Aerocivic - how to drop your Cd from 0.31 to 0.17

[M_a_t_t]

Racing has been a somewhat common topic on here. Since one of the main focuses on this forum is aerodynamics racing and speed are commonly discussed. Aerocivic has disclosed his calculated new top speed, I think it was like 150, somewhere in this thread or on his website.

There is pretty much always a negative pressure in the wake of the car. So maybe theoretically, but in the practical world I would say no.

The semi idea has already been proposed. They also already have flaps at the back of semi trucks for the same reason. I think freebeard has posted the images of the trailer with the inflatable tail. I might be mixing people up though.

The back side of the sphere is curved (naturally) and so it creates lift, which means an even lower pressure in the wake. Thus more drag. Thats my interpretation anyway.

Smaller displacement and not a high performance racing version is typically a good recipe for any car. You just don't ever see a McClaren with a smaller than stock engine. I would say the closest to a racecar with an "eco" trim is the first gen insight, maybe a few other, but mainly the insight. It's pretty light. 1900lbs with battery and everything. Has a pretty low Cd, .24 I think I've read around here. Something you may not be considering is the effect of downforce at speed. The lowest drag would have neutral, no lift and no downforce, the farther you get from neutral the more drag you have.

One of the biggest contributors to fuel economy (when we drive them, not the epa rating) is how you decelerate. I bet if you drove the McClaren in full ecomodder mode (EOC, etc.) you would atleast be close to a smart car. Not the hybrid version, the gasser.

[MetroMPG]

Originally Posted by CeeforCitroen

I've been thinking - fuel economy aside - these aerodynamic mods are going to make the car faster. Isn't it every boy racer's dream to own a fast Honda civic - far more than the drag inducing body kits and spoilers. Imagine this on a Type R. The Fast and The Furious would have nothing on it!

That's the issue though: no boy racer is ever going to covet the AeroCivic because its modifications don't make it look anything like the kinds of cars in the Fast and Furious!



Different = bad.

[aerohead]

Originally Posted by CeeforCitroen

I've been thinking - fuel economy aside - these aerodynamic mods are going to make the car faster. Isn't it every boy racer's dream to own a fast Honda civic - far more than the drag inducing body kits and spoilers. Imagine this on a Type R. The Fast and The Furious would have nothing on it! Now, before I get banned from the forum for suggesting anything as fuel wasting as racing. I will add that as the drag coefficient goes down, the stock gear ratios become unnecessarily low now you don't need to be so far up the power band to fight Aeolus.

I have also noticed that a lot of cars (usually hatch back like my Citroen C2) just stop suddenly at the back. The air just kind of clashes together like a judge's hammer on a bench instead of "pinching" the car forward. The front of the car parts the air which needs power, but the air gains some potential energy, and you can reclaim some of it from the back end.

Could not even semi trucks have some sort of fold out or inflatible pyramid where the back trailer doors are? It would tuck back in when in town to not make the vehicle longer than it needs to be.

Another thing that is confusing is an article on Wikipedia that lists drag coefficients but a hemisphere is displayed as a lower drag coefficient than a sphere. This would seem to be wrong to me unless the hemisphere is side on - but there was an arrow pointing to the round side so I'm assuming that was faced into the wind.

Another weird quirk is whenever a car company puts out an "eco" car which has the aerodynamics of a shed. Which do you think has the most drag? A Smart Fortwo or a McLaren F1? Is there a speed at which the McLaren F1 would get better fuel economy?

1) Hucho wrote in his textbook that, a 30% drag reduction would essentially produce a 10% increase in terminal velocity.
2) And rather than engineer more powerful ( heavier) brakes, the automaker would limit top speed with over-driven gearing, which would not allow the engine to reach torque and power of it's engine map, capable of exceeding a safe target velocity, while allowing the better mpg.
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3) The abrupt truncation of the car's body dates to the mid-1930s research of Fachsenfeld and Kamm at the FKFS, next door to Daimler-Benz. One simply takes the streamline body of the long-tail and chop off the tail wherever one likes, to satisfy ' practicality.'
4) For the distance allowed by the existing amount of tail, the aft-body airflow is provided a deceleration ramp, on which velocity is traded for increasing static pressure.
5) Wherever the tail is chopped, it remains at the highest possible pressure for that particular length.
6) This higher pressure imparts a higher base pressure, lower pressure drag, and lower overall drag. This IS by definition, 'STREAMLINING.'
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7) The Fachsenfeld' inflatable semi-trailer tail dates to circa 1935. It uses exactly the mechanism mentioned above to deliver a tractor trailer with a drag coefficient potential below Cd 0.20. The efficacy of the technology has been demonstrated over and over again. I've used it myself.
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8) Be careful of Wikipedia. None of there material is vetted, as one might imagine, as far as scientific rigor is concerned. Whatever drag table presented the drag coefficients you posted lacked caveats, with respect to Reynolds number effects. As a smooth sphere, a ballistic cannonball, will have a fraction of the drag of that of a ping-pong ball, all else being equal.
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9) As to the Smart Fortwo vs McLaren F1, ultimate fuel economy would include many metrics beyond that of aerodynamics. I've never seen a drag coefficient published by McLaren for the F1. Nor it's frontal area. At Cd 0.38 for the Smart Fortwo, and it's frontal area, it may be possible that the two cars are aerodynamically identical in CdA. If at same weight, rolling resistance, power, and mechanical efficiency, they might be of equal mpg. Road velocity would not be a factor.

