure at 80 kph. Left: no splitter. Center: with splitter. Right: difference.

Hypothesis and Testing
 
So, I’ve got a problem here I want to investigate: high pressures on the engine undertray where most examples I’ve read about have lower pressures. Where to begin?
 
When you investigate something like this, a good place to start is fundamental principles. I know that velocity and pressure are related, and that as pressure goes down, velocity goes up. So the velocity under my car’s engine undertray must be slower than on other cars I’ve read about. I also know that faster velocity and lower pressures on the engine undertray will contribute to reduced front lift—so I want lower pressures there. Can I modify the car in such a way that I can achieve this?
 
As a second step, I’ll compare my car with others. For example, here are the stock undertrays on some cars built within the last few years:

2023 Honda HR-V

2023 Nissan Ariya

2023 Volkswagen ID4

2024 Ford Mustang GT

And here’s mine, the stock undertray on a 3^rd-generation Prius:

There’s quite a bit of difference in the smoothness of these trays: mine is significantly rougher and has more openings, and this roughness might explain the high pressures there compared with smooth trays.
 
I know from other testing I’ve done and from reading a lot that one way to get lower pressures under a car is to fit an air dam (front spoiler)—and that’s a potential solution here (in fact, you can see two small spoilers on the HR-V undertray above). But, I also know from my splitter testing that reducing the volume of air flowing under my car has unwanted effects on the diffuser pressures, lowers pressure on the hood (increasing lift), and doesn’t change engine undertray pressures much. Additionally, adding an air dam will increase the car’s frontal area and may not change drag area much if at all.  I think instead the way to go here is to test a smooth panel under the engine:

Results
 
I measured pressures on the tray centerline at the front, center, and rear of the added Coroplast panel (it was taped completely for testing; the picture above is just a mockup in my garage). Pressures were measured again in the same locations with no panel. Since it’s January and a lot colder than my summer testing of the splitter, pressures here are more exacerbated (colder air is denser, and the dynamic pressures are a function of density). The results at 80 kph, averaged from two directions to account for wind:
 

	Stock Undertray	Smooth Undertray	Difference
Front	+20 Pa	0 Pa	-20 Pa
Center	+50 Pa	+ 20 Pa	-30 Pa
Rear	+40 Pa	-10 Pa	-50 Pa

 
That’s quite a difference from simply smoothing part of the undertray! And it turns out my hypothesis is correct: a smoother undertray on my car develops lower pressure than the bumpy stock tray.
 
Build
 
Since I had to jack the car up anyway to rotate tires as part of its 145,000-mile service, I decided to build a smooth tray. I used the same HDPE sheet I added to my truck as an air dam. HDPE, or high-density polyethylene, is the same polymer used as plastic wrap for food—and the base molecule, or monomer, is the same as in milk jugs in the US, which are typically made of low-density polyethylene. LDPE is what we call a “linear” polymer; the long molecular chains aren’t linked together, and the resulting material has a lower melting temperature and is less strong than HDPE, which has branching and secondary bonds holding the chains together (“crosslinking”). For the same dimensions, HDPE is a stronger material than LDPE, will withstand greater temperature extremes than LDPE, and is readily available in long rolls specifically for building air dams.
 
Since I’ve got a stock undertray that is robustly factory mounted, I decided the easiest thing to do here is simply bolt an HDPE panel to it. While I was at it, I decided to extend the panel on each side in front of the wheel spoilers, across nearly the full width of the car:

While I was under there, I added a bracket to ensure the factory spoiler holds up. I had to stitch all that together after a run-in with a raccoon in Indiana several years ago; rather than send the bumper cover, spoiler, and grill to a landfill and spend thousands of dollars on what amounts to cosmetic damage, I decided at the time to simply repair it, first with zip ties and then fiberglass. It’s held up now for 6-7 years, so I think that was the right call.
 
As with a lot of effective aero mods, the new tray is completely invisible to most observers.