Two decades before BMW launched its first production EV, there was the E1 Concept.
Energy Density and Specific Energy are the twin holy grails for any automaker wishing to bring a viable electric vehicle to market. These two units of measurement are often confused, even by people who really should know better(1). In simple terms, energy density is the amount of energy that can be stored in a given volume, whereas specific energy is the amount of energy that can be stored in a given mass. The S.I. unit for the former is joules per cubic metre and for the latter is joules per kilogramme. In the context of electric vehicles, the energy component is more usefully measured in kilowatt-hours, since the joule is a very small unit of energy(2).
Petrol has a specific energy(3) of around 12.5kWh/kg. Diesel is slightly lower at around 11.5kWh/kg. These numbers might appear meaningless in isolation, but compare them with the specific energy of a traditional lead-acid battery, which is a tiny 0.04kWh/kg and you can immediately see the problem faced by EVs in the past.
To contain the same amount of energy, lead-acid batteries would weigh more than three-hundred times as much as petrol. That is why science has tried to develop new forms of batteries with a much-improved specific energy. The latest lithium-ion batteries used by Tesla contain something around 0.25kWh/kg. This is, of course, a huge improvement over lead-acid batteries, but still only one-fiftieth of the specific energy of petrol. That is why EVs, despite their mechanical simplicity, tend to weight so much more than ICE vehicles.
That’s enough science. The subject of today’s piece is the BMW E1, the Bavarian automaker’s first attempt at producing a viable EV. It was developed partly in response to a Californian state law which required that 2% of each manufacturer’s vehicles sold in the state from 1998 on would be zero-emissions models(4).
The E1 was developed by a team headed by Dr Ing Klaus Faust under the codename Z11 and was unveiled in September 1991 at the Frankfurt motor show. It was driven by a rear axle-mounted electric motor that developed power of 45bhp (34kW) and torque of 111 lb ft (151Nm). A 19.2kWh sodium-sulphur(5) battery pack was installed under the rear seats. It measured 864mm (34”) wide, 457mm (18”) long, 330mm (13”) high and weighed around 200kg (440lbs), contributing to a kerb weight of 880kg (1,936lbs). Both battery and electric motor required water-cooling. Recharging fully from a 240 volt 13 amp domestic supply would take between six and eight hours.
The E1 had an aluminium floorpan and body frame clad in plastic panels. It was a three-door hatchback, a short but wide and tall monobox shape with a low nose and bonnet that swept up to a steeply raked windscreen. The E1 was 3,460mm (136¼”) long, 1,648mm (65”) wide and 1,500mm (59”) tall. It achieved a drag coefficient of 0.32, which was very credible for such a car at the time.
Despite its unique (for BMW) profile, the E1 reprised the marque’s traditional styling motifs such as the twin-kidney front grille (one of which provided access to the integral charging cable) and a ‘Hofmeister kink’ C-pillar. The prototype was finished in an orangey-red mettalic hue, with a funky orange and grey steering wheel and upholstery inside.
The E1 was designed to seat four in comfort, albeit over the relatively short distances dictated by its battery range which, realistically, was somewhere between 100 and 150km (60 and 100 miles) depending on driving conditions(6). The E1 featured regenerative braking to improve its range.
The car was equipped with side and front impact bars and ABS, albeit with drum brakes fitted all round. One unusual feature was that the wheels differed in diameter, with 14” on the front and 16” at the rear, fitted with skinny low-rolling-resistance tyres. The alloy wheels had integral brake drums, which saved about 20kg (44lbs) in weight. The E1’s rearward weight bias meant that no power assistance was required for the steering, which was light and direct. Suspension was by MacPherson struts at the front and trailing arms at the rear.
One specific issue with the sodium-sulphur battery was that it operated at a temperature of around 300°C (572°F). If allowed to cool for more than two days unused, the battery’s electrolyte would solidify. It could be recharged from this state, but this placed great physical stresses on the battery. After about ten such cycles, the battery would fail. Even in regular, normal use, the battery would be unlikely to last longer than a year and the cost of a replacement was around £15k, making the E1 instantly unviable for commercial sale.
The EV technology underpinning the E1 was in its infancy and it would take another twenty years before EVs became a viable proposition. In other respects however, the E1 was a remarkably forward-looking vehicle. Fitted with a small rear-mounted internal combustion engine, it would have made a spacious, practical and attractive city car. Sadly, the single prototype built met an unfortunate if unsurprising end: it was destroyed by a fire that started while it was being charged.
There was a second-generation E1 prototype in 1993, developed under the codename Z13. This was a more conventional looking car with both EV and hybrid powertrains, the latter utilising a BMW K1100 motorcycle engine. Like its predecessor, it remained just a concept.
(1) Yes, automotive journalists frequently mix up these terms.