[freebeard]

The front of the car parts the air which needs power, but the air gains some potential energy, and you can reclaim some of it from the back end.

You misspelled entropy.

[aerohead]

Originally Posted by freebeard

You misspelled entropy.

I believe that the use of potential 'energy' is correct usage, as it's contextually consistent with the Bernoulli Principle of equipotential energy of the flow environment.
By displacing the air, the passage of the car through the troposphere, potential energy of static pressure is converted to kinetic energy of velocity (as per the Bernoullu Equation) and all but surface-friction drag heating of the air ( unavoidable) can,potentially be recovered as pressure, if the the car is 'streamlined.'
The degree of body mutilation and 'non-streamlined' shape would directly affect ' entropy', as any flow lost to flow separation and turbulence is, by definition, direct heating of the troposphere through viscous attrition, an irreversible energy loss.

[freebeard]

Just being hard on the noob. [ ¯\_(ツ)_/¯ ] (sorry CeeforCitroen)

So you're saying.... we can both be right?

[CeeforCitroen]

I did find a link you might find interesting based on the semi truck idea
www(dot)yankodesign.com/2017/12/13/hey-elon-heres-an-idea-for-your-semi-trucks
Interestingly I was looking up about the Tesla Semi, because electric power has often been a challenge for long range vehicles. I'm surprised it's still only a single speed transmission as well because while electric motors generally get away with less gear range, as vehicle weight increases, a choice of ratios generally come back into the picture which is why a semi generally has more gears than a motorcycle. (I've drafted a write up of the efficiency of electric motors at different loadings and come to the conclusion that motors with high DC resistance and low mechanical friction should be lightly loaded, and vice versa for low resistance but high mechanical friction). I would be interested to know how the motor performs towing 80,000 lb up a hill, compared to coming back to the dock empty, because I find it hard to believe you would get optimal range using the same gear ratio for each task.
As for McLaren F1 fuel economy, usually with supercars and hypercars the efficiency island for the "over sized" engine puts you well over most speed limits. At which case the air drag is more than optimal if you can't cruise at peak efficiency until over 100 mph. For most city driving it would be like using the engine to run an electric toothbrush. A McLaren F1 body with a Smart Fortwo engine would do rather well - if a little boring to drive on a track. The other factor is the speed range the two cars are generally used.

It would seem to me though as speeds head north (possibly beyond the top speed of the Smart) that the McLaren would drink less petrol. Also whether the low mpg figures of these cars are measured puttering to the shops of caning it round the Nurburg Ring. Perhaps one of these cars should have a tiny engine to putter along to the track day and then switch over to the large engine to have some fun!

I'm curious as to where the fuel goes in some of these cars because 15 mpg US (18 UK) is what my Citroen C2 would get if I put it in 3rd or 4th, pushed the pedal to the floor and also put my other foot partway on the brake (or drove up a hill) enough to stop the car from accelerating. I mean a McLaren isn't exactly a 4 ton 2 lane wide garden shed, so where does the fuel go? Is the engine fighting itself? If I was to tow one with my Citroen with the McLaren in gear would the engine braking of the large engine cause my car to consume the same fuel? I'm assuming the "waste fuel" in an uneconomical large engine just goes to moving itself round - just like a 7 litre V8 geared to move a scooter at 30 mph at 4000 rpm is mainly fighting its own resistance and a tiny bit is used to move the scooter.

[freebeard]

Pressure drag. It goes up with the square of the velocity.

Before society got a stick up it's b*tt, you could drop a fiberglass 917 splash copy on a Beetle floorpan and get 0.40 to 0.30 drag reduction. Fine for traffic downtown if you can handle the ingress/egress and lack of stowage.

edit: You have five posts now, you can use the Link button: https://www.yankodesign.com/2017/12/...r-semi-trucks/

My proposal for that has a solid panel which makes the roof of the diffuser.

[CeeforCitroen]

I'm curious now as to whether a cyclist will go faster if they wear one of those on their back.
Here's a video of a lorry with one in action. It shouldn't be too hard for a fleet of trucks to save masses of diesel this way, though I have no idea what the savings will be at 55 mph

https://www.youtube.com/watch?v=SCKbQ8irR3A

[freebeard]

Cyclista have fastback helmets.

The designs you are finding are crippled by the need to interface with loading docks and the swing doors. The design I showed incorporates a diffuser. It could have fences and/or suction ports.

Originally Posted by basjoos

I sketched out a design for a removable boat tail that doubles as a cargo carrier that mounts on the back of the prius.

Like the Beatles.... We'd all love to see the plan.