Impressive aero attention to detail

Discussion in 'Gen 4 Prius Main Forum' started by DieselHybrid, Mar 2, 2016.

  1. DieselHybrid

    DieselHybrid Junior Member

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    As an aero engineer I truly appreciate the effort Toyota has expended with their '16 Prius.
    Toyota spent a great deal of time in their rolling wheel wind tunnel to achieve a claimed Cd of 0.24.

    Here are some shots of the under-appreciated aero details found on the vehicle's underside.
    In sharp contrast, I looked at the underside of a F-150 & was dismayed at the complete disregard for vehicle aerodynamics.

    Come on people! In terms of reducing aero drag, underside work is really low hanging fruit!


    IMG_0724.jpg IMG_0725.jpg IMG_0723.jpg Prius_2016WindTunnel-1200.jpg

    In the near future, I hope all cars/trucks will have the '16 Prius' level of underside detail to reduce aero drag.

    Notes for each picture:
    Pic 1. Forward underside detail. Underbody panels cover all but the exhaust tunnel. Nicely done!
    Pic 2. General airfoil-shaped exhaust helps contribute to upslope of diffuser-like rear complete with vortex channels. Also note- independent rear suspension!
    Pic 3. Close-up of NACA-like ducts with vortex channels- yet with no openings. Purpose unclear? Also note flattened aluminum chassis braces. Weight & Aero conscious- nice touch!
    Pic 4. Toyota press-release pic of Prius in their rolling wind tunnel. Note rotating wheels! Also note- nicely reduced tire/wheel gaps! After-market lowering springs should reduce the quite large gaps present in stock Prii nicely.
     
    #1 DieselHybrid, Mar 2, 2016
    Last edited: Mar 2, 2016
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  2. CR94

    CR94 Senior Member

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    It's surprising that all that attention to aerodynamic detail produced such a tiny improvement in drag coefficient vs. the preceding generation. Reducing the height helped as much or more in reducing drag.
     
  3. JohnF

    JohnF Active Member

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    I'm all in favor of improving aerodynamics, as long as it does not unduly impair functionality or add a lot of weight or cost.

    But it's important to keep aero in perspective:
    - The Cd is a sort of first order approximation of aero drag (actually form drag, there is also surface drag). The theory assumes the object is moving through still air, whereas in the real world the car may experience winds from directions other than straight ahead. The apparent wind direction may change as the roadway curves. There will certainly be vortices and eddies thanks to other traffic, especially semis, and wind bouncing off features of the passing landscape. These will disturb the airflow over the car's body. Incidentally, the situation is much simpler for a plane, because its much higher speed means that drag calculations can assume that the air is effectively passing over the plane from dead ahead.
    - Even in this simplified model, total form drag is Cd x effective frontal area. So reducing the Cd from 0.25 to 0.24 is equivalent to reducing the frontal area by roughly 4%.
    - Reducing aero drag by 4% does not increase mpg by 4% because aero drag is only one of the energy costs of moving a car down the road. The simple model predicts that drag increases as the square of the speed, so it becomes a larger factor at higher speeds: frictional losses in the engine/drive train and tire flex losses apparently increase less rapidly with speed.
    - Because drag increases as the square of the speed, simply slowing down slightly can have a far greater effect on mpg than improved aero. Wayne Gerdes on cleanmpg published a speed vs mpg plot for the 2016 Prius ECO. His data shows an increase in mpg from 56 to 64 simply by slowing down from 65mph to 60mph. That's roughly 13% improvement. Incidentally, at 55mph it goes up to 72mpg, and the gains from slowing down further decrease, as you would expect.

    So it seems to me that slowing down is really the low hanging fruit here.

    But I still like good aero!!!
     
    #3 JohnF, Mar 3, 2016
    Last edited: Mar 3, 2016
  4. Mendel Leisk

    Mendel Leisk Underfoot

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    Looks like they fixed the oil change door:

    Capture.JPG
     
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  5. DieselHybrid

    DieselHybrid Junior Member

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    JohnF-

    Agree with everything you said, but good luck trying to get any metropolitan drivers to slow down (not to mention stop texting & driving!) :) Drive slow in the DC area & you're liable to get shot in a road rage incident! Humans behind the wheel can often be dangerous & unpredictable.

