Mini vs BMW i3 aerodynamics.

A

AndysComputer

Well-Known Member
I read the pure electric version of the i3 has a CD of 0.29
It has a frontal area of 23.67
This gives it a CDA of 7.1.

The Mini has a CD of 0.3 as far as I know.
But the frontal area is 21.3sqft.
This gives a CDA of 6.39.

So this is why, despite its carbon fiber structure (and much higher cost) giving an 80kg weight saving over the Mini, it’s less efficient at moving through the air and thus has less range on the same battery…

The fact the i3 has the same 114mile EPA range as the Mini has me calling shenanigans on their whole testing methodology as they just use a rolling road as far as I know so aerodynamics don’t even come into it. No wonder car companies make such little effort there. Sounds like a legal version of dieselgate….
 
I have not read up on the aerodynamic efficiency of the i3 compared to the Mini so that is interesting. I agree, they seem to pull these EPA/WLTP figures from a hat rather than from real world testing, though the figure for the Mini isn’t as far from the truth as on other EVs.

I’m fortunate that we have both a Mini Cooper SE and an i3 in our household. In my view the Mini is a much better looking car overall, but the Mini seems more brick like to me, with a stubby front end rising to a very short, steep windscreen compared to the long, raked screen on the i3. I wonder if the aero on the i3 changed with the pretty minor facelift in 2019. The lower bumper was redesigned around the auxiliary lights, but I don’t think it’ll make a huge difference. It is worth noting that the current BEV i3 (120Ah) introduced in 2019 has a battery capacity of 42.2kWh (37.9kWh useable) and has a WLTP range of 193 miles. In the real world it’s not difficult to achieve 150 miles range from a full charge, sometimes we get more, sometimes a little less. In my experience to date, the i3 is more efficient than the Mini, seeing a consumption figure of around 3.7miles/kWh in the i3 and 2.9miles/kWh in the Mini. Both driven by me in the same way. I’ve posted previously about very poor range in the Mini while driving in green mode very conservatively on the motorway in wet and windy weather and getting around 78 miles from a full charge, arriving home with 4% charge and 2 miles range remaining, the consensus being that the wind on my return leg may be the culprit. We’ve done the same trip in similar conditions in the i3 and still had 50 miles range when we got home. I know others on the forum see higher efficiency figures in their Mini, but so far I have not. What I do see is that for a 30% larger battery I get around 50% increase in actual range in the i3.
One reason for the higher energy use of the Mini, other than the car encouraging spirited driving may be that it uses the motor from the slightly more powerful i3S rather than the standard i3. I have to say that from driving both the Mini and the standard i3, the acceleration of the i3 doesn’t feel much less than the Mini; though the Mini definitely handles far better than the i3. The Mini is just more fun. It’s also much better equipped than the i3. The only extras added to our basic i3 were the sunroof and the park assist pack and it cost almost the same as my fully loaded level 3 / iconic.
 
Nice that you have both, when we first moved to Texas from NYC I hoped to buy a used say 3yr old i3 for my wife but couldn't find any within 500 or so miles so we just got a regular gas car. She didn't care about cars at the time and never had any interest or attachment to her first car but kept pointing out Mini's on the road so when I got the chance to essentially swap her by then 2.5 year old Kia Soul for a brand new base model Cooper SE at no cost I did. So now she has a car she's interested in. She was rather less than happy when I pointed to an i3 on the road and told her that was what I was going to buy if I could have found one as she think it's hideous. She was more speechless when I pointed to the facelift Chevy Bolt and said if the rebates still existed I would have bought one of those as ugly wasn't a strong enough word in her opinion. So each to their own. Personally I really like a white i3 but I also like our Mini.

Your thread on the range you got on the motorway that time is one of the things that inspired me to start testing and making videos on the Mini's range as I was alarmed by your numbers.

From some limited testing I have done my current thinking is this:

At 70mph (and presumably slower at each end of the trip) but driving with economy in mind at 21degC, dry road and next to no wind you would have 120 miles of range or thereabouts according to my experience.

But you had a trifecta of compounding problems:

1. Running the cabin heater due to -5degC outside temp you would burn through ~2.2kWh or 7.5% of the battery per hour. Assuming you drove for about 1hr 10mins that would mean you lost 8.5% of the battery to the heater. So you were operating on 91.5% worth of battery or 110 miles of range.

2. Bjorn Nyland shows the Mini used 18% more energy in the wet at just over 70mph so that knocks down your maximum range to 90 miles.

3. A 22mph headwind reduces the range of a Tesla Model 3 by about 19 % so if we apply that to half your journey and call it 9.5% then your max range is 81.5 miles.

