IEEE Spectrum November, 2009 - 26

battery-powered EVs approaching the
market, such as Mitsubishi's iMiEV subcompact car, which is expected to operate at three times that efficiency.
The Nègres argue that although lithiumion-powered EVs can beat the AirPod's
efficiency by a large measure, they make
for unrealistically expensive cars. A lithium battery for the AirPod would cost
almost as much as MDI will be asking
for the entire vehicle. Most city cars on
the market today use cheaper but heavier
and less efficient lead-acid batteries. As a

and €35 000-a steep jump up in price
from vehicles of similar size with internal
combustion engines, which can be had for
less than €10 000. So the AirPod is economically competitive with conventional
vehicles of this sort, and critically, it meets
Air France's modest needs-according to
Bouteyre, a top speed of 30 to 40 km/h
and a range of 10 to 20 km per day.
Motorists, however, have come to
expect much more from their cars.
That's why some of MDI's critics think
that automakers should be focusing not

double duty: the Airpod's engine uses two linked cylinders. compressed air flows into the
smaller cylinder first at a constant pressure of 20 bars [1]. When the smaller piston bottoms
out, the intake is closed, and the air in the small cylinder expands, flowing into the larger cylinder
[2 and 3]. both pistons then move to exhaust the expanded air [4], and the cycle begins again.

illustration: jason lee

result, estimates MDI, those EVs actually
suck more energy from the grid for each
kilometer traveled than does the AirPod.
Doug Nelson, an expert in hybrid vehicles
at Virginia Tech, in Blacksburg, can't confirm that assessment, but he agrees that
pneumatic vehicles could be a winner for
the city-car segment if lithium batteries
remain too pricey.
The promise of a cheaper form of clean
transport is indeed what convinced Air
France to consider AirPods, according
to Jérôme Bouteyre, the airline's director of facilities management. EVs that
Air France has tested as runabouts for its
maintenance crews cost between €15 000
www.spectrum.ieee.org

on air-powered cars but on pneumaticfuel hybrids. Unlike cars running on
compressed air alone, the greater power
available from pneumatic hybrids would
suit full-function, highway-capable vehicles. And compared with today's hybrids,
whose battery-equipped drivetrains cost
three times as much as an ordinary gasoline engine, pneumatic hybrids could be
priced for economy shoppers and for the
developing world. "We have a system that
provides 80 percent of the benefit and
costs maybe 20 percent extra," says Lino
Guzzella, a professor of mechanical and
process engineering at the Swiss Federal
Institute of Technology, in Zurich.

With support from German autocomponents giant Robert Bosch, Guzzella
is testing small engines that use a
20- to 30-L compressed-air tank to rival
the performance of power plants twice
their size. That tank is filled with air pressurized to 20 bars using braking energy
or spare engine power. In this novel variation on conventional turbocharging,
that high-pressure air is then fed back
into the engine along with extra fuel to
deliver precise bursts of power. "We are
able to burn the fuel in a much more efficient way," says Guzzella [see sidebar,
"Turbocharging the Turbocharger"].
MDI is working on an even simpler
hybridization scheme, first successfully
employed in 1901 to extend the range
of pneumatic torpedoes. The idea is to
add a small fuel burner upstream of the
engine to warm the air released from
the tank, increasing the air's volume
and thus reducing the amount required
to charge the cylinders. MDI claims
this dual-mode system will triple the
AirPod's range, while consuming just
0.56 L/100 km of gasoline (420 mpg).
Clodic, however, questions whether MDI
possesses the discipline and resources
to see this hybrid system through.
MDI's prospective industrial partners, Tata Motors and Air France, seem
to have a wait-and-see attitude. Air
France, which originally expected to
begin testing AirPods in May and was
still awaiting delivery as this article
went to press, isn't worried, according
to Bouteyre: "Either it's built and corresponds to our requirements, or it's not
and we won't try it. We have nothing to
lose." The head of corporate communications for Tata Motors, Debasis Ray, says
his company is licensed to use MDI's
technology in India, "only as and when it
gets ready. We are not engaging beyond
this at this point in time."
Guy's response to the skeptics? He says
that there is no such thing as a truly independent expert for assessing novel vehicles: "The only evaluation possible is on
the road." True enough, but that's only an
option if others can acquire examples of
the finished product. And in all the time
that MDI has been working on vehicles,
no product has emerged. In the computer
industry that's known as vaporware, an
appellation that seems oddly appropriate
for a car that runs on air.
o
To ProbE FUrThEr To see an online
slide show surveying other micro-size cars,
go to http://spectrum.ieee.org/micro_cars.
NovEmbEr 2009 * iEEE SpEctrum * NA

http://spectrum.ieee.org/micro_cars http://www.spectrum.ieee.org

Table of Contents for the Digital Edition of IEEE Spectrum November, 2009