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Part 11: Build a Reliable Light Weight Ignition System for Your VW Or Subaru Aircraft Engine

By Ron Wegner

SPECIFICATIONS

The VW engine was originally designed, and used, to power two seat training aircraft between the great wars, before it became a car engine. In re-birthing it as an aircraft engine, we have usually had to change both ignition and carburettor systems.

One of the earliest conversions used the Scintilla Vertex magneto replacing the distributor, and later, a single aircraft magneto was used, direct driven off the rear of the engine. This has been expanded to dual ignition using a secondary electronic system, with an extra sparkplug installed in each cylinder head. Sometimes the latter has resulted in cracked heads, and has always resulted in increased weight which our little aircraft can ill afford.

Modern spark plugs are very reliable if normal servicing is carried out, but in a single ignition system using a magneto or coil and distributor, if one condenser or one set of points, or one coil fails in flight, it can spoil your whole day!

My system overcomes these problems by providing four separate ignition systems, one for each cylinder. See Fig. 1. If any interface/ coil combination fails or blows its fuse, the remaining three cylinders are unaffected.

Part 11 Figure 1
Part 11 - Figure 1

The system uses a standard 009 VW distributor which has centrifigal advance (modified), but no vacuum advance. The rotor and distributor cap are deleted, and an aluminium cap fitted on top. The points plate is changed, and four photo interrupters mounted, each spaced 90 apart, and activated by a rotating aluminium drum with a " window" in its edge, attached to the distributor spindle. The photo interrupters are connected to four electronic interfaces, each feeding its own coil.

The coils are secondhand units from a Yamaha 650 Special motorcycle, available from any motorcycle wrecker for around $25 each. Because of the design, each coil is charged up, then fired, in seqence, so when the engine is running, the whole system draws only 1.8A from a 12V battery, and continues to work OK down to 10.7V.

WEIGHT

The weight of the system. is as follows:-

RELIABILITY

Refer Fig.2. The system was given an extensive bench trial of over 250 hrs continuous, using an electric motor to drive the distributor at 1500 rpm,(equals 3000 rpm engine). With no lights on at night in my workshop, the sparks attracted and electrocuted dozens of insects! The system was then installed in my Teenie Two, 19-3282, and has performed faultlessly for three years and approximately 100 flying hours. Hand cranking is very easy; my engine usually starts third pull, hot or cold.

Part 11 Figure 2
Part 11 - Figure 2

COST

The cost of the system was as follows:-

Total = $358 plus approximately 4 hours lathe work.

INTERFERENCE

The system produces no electrical noise in the 12V wiring, and provided you mount coils and interfaces in a metal box and use shielded HT leads and plugs, no hash in the radios. See Fig.3. I mounted mine on either side of the fuselage inside the streamlining cheeks behind the cylinders, 2 coils and interfaces on the port side for Nol and No2 cylinders, and 2 on the starboard side for No3 and No4, along with the rectifier/regulator for the generator.

Part 11 Figure 3
Part 11 - Figure 3

OTHER FEATURES

The system also has a "cylinder test" feature, consisting of a panel mounted single pole nine position rotary switch, the positions being OFF, 1, OFF, 2, OFF, 3, OFF, 4, OFF. At position 1, No1 cylinders interface is disabled, ditto for 2, 3, and 4. Once the engine is warmed up, revs are set to 1000 rpm, and the switch cycled through the 9 positions. As each cylinder is disabled, the RPM should drop by the same amount, indicating that each cylinder is doing the same amount of work to maintain 1000 rpm. If the drop on one cylinder is less than the others, it indicates that cylinder is not working as hard - can be caused by leaking valves or rings, weak spark plug or faulty HT lead, or a faulty interface. In flight, if you have one cylinder misfiring, the switch will help you pinpoint which one it is.

Each interface also has a tacho output to feed into my homebuilt tacho. It weighs 2.5 Ozs, accurately indicates to 5000 rpm, and costs about $40 to build. There will be an article on that in the future.

This system could also be used on the Subaru engine, with the Subaru distributor modified same as the VW.

POWER

To power the system, plus radios etc., I have developed three different generators, all direct driven off the flywheel holding bolt at the rear of the VW engine. See Fig.4.

Part 11 Figure 4
Part 11 - Figure 4

MK 1

This system, fitted to my Teenie 2, uses a modified Kubota dynamo, with a rectifier/regulator of my own design. Following are the specifications:-

Cost:

MK 2

This one is for those of you who want to "build it yourself". It uses the same rectifier/regulator as the MK 1. The stator is a second hand unit from a Honda D-10 motorcycle, available from motorcycle wreckers for around $50, and rewound. The rotor is made from a galvanised steel fence post cap, and the magnets are small rare earth types available from Jaycar. The housing is aluminium, and the spindle made from bright steel, running on two sealed ball bearings.

Specifications:

Cost:

MK 3

This system uses a second hand 12V Denso car alternator as fitted to Daihatsu cars, models CB, EF, EL, & ED10. I purchased mine from a wrecker for $50, dismantled, checked, and reassembled. The two aluminium lugs were sawn and ground off, the pulley replaced with the driven half of the coupling. The alternator has its own internal regulator.

Specifications.

Cost:

The MK 1 and MK 2 generators, having no sliprings or brushes, create no noise in the electrical system or the radios. Provided the brushes and sliprings in the MK 3 are in good order, and the usual suppression capacitor is connected to the output terminal, it will also be noise free. The MK 1 and MK 2 provide enough output at 1100 rpm to carry the ignition load. The MK 3 does the same at 1300 rpm.

In my next article, I will explain how the ignition system works and give you a material list of electronic and mechanical parts required.

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