DIY dynastart (starter/alternator in flywheel) - Fuel Economy, Hypermiling, EcoModding News and Forum

JohnAh · 08-12-2012, 10:15 AM

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...

JRMichler · 08-12-2012, 05:09 PM

The stator from an induction motor is a stack of thin laminations. If you are making a motor, you can use as many or as few of those laminations as you want.

One of the problems with a pancake motor (large diameter relative to length) is that it has large losses due to the end turns plus 3D magnetic field effects. They are not efficient.

Large neodymium magnets are a real challenge to handle.

IsaacCarlson · 08-13-2012, 02:10 AM

Yes, large neo's are very hard to handle and require extra care. If they get within a foot of anything magnetic, anything can happen. BE CAREFUL.
They can crush hands/fingers. Sometimes they can take them right off. Be watchful of the repelling force as well as the pulling force.

JohnAh · 08-13-2012, 06:01 AM

Without making any serious misstakes with neo-magnets I have realised that a man should NEVER carry a big magnet in each front pocket of his pants...

Interesting point abot losses in a pancake motor! I didn´t think about that but it seems very reasonable. Without an advanced workshop this approach is impossible anyway. The golfcar/forklift motor approach is probably a lot more DIY-friendly but but I guess that solution demands different gear ratios when working as a starter or as a dynamo. The low gearing needed for starting will force the motor to very high speed when the engine is running fast. This will increase wear of the electric motor and perhaps even cause it´s rotor windings to move... Maybe it´s not that bad to have a separate starter and alternator after all...

ecomodded · 08-14-2012, 11:22 PM

It would be nice if most automatics could coast jump start ,it seems most cannot at lower speeds, for them its the starter or nothing.
For me with my manual transmission car i bump start exclusively when coasting.
I need a electric brake booster to keep my brakes fully functional while engine off coasting.
A little electric brake booster should not be too complicated or expensive, ideas welcome ..

JohnAh · 08-15-2012, 06:33 AM

Electric brake booster, isn´t that what many modern cars with ABS-brakes have? I know some still have the vacuum servo but I think there are others that have a electric hydraulic servo together with the ABS unit.

I guess an electric vacuum pump wouldn´t need to be a very big thing, if the car have an ordinary vacuum servo.

I prefer to REMOVE things from a car to make it lighter but a vacuum tank may be a good backup and an easier alternative to electric pumps. Some time ago I had a rare russian car (a restoration object without engine), a ZAZ965 from 1967. One interesting thing about these cars was that probably all of them were equipped for disabled drivers. There was a hand-operated clutch lever and a vacuum brake servo operated from a lever at the steeringwheel. To improve safety there was a quite big vacuum tank under the backseat.

Adding such a tank should be a cheap and easy modification. A vacuum gauge at the dashboard will give an early warning for when it´s time to start the engine of safety reasons. If the one-way valve is mounted directly at the brake servo, another valve must be mounted between the vacuum tank and the intake manifold. I also guess there should be a calibrated hole/orefice so the engine doesn´t have to empy the tank too quick (risk for stalling due to lean mixture).

ecomodded · 08-15-2012, 12:17 PM

I have found some good info on brake boosters today, it seems some newer cars use hydraulic pressure generated by the power steering pump rather than engine vacuum to provide the power assistance.
Most of the gasoline cars brake boosters use the engine vacuum with the assistance of a rubber hose that comes from the intake manifold.
On diesels, the engine does not create enough suction pressure by itself for the brake booster to function. Instead, engine power is used to drive a diesel vacuum pump to create the required pressure.
The pump is driven mechanically through either a gear drive or a port on the rear of the mounting flange.

JohnAh · 08-16-2012, 05:07 PM

@ecomodded: Oh, offcourse! I didn´t think about diesels... I have seen one with a small belt-driven vacuum pump for an ordinary vacuum servo. I guess such a pump could be modified to be driven by an electric motor and it will only need to be activated when the engine is turned off and the servo is running out of vacuum.

But I would personally try a vacuum tank first. I guess it doesn´t need to be particulary big to give margin for several safe brakings.

Talking about preassure/vacuum, long ago they used compressed air to start diesels and gunpowder to start airplane engines but I guess none of those methods would be suitable for P&G eco-driving...