These are my back of envelope calculations and I suspect the Mini is much more affected by a headwind than the Tesla, or maybe you had a higher wind speed.
But my calculations don't seem to be too far off as my 81.5 miles vs your 80 (78 traveled + 2 left) are reassuringly close although I don't know what your wind speed was or even outside temperature. This gives me hope about what I'm trying to understand and then explain to others...
 
That’s a great piece of work you’ve done and gives me hope for better range on a better day. I expected 100 miles of range in those less than favourable conditions, but clearly that didn’t happen. I’d be happy enough with a real world 120 miles range. I didn’t think the heater would have used that much since the external temp was 50F and the car was preconditioned for departure when plugged in. That being said, the AC was on due to fogging on the windows, so it would’ve used more energy.
I will try it again on a dry, calm day when I get the chance just to see how it does. In truth, my daily routine means that I’ll need to charge once a week on average, so the Mini is ideal. It was just a poor choice of day to try a range test I think.

I’m glad your wife loves her Mini. Mine adores her i3 and she wasn’t a big car fan previously either. She wasn’t initially sold on the looks either, but it was the driving position and driving experience that hooked both of us. The funky rear doors are also a nice feature. Our i3 is white and my wife has named it Finn, after FN-2187, the rogue stormtrooper from The Force Awakens.
 
I hope the testing work and calculations I do will be (A) vaguely correct, and (B) useful for other people as the more we know the more accurate our expectations will be. And this EV stuff is all new to everyone.
I should say my numbers above are based on pre-conditioning the car, so the 2.2kWh (my current number but subject to more testing) is just to maintain the temperature. The Mini does have a heat pump which helps heating efficiency, but I am wondering how well it works and as temperatures get colder how much resistive heating is added to the mix...
Bjorn in Norway got 100 miles range at 120kph (what's that about 72mph?) but I don't remember what the temperature was. He is also testing from motorway charger to motorway charger as far as I can tell but my numbers look slightly better as I have a couple of miles at each end of the test getting to and from the motorway. But to be fair I think that's more realistic for most people as they don't live on the motorway and their destination is typically not on the motorway itself either. But for road tripping between chargers on. Motorway his number is more useful. I'll get a chance to try that out when I road trip our Mini which will essentially be charger hopping to Austin and back although the hops will be longer than I'd like as Texas isn't exactly a charger desert but it's also not the East or West coast...
 
I’ll definitely update you when I get the chance to do another trial run of the same route to see how close your calls are. We’re due to go there for a Scotland V England rugby match in just under four weeks but if the game goes ahead, (currently we’re not allowed spectators in stadiums at sporting events in Scotland), we’ll probably use the i3 as it’s bound to be freezing and it has enough capacity to comfortably make the trip while allowing preconditioning without being plugged in so we come back to a warm car after the game. If there’s a chance to make the trial run sooner I’ll let you know.
 
Thanks, I'd be curious as to how that works out.
On that note, does your i3 have a heat pump?
I know they become less efficient the colder it gets such that if it's a bit below freezing the car may simply use resistive heat anyway as the power draw for heat produced it the same.

On an even bigger side note, as a Welshman in Texas I'll try and not sound jealous about your ability to go to a match...
 
The official document about the i3 heating system posted to my thread re battery preconditioning suggests the heat pump can suck up to 2.5kW (and give 5kW of heat) so my 2.2kW initial estimate for the heating system, might not be far off given I was testing at -5degC and it is supposed to work down to -10degC...
 
No heat pump in the i3. It’s optional in those and the one we bought was a dealer order which couldn’t be amended and they hadn’t selected the heat pump.

Good luck to the Welsh lads this year. Defending champions have to be fancied but it looks on paper like it’ll be a very close championship. We went down to Cardiff for the Wales v Scotland game three (EDIT:it was actually four) years ago and it didn’t end well for us. That was a long trip home! Should be a good game when it comes.
 
SpeedyRS said:
I have not read up on the aerodynamic efficiency of the i3 compared to the Mini so that is interesting. I agree, they seem to pull these EPA/WLTP figures from a hat rather than from real world testing, though the figure for the Mini isn’t as far from the truth as on other EVs.
Click to expand...

EPA and WLTP are very similar in their testing. They set up a rolling road with the cars parameters and run till the battery is dead. They dial in different parameters (eg. speeds, acceleration etc) to simulate real driving. They average the highway and urban cycles to get the result. The main differences are the speeds and percentages of highway/urban. The EPA have 5 tests, of which most manufacturers only ever do 2. In this case, the EPA takes the result and multiplies by a fudge factor of 0.7 to get a "real world" value. This 0.7 is applied to every car that does 2 tests even though logic dictates that no two cars will behave the same in lower temperatures. This is the main issue. WLTP doesn't do this, and their range estimate is far better for average to good conditions. It is quite easy in the MINI to reach the WLTP range if you drive in good conditions and have a similar breakdown of highway to urban use. If you drive in excellent conditions, or do more urban than highway, you can exceed the WLTP significantly as shown in a recent video posted in this group.