    In the absence of a paradigm shift in people's driving habits (or better yet, autonomy that removes the unpredictable human element out of the equation entirely) cleaning up a vehicle's underside is pretty low hanging fruit. Just one glance at the underside of the #1 selling vehicle in the US (Ford F-150) will convince you that so much more can be done.

    Other low-hanging fruit would include replacing the dreadful side mirrors with conformal cameras- as Tesla has unsuccessfully lobbied lawmakers to consider. Luddites, money, & politics often stand in the way of progress.

    While the claimed Cd = 0.24 of the new Prius may not sound like a big improvement over the Gen 3's (contested & challenged) claim of Cd = 0.25, Toyota has entered an area of diminishing returns. Heck, 0.24 is what the Tesla Model S claims (in a vehicle that does not have conventional engine heat radiating related drag)

    Dr. Teddy Woll, director of vehicle aerodynamics at Daimler, opines that "a Cd of 0.2 represents the 'sound barrier' for production cars."
    Ref: "A Drag Coefficient of 0.2 Represents the Sound Barrier" - ATZ online

    To be fair, the Gen 4 Prius is a significant step ahead of the commendable Gen 3 wrt aerodynamics.
    Based on my frontal area estimates (& Toyota's claimed yet unverified Cd's):
    Gen 3 Cd*A = 0.25 * 25.97ft^2 = 6.493ft^2
    Gen 4 Cd*A = 0.24 * 26.05^ft2 = 6.252ft^2 which amounts to a significant ~3.71% reduction (if true)

    (Note to CR94: even though the Gen 4's ride height has been reduced, the overall frontal area of the Gen 4 is marginally larger than the Gen 3. If Toyota had not reduced Cd (as they have claimed)- the Gen 3 would have had an effectively lower Cd*A than the Gen 4! Conversely, a Gen 3 owner could potentially achieve a lower Cd*A than the new Gen 4 by simply lowering the ride height (1.2" lowering springs).
    Which happens to be my 1st planned mod for my Gen 4!

    But, as JohnF has correctly pointed out, how often do cars experience clean, head-on free stream conditions?
    It would be interesting to test vehicles at different yaw angles in a wind tunnel.

    Largest low-hanging fruit:
    Autonomous drafting could allow for perhaps the most significant reduction in vehicular drag. Consider migrating geese, or bicycle pelotons- except with autonomous driving cars in close proximity... sure there will be huge technological (& political) obstacles to overcome, but imagine the incredible potential!

    Until then, please slow down, please get off your devices, & drive safe!
     
    #5 DieselHybrid, Mar 3, 2016
    Last edited: Mar 3, 2016
  6. bwilson4web

    bwilson4web BMW i3 and Model 3

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    #6 bwilson4web, Mar 3, 2016
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  7. JohnF

    JohnF Active Member

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    @DieselHybrid:
    I've given up trying to get other drivers to do anything, certainly not get them to slow down! On the highways I just stay in the right lane (unless there is a jam) at 50-60mph, often on cruise control, and MYOB. Much of my driving is on the coastal corridor between eastern CT and NYC, and on into NJ, which is fairly "metropolitan". I've been driving this way for nearly 10 years, no problems. Much of I-95 through NYC is zoned at 50 or 55, which slows traffic down to 60-65 maybe. Perhaps drivers in this area are more considerate (I never thought I'd say that!).

    As for autonomous, did anyone notice the news on the Google car accident this week? It got confused by some sandbags on the shoulder and ran into a bus. And the GPS maps that directed people into bodies of water, etc. Just think of the opportunities for hackers to write viruses. "Trust your life to the internet, you'll be fine!"

    Autonomous drafting? Only if debris in the road and truck tire blowouts are also autonomously controlled. In my younger days I got a
    LOT of experience drafting in pelotons. And drafting on my bicycle behind scooters, mopeds, semis, school busses. I learned that you have to be really really close: inches behind a bicycle, and at most 2 feet behind a semi trailer. Any further behind a semi and the blast of air coming down off the roof hits you and pushes you out of the pocket. Forget drafting cars, there's no useful draft behind them.
     
  8. thunderstruck

    thunderstruck Active Member

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    Best trucks to draft behind are the tanker trucks. I've always had better results with them which seems to be due to the rounded shape of the back of the truck. I'd just watch my GPH on Scangauge to figure when I was in the sweet spot since consumption would drop substantially. Even without drafting I've been able to achieve good results in my car (GTI, considering Prius) by just slowing down a little.
     
