GM disputed Prius Aerodynamics - any follow up?

This is actually encouraging. GM is intensively studying the Prius. We can all hope that they learn how to build better cars, starting with matching the capabilities of the Prius.

My guess is that both GM and Toyota are being honest. GM reports the results of their tests in their environment. Ditto Toyota. Until there is something like an EPA standard wind tunnel procedure, different results are to be expected because the tests are not exactly the same.

 

RobH,
I think you are right that GM is not outright lying, but I expect them to have the same effect with spin and ambiguity.

Question: If the EPA were to choose a test protocol for a standard, do you think they would turn the wheels (like Toyota) or keep them stationary (like GM) ?

 

It would probably be a computer simulation

There is a whole host of real world difficulties in doing wind tunnel testing and comparing the data from different tunnels. One is the cross sectional size of the tunnel compared to the size of the object being tested.

 

A wind tunnel test is a very artificial environment. The first thing I'd try would be different angles toward the wind. How do side winds impact the performance? Turbulence such as could be expected on a crowded highway? Different road surfaces?

Spinning the wheels is an obvious thing to test, but until it's tested we really can't say if it's worth having in a standard.

 

Toyota claims that the Prius is 0.25 but no information has been given on which wind tunnel that was tested and under what conditions.

From talking to the people who did the tests at GM, they tested the Volt and the Prius in their wind tunnel under the same conditions. The conclusion was the Prius had a drag coefficient of 0.29 while the volt measured 0.28 from memory. In both cases, the the wheels were not rotating and the ground was stationary (no moving belt).

This does not mean that 0.25 is a lie from Toyota. The drag on a vehicle is generally higher when testing without rotating wheels and with a stationary floor. So when the vehicle is moving down the road, the wheels are rotating and the wind is moving at the same relavent speed to the road. It could be the case that the Cd of the Prius on road 0.25. How Toyota measured this is not not known and they could be using computer simulations to calculate what this effect is. Other ways to test that could be to use coast down testing but it is very inaccurate. It is typical to see a 30-40 count reducing in Cd when a moving ground and the wheels are rotating in a simulation.

So in the case of the Prius verus the Volt, GM found the Cd on the Volt was lower for the same test conditions. This would then imply that the Cd of Volt would be lower in real world conditions as well. In any case, both are very aerodynamic vehicles

 

I'm sure the engineers who turned up the difference were offended by the claims from Toyota. The marketing people who heard about the results thought that it would be worthwhile to publicly challenge Toyota.

The engineers did their thing. The marketing people did their thing. Now we get to do our thing - weigh the claims. And my judgement is that it isn't worth worrying about. Both the Prius and the Volt are light years ahead of a Nissan Cube in terms of aerodynamics. And now I know that not all wind tunnel results are comparable.

 

Yup....The inmates run the asylum over there!....

 

That does say 23 mpg when i look at it.

 

I travel at lower speeds and aero drag goes as the square of the speed, so I've always figured that Cd isn't very important to me - as long as I have something a bit more aero than the Nissan Cube (I wonder what the Cd of the Cube is?). But this discussion started me thinking:

The Cd's GM measured in their wind tunnel were actually 0.30 for the Gen III Prius with 17" wheels and 0.28 for the Volt:
Chevy Volt’s Coefficient of Drag is 0.28, Beats Prius and Insight
Now let's hope that they made the correction for the larger frontal area of the Prius with the 17" wheels vs the 15" wheels. And let's assume that the Prius with 15" wheels would also have a Cd of 0.30 in the GM tunnel. And finally let's assume that the Prius and the Volt have the same frontal area (I have no idea).

If a Volt is driving at 65mph in a no-wind situation, how much slower would I have to drive my Prius Two (15" wheels!) to reduce my aero drag to the same level?

Aero drag force goes as the square of the speed, so I would have to ease off just slightly to 62.8mph, or 2.2mph slower.

But wait, what I really care about is mpg, not aero drag! Slowing down also reduces frictional losses in the engine and drivetrain, losses due to tire flex, and of course losses due to real-world slowing and accelerating due to traffic (slowing down means you don't have to vary your speed as much).

So to compensate in terms of mpg for the Prius having a Cd of 0.30 (according to GM) rather than the 0.28 of the Volt, I would have to slow down less than 2.2mph, how much less I don't know.

Before the Volt owners get all excited here, this part is not a comparison of Prius mpg with Volt mpg, it's determining how much a Cd of 0.30 versus 0.28 is likely to affect mpg of the Prius.

EDIT: Looked up the Nissan Cube, and the 2009 has a Cd of 0.35, much better than I expected. Not sure whose wind tunnel was used though!
[ame]http://en.wikipedia.org/wiki/Automobile_drag_coefficient[/ame]

 

Actually I think it's a plug for the Volt.

 

You have stumbled upon an example of the reason they use wind tunnels to determine Cd instead of taking a survey of car forum readers.

Rounded corners go along ways towards improving the aerodynamics of a brick.

 

As a former cube owner I can say while the design does have limitations at hwy speeds, its interior volume and urban "run-about-ness" were awesome.