I'm Baffled By an Old Automotive Engineering Mystery and I Need Some Help

I hope you’re ready to dive into some really timely content, getting to the bottom of an automotive design decision that hasn’t really been seen since, oh, 1930. But it’s one I’ve always been drawn to, and yet one I’m not sure I understand. It’s a design conceit usually called the coal scuttle bonnet, and it’s likely best known on Renaults from around 1904 to 1930.

The genesis of the design seems to come from very early Renaults—1900 to 1904 or so— that used side-mounted radiators positioned on an engine cover with a sloping front:

Later, around 1904, Renault started building cars with single full-size radiators, but mounted them behind the engine, acting as a sort of firewall/dashboard. The shape of the hood retained the “coal scuttle” look:

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Oh, it’s also probably worth mentioning just what a coal scuttle actually looked like. There’s a few different styles out there, but the kind that seems to resemble these old Renault hoods the best is one that looked like this, a little metal box with a sloping front door.

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These hoods do look sorta like those, I agree.

I should also mention that I love the look of these coal scuttle hood cars; they look oddly sleek and clean compared to so many other cars of this era. Take a look at this 1928 Renault 40 CV Type MC:

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I mean, that’s a fantastic look, right? And that alone may be why Renault stuck with this design. The mystery, though, is why would you design a car with the radiator behind the engine like that? Isn’t it terrible for cooling?

Look at this picture of Louis Renault himself in one of these cars:

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That’s a big car with a pretty big engine under that hood, and here’s how much cooling airflow the car gets:

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That’s not a lot. We’re looking at, what, 10 to 20 percent of that radiator’s frontal area is actually exposed to any airflow? Most of that radiator is trapped under the hood there with the hot engine, and those side grilles cant be getting nearly as much airflow as a front-facing grille.

Why did they choose to do it this way? It seems so counter-intuitive.

I can think of a few possibilities:

• Styling

• Aerodynamics

• Packaging

• Using the radiator for cabin heat

• They just didn’t need much airflow back then

Styling could be the reason, and that could be enough. I’m skeptical of aero, because, well, look at the rest of that car, for example. It’s basically got an open parachute on it.

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Packaging is a thing, sure, and other cars with behind-the-engine radiators certainly did it for those reasons. Look at the Fiat Topolino, for example:

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...or the DKW:

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...but even in these cases the internal configuration still leaves a decent amount of exposed area for the radiator to get airflow from the grille—a grille these old Renaults lack.

Plus, the size of most of these old Renaults (and the other various makes that used a similar layout, like, um, Charron or Arrol-Johnston) meant that having to cram a radiator into a small area wasn’t really an issue.

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As far as using the close-by radiator to heat the cabin, I have yet to find evidence that this method was actually used. I’m hoping I’ll find something yet that confirms it, though, because I love the idea.

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Of course, these cars did actually work. Early Renaults were even prized for their durability. “The car that will last forever,” Renault’s ads billed back in 1908. “Renault was making cars so sturdy that they were all but incapable of wearing out,” Automobile Quarterly remembered in a glowing retrospective published in 1963. That’s Volume 1, Issue 4, if you want to look it up. So maybe airflow just wasn’t that big a deal when you’re going 30 MPH with minimal traffic to cause you to idle in the heat?

I reached out to Renault, to see if anyone in its heritage department might be able to give me some insight, but, shockingly, they didn’t respond to this pressing matter.

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I guess this is all to say that I’m opening it up to you, the smartest, smelliest group of automotive minds in the world: Why did Renault (and the other coal scuttle bonnet builders) make this decision? Is there some advantage I’m overlooking? Are the perceived limitations of airflow and cooling just less of a big deal than I think?

Is it all about style? If so, I’m okay with that, as I do love the style. I just want to know more. I want to know why these were a thing for so long, at least for Renault.

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Maybe this article will goose them into responding. Until then, let’s speculate, wildly!

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About the author

Jason Torchinsky

Senior Editor, Jalopnik • Running: 1973 VW Beetle, 2006 Scion xB, 1990 Nissan Pao, 1991 Yugo GV Plus • Not-so-running: 1973 Reliant Scimitar, 1977 Dodge Tioga RV (also, buy my book!)