I have a 1975 Saab V4 with frewheel in it´s gearbox making "pulse & glide" nice and easy. Having the engine idling for perhaps 1/3 of the time is offcourse a waste of fuel but turning it off every cycle adds a lot of wear to the starter. Using the clutch to start the engine is possible but would add tedious operations since I have to lock/unlock the frewheel.
The perfect solution would be to have a durable and efficient starter, preferably combined with an alternator. I come to think about the 1950´s dynastart in smaller vehicles, as well as modern hybrid systems.
So how to design a modern DIY dynastart to any average car? I see two solutions: Add a suitable starter/alternator to the crankshaft via a toothed belt instead of the standard alternator, or re-designing the flywheel to become a modern brushless dynastart.
The first solution is probably the easiest. A good quality DC-motor from a forklift or golfcar with proper gear ratio should also make a good dynamo with help from a bunch of relays, rectifier diodes and a switch-mode power converter. A possible problem is that there may be a need for different gearing when changing from starter to dynamo mode.
The second solution have about the same need for special electronics as above but also involves a lot of machining in the workshop. On the other hand a brushless design with a permanent magnet rotor will never wear out, have a minimum of losses, weigh less in total and see no broblem with gear ratio.
With english as my secondary language and with text only I find it difficult to describe how I imagine the mechanical design but I´ll give it a try:
The flywheel is modified around it´s perimeter by milling out 5-10 wedge-shaped recesses where flat neodymium magnets can rest. A equal number of flat wedge-shaped pole-pieces from soft iron is lightly secured by stainless screws and spacers secured by glue. The flywheel is now converted to a strong multi-pole magnetic rotor! If the milling is done on the backside of the flywheel it may be possible to reduce the diameter without disturbing the clutch disc.
Around the flywheel is a stator made from soft iron sheet metal machined by EDM and wound with copper wire like any 3-phase electric motor/alternator. It would save a lot of work and cost to use the stator from a commercial electric motor but they are generally quite long compared to the diameter and will not fit between the engine block and gearbox.
I guess the solution above may be exactly what hybrid cars like Toyota Prius have but the question is how difficult it would be to make a DIY version of such a dynastarter? Long ago I made a series of successful and efficient 10W brushless 3-phase electric motors much like the description above but there´s offcourse a big difference between 10W and perhaps 1kW...
Will such a design have a chance to fit in the narrow space on an average car? For my Saab I guess the "scrapheap challenge approach" with a golfcar or forklift motor is the only justifiable solution but it´s always nice to think about more sophisticated solutions...