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Old 09-14-2015, 09:55 AM   #1 (permalink)
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Why hydrogen powered cars will have poor drag coefficients

Why? Well, have a gander at the gaping maw on this thing:


(Toyota Mirai hydrogen vehicle: a face only a mother could love)

Hydrogen vehicles need big grilles for cooling and also to take in air to process in the fuel cell:

Quote:
...cooling is a challenge, because fuel cells run at far lower temperatures--perhaps below 100 degrees Centigrade--than do internal-combustion engines.

The huge temperature differential between ambient air and the temperature of the engine coolant means [IC] engines are easy to cool: You can shed heat more easily from a given amount of airflow.

To shed heat from the [fuel cell] stack coolant at the lower temperature differential requires a greater amount of airflow--hence the large cooling inlets.
Source: 2016 Toyota Mirai Hydrogen Fuel-Cell Car: A Few Things We Noticed

And the Mirai's drag coefficient?

0.293

Oof! Not exactly class-leading. In fact, pretty far from class-leading.

One of the great things about (battery) electric cars is they don't need big cooling systems or exhaust systems, so you can dramatically cut drag using smooth underbodies and no (or much smaller, or fake) grilles.

Tesla Model S is a good example:



The Model S actually has a pretty big grille on its snout, but it's fake. It's just there for styling.

The car's 0.24 drag coefficient is actually top of the class for a production vehicle.

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Old 09-14-2015, 12:43 PM   #2 (permalink)
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One of the dirty secrets of FCEV's is they are quite intolerant of heat. And while they are more efficient than an ICE (which is damning with faint praise!), they are a lot less efficient than EV's. So, the heat they generate has to be shed much more effectively than we are used to.

I am not remembering the temperature limit for FCEV's, but it is surprisingly low - and this requires the very large cooling intakes on the Mirai.

So, the aero penalty for cooling on ICE's is roughly 10% (if I remember correctly), and it would be higher for FCEV's.
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Old 09-14-2015, 12:44 PM   #3 (permalink)
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I can't see the word "maw" without immediately thinking of the original Halo.

Actually, I had been wondering why the Mirai had such large grilles and horrible drag coefficient. Now I'm curious what it looks like under the hood.

Edit: Oh...after seeing that interior picture, it's clearer how much Toyota is aping the Mirai with the new Prius. You would think they would have taken the universal criticism of the Mirai's looks to heart.
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Old 09-14-2015, 01:12 PM   #4 (permalink)
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Whoa--also, there's this:

"Over 400 miles of driving, the Mirai averaged 56 miles per kilogram of hydrogen, or 57 MPGe."

Are hydrogen cars typically that inefficient?
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Old 09-14-2015, 01:40 PM   #5 (permalink)
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Hydrogen is a red herring. The sooner we ignore it, the sooner we get to real solutions.
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Old 09-14-2015, 02:12 PM   #6 (permalink)
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Heat sink hood?
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Old 09-14-2015, 02:29 PM   #7 (permalink)
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Quote:
Originally Posted by Vman455 View Post
Whoa--also, there's this:

"Over 400 miles of driving, the Mirai averaged 56 miles per kilogram of hydrogen, or 57 MPGe."

Are hydrogen cars typically that inefficient?
That does appear to be pretty typical. But what's really interesting is that the next generation Prius will likely be just as efficient as the Mirai.

Why would anyone choose the Mirai over the Prius?

The only advantage is tailpipe emissions, but then you could simply get the PHEV version and be in a win-win scenario.
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Old 09-14-2015, 05:35 PM   #8 (permalink)
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Quote:
Originally Posted by ksa8907 View Post
Heat sink hood?
Maybe somebody at Toyota should look into the aeronautical history books for the British Supermarine racer planes that used flush-mounted skin radiator tubes along the fuselage sides as radiators?

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Old 09-15-2015, 12:34 PM   #9 (permalink)
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Quote:
Originally Posted by Vman455 View Post
Whoa--also, there's this:

"Over 400 miles of driving, the Mirai averaged 56 miles per kilogram of hydrogen, or 57 MPGe."

Are hydrogen cars typically that inefficient?
Yes. They are lousy. There are TWO compressors onboard the Mirai - which is just bizarre.
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Old 09-15-2015, 05:13 PM   #10 (permalink)
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flush-mounted tubes

Quote:
Originally Posted by Old Tele man View Post
Maybe somebody at Toyota should look into the aeronautical history books for the British Supermarine racer planes that used flush-mounted skin radiator tubes along the fuselage sides as radiators?

I think Isuzu is doing this with their 'Glider' semi tractor.

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