  9. DieselHybrid

    DieselHybrid Junior Member

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    A bit off topic, but what the hey...
    Having been a cyclist for +30yrs I have become very aware of energy use & the benefits of drafting.

    Mercedes & Volvo & others have been working on semi-autonomous truck drafting with adaptive cruise control for years. Here are a couple cool videos on wirelessly-linked semi-autonomous truck drafting:
    Peloton Technology

    Clearly the highest drag reduction occurs at Nascar-like bumper-to-bumper distances. However, drafting distances don't have to be as close as you mention.

    As Peloton demonstrates in their second video, there are tangible fuel economy benefits for tractor-trailers at greater drafting distances (in this case 12m results in a 4.5% lower fuel burn for the lead & 10% lower fuel burn for the wing). I'm glad to see there are like-minded folks out there trying to reduce our consumption of limited resources (btw, love the name of the company: Peloton-Tech)

    Terms such as "Platooning," "Collective Action," "Future Convoying" - all refer to interlinked vehicles safely drafting at distances from 12m- to as little as 3m. Pretty cool!
    Platooning: The future of freeways is lining up - CNET
    BBC - Future - Robot truck platoons roll forward

    Driving is mostly a 2-D problem. If geese (with the brain the size of a pea) can migrate vast distances via precisely positioning in the vortex wake updraft of leading geese (in 3-D space)- I'm sure engineers could soon provide a mainstream solution for semi-autonomous trucks & cars.

    All this will require overcoming many technical & legal obstacles- but it could potentially make driving much safer. Semi-autonomy will be the first step. Both will be required to help relieve congestion & make transportation safer. We have a lot of idiots on the road in the DC area & nearly nightly news reports of road rage incidents. Unless our drivers develop the sense (& courtesy) of a goose- we'll need autonomy.

    Drive safe!
     
    #9 DieselHybrid, Mar 3, 2016
    Last edited: Mar 3, 2016
  10. JohnF

    JohnF Active Member

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    From my "research", I would say the best draft pockets are behind something that goes all the way down to the ground. Moving company semis, some of UPS's double trailers, mobile homes, etc. You don't get the massive blast of air flowing UNDER the trailer box. Possibly the new aero aids, like skirts under the trailer, would help a "draftee" - I have no data on them. I never had much luck with tanker trailers.** On a bicycle, you can feel the air currents very well.

    For a different reason, cattle trucks are a poor choice, as are reefers defrosting their trailers on the road after offloading their freight.

    Drafting a semi with a car is really dangerous if you are close enough to be in the pocket (and pointless if you are not). I tried it once for 30 minutes in my Subaru going as close as I dared and the ScanGauge showed no improvement in mpg. I suspect any reduction in drag is offset by all the accelerator adjustments needed to stay the same distance behind the trailer as it changes speed due to traffic and hills. And the mental effort is exhausting.

    **- Part of the problem is that it's wiser not to be directly behind a semi, especially on a bicycle. It's better to offset to one side (the left side if you're drafting in a car) so you can see a little of what's coming up ahead, react a little sooner, and flick around the trailer if is slows suddenly. If you're towards the edge of a tanker trailer, you are in the blast of wind coming down the side.

    Merged.

    All of this seems like reinventing the wheel. We already have an autonomous drafting system that's well worked out and rather safe (though safety is always a work in progress). Drafting is at roughly 2 meters. Metal tracks take care of guidance. It's called a "train". Modern adoption of ship/train/truck containerization has largely eliminated reloading of freight for distribution.

    As for moving people, trains, subways/light rail, buses, trams are all very aero and fuel efficient compared with moving the same number of people in cars, 1 person to a car. Car congestion is a huge problem in our metro areas in so many ways, and autonomous cars seem unlikely to solve traffic and parking issues. When flying low in a landing pattern over a big city, I often marvel at how much of its area is pavement.

    As for driving, it's a 3-D problem, not just 2-D, certainly for fuel efficiency and probably also for safety. For example, I have two choices when heading to NYC: I-95 or the Merritt Parkway. Barring traffic tie-ups, I always get great mpg on I-95 because it is gently rolling terrain, whereas mpg suffers on the Merritt due to steep sharp hills: lots of load going up, can't recoup that on the downhills (either recapture it inefficiently by regeneration or speed up and burn energy to aero drag). The two routes look similar in 2D.