(2) A joule is 1 watt-second, in other words, one watt of power supplied for one second. Hence, 1 kilowatt-hour equals 3,600,000 joules.
(3) It is more precise to talk in terms of specific energy (sometimes confusingly referred to as specific energy density) than energy density because the volume of liquids (petrol and diesel) and gases (hydrogen) varies with temperature, but the mass (or weight) remains constant. Incidentally, this is why Formula 1 cars are fuelled by weight and not volume.
(4) The law would only cover manufacturers selling more than 30,000 vehicles annually in the state, so would not affect BMW. However, the company was sufficiently concerned about the potential spread of such legislation to develop an EV. This was also the motivation behind the General Motors EV1 project. In any event, the law was later repealed after intensive lobbying by US automakers.
(5) Sodium-sulphur batteries were the best available at the time, with a specific energy of between three and four times that of a traditional lead-acid battery. In this case, the specific energy was about 0.1kWh/kg.
(6) BMW rather optimistically claimed a range of up to 155 miles (250km) for the E1.
Driven To Write 
Another great article Daniel. That would be the last BMW not fitted with disc brakes, and the first BMW not fitted with disc brakes since the ’57 – ’59 BMW 600, (which was the other 4 seat monovolume rear engine BMW.)
The 600’s rather better received successor, the 1959-65 700, also had drums all round.
In the late Eighties there was a nearby ‘very old car’ museum that I visited every now and then.
Amongst their exhibits was a Detroit Electric ‘Grandma Duck’ car.
One day when I came there someone from the museum staff was driving around in the Detroit Electric and when he saw me he stopped and we got into conversation.
He told me that there were plans for a congress on electric mobility featuring all the usual suspects in their museum. The organisers of the congress had insisted the Detroit Electric was removed from open display because otherwise it would have become clear how little progress had been made since the days of this car.
The Detroit Electric had a 6V battery of 12.5 kWh capacity and could do 80 kph for an hour.
Compare this to a Nissan Leaf of Renault Zoe progress is a lame duck.
The problem of battery based mobility is in the batteries. They rely on the principle of the electrochemical series and there are only a limited number of materials that can be used to create a battery and all of them have been extensively tried and tested.
The only way of progress currently left lies in production processes packing more of the stuff into a tighter space. Bosch leads the field in this area and for them the boom in electric bikes is a gift from heaven. They can try and make a couple of thousand bike batteries in a new process which in case of a failure are replaced under warranty for very little cost – this would not be possible if they had to use cars as an experimental field.
Thanks Daniel, and Dave for the important factual information about battery technology. There has also been progress in motor tech, for now restricted to the few who can afford it. Here’s an in-depth look at how Lucid has beat Porsche and Tesla for range and efficiency. The video is cued up to one of the more interesting parts that gets to the heart of the matter, but you’ll likely want to watch all 72 minutes, especially if you don’t have a background in electrical engineering.
While the viewpoint is biased, the science is presented in a forthright manner that promotes understanding and learning for the layperson. A remarkably simple yet clever mechanical innovation is also discussed.
There doesn’t seem to be any reason why this technology won’t be subject to economies of scale and become cheap enough to appear in products more of us can afford.
The ways to improve the efficiency of an electric motor were long known, it was the means to make them reality that weren’t available.
Stronger magnetos – neodymium is the magic word, tighter air gaps – motors can be made to tighter tolerances than ever before. Tight tolerances by the way rule out hubs with integral motors.
That’s another experimental field where electric bikes are helpful for the same reasons as with batteries.
80% Neodymium production is currently sourced from China, but brushless AC induction motors that don’t require permanent magnets are also being used in some EVs.
Here’s an article on the axial flux motors currently used by Ferrari, Koenigsegg, and Mercedes, also claiming significant advances in energy density and specific energy output.
https://www.autoblog.com/2022/08/20/axial-flux-electric-car-motors/
Was there any truth to the supposed legend that BMW were actually thinking of putting some form of Z13 into production as rival of sorts to the upcoming Smart (if there was an awareness of the latter at the time) before they soon went in a different direction?
https://www.motor1.com/news/226055/bmw-z13-concept-we-forgot/
Thank you Daniel for this article today. You have managed to convey maximum knowledge with the minimum necessary words.
Good morning all and greeting from Ireland, our new home! Please forgive me if my below-the-line contributions are a bit erratic for the time being. We are still surrounded by packing cases and it will be a few weeks before we are back to any semblance of order.
Hope you enjoy setting up the new abode
Good morning, Daniel. Checking in from Tokyo.
Thanks for today’s article. There was an electric BMW before the E1, the 1602e. If memory serves me right it was used during the ‘72 Olympics in Munich.
Top tip if you like books about cars: Daikanyama T-site. They have a fine selection there.
