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-   -   New Diffusers (https://ecomodder.com/forum/showthread.php/new-diffusers-37728.html)

fastflyer 08-05-2019 09:57 AM

New Diffusers
 
For several days I have been seeing cars with this type of "Diffuser". And I would like to know if someone understands its operation or knows of any study. Well, I've only seen it in modern German cars.

https://www.diariomotor.com/imagenes...eba-mdm-05.jpg

https://www.diariomotor.com/imagenes...-prueba-33.jpg

https://www.diariomotor.com/imagenes...eba-mdm-01.jpg

Pics from www.diariomotor.com

Vman455 08-05-2019 10:45 AM

Everything you can see there is cosmetic. A diffuser is a panel under the car angled so that it forms one side of a divergent duct with the road surface the other. There's no way of knowing if this car has a diffuser without seeing underneath it....

Snax 08-05-2019 12:08 PM

Diffusers are also largely considered ineffective if airflow under the car is also not smoothly managed. I.e., turbulence from under the car remains turbulence behind it, but if smooth flow can be maintained, not only can these create downforce, but reduce wake size. In most production cars as Vman455 already stated however, they are cosmetic, and even most people adding them onto their street cars are really just playing dressup without the underbody work to go with it.

Snax 08-05-2019 12:14 PM

1 Attachment(s)
Looking at the rear of my i3, you can see a legitimate effort at an OEM diffuser. The tray extends all the way up to the rear axle and is preceded by relatively smooth body work ahead of it.

I'd hesitate to say that down force of any significance is created or intended by it, but the extra hole mine has for the gas generator cooling has a 0.01 hit on the drag coefficient of the car. Pure battery i3s don't have that hole.

Vman455 08-05-2019 06:02 PM

Interestingly enough, the only car on sale outside of exotics that has a diffuser panel that looks the way most people think diffusers look--that is, one with a row of straightening vanes--is this:

https://ecomodder.com/forum/member-v...101-162607.jpg

That would be the underside of the current Nissan Leaf.

freebeard 08-06-2019 01:56 AM

An aerodynamically efficient difusser will look like this:
https://ecomodder.com/forum/member-f...80-silver4.jpg
The production vehicles have been worn off to the point were the average customer won't be constantly high-centering everywhere.

rob.e 08-07-2019 04:55 AM

has anyone fitted a DIY diffuser and seen improvements?

its something i'm considering for my car. it has pretty good /flat floor already but some parts at the rear bumper could be better if there was a smooth panel there.

that part of the underfloor already slopes upwards so easy to fit a "boy racer" ebay flat panel with straighteners. Do you think it'll make a noticable difference?


I'm thinking about something like this:
https://www.ebay.co.uk/itm/1X-Abs-Un...9/303179537634

https://i.ebayimg.com/images/g/0NUAA...Ej/s-l1600.jpg

freebeard 08-07-2019 11:47 AM

It depends on the car. The entire undercarriage is maybe 25% of the total. The delta in overall Cd is twice the delta in fuel consumption IIRC. IOW 10% better drag is 5% better mileage (kilometerage?).

Other areas that might see improvement could include rear-end lift, but it's going to depend on the vehicle.

euromodder 08-08-2019 04:16 PM

Quote:

Originally Posted by fastflyer (Post 603913)
For several days I have been seeing cars with this type of "Diffuser". And I would like to know if someone understands its operation or knows of any study.

Hucho ...
And from his studies, that is far too steep an angle to be beneficial.

For it to work in reducing drag on regular cars, a diffuser would need to angle up only a few degrees, say like 5°

Also depends on length, height of the underbody, and shape of the rear of the vehicle - see the drag rise on an estate ...

https://i.pinimg.com/originals/33/88...3116909625.jpg

fastflyer 08-08-2019 04:25 PM

Yes, with a smooth undertray it works.
But with a normal car is there any benefit?

freebeard 08-08-2019 06:19 PM

'How about no? Does no work for you?'

