[mattW]

Hey sharkey,
Nice post! Yeah the poll was just to get an idea of how far people were willing to go, and to spark some discussion on the topic. I think its general enough not to get too complex and you can select more than one option. You've done some nice work on your rabbit, have you got any photos of the rear air deflector? I don't know how much that darth vader body kit will do for EV PR... that thing is huge! And i doubt it would be as effective as just a nice rounded front like the EV1 or a streamliner. You want your car to be as close to a tear drop shape as possible. There is a nice quick outline of what the insight designers did for aerodynamics at http://www.insightcentral.net/encyclopedia/enaero.html

 

[mattW]

Check out this pick-up, sounds like its had the kind of treatment you are envisaging (except i don't know what turbulators are):

The owner says he got 28% better fuel economy for his efforts (story at http://www.evworld.com/article.cfm?storyid=870) so it sounds like its definately worth it. If it were an EV with a 40 mile (60km) range it would go up to 51 miles (82kms). I don't think you would get such a drastic change with a car but it shows you the difference aerodynamics can make.

 

[mattW]

Well this is getting interesting, i've been reading more on aerodynamics at http://autospeed.com/cms/A_2455/article.html (really great article) and found this table:

As you can see the biggest factor affecting aerodynamics for this car (a volvo under development) is the cooling package! In an electric car (depending on the motor and controller) this is almost completely unnecessary since the motors are so much more effecient. By just blocking off the majority of the airflow into the engine with a nice rounded curve you could decrease drag by a third! That and the other aerodynamic adjustments discussed earlier i think would make a pretty reasonable increase in range. Enough to be worth the effort anyways.

 

[mattW]

Since the Pusher needs cooling air to enter the grille, I've fabricated custom mounts and cut down the rear air deflector from a 1980's Oldsmobile station wagon to catch a bit of the air coming off the rear of the roof and direct it down the slope of the rear hatch. No scientific data on that modification, but anything that smooths the air flow behind the car can't be bad.

Has your deflector made a noticable difference to your stability at high speed? A spoiler works by deflecting air upwards which results in a reaction force downwards on the back of the car. Your deflector is doing the opposite, deflecting air down and lifting the back up... Its possible to reduce the drag of a car but increase the lift which results in a effiecient but dangerous car. I don't mean to be critical but i don't want you spinning off the freeway at speed.

 

[mattW]

How good are E-meters and that kind of thing at measuring your effeciency? (i was going to say fuel economy but i guess it would be amp economy ). Or i guess the bigger question is; if i was making aerodynamic changes what would be the best (electrical) diagnostic tools to test their effectiveness. You would obviously want to do before and after trips on the same stretches of road at the same speed but what/how would you measure to tell the difference?

 

[mattW]

Well it might be a while yet, i'm still in the research/learning/saving phase of EV construction but judging by the success of other people's aerodynamic adjustments i am definately planning to do some changes and will also try to post the before and after stats. If all goes to plan i should be starting with a honda CRX. Depending on how ambititious i am at the time i intend to work on the undercar airflow with an ABS plastic belly pan (similar to http://www.autospeed.com/cms/A_2456/article.html but bumper to bumper) possibly larger wheel guards or even rear wheel covers (aka insight) and maybe even a custom fiberglass bodykit that smooths out the front air intake and bumper (using the same method as http://www.geocities.com/silverkit98/BodyKit1.html but looking more ev1-like than street racer). These will obviously be the finishing touches to the car and may or may not happen but a big part of the appeal of electric cars for me is their efficiency and so things like aerodynamics and weight reduction really appeal to me.

 

[mattW]

I am also curious to ask people who already have working EVs how hot does your motor and controller get? Can you touch them after a period of driving? how much airflow do you think is necessary to keep them from overheating? If decent airflow was required it would definately affect how far you could go with fitting a belly pan because if air is to flow through the engine bay there needs to be somewhere for it to go, whether its air vents on the hood or from underneath the car (most cars vent under the car). So a belly pan would prevent air from flowing through the motor compartment unless you made room for vents in the pan. You can read about airflow through the engine bay here, the site is to do with getting the most out of your turbo but it has implications for electric car design. Basically we can assume that if 85% of an ICE is lost as heat compared to 15% in an electric car then there is probably 5-6 times more airflow in our cars than is required to keep the components cool. For the most aerodynamic car we want to minimise the flow to the minimum possible level without the motor or components overheating. Adding a small belly pan, only on the front section of an already aerodynamic prius in the site i linked in a previous post increased fuel economy by by 10% at a cost of just AUD$66. I hoping this thread will help people realise that they can increase their range and top speed considerably without the cost and weight of more or bigger batteries.

 

[mattW]

If you place a scoop on an area with attached flow you create an area of turbulance behind it, look at the area behind the hood scoop of this WRX undergoing DIY aero testing. I think your idea sounds good john, will you be using the existing grill or creating your own ports? Also it would be good to get a photo of the front end of your car so we can see what you mean. Sharkey do you have a photo of under the hood that would show the ducting or are there batteries in the way?

 

[mattW]

Greenflight you could also try to measure your power output at the same speed before and after. Has your overall top speed improved? If your speed is limited by the revs of the motor it probably wont make a difference but if you were struggling to get enough torque to get to higher speed then you should have a higher top speed with the changes you made. You could also try getting up to a specific speed (the higher the better) and then let the vehicle coast to measure the deceleration. This experiment and the maths needed are described in detail here. You definitely need to be doing testing to make sure what you have done will make a practical improvement to your efficiency. Aerodynamics is a hard thing to predict and sometimes is very counter intuitive. I would say that if you can't find a measurable improvement after adding something then its probably not worth the extra weight.

 

[mattW]

Ok I don't think I'm understanding this properly, as far as I know force occurs due to a pressure differential and a sloping belly pan would have positive pressure since it would be deflecting air downwards, and since there would be less pressure across the bonnet (/hood) and behind the pan the total force would be upwards and backwards. I try to explain my understanding of it visually in this dodgy flow diagram attached, the green line is where i would image the force to be acting. Is there something I am missing? what would be creating down-force?

 

[mattW]

I understand how that would work with a gust of wind at the front of the car; Pressure would build at the front compared to the back but then the pressure difference would 'suck' the excess air from the high pressure to low pressure zone. But when the car is moving the pressure at the front of the car is being constantly renewed by new air crashing into it. The only reason the air would accelerate would be because the pressure differential was maintained which means that there is still high pressure at the front. I'm not trying to argue, I'm just struggling to understand how it works...

 

[mattW]

Hey guys I was thinking we could start a Wiki topic about aerodynamics. One section on the theoretical side of things and the other on practical projects for people to undertake. I just realised that this topic is approaching 50 posts and appreciate how much information is on here, it would be good to get it into an easily accessible form. Would you guys be willing to add some of your wealth of knowledge?

 

[mattW]

Hey guys, we're been doing all wrong, trying to make subtle changes while keeping with the style of the car... we should be ashamed. THIS is what we need to do:

And look at the European syling on the front here:

i'm...just...speechless!

 

[mattW]

But on a more serious note I just found a forum for people trying to up their mpg with a whole section on aerodynamics with 11 pages worth of topics. Check it out here, looks like a good resource.