Part 11: Build a Reliable Light Weight Ignition System for Your
VW Or Subaru Aircraft Engine
By Ron Wegner
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
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.
The weight of the system. is as follows:-
- 4 coils plus 4 interfaces = 1.22kg. (21b ll ozs).
- Distributor and clamp. = 0.511kg. (1 lb 2 ozs).
- Total = 1.73 kg. (3 lb 13 ozs) - (a single aircraft magneto
weighs 3.18 kg (7 lbs)!)
- 12V 4 AH gel cell battery = 1.7 kg. (3 lb 12 ozs).
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
Part 11 - Figure 2
The cost of the system was as follows:-
- 4 second hand coils = $100
- Second hand distributor = $50
- Aluminium for rotor and cap = $15
- Electronic parts = $170 + $23 for dalo pen, persulphate
etc, if you don't already have them.
Total = $358 plus approximately 4 hours lathe work.
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
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.
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
This system, fitted to my Teenie 2, uses a modified Kubota dynamo,
with a rectifier/regulator of my own design. Following are the specifications:-
- Weight of alternator, coupling, mounting, & rectifier/regulator = 1.73 kg. (3 lb 13 Ozs)
- Length behind engine rear face. = 130mm.
- Diameter. = 100mm.
- Output. 12V @ 3.5A at 3000 rpm, regulated to 14V.
- 1 new Kubota dynamo = $420.(October 02). Second hand OK if you can find one.
- Parts rectifier/regulator = $50.
- Winding wire, aluminium, steel. = $55.
- Total = $525 + approx. 6 hrs lathe work.
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.
- Weight of alternator, coupling, mounting & rectifier/regulator. = 2.04 kg. (4.5 lbs).
- Length behind engine = 140mm.
- Diameter of alternator = 95mm.
- Output - 12V @ 3A at 3000 rpm, regulated to 14V.
- S/H stator. = $50.
- Rectifier/regulator parts. = $50.
- Rare earth magnets. = $30.
- Cap. = $5.
- Winding wire, aluminium, steel & bearings = S80 + approximately 8 hrs lathe work.
- Total = $215.
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
- Weight of alternator, coupling & mounting = 2.95 kg (6.5 lb).
- Length behind engine = 175mm.
- Diameter = 125mm.
- Output 12V @ 8A at 3000 rpm, regulated to 14.2V.
- S/H alternator. = $50.
- Aluminium and steel. = $40.
- Total = $90 + approximately 4 hrs lathe work.
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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