Tahoe_Hybrid 08-10-2019 12:32 AM

Quote:

Originally Posted by rob.e (Post 604030)
has anyone fitted a DIY diffuser and seen improvements?

its something i'm considering for my car. it has pretty good /flat floor already but some parts at the rear bumper could be better if there was a smooth panel there.

that part of the underfloor already slopes upwards so easy to fit a "boy racer" ebay flat panel with straighteners. Do you think it'll make a noticable difference?


I'm thinking about something like this:
https://www.ebay.co.uk/itm/1X-Abs-Un...9/303179537634

https://i.ebayimg.com/images/g/0NUAA...Ej/s-l1600.jpg


they got it on ebay US considering the exchange rate you still win

https://www.ebay.com/itm/4-Fins-Car-...EAAOSwWrxcMypk

aerohead 08-10-2019 12:20 PM

benefit
 
Quote:

Originally Posted by fastflyer (Post 604160)
Yes, with a smooth undertray it works.
But with a normal car is there any benefit?

without paneling upstream of the diffuser,you'd just have turbulence coming at it,for zero gain.:(

rob.e 08-12-2019 03:44 AM

Quote:

Originally Posted by aerohead (Post 604271)
without paneling upstream of the diffuser,you'd just have turbulence coming at it,for zero gain.:(

yep, thats the plan. the honda has pretty good underfloor aero already from the factory, but yes i'll be looking to see if there are any areas that could be improved.


From the honda press blurb:
"Aerodynamic efficiency is the very basis of the Civic’s design and is crucial to achieving exceptional high-speed stability and efficiency. As well as a low drag mono-form design, the Civic also uses a full-length bumper-to-bumper undertray with flared leading edges to manage the flow of air under the car. Together with larger covers on the rear suspension, this undertray adds to stability, particularly when overtaking long vehicles on multi-lane roads"

Mine has something like a bodykit fitted which at the rear, seems to jut into the underfloor flow, dealer fitted honda option i believe. I either need to pull this part off (which i expect will leave holes and give me paintwork issues to resolve) or use something like that ebay diffuser to bridge the step and make it smooth.

Joenavy85 08-12-2019 01:41 PM

Only way you'll truly benefit from one of these (without doing a ton of testing) is to have it extend forward of the rear suspension so that your rear bumper isn't acting like a parachute. Other than that, as has been said, you need to smooth out all the airflow ahead of it first. If you already have the full undertray there likely won't be much improvement.

Vman455 08-12-2019 06:17 PM

One example of undercar airflow treatments specifically called out by Hucho in Ch. 4 (I don't have the book in front of me, so I apologize for not citing the section number) is on the 1996 Opel Calibra. There's an illustration that shows the results of air dam testing, etc. and part (d) has a figure of the rear diffuser panel. It stops short of the rear bumper cover, with a pretty substantial gap between them, but the text states that the diffuser's effectiveness is still increased by removing the center section of the front airdam and allowing more airflow under the car--both of these struck me because they run counter to conventional aerodynamic wisdom. Things don't necessarily have to be perfectly smooth to get some benefit; and, I think people tend to overestimate the "parachute effect" without any real information to back it up because we "feel" like it should be significant. It might not be, though--you'll have to test to find out.

I'm reminded also of the Audi A2 study from 2011; a completely smooth floor showed a 20-count drag reduction in a tunnel with no moving ground; that was reduced to just a 6-count reduction in CD in a tunnel with moving ground and rolling wheels. The stock A2 has a fair amount of paneling underneath, but with a substantial central opening for the exhaust and openings around the wheel housings, all of which were covered to get that -0.006 CD delta:

https://www.a2oc.net/community/index.../b8-jpg.11479/

We're all really just shooting from the hip here. If you want to know if that eBay diffuser does something, the only way to find out for certain is to buy one and test it. Of course, then you'll run into the issue of noise in your results and how to reduce it so you can ascertain anything meaningful, which will likely be very difficult.

Joenavy85 08-13-2019 09:56 AM

Quote:

Originally Posted by Vman455 (Post 604422)
It stops short of the rear bumper cover, with a pretty substantial gap between them, but the text states that the diffuser's effectiveness is still increased by removing the center section of the front airdam and allowing more airflow under the car...... I think people tend to overestimate the "parachute effect" without any real information to back it up because we "feel" like it should be significant.

