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basic aerodynamics for i3

Thu Jan 23, 2014 12:47 pm

First off, I like the i3. As a city car (say London within the M25) it makes a lot of sense. However, having driven both REx and BEV models both on 24h test drives using same commute I have come to the conclusion that it is not quite as efficient as it might be.

Around town, at or below 40 mph, aerodynamics have little impact on a vehicle's range.

This post is about basic aerodynamic drag so I'm not including the complexities of interference drag (e.g: between mirrors and bodywork or turbulence created by wheels) nor surface friction concerns or Lift induced vortices.

We'll stick to good old Bernoulli's principles and Frontal Area.

BMW claim the i3 BEV has a Cd (non dimensional drag coefficient - basically describes how the car's shape affects drag) of 0.29 and Rex has 0.3Cd - doesn't sound too bad until you multiply that by the frontal area: CdA

This makes a lot of sense as now you can compare a huge variety of cars side by side.

OK so that makes the CdAs for the i3:

BEV: 0.690 m2
REX: 0.710 m2

Now compare this with other cars like:

1990s Honda Insight (2 seater hybrid): Cd = 0.25 but CdA = 0.474
1999 - 2005 Audi A2 1.2 TDI: Cd = 0.25, CdA = 0.544
1980s Toyota MR2 (2 seater) : CdA = 0.539

So the i3 is about as aerodynamic as a 1990s Toyota Camry or Subaru Impreza!

Even the 1995 BMW M3 is better: CdA = 0.629 m2 and possibly my least favorite car (the 2004 Prius) is 0.580 m2

The best examples in the noughties seem to be the VW XL1 (2 seater) with CdA of 0.279 and the German Loremo prototype (2+2): CdA = 0.250 m2

Sadly the XL1 is no People's Car and might better be re-badged the EB XL1 (a cheap £100k Bugatti !) Sadly Loremo still seems to be in the doldrums.
Last edited by ecoangel on Thu Jan 23, 2014 1:03 pm, edited 3 times in total.

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Re: basic aerodynamics for i3

Thu Jan 23, 2014 12:59 pm

Next , so what? How does this affect my EV range?

basic drag equation:

Drag = 1/2 rho CdA (speed squared)

In metric:

rho is 1.225 kg/m3 i.e: standard atmosphere air density at sea level - just like in NL !).

At 62 mph (100 km/h): we'll use 27.74 m/s:

Drag = 326.25 N for i3 BEV

and 335.6 N for REx

Now lets convert to work done per unit of time ie: Power

In simple terms:

Power = Force x Velocity

which in Metric is Joules per second or Watts.

This gives a basic aero calc for the i3 BEV of 9.06 kW and i3 REX of 9.323 kW.

** NB: we are not including friction of road surface, tyre efficiency or fact we are restricted to 1 gear ratio with associated mechanical losses.**

Using another car from above eg: A2 requires 7.14 kW at same speed.

Around town the i3 wins hands down over an ICE but go on a faster / longer commute and physics might mean a re-think both ny consumers and BMW aerodynamicists!

The good news is the REX is only 3 % worse in aero terms than the BEV.

This tallys as the BEV I drove had the 175 rear tyres as did the REX and both gave just over 60 mile range over a 29.1 mile commute in 5 deg C , 10 kts wind and little hill climbing - averaging around the same 38 to 40 mph and 19.6 kWh EV consumption.

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Re: basic aerodynamics for i3

Thu Jan 23, 2014 3:24 pm

ecoangel wrote: OK so that makes the CdAs for the i3:

BEV: 0.690 m2
REX: 0.710 m2
So, the difference is down to the tyre sizes, which become the same as soon as the BEV is specified with any of the optional wheels?
2014 BMW i3 BEV deposit placed 19th Dec 2013
Delivered in Melbourne 4/12/2014. Arrived locally 11/12/14, with 24km on the clock.

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Re: basic aerodynamics for i3

Fri Jan 24, 2014 3:42 am

Hi Ecoangel,

while I agree that the Cd of around 0.3 is a sign of BMW not trying very hard (alas, I have to admit), the fact remains that the i3 is quite a bit larger than the A2 (ca. 10% frontal area).
Whether that is good or bad is debatable...

Thanks for sharing your insights,

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Re: basic aerodynamics for i3

Sat Feb 15, 2014 1:48 pm

A little tip from driving my Active E for two years in Southern California and 45k miles later...

If you need to go somewhere and you barely have enough charge to get there, try this method, it really works:

1) Drive slower - The faster you go the more drag you'll get.
2) Draft behind a semi truck, not too close to be dangerous but 3 or 4 car length behind.
3) Drive slow uphills and let it recharge down hills.

If you have a full charge and only need to go somewhere close, forget all that and enjoy the ride.


By the way, I just signed up on this Forum.

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