Impressive aero attention to detail

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.

 

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!!!

 

Looks like they fixed the oil change door:

 

The active air inlet vanes are good for ~2-3%. I ran a test on the Gen-3 a couple of years ago and scored a solid MPG improvement: Why mileage gets worse in winter | Page 5 | PriusChat








Bob Wilson

 

@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.

 

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.

 

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.

 

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

 

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

 

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

 

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

Bob Wilson

 

this is gen 3 prius?

 

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

Maybe the TRD kit



PRIUS -AERO PARTS- | TRD

Spoiler is nice

 

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

 

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

 

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.