In my mind it's similar to the bed of a pickup.
A simple wing extension off the cab, while still having a significant gap to the tailgate, will show improvements, but a full aeroshell over the bed will be the best.

aerohead 08-14-2019 11:23 AM

parachute
 
Quote:

Originally Posted by Joenavy85 (Post 604398)
Only way you'll truly benefit from one of these (without doing a ton of testing) is to have it extend forward of the rear suspension so that your rear bumper isn't acting like a parachute. Other than that, as has been said, you need to smooth out all the airflow ahead of it first. If you already have the full undertray there likely won't be much improvement.

I've seen this argument through the years,and I believe it's based in urban myth or something.
A 'parachute' requires a 'clean',unobstructed flow source,which a rear bumper would not have available to it, unless a car was fully paneled,with laminar flow all the way underneath to the bumper,and the bumper extended below the elevation of the belly,into the laminar flow.
Without an operable diffuser,the leading,hollow face of the bumper would,simply be in train of a captured pool of turbulent air,traveling along with the vehicle.
I have seen some Ford cars whose rear bumper fascias 'quake' on the highway,but it's not do to 'parachuting,but rather due to the turbulent environment it's residing within.This should have been caught inside the aero-acoustic chamber.A true design oversight.Some webbing added to theinjection molds could have 'tuned' the natural frequency away from that turbulent signature,and any harmonic of it.

aerohead 08-14-2019 11:31 AM

pickup bed
 
Quote:

Originally Posted by Joenavy85 (Post 604470)
In my mind it's similar to the bed of a pickup.
A simple wing extension off the cab, while still having a significant gap to the tailgate, will show improvements, but a full aeroshell over the bed will be the best.

It is similar.It would be an inverted,rear-facing step,with reattachment,and a captured vortex if properly designed.You're losing some pressure recovery potential due to the vorticity,however,it's way out ahead of nothing.

Joenavy85 08-14-2019 01:43 PM

Quote:

Originally Posted by aerohead (Post 604547)
I've seen this argument through the years,and I believe it's based in urban myth or something.
A 'parachute' requires a 'clean',unobstructed flow source,which a rear bumper would not have available to it, unless a car was fully paneled,with laminar flow all the way underneath to the bumper,and the bumper extended below the elevation of the belly,into the laminar flow.
Without an operable diffuser,the leading,hollow face of the bumper would,simply be in train of a captured pool of turbulent air,traveling along with the vehicle.
I have seen some Ford cars whose rear bumper fascias 'quake' on the highway,but it's not do to 'parachuting,but rather due to the turbulent environment it's residing within.This should have been caught inside the aero-acoustic chamber.A true design oversight.Some webbing added to theinjection molds could have 'tuned' the natural frequency away from that turbulent signature,and any harmonic of it.

So the turbulence is similar to what you get in the bed of a pickup with the tailgate up, one big oval cushion of swirling air?

Frank 09-05-2019 01:10 PM

re Diffusers: they're typically used to increase down force which I always thought would increase drag. Unless you're racing I don't understand the point: or is my understanding incorrect?

freebeard 09-05-2019 08:04 PM

Wake management. I labor under the possible misapprehension that a diffuser nets downforce without increasing drag.

Frank 09-06-2019 05:03 AM

You may be right! If well done that seems to be the case: https://en.wikipedia.org/wiki/Diffuser_(automotive) although I suppose actual testing might reveal the truth.

Love your haiku BTW... :)

Oceanic815 09-06-2019 09:55 AM

Quote:

Originally Posted by rob.e (Post 604030)
has anyone fitted a DIY diffuser and seen improvements?

its something i'm considering for my car. it has pretty good /flat floor already but some parts at the rear bumper could be better if there was a smooth panel there.

that part of the underfloor already slopes upwards so easy to fit a "boy racer" ebay flat panel with straighteners. Do you think it'll make a noticable difference?