The EPA range is probably designed more the the US conditions, which has harsh winters. I live in Oz, in a sub-tropical city. This is the definition of good conditions, and I routinely beat the WLTP. It goes without saying that every day I smash the EPA rating.

One interesting thing to note is that the temperature has a direct effect on the air friction. The number of atoms (and hence density of the air) is directly proportional to the temperature (PV = nRT). At freezing, the temp is about 273K, and 27C is about 300K. There is (approximately) a 10% difference in density, so at the same speed (and air pressure), the air friction is 10% higher at freezing than at 27C. Air friction becomes dominant at higher speeds, so at highway speeds, you will use maybe 8 - 9% more energy in the cold from air friction alone. This is before taking the heater into account.
 
Two things that annoy me about the EPA test is that it’s on a rolling road and thus I can assume aerodynamics don’t come into it.
The second is that while they do use the AC they do not use the heater and while that didn’t matter for an ICE car it does for an EV.

These two factors have me concerned manufacturers won’t put as much effort into those two areas as they could because it’s not something they will benefit from in advertising and the customer isn’t clued up enough to know about.
Ergo they buy a car with what they think is sufficient range, find it’s terrible in cold weather (and won’t know if a heat pump is beneficial and by how much) and will also wonder why their 300 mile range pickup truck with the aerodynamics of a brick struggles to get 200 on the highway and even less in winter…
 
vader said:
One interesting thing to note is that the temperature has a direct effect on the air friction. The number of atoms (and hence density of the air) is directly proportional to the temperature (PV = nRT). At freezing, the temp is about 273K, and 27C is about 300K. There is (approximately) a 10% difference in density, so at the same speed (and air pressure), the air friction is 10% higher at freezing than at 27C. Air friction becomes dominant at higher speeds, so at highway speeds, you will use maybe 8 - 9% more energy in the cold from air friction alone. This is before taking the heater into account.
Click to expand...

Thanks for this!
 
AndysComputer said:
Two things that annoy me about the EPA test is that it’s on a rolling road and thus I can assume aerodynamics don’t come into it.
The second is that while they do use the AC they do not use the heater and while that didn’t matter for an ICE car it does for an EV.

These two factors have me concerned manufacturers won’t put as much effort into those two areas as they could because it’s not something they will benefit from in advertising and the customer isn’t clued up enough to know about.
Ergo they buy a car with what they think is sufficient range, find it’s terrible in cold weather (and won’t know if a heat pump is beneficial and by how much) and will also wonder why their 300 mile range pickup truck with the aerodynamics of a brick struggles to get 200 on the highway and even less in winter…
Click to expand...

While the EPA isn't perfect, I do believe they are accounting for aerodynamic drag.

Manufacturers are required to submit coast down data (I think the EPA calls this "road load"?) which is three coefficients for an equation that describes both the aerodynamic drag and rolling resistance.

They should easily be able to incorporate this into their rolling road testing
 
AndysComputer said:
Two things that annoy me about the EPA test is that it’s on a rolling road and thus I can assume aerodynamics don’t come into it.
Click to expand...

The rolling road does take aero into account. You setup the resistance depending on the specs of the car. You input the CdA so that the resistance is increased in proportion to the air friction. Rolling resistance is a constant - F = mgr (mass x gravity x rolling coefficient), and work (energy) is F x d. You use a constant amount of energy no matter what speed (within reason). The power is F x v, which increases, but for a shorter time. You would get a range maybe 5 times higher if you didn't account for air friction.

I downloaded the EPA test result - although can I find it right now, no. From memory it was something like 147 miles freeway till exhausted, and 167 miles urban. The ratio is (again from memory) 40% freeway, 60% urban. This gave an average of 157 (my example 159) miles, which when multiplied by 0.7 gives 110 miles (for the 2020 SE).

So, they actually got over 150 miles per charge, and in good conditions so can you. The 0.7 is meant to make it more "real world". What it ends up doing is making sure the figure is pretty much always wrong. The Taycan is another car which has a ludicrously low EPA result for the same reason.
 
vader said:
The rolling road does take aero into account. You setup the resistance depending on the specs of the car. You input the CdA so that the resistance is increased in proportion to the air friction. Rolling resistance is a constant - F = mgr (mass x gravity x rolling coefficient), and work (energy) is F x d. You use a constant amount of energy no matter what speed (within reason). The power is F x v, which increases, but for a shorter time. You would get a range maybe 5 times higher if you didn't account for air friction.

