What BMW’s decision to incorporate a gas engine into its new, electric i3 says about batteries.
Tight fit: The range-extending gas engine sits next to the electric motor between the rear wheels of the BMW i3.
In the last few weeks BMW has made a lot of the
claim that its new i3, an electric car it will start selling in the
fall, is a purpose-built electric vehicle (see “Tesla Versus the Luxury Automakers”)
. Unlike the electric cars made by many other automakers, it isn’t just
a gas-powered car with the engine and fuel tank swapped for an electric
motor and battery back. It’s a new design, with the battery pack and
motor kept in mind from the beginning. And engineers went to great
lengths to lighten the car to extend its battery range.
But
the i3 isn’t really a purpose-built electric car. It’s a purpose-built
plug-in hybrid. BMW designed it so that at the back of the car, between
the rear wheels, there’s a compartment for a gas-powered generator
(originally designed for scooters) which can recharge the battery as you
drive. In the battery-only version, that space is wasted. In a purpose
built battery-only electric car, it could have been used for more
batteries or more space for luggage.
The
difference matters because of what it tells us about batteries. With its
design choices, BMW is basically saying that battery technology isn’t
good enough yet. It doesn’t buy the argument of other automakers (save
Tesla) that a range of about 100 miles is enough to attract large
numbers of customers. BMW, which expects most customers will by the
version of the i3 with a gas generator, is essentially saying the same
thing as GM, which builds the gasoline-enhanced Volt, although BMW is
using a much smaller engine and gas tank. Customers are going to want
more range than batteries alone can affordably provide.
Below the image are the numbers that show what BMW was thinking.
Empty space: Without the range extender, a lot of space between the wheels of the i3 goes unused.
A gas tank about the size of a brief case can
nearly double the range of BMW’s i3 from about 145 kilometers (90 miles)
to 270 kilometers (168 miles). The gas engine also adds about $3,650 to
the price of the car, boosting it, according
to reports, from $41,350 to $45,000 in the U.S. Although the engine
adds weight and cost, affecting performance, doubling the size of a
battery pack would be more expensive and add more weight.
What
would the tradeoffs have been of doubling the size of the battery pack
to get a similar range? Automakers generally keep quiet about the cost
of batteries, but current estimates suggest it would cost about $11,000
to double the size of the i3’s battery pack, far more than the price
increase for the gas generator. It would also add another 230 kilograms
of weight, nearly twice the weight of the range extender system.
So
adding range with batteries would cost more, and it would weigh down
the car more, and presumably would affect the performance of the car
more.
These calculations will change as
batteries come down in price and weight (the two go hand in hand,
because cost comes down as battery makers find ways to reduce the amount
of material needed). If batteries get cheaper than around $200 per
kilowatt hour, extra batteries start to look competitive with the
extended-range gas engines—especially if you factor in the offer
advantages such as quieter operation and reduced maintenance.
But
even at that cost, a challenge remains. One great thing about the
range-extender concept is that you can refill the gas tank in a few
minutes at ubiquitous gas stations, which has got to be reassuring to
drivers. Charging stations exist that can recharge the BMW to 80 percent
capacity in half an hour, but they’re uncommon. Tesla Motors has
superchargers that are twice as fast, and it has demonstrated battery
swap stations that are faster than filling up a gas tank. So we know
that it’s technically possible to get an all-electric car back on the
road quickly. It will take some time, though, before such charging
stations are close to being as readily available as a gas station.
Has
BMW hit on a better range-extending concept than GM? The i3 has a much
longer battery-only range than the range-extended Chevrolet Volt or the
upcoming Cadillac ELR, which travel about 35 miles on a charge, but can
go 300 miles more with electricity from the gas generator. The 90-mile
battery range of the BMW will allow far more drivers to use batteries
alone for commuting, recharging by plugging in at home and possibly
never needing to go to the gas station. But the range-extending gas
engine doesn’t provide enough range for the i3 to be a general
replacement for gas-powered cars. It’s more of a second car, or one for
commuting only, not road trips.
It will also be interesting to watch Tesla, which seems to be driving down battery costs faster than others. It’s betting on pure electric vehicles that have a range on a charge close to that of gasoline-powered cars—its Model S can go 265 miles on a charge. Tesla claims it can make a $35,000 car with a range close to that of a Model S within a few years, by which time it plans to have an U.S. wide fast-charging network in place (see “Forget Battery Swapping: Tesla Aims to Charge Electric Cars in Five Minutes”). If its plan succeeds, electric cars with gas range-extenders could prove to be only a short term fix, and we’ll look back on them in a few decades as a historical curiosity.
http://www.technologyreview.com/view/517936/why-bmws-i3-electric-car-is-really-a-plug-in-hybrid/
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