    There have been lots of exciting ideas for vehicles that for various reasons just have not worked out. Planes that can morph into cars, for example. I think autonomous control is one of those. Driving has become one of those activities like washing dishes or doing laundry that some people (not including me!) just don't want to expend any concentration on, and therefore don't pay attention and are a hazard. I think a better solution would be to divert the effort that is being devoted to autonomous control to improving the functionality of public transportation and expanding it. After all, when you are riding on a train or bus your mind is just as free to text, phone, surf, or read as it would be in an autonomously controlled car, minus the risk that a dropout of the internet, computer freeze, or virus will end ALL activities for you.
     
    #10 JohnF, Mar 4, 2016
    Last edited by a moderator: Mar 5, 2016
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  11. DieselHybrid

    DieselHybrid Junior Member

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    Very interesting rants on the dangerous practice of drafting, & the benefits of public transportation...

    btw the rolling resistance of steel on steel (trains on rails) is insanely low. When it comes to energy efficient transportation, it is impossible to beat trains & bicycles. Trains work great in socialized Europe (where cities are closer & smaller)- but not so much in the suburbs & larger distances of the USA. But, if you can find a way for more Americans to abandon the 'burbs & make better use of public transportation- good on you! It just seems folks have been lulled into wasteful ways of living in this country. One of the curses of cheap energy? Anyway...

    As the OP I feel the need to bring this back on topic:

    Notice the last picture of the yellowish '16 Prius in the wind tunnel? The massive wheel gaps are pretty much gone- either through mounting the car low in the tunnel or through the use of lowering springs- I don't know.

    Either way, reducing the wheel gaps must help lower the Cd enough for Toyota to actually run their Prius in the tunnel in that configuration (at least their marketing picture demonstrates this). Additionally, lowering springs would help reduce frontal area (Cd*A). Even a small reduction should translate into increased FE at hwy speeds. Every little bit counts for us hyper-milers.

    I'm searching for lowering springs for the '16 Prius in an effort to improve (or match) Toyota's claimed Cd = 0.24.
    If anyone finds them, please let me know!

    Thanks in advance.
     
  12. NorthernHSD

    NorthernHSD Member

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    Interesting video related to this topic. Seems that there are other factors too than just looking claimed Cd numbers.

     
  13. krmcg

    krmcg Lowered Blizzard Pearl Beauty

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    I'm also working the springs. I'll let you know what I find.
     
  14. Tideland2005

    Tideland2005 Member

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    It would be interesting to measure the aerodynamic efficiency of the Gen 4 vs the Gen 3 by
    70-0 coast down tests on level road.

    Thanks Everyone for this very good thread.

    Steve
     
  15. bwilson4web

    bwilson4web BMW i3 and Model 3

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    In the "Test Car Database" at EPA.gov.

    Bob Wilson
     
  16. bisco

    bisco cookie crumbler

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    this is gen 3 prius?
     
  17. cjecpa

    cjecpa Member

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    Wonder how much better you could get the air flow with lowering and wheels?

    Maybe the TRD kit

    [​IMG]

    PRIUS -AERO PARTS- | TRD

    Spoiler is nice

    [​IMG]
     
  18. bisco

    bisco cookie crumbler

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    wouldn't making the nose a little pointier and losing the fog light scoops help?
     
  19. bwilson4web

    bwilson4web BMW i3 and Model 3

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    IMHO, no. A pointer noise doesn't really solve any problems at these speeds and might lead to some nasty flow separation problems 'in real life.' Then there is the problem with parking when you can't tell where the front is. The fog light effects too little air flow. However, there may be some better 'low hanging fruit.'

    I've long felt the bumper air inlet should be more under the bumper, possibly with a scoop that articulates yet does not break upon hitting road debris. Articulating so it can be fully closed yet open enough to handle mountain climbing from Death Valley with a tail wind. But what to do with the waste heat air. I'm thinking the front wheel wells with possibly a flairing to minimize mixing with the side-flow air.

    I also like rear wheel well covers to minimize turbulent air leak out the side.

    Bob Wilson
     
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  20. Topp Cat

    Topp Cat Member

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    Gen 4 is 1 inch lower drop the gen3 CD. number be almost the same! It be better if Prius uses the setup in the Jetta hybrid where it completely dis engages the drive train when coasting! Freeway On the same route from S.F. To Las Vegas , Jetta Hybrid, Chevy Volt and Prius one stretch its a good 12-14 miles down hill the Jetta coasts 75+mph no assist , where the volt and Prius need assist to keep 70 mph.
     
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