BMW i3 Range Extender is Nearly Flat


BMW i3 Has A Unique Extended Range Solution (i3 Concept Coupe shown)

BMW i3 Has A Unique Extended Range Solution (i3 Concept Coupe shown)

We know that the i3’s range extender is based on BMW’s C 650 scooter engine. This engine, in the scooter version is rated at 65 hp @ 7500 RPM. However, as the range extender, it is de-rated to 35 hp. Presumably when used as a range extender the engine will be running at a lower engine RPM due to NHV considerations.

First reaction would be that a typical transverse mounted 2 cylinder would be too tall. One might think that BMW’s boxer engine would be a better pick for this application (picture an old VW Beetle engine with 2 cylinders removed) as it would fit easily under the rear deck.




Having owned a K1200S motorcycle, I know that BWW likes to lay the cylinder down more than on a typical transverse mounted engine in order to keep the bike’s center of gravity low. In the case of the C 650 we find that the cylinders are laid down a full 70 degrees from vertical.

A quick look at the C650 scooter tells us why the engine is laid down so far: scooters are a step thru design.



C650 Cutaway

C650 Cutaway

One can see from the cutaway of the C 650 engine how radically flat this engine is for a conventional 2 cylinder. Other things to note are the balance shafts.

The engine owes its low position and therefore low centre of gravity to its cylinder bank, which is inclined to the front through 70°.The characteristic sound and low vibration levels are the result of the 90° crank pin offset, 270° ignition spacing, and two balancer shafts driven by spur gears.

An electronic fuel injection system supplies the four valves under the two overhead camshafts. Oil is supplied from a dry sump with double oil pump, and an efficient cooling concept optimises the thermal equilibrium in the 2-cylinder engine.”

As a Volt owner, I can say that this BMW i3 is a very interesting offering. Range should be between 80 to 100 miles with only 22 kWh of battery (vs 24 for the Nissan Leaf) due to its light weight, carbon composite design. Also, with 15 more kw of power (135 kW) and 1000 lb less weight than the Volt, this little EV should be a blast with a 0-60 time of 7.2 seconds!


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20 Comments on "BMW i3 Range Extender is Nearly Flat"

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A range extender can run at a lower RPM and power output because it only needs to output a little more than the *average* desired output of the vehicle. My “100hp” engine does 100hp in spurts when I need it, but usually chillaxes at around 20hp on the highway, and less than that when cruising at city speeds. So, a range extender could run at 25hp and still continue to top off the battery while allowing the vehicle to cruise at 70mph. This is done to maximize the efficiency of the range extender. NHV is a close second consideration, but not the first reason.

Bottom line, an EV power train allows the generator to be specialized and lower power than an ICE power train that must do everything and meet peak power output demands.

The concept is interesting, but I am worried about its practicality. With only 35hp I think it will essentially result in a “limp home” mode. Yes, it is better than being stranded with a dead battery, but it isn’t something you can take on a road trip. The Chevy Volt’s engine is rated at 84hp, and if you attempt to climb long inclines with no battery left, you end up limited to 30mph. That’s with 2.5 times the power of the BMW i3. I would be that there will be a limited speed you can travel once you have depleted the battery.

As they say on Wikipedia, “citation needed”! You’re definitely going to need to support that ludicrous 30mph data point. In my Volt I can blast up long inclines far faster than that, in gas mode. Sure, the engine’s making a racket, but it goes.

Really good article George. As a Volt owner, I bought into the range extender concept but quickly saw that I needed it a lot less than I thought I would. I still like the idea of a BEV and a PHEV in every garage but think there might be a larger market for an efficient specialized lower power extender for BEVs. Following Tom Maloughney’s blog and what he has done has partly brought me to this conclusion. This is only the start. As EVs progress, I can see this treated more like an accessory especially if the weight can be significantly lowered and cost can be reduced to a $3500 option.

Thanks for the mention Mark. Reading comments like that really inspires me to continue my efforts 🙂

BMW is offering too little for too much and too late.

Amen brother.

Hey George! Great article, and thanks for mentioning me in your comment above 🙂 Indeed I’m following the BMW i3 development very closely, like I was following the Volt development in 2010. I don’t have an i3 on order, as they aren’t accepting orders yet (except for some sort of pre-order list in Germany). So I’m not yet committed to the i3, and frankly don’t really need to make a decision until Dec 22nd when my own 3 year lease runs out. And even though I do like the i3, I don’t like it enough to BUY it (and keep for 10 years) so I’ll likely do another lease. But I’m glad to see you following me over to the i3 🙂 I’m also starting to reconsider the Cadillac ELR, which I had dismissed after Detroit, but pricing will be a major issue there. One general observation: it seems like Volt owners in particular seem the most comfortable with degraded performance when in gas mode — because the Volt already does that! Sure, the Volt has got good power under gas mode, but it is *NOT* as good as EV mode, and so we’ve already become used to degraded performance.… Read more »

“Volt owners in particular seem the most comfortable with degraded performance when in gas mode — because the Volt already does that!”

That I suppose is what BMW would like the world to think but the reality is otherwise. My Volt has EXACTLY the same performance in EV mode and what you call the gas mode.

Our Volts are not exactly the same, but they are so good you really hardly notice. We are talking about a slightly different beat here than what we have in our Volts. As EVs advance, IMO, you will see three types. One that is designed around the gas engine like the Ford Fusion, C-Max, Honda Accord, Toyota Prius. The unique category found in the Volt, though expect to see this downsize to a three cylinder in the future with slightly less performance than what we have now but still and all-round-respectable solution which you point out. That is definitely noteworthy for those who have not experienced the Volt. And the third being a low power extender for the 75+ mile BEV. The i3 as Geroge points out is touching on this and Volt drivers will help educate the BEV market along with crafty BEV innovators like Moloughney who cant wait for the manufacuters to catch up with their ideas. I think the i3 has made a move in the right direction, though BEV owners will be looking for even smaller,lighter, less efficient extenders for maybe only 5% of their driving needs. This is a totally different beast from the Volt,… Read more »

Beech, I think you and I have different definitions of “exactly”.

Compared to EV mode, in gas mode the Volt has slightly less acceleration and horribly worse NVH. Once you get used to that bottomless well of silent, electric torque, it’s nigh impossible to see gas mode as anything but a stopgap to get you back to EV driving nirvana.

Just so my point isn’t lost here, AS A VOLT OWNER I think we are more willing to accept degraded performance in gas mode, especially in a car that has a 90-100 mile EV range. So bring it on, BMW!

Hummm, seeing as BMW has already had spline problems I wonder if they are going to have belt problems, especially this extreme speed this engine runs at.

If I was trying to shoehorn this engine into a car, I’d consider direct dirve to the generator first and get rid of those belts an cone pullies, and run the thing at 120 Hz, 7200 rpm.

It does not appear as the I3 is capable of going beyond it’s 185 mile range. So those hoping it would be a Volt replacement may be disappointed. The Volt’s range is unlimited. The i3 does not seem to have a strong enough motor and from vague descriptions, they kind of tell you that you cannot go beyond the range, even if you fill up the gas tank again. The i3 may shut you down at the end of it’s range in order to protect the electric drive components that are designed to operate at higher rates. This is why they will come rescue you with a charge, and not with a gallon of gas.
I still welcome the new BMW EV. It should give EV’s more recognition.