Thursday, June 14, 2007

AV - Reliable Ignition

Thanks to space-age electronics the ignition system on most modern-day automobiles can deliver 100,000 miles of service - - about 2,000 engine-hours - - and do so without any maintenance at all. Such ignition systems are superior to magnetos in every way except one: they require a source of electrical energy. Solve that problem and you can enjoy the benefits of a reliable, zero-maintenance ignition system for a scant fraction of the price of the typical magneto.

Fortunately, solving that particular problem is rather easy since a fellow named Henry Ford showed us how to do it. He installed magnets on the rim of a flywheel and surrounded the flywheel with a series of coils. So long as the engine was spinning you would have electrical power. That was in 1897, by the way.

Nowadays we have rare earth magnets the size of a dime that are a million times more powerful than the magnets Henry used on his Model T. Which means we can make a 40A dynamo smaller than a pie pan and weighing less than five pounds. Install that on the crankshaft of a VW converted for flight and you’ve solved the power problem. Now you can enjoy the benefits of a reliable, zero-maintenance ignition system for a scant fraction of the price of the typical magneto. (Why does that sound so familiar?)

Here’s how I did it.


The first task was to design a method of holding the stator coils perpendicular to the axis of the crankshaft. This is dead simple if you want to install the dynamo on the flywheel end of the crankshaft but a bit more difficult if you want to drive it from the pulley hub. One reason for the difficulty is that all VW crankcases are not idential in this area, so the first step was to get rid of any casting flash or as-cast (ie, un-machined) surfaces that might prevent the stator plate from mounting correctly. (The drawings show how to install the dynamo on either end of the engine.)


I won’t get into the design of the stator plate nor drive hub here. There are too many drawings and pictures for a blog. Basically, you simply print-out the patterns on an accurate printer, glue them to aluminum of suitable thickness, cut them out and file the edges to split the line. Vertical spacing is automatically accommodated by using aluminum plate of the proper thickness. To keep assembly simple I used rivets, which you may hand-set if you wish.

You’ll need a lathe for the hub but perhaps we can talk someone in making the things. I’ll post the drawings and photos over on the Chuggers Group when time allows.

The stator plate is held in place by fasteners using threaded bores already existing on the engine; basically you simply bolt it on.


Once the stator plate is attached to the crankcase you simply bolt the stator to it. The stator and rotor are available from Great Plains Aircraft Supply Co. but may also be found at motorcycle junkyards and after-market motorcycle suppliers.


The drive hub is turned from a 2" thick slice of 4" dia. 6061-T6 bar stock. I've made a couple from tooling plate but it doesn't machine very well. I've even melted down some old pistons and cast one. Which taught me how much I didn't know about casting :-)

This is about the tenth iteration of the design and is sized so it can be turned on one of those 7x10 hobby-lathes... if you have the determination :-)

The drive hub is a light press fit onto the nose of the crankshaft, indexed by the Woodruff key. (Making the keyway is one of the Tricky Bits. It may be filed but I bit the bullet and made up a 6mm broach.) This is a relatively fragile part; if you drop it, it will bend. I used a dial indicator to check the run-out and found about +/- .0015, which is pretty good. I’ve no idea what the maximum allowable might be but clearly, little or no wobble is best.

The hub is attached with the stock VW pulley bolt and warpy washer using thread-locker then torqued to 30 ft.lbs.


Be careful here. The six magnets inside of the rotor have enough pull to chop off the end of your finger if you get it in the wrong place. There’s a bit of slop in the bolting holes to allow for alignment. The bolts are commercial stainless steel 1/4-28, half an inch long having drilled heads. They are installed with thread-locker, a regular thickness AN960 washer, an external tooth star washer, torqued to 5 ft.lbs. and should be safety-wired in pairs.

The part shown is as-received from Steve at Great Plains. In a real installation it would be given a light coat of flat black paint on which the engine's timing marks would be stenciled with white paint. TDC, static firing point and maximum advance point are also marked with etchings in case the painted marks gets removed. The stencil is included with the drawings but how to make the thing is up to you. (I used the silk-screen process; a lot of work for such a tiny stencil.)


As shown, the unit weighed 4-3/4 lbs, not including the pulley-bolt & washer.

I’ve now done a few of these, making hubs and stator plates of different dimensions to suit different engine-mount configurations. If you are careful with your dimensions there won’t be any alignment problems. If there are, you’ve a bit of slop in both the stator and the rotor. If you use shims to align the rotor whilst bolting-up, be sure they are brass, wood or cardboard. If you use steel you’ll be in for a surprise :-) (The unit shown is a hangar queen, a mish-mash of parts used solely for illustration.)

Depending on who you talk to this dynamo is good for up to 40 amps. And perhaps it is. But at an rpm where your prop is most efficient you’ll probably see only ten to twelve amps. Which isn’t bad; at that level of output it should run cool and last just about forever.

Ten amps at a nominal twelve volts is 120 watts. Given its small size the thing is probably about 50% efficient meaning it will take about 240 watts of mechanical energy to give you 120 watts of electrical energy. The difference will appear as heat.

Two hundred and forty watts is about a third of a horsepower, which you should include in your overhead or pumping losses. Unlike a belt-driven alternator, the coaxial arrangement provides a symmetrical load of a magnitude sufficiently small so that bearing wear is not an issue.

The rectifier/regulator should be mounted on the firewall and provided with a dry air-blast for cooling. This is the only part prone to failure and that usually happens because of excessive heating. Ensure a good ground.

There are a lot of details I've not bothered to cover here, such as when using a fuel pump (not a problem) or using the fuel pump's opening as your oil filler (ditto). Some of the details will be included with the file of drawings & photos, whenever I can find time to upload them. The purpose of this post is to explain how to provide a reliable source of electrical energy so you can run a modern, inexpensive ignition system... along with your stereo :-)