I downloaded the EPA test result - although can I find it right now, no. From memory it was something like 147 miles freeway till exhausted, and 167 miles urban. The ratio is (again from memory) 40% freeway, 60% urban. This gave an average of 157 (my example 159) miles, which when multiplied by 0.7 gives 110 miles (for the 2020 SE).

So, they actually got over 150 miles per charge, and in good conditions so can you. The 0.7 is meant to make it more "real world". What it ends up doing is making sure the figure is pretty much always wrong. The Taycan is another car which has a ludicrously low EPA result for the same reason.
Click to expand...

Question from the peanut gallery: do you know if the 0.7 factor specified by the EPA, or chosen by the manufacturer?
 
AndysComputer said:
Bjorn in Norway got 100 miles range at 120kph (what's that about 72mph?) but I don't remember what the temperature was.
Click to expand...
Yeah, I just happened to reference Bjorn’s range tests the other day…in dry, 66F-degree weather, he traveled 147 miles at 56 mph and 101 miles at 74.5 mph, FWIW.
 
CuriousGeorge said:
Question from the peanut gallery: do you know if the 0.7 factor specified by the EPA, or chosen by the manufacturer?
Click to expand...

If I remember correctly, the 0.7 is chosen by the EPA but the manufacturers can choose to do more in depth testing to get an alternate "fudge factor". I'm pretty sure Tesla chose to do the more in depth testing which is why their range ratings are less conservative.

I also read that the F150 lightning was tested with weight in the bed to better reflect real world use but don't think that has been confirmed. It at least seems like manufacturers have some influence.
 
AndysComputer said:
I read the pure electric version of the i3 has a CD of 0.29
It has a frontal area of 23.67
This gives it a CDA of 7.1.

The Mini has a CD of 0.3 as far as I know.
But the frontal area is 21.3sqft.
This gives a CDA of 6.39.

So this is why, despite its carbon fiber structure (and much higher cost) giving an 80kg weight saving over the Mini, it’s less efficient at moving through the air and thus has less range on the same battery…

The fact the i3 has the same 114mile EPA range as the Mini has me calling shenanigans on their whole testing methodology as they just use a rolling road as far as I know so aerodynamics don’t even come into it. No wonder car companies make such little effort there. Sounds like a legal version of dieselgate….
Click to expand...

The whole point of the CFRP life module was to provide R&D for future vehicles and factory tooling at 1/3 of the price. Fuel efficiency is secondary to manufacturing costs and that's why we have the EV-ICE retrofits to reduce the capital investment burden. If anyone can confirm the Cooper SE is using a NCM 622 CATL prismatic battery pack (needed a flatter pack) compared to the 94Ah Samsung SDI 622 for the 2017-2018 i3. I'm guessing that the 120Ah i3 went to Samsung SDI NCM 811.

Anyways, BMW Rule 60 (seniors executives must retire by 60) was a tragedy in 2015+ and a lot of great personnel moved on.
 
I had to go on a site visit today so had the chance to give my SE another run. It’s a similar run to the one I did previously in terms of road types and distance. The weather today is beautiful; sunny and pretty still, so pretty good conditions for another test and a massive difference to the wind and rain which accompanied my last attempt. I preconditioned the car before leaving and it had just finished charging for my departure time so the GOM was showing 100% battery and a forecast range of 101 miles when I set off. Ambient temperature was 4.5C when I left home and had risen to a balmy 6C when I got home, almost time for shorts in Scotland!

My trip was to a port around 35miles from home and 90% of the distance was on motorways or dual carriageways with a speed limit of 70. I set cruise control at 66mph and was in mid mode throughout. Climate control was on at 20C but I didn’t need AC so it was switched off. I drove normally, not too fast and not trying to keep power use down so I accelerated as the car wants you to accelerate. I didn’t need the seat or steering wheel heaters either.
My total trip distance after stopping off somewhere on the way home was 77.9 miles, and when I got home the GOM showed 29% battery remaining, 25 miles range left. It also showed my consumption as 3.7 miles per kWh. Doing my own calculation based on the usable capacity of the battery I got a whisker under 3.8miles per kWh (3.796 miles per kWh). I calculated that at that rate my theoretical range was just under 110 miles. It’s a much better result than my previous attempt which almost used all available battery capacity to do a very similar trip in terms of roads and distance and illustrates just how big an impact wind and rain have on range. It was colder today than it was then, but the fact that there was very little wind and it wasn’t raining seemed to make a difference. I’m much happier than I was after the last longer trip I did as 110 mile range is much more in line with what I expected when I bought the car.
 
SpeedyRS said:
Ambient temperature was 4.5C when I left home and had risen to a balmy 6C when I got home, almost time for shorts in Scotland!
Click to expand...
Great report--certainly a better day for a long drive. However, everyone knows Scots don't wear shorts--they don't lend themselves to displaying tartans very well :)
 
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