I'm thinking about something like this:
https://www.ebay.co.uk/itm/1X-Abs-Un...9/303179537634


https://i.ebayimg.com/images/g/0NUAA...Ej/s-l1600.jpg

I have that ebay diffuser; it has been on the Prius since May 2018. I have noticed nothing significant from it other than the occasional comment from car enthusiasts. I consider it cosmetic only.
https://i983.photobucket.com/albums/...pscef80lll.png
stupid watermark

aerohead 09-07-2019 12:32 PM

diffuser
 
Quote:

Originally Posted by freebeard (Post 606164)
Wake management. I labor under the possible misapprehension that a diffuser nets downforce without increasing drag.

The 2.8-degree 'long',and 4-degree 'short' diffusers prevent underbody separation,allowing pressure regain at the wake,increasing the base pressure and lowering pressure drag.
Anything steeper than these will generate separation and drag,while telegraphing low base pressure under the diffuser to create downforce.It's a trade off.
Ferrari has drag-reduction valves in their diffusers that can be turned on and off,depending on straightaway high speed,of cornering ability.

rob.e 09-09-2019 04:29 AM

Quote:

Originally Posted by Oceanic815 (Post 606185)
I have that ebay diffuser; it has been on the Prius since May 2018. I have noticed nothing significant from it other than the occasional comment from car enthusiasts. I consider it cosmetic only.
https://i983.photobucket.com/albums/...pscef80lll.png
stupid watermark


ha ha.. ok thanks for that. i'm not expecting anything groundbreaking - did the prius already have a flat area there under the floor? my honda has a lip there that can't be good for aero so i'm using the diffuser to bridge that gap, logically it should be better but i'm not expecting anything major :)

Tahoe_Hybrid 09-09-2019 11:35 PM

Quote:

Originally Posted by freebeard (Post 604075)
It depends on the car. The entire undercarriage is maybe 25% of the total. The delta in overall Cd is twice the delta in fuel consumption IIRC. IOW 10% better drag is 5% better mileage (kilometerage?).

Other areas that might see improvement could include rear-end lift, but it's going to depend on the vehicle.

the lift will make the vehicle lighter like inflating your footballs with helium...

freebeard 09-10-2019 02:29 PM

Quote:

Quote:

Originally Posted by myself
Other areas that might see improvement could include rear-end lift, but it's going to depend on the vehicle.

the lift will make the vehicle lighter like inflating your footballs with helium...
Since it's a reduction in lift (considered as an improvement), it would be like inflating your football with methane.

benbenben 09-12-2019 07:31 PM

Quote:

Originally Posted by freebeard (Post 606433)
Since it's a reduction in lift (considered as an improvement), it would be like inflating your football with methane.

.
Well, no.

Methane (CH4) may not have the extremely low density of H2 or He, but it is definitely less dense than standard atmosphere at stp. Nitrogen and Oxygen are both more massive than Carbon AND they are diatomic, N2 and O2. The four little hydrogen atoms along with the smaller single carbon means methane at stp is around half the density of regular air at stp.
.
You could use argon or sulfur hexafluoride for analogies requiring a gas more dense than regular air.

freebeard 09-13-2019 12:22 PM

I thought it was methane that kills farmers in manure pits.

teoman 09-13-2019 04:36 PM

Methane is more of an asphyxiant. Half the density of air.

In confined spaces if ignited it can go kaboom.

benbenben 09-14-2019 04:46 AM

Quote:

Originally Posted by freebeard (Post 606619)
I thought it was methane that kills farmers in manure pits.

While that certainly makes it a heavy topic, it doesn't mean CH4 is more dense than air at typical atmospheric (or tire) pressure and temp..
I haven't even heard of this so I am not well versed in the phenomena, but I suspect a couple things could be at work in those cases.
First, not all the oxygen needs to be displaced to cause unconsciousmess. Unconsciousness is pretty quick with less than 4% O2. One or two breaths. Falling unconscious would of course make death more likely as it brings the face closer to the source of the displacing gas and it removes the best chance for getting clear of the anoxic area.
Second, the manure may be actively emitting methane. Even being less dense, there may be enough especially in a pit to displace oxygen below levels needed for consciousness.
Third, the pit and methane being emitted might be cooler than the surrounding air. It wouldn't have a temperture difference sufficient to make methane more dense than air, but it might offset the difference in density a little.

freebeard 09-15-2019 02:18 AM

Likely the second one. I wasn't trying to take things off topic.

How about those new difussers, eh?


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