i've been thinking about this issue too. but i would take a much different approach than trying to integrate a custom hybrid system within an existing gas car.
let me back up for a second to show where i got my idea:
if you're familiar with existing electric vehicles (EVs) (conversions, mostly), you know that they're often limited in range - 20-50 miles is common. to permit the cars to take longer trips, several industrious EV owners have constructed custom trailers which either (a) carry extra batteries, (b) carry a generator to charge the batteries while driving (essentially making it a gas/electric hybrid), or (c) have a self-contained internal combustion drivetrain that actually pushes the vehicle (also essentially a gas/electric hyrid).
here's an example of (c).
the 944 is a pure EV. the trailer is a fully functional front half of a front-wheel-drive vw rabbit with an automatic transmission, self-contained with shortened exhaust system & fuel tank. the driver has an extra set of controls in the porsche to start & "drive" the pusher trailer once out on the highway. <a href="http://www.jstraubel.com/EVpusher/EVpusher2.htm">here's his web page describing the set up</a>
what i would do is exactly the opposite:
- my "hybrid" would be a small internal combustion (IC) car
- attached to an electric "pusher trailer"
- the trailer would be for sub/urban use only
- electric-only propulsion up to about 30 mph
- with optional regenerative braking
- and plug-in and/or solar recharging
- added benefit: swappable between multiple IC "host" vehicles
i've done a small bit of reading. a 48 volt system with a 10 hp electric motor would suffice for a car the size & weight of a metro (remember the torque advantage of electric motors - don't need high hp).
i already have manual steering, so i can get "pushed" with the metro's engine "off" and still steer with no difficulty. brakes are power assist, but they aren't hard to use un-assisted.
around town, drive it like a prius: gas engine off, pure electric drive up to around 20-30 mph. above that, switch the IC to "on", pop the clutch and continue driving if you need to go faster. disconnect and leave the trailer at home for hwy trips.
no plans to make this, just like thinking about it.
was going to post a page on my site with this idea. maybe i still will eventually.