The 1602e was made in two examples and its range was even large enough to lead the marathoncrunners along their route! 43 PS, 12.6 kWh
I’m glad you’ve arrived safely, Daniel – and welcome to your new home. I suspect that it’ll be a bit disorientating at first, until you get settled in.
Re the E1, here’s a look at what must be one of the later ones. It’s very professionally put together – it must use parts from other BMWs. I like the retractable cable – it’s like the one on my vacuum cleaner! You’d have to be careful to keep hold of the plug, or I guess it could whack the bodywork as it retracted.
I’d watch the video with the volume low / off, as I found the music irritating. However, it’s worth turning it up when the car moves, as I think it makes a cool sound.
The retractable cable is an early form of range extenter and meant to propel the car in case you’re stranded with an empty battery.
You tie the cable to a fixed object and by retracting the cable you move the car towards the object.
Of course – it all makes sense, now. 😂
Brilliant, Dave! 😁
Best bit: the red MG Maestro style seat belts!
If the volume of petrol varies with temperature, should I avoid refuelling my car in the middle of the day?
Hi Jonathan, without any evidence either way, I imagine the temperature of the fuel in underground tanks at a filling station would be reasonably stable.
A couple of decades ago the recommendation was not to completely fill a tank on a summer day because the expansion of the fuel would lead to spillage.
Nowadays there are expansion reservoirs preventing this.
I had this happen on my barchetta once. There was so much ‘expanded fuel’ that it was even pressed into the charcoal container for fuel vapour. The soaked charcoal was impossible to dry out and the canister had to be replaced (half a day of work because it is inaccessible in the front right hand wheelarch).
Thank you both!
An excellent article Daniel. The appliance of science… Think someone has used that saying before 🤔
Best of luck in your new home too 👍
Internal combustion engines are notoriously inefficient, with a paltry 20% of the energy released from petrol actually turning the wheels (diesel a bit more). For a battery, up to 90% of the stored energy ends up turning the wheels. So, pedantically, in real terms the difference in specific energy falls from 50x to a mere 11x. But then as the EV is heavier (and only gets nominally lighter as range depletes) the available energy won’t push the vehicle so far. Then, away from mere dynamics we come to how much energy is used to drill, refine and transport fuel for the ICE vehicle, and how much energy is used to produce the fuel source and generate the electricity to charge the EV batteries. Is my head beginning to hurt? Fill her up with 4 star mate! It was all so easy.
So, pedantically, in real terms the difference in specific energy falls from 50x to a mere 11x.
This is crucial. The gap being “only” one order of magnitude has motivated a lot of research on battery science because it feels bridgeable. On the other hand, electrified cars are far from being the only goal here. Batteries are being developed at fast pace also for stationary storage applications, where weight isn’t a concern, and for handheld devices, where the amount of energy stored matters, not so much the rate at which it’s delivered. EV batteries are in the most unfavourable corner of the graph, needing both energy and power density while having to keep weight down.
As far as I know, upcoming EV batteries are focusing on improved chemistry and packaging but also on cooling and failure management, since safety has to catch up with increasing energy densities and ever higher charge/discharge rates.
And in terms of safety, a development that might go mainstream is actually solid-electrolyte batteries, which greatly reduce the risk of explosions. The frozen electrolyte in the E1’s battery was in fact a sign of things to come!
Crucially, a gap of “only” one order of magnitude seems bridgeable and this has motivated a lot of research on battery science. Electrified cars, however, are far from being the only goal of battery companies. Batteries are being developed at fast pace also for stationary storage applications, where weight isn’t a concern, and for handheld devices, where the amount of energy stored matters, not so much the rate at which it’s delivered (power). EV batteries are in the most unfortunate corner of the graph, needing both energy and power densities while having to keep the weight down.
As far as I know, upcoming EV batteries are focusing on improved chemistry and packaging but also cooling and failure management, because higher energy/power densities are a challenge in terms of safety too. In terms of safety, one of the developments that might go mainstream is the use of a solid electrolyte, which reduces the risk of explosions even at extreme charge/discharge rates. After all the frozen electrolyte in the E1’s battery was actually a sign of things to come!
I wish my science teachers had been this clear… Thanks Daniel! I liked the first E1 immediately and have never stopped liking it. The second gen looks slightly less well resolved to my eyes:
(Images: https://www.autovanmorgen.nl/vergeten-concept-de-bmw-e1-uit-1991)
Wikipedia and the source of the picture above peg the development code of the second gen at Z15, by the way. Not that I’d take either at face value, but I thought I’d mention it.
I hope your move wasn’t too stressful, Daniel. And that you’ll enjoy the seaside… If things go to plan, I’ll be moving away from the sea sometime next year… I’m going to miss it.
