Wednesday, September 29, 2010

CarBEN EV5 Open Source Project Part 3 - Updated 12 Jan 2011

CarBEN EV5 Part 1

CarBEN EV5 Part 2

CarBEN EV5 Part 4

The single entry is probably the most controversial feature of the CarBEN EV -- it has to do with weight savings and surrounding safety structure.

It's not like the the benefits aren't well worth the minor sacrifice: the CarBEN EV could well be the most efficient car yet made, and it could be one of the first electric cars to have a range of 400 miles (or more) on a single charge.  If I was able to take part in the X-Prize, the CarBEN EV would have held the most people of any car in the contest.  It might have a Cd under 0.14 and weigh less than a ton; hopefully less than a ton with the driver onboard.

I'm serious about these goals, and I have to make choices that save weight, while not diminishing safety, and yes, body gaps add aerodynamic drag.  The Bionic increased the Cd from 0.095 (the early blue clay model that I am starting with) up to 0.19.  The main reasons for much of this increase is the uncovered wheels and the cooling for the diesel engine.

Since about 97% of all accidents involve impacts on the front and sides of the car, I want to have maximum protection in those areas.

Since a square encloses the most area with the least perimeter (except for a circle, naturally), it is the best shape to make a car with a given frontal area, and it gets the most usable interior volume.  The Mercedes Bionic/Boxfish model provides an amazing opportunity: it combines an amazingly low coefficient of drag (Cd) in a shape that is nearly a square in the frontal area.  This makes it possible to have comfortable seating for 5 people in a car less than 14 feet long.

A compact car can be much lighter and stronger, and still keep the frontal area down to ~25 sq ft (2.323 sq m).  If the Cd of CarBEN EV is 0.14, then the effective frontal area (CdA) is 3.5 sq ft (0.325 sq m).  And it is possible to get the Cd as low as 0.11 or so, and that would lower the CdA to 2.75 sq ft (0.255 sq m).

These would be unprecedented drag numbers for any car, let alone one that seats up to 5 people.  Having an electric drive train also contributes a lot to this packaging efficiency: the electric motor is much smaller than an equivalent ICE and it's transmission (an electric motor only needs a reduction gear -- or can be direct drive!) and they need just a fraction of the cooling air flow.

And here's one of the reasons where the aero and the aero shape enter into why the entry door is in the back: since truncating the back of the shape (called a Kamm back) makes the vehicle makes it much more practical, and has a very small increase in drag (and the Boxfish model achieves it's staggering Cd of 0.095 with a Kamm back), and this is where a small fraction of the accidents occur anyway, this is where I chose to put the main entry door.

Side doors add weight and reduce the safety; by cutting big holes in the structure (think about a large box beam web) which then has to be reinforced all around the perimeter, and the door itself has to have a similar frame all around the perimeter, and you add the hinges and if you want to have as much strength as possible, you need 2-4 latches (instead of the usual 1).  Adding the latches, means that you gain back some/much of the strength you had with no side doors, but it will weight more.

Since I would need a rear hatch door anyway if I put in a side door; I can save a lot of weight and get the safety protection even better than most cars.

Another aspect of the aero that affects many other things, including the seating arrangement: the tapered shape required for ultra low drag means that conventional rows of seats is not the best way to fit everything in.  Since the electric motor is so compact, the driver can be moved forward between the front wheels, opening up more room.  And the staggered seating means that even more legroom is available by angling your legs off to the side.  So, the CarBEN EV fits 5 comfortably, in a package that most cars fit 4 less comfortably.  The mesh seats are also a big part of this.

On the asymmetrical seating -- basically, the most the weight would be unbalanced is about 300-350 pounds (the two "extra" seats are for shorter adult/kids), and that weight is on the higher part of the road crown; and away from the much rougher right side edge. I've only ever had to replace wheel bearings and the like on the right side of any of my cars. The battery pack in the floor is 800-900 pounds, and since most cars have the driver on the left -- and most often the driver is the only person in the car; so, most of the time the CarBEN will be in total balance! On the other hand, most cars are usually out of balance by up to 250 pounds (or more).

I think I've shown that the choices I've made so far, are aimed at achieving unprecedented ultra-efficiency, in a compact, very practical people moving machine.  Since the most import part of that function is just that: moving people with safety, the small inconveniences of slightly more effort getting in and out of the car are more than offset, if I can get anywhere near the performance I think are possible.  Form follows function, and I think the CarBEN EV can function at a very high level, indeed.

As Oliver Kuttner says: you must get the physics right to get to higher efficiency; and all design choices affect the efficiency. Using less energy is my focus, and that is where I cannot compromise.

After I get a prototype and running, I hope to experiment with rigid wheels and solid (non-inflatable) tires and regenerative shock absorbers.  The solid tires and rigid wheels could be much lighter weight (which counts double to weight losses anywhere else), and they could have vanishingly low rolling resistance, and they would pass along most of the energy to the regenerative shocks; making their effect greater than it would be with conventional tires.

The ride quality could actually be better than with conventional tires, since light wheels makes the system more compliant (they move rather than moving the car), and the suspension can be fully tuned and damped to match the wheels.

This could help get the energy consumption even lower than 100Wh/mile, and that could extend the range, as well as recharging the batteries (a bit) from the energy regained from the shock absorbers (instead of wasting it as heat).  Every little bit counts.


Here's the latest video animation: Final Design Intent Video

Here are the newest images and of the SketchUp model.  If you want a copy of the model, I'd be happy to email you a copy!

Some pictures of a 1/12th scale (1" = 1') model of the eggcrate frame similar to what could be used to lay the fiberglass shell:

Creative Commons License
CarBEN EV5 by Neil Blanchard is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.


  1. Just now catching up on this project. I think the decision to go to wider A pillars was a good one for visibility.

    I can't understand your decision to sharpen up the front corners in the latest designs, however. Those will need to be smoothed to improve flow over the sides of the car.

    I love the hatch-entry design, but have some concerns about structural integrity in the event of a roll scenario. You'll need extremely stout locking mechanisms to hold it all together if it comes to that.

  2. Sorry I didn't respond earlier.

    The current windshield is from the Smart Car, so it is glass and available with wipers already set up.

    The "hard chines" on the front fenders were inspired by the new images I got of the Mercedes Bionic model, that has an impressively low Cd of 0.095. There is now a low wedge shape which pushes more air up and the then splits it to the sides. This seems to work well on their model, so why not stay with what works?

    The highest part of the roof is a roll cage and the driver's seat is supported by two steel tubes up the the roof; just forward of the hatch hinges, so I think the roof is very stout!

    Sincerely, Neil

  3. Could you reduce frontal area by seating the passengers lower down? By that, I mean lowering the back of the seat, whilst keeping the leading edge of the seat base at the same height. This brings the passengers' heads down at the expense of a sharper bend in the knees/hips. That's how the original Mini was such a marvel of passenger packaging - you pretty much sit on the floor in it! Two inches lower on the passengers' heads brings your roof down two inches and aero drag reduces accordingly.

  4. Hi Andy,

    Thanks for your comments.

    Yes, lowering it would save some area, but I'd lose a lot of interior volume, and it would have to be longer to make up for the lose of legroom. The interior volume of a squarish frontal area is much higher than a lower rectangle of the same area.

    Reducing the Cd is a much higher "payoff" than reducing the area. The Car BEN EV has a frontal area of just 25.14 sq ft (2.243 sq m) and the Cd of the blue Mercedes model I based it on is just 0.095. So the reducing the frontal area has much less effect than lowering the coefficient.

    Sincerely, Neil

  5. This is a minivan. And who drives minivans- families. I love the rear exit and seating. The middle aisle cuts down on the "He's touching me" complaints and I have to go into the back anyway to buckle the kid's car seats. With your arrangement I can get them in and deal with seating as I move toward the front instead of going around to multiple doors.

  6. Greetings Dowlan,

    Yes, it is either a "hot hatch" or a true minivan -- it is just a bit larger than my Scion xA in all dimensions.

    I think the rear hatch door entry is workable, and the main benefit is the increased structural integrity in a front or side crash, with lower weight than if there were side doors that need both the door and the opening reinforced.


  7. Batterystoreonline.comJune 13, 2011 at 7:54 PM

    I have access to Lithium batteries, chargers and ideas.....  I love cars, have two Prius' and love them.... and would love to visit - although I must tell you -  I LOVE the ICE concept vs PHEV .... knowing I dont like using nuke or coal electricity.

    (D Brennfoerders Cousin)

    Look forward to working with you.

    Oh, and a Smart Car is the Dumbest car I know of in comparision to your ideas.... and seat and transportation of PEOPLE......

  8. Thanks for the reply.  At the moment, I am immersed in getting the full sized shell built, and I'm trying to figure out how to best use the Phlatprinter 3 to make the ~2" thick sections, to build up the chassis.  There are about 85 ~2" layers.

    Where are you located?  What's your name?  It is a small world.

    I'm using the windshield from the Smart car.  And yes, the Toyota/Scion iQ is a much "smarter" design, in many ways.


  9. Hey Neil.

    This has got to be one of the best solutions to front wheel skirting I've ever seen.  Keep up the good work.

  10. Thanks -- I hope it can be made to work.  I think it can, but the devil is in the details.


  11. Wow it makes me so happy to see somebody who was involved in developing the Aptera (Just had my heart broken 20 minutes  ago catching up) working on an open source vehicle.   I was trying to help USW market/license "V-Fuel" vandadium redox batteries but seem to have missed the boat, hopefully I missed it to somebody interested in selling the technology, not killing it.  I'm following you now, maybe you can help me feel better about the world.

  12. my avatar is from a bottle of unfortunately branded sunblock i saw in china town, can't seem to get google to change it.

  13. Andy, I'm flattered but I was not involved at all with anything on the Aptera.  I'm a long time fan, and a moderator on the *un*official* fan site.


  14. I misread your post, then, I apologize.  I got mad at them last year when I tried to connect Aptera with the University of New South Wales, and their purchasing people didn't react in a professional way, not even for an OEM, who can often be impetuous.Very typical trajectory for innovation.  Slow organic growth, grassroots popularity built on technical sophistication and know-how, absorption into a stagnant industry,  executive gamesmanship to out-maneuver the science and tech founders, lose their technical and moral center and run company aground focusing on marketing and "appearing legitimate (read: unnecessary overhead to appear powerful)" for a technology and ideologically driven segment.

  15. so what do you think of that westinghouse residential wind turbine?  Pretty sweet, no?

  16. I am not familiar with it -- I'll look for it.  Or, do you have a link?