For those interested in computers: Mac website sixcolors.com did a series of podcasts two years ago about notable Macs. One of those was the Powerbook 5300, which sported Lithium Ion batteries – cutting edge back then:
“It gets worse. The PowerBook 5300 shipped with a cool new battery technology, a Sony-supplied Lithium Ion battery. You can probably guess what happened next—this was the Galaxy Note of its era. A few of the Lithium Ion batteries caught fire, although fortunately not while in the hands of consumers. Regardless, Apple had to recall the 5300’s batteries and replace them with less efficient Nickel Metal Hydride batteries.
Talk about a messy roll-out: Here’s our new laptop! It caught on fire, wait, we’ll be right back! Now here’s our new laptop, now with 70 percent less battery!”
(https://sixcolors.com/post/2020/08/20-macs-for-2020-17-powerbook-500-5300/)
Tricky things, batteries. I think I’ve heard experts say that it’s something of a miracle that more batteries don’t explode in our pockets, given the number of these we surround ourselves with every day.
Users of lithium polymer (LiPo) batteries are facing a similar problem.
Spontaneous inflammation is a problem in the RC model fraternity. There are special bags available into which the batteries should be placed before the charging cable is connected.
All alcaline metals are easy electron donors, the more so the smaller the atom is. Lithium as the smallest atom of them is an ideal electron donor in batteries. The problem is that lithium is not picky about the recipient of its electron, it might be atmospheric moisture which then splits into hydrogen and the lithium equivalent of caustic soda. The process creates lots of thermal energy that’s enough to ignite the hydrogen or anything inflammable around it. Boom!
You wouldn’t want firefighters to use water on a battery with a crack after an accicent.
Handling lithium in industrial processes needs clean/dry room facilities to keep out the moisture. And
Off topic, but the article linked above made me feel old.
“Why you need an ultra fast internet connection with ISDN”
That’s right, lithium fires are difficult and expensive to put out because water isn’t the solution. Which reminds me of the story (which I could never fully verify) of the terrible accident at 1955 Le Mans: the large amount of alloyed magnesium in the body of the Mercedes SLR made it very hard to fight the fire with conventional methods (the only ones at hand in the circumstance)
God, I remember ISDN. And dial up. Beee-eep–scree-eeee-eeee-eech.
Apart from the fact that it’s killing the planet by re-releasing the carbon that had been pulled out of the atmosphere over millions and millions of years, back into said atmosphere over the course of about a century, fossil fuels are spectacularly good at transporting energy. Part of that has to do with the infrastructure being built and refined over that same century, so that will just take time with new energy sources.
If only electric energy (whose use is so efficient partly because the inefficiencies are elsewhere in the electricity production chain) could be contained in some fluid that’s less fiddly than hydrogen, which would fit into the current petrol infrastructure. Just substitute “electroil” (TM) for petrol and do away with all that expensive new charging infrastructure. There. Solved it. 😁
With the dangers of batteries having “thermal episodes” and the exotic materials needed to produce them, they really don’t look like the best long term solution. Electrifying the world outside of Europe, the US and China seems like a sisyphean task.
Maybe we should not put photo voltaic modules on our roofs but photo synthetic ones.
These yet-to-be-invented modules contain algae producing fuel from sunlight – should be easy to do by genetic modification.
The algae don’t care whether they produce cellulose or BP five star. Then you could directly convert atmospheric carbon dioxide into fuel which is converted back to CO2 in your car.
There would be green modules for BP fans, red ones for Exxon addicts and yellow ones producing supercortemaggiore.
(this reminds me of the story from VCD development at Citroen, the car that later became the DS. Levèbvre and Bercot encouraged everybody to come up with fresh ideas and one young engineer proposed luminescent bacteria along the B posts for interior lighting. The presentation of the idea resulted in one of Levèbvres famous outbursts of rage).
Now I know that I’m getting old.
First I remember ISDN and then I find out that ‘supercortemaggiore’ wasn’t used since the mid-Seventies.
Excellent, Dave! If only it were possible…
I think this explanation of energy density is a good one. That said, these things soon become out of date as technology progresses. I’m still of the view that having a diverse range of fuel sources available for vehicles makes sense. That said, ICE aren’t being banned in future – we can still have hybrids.
Actually, here’s a version that takes efficiency and other factors in to account. I find it fascinating, if a bit mind-bending. As with petrol and diesel, whether EVs make sense depends on what you intend using them for.
Thanks Daniel for a very interesting article, and the commentariat for erudite informed and interesting discussion.
I think you mentioned elsewhere that you were moving to Cobh, I wish you the best of times in your new location.
I visited Cobh about 25 years ago when we were doing some work for Irish Steel (Ispat as it later became). Less enlightened times then but I would not want that connection today, it seems they were not good neighbours and there is still environmental impact from the site today even though it closed many years back.
Having finished business fairly quickly I had a spare afternoon and a hire car so spent some happy hours exploring the region. Not the best of weather but I gather that is the proper ambience. A lovely region.