tech18

The ET Ignition vs. the Battery/Coil Ignition System

Lucas, in all their wisdom designed in the early 1960's an ignition
system for British competition motorcycles called the Energy Transfer
Ignition System (ET). I have been and still am a negative voice about
this system but many of you have such bikes and probably don't
understand how they work or the advantages (?) or disadvantages of
such a system.

My personal experiences are with my 1966 BSA Hornet that came stock
with the ET system. While searching out the gremlins in the never
rebuilt motor I focused on the ET system. What I found was
questionable quality of components, a need for extra good connectivity
due to the systems lower voltages for lighting, and a un-encapsulated
stator with old and cracking insulation on the wiring. Additionally,
the timing was critical as there exist only a very narrow window for
the initial advance setting. I made the system as good as possible,
replacing the Lucas coils with Japanese coils, all to no avail. The
gremlins were still there. The problem turned out to be a leaky head
gasket between the two cylinders. The problem was found at engine
re-build time but the investigation of the ET system convinced me that
it had to go.

The Battery/Coil Ignition

It is , of course, the operational differences in the two systems that are interesting. The battery/coil ignition system uses a battery to provide current to flow through the coil, a coil to generate the spark, a set of points to time the firing of the plug, and a condenser to keep the points from burning when the points are changed from closed to open. The condenser also provides an oscillator effect with the coil after the points open, enhancing the primary voltage swing which can increase the secondary voltage output of the coil.

With the points closed the current from the battery flows through the
coil. With this flow of incoming current a magnetic field is generated
in the coil which stabilizes after some time. At the correct timing
point the points open and the magnetic field collapses. That collapse
of the magnetic field induces a voltage into the coils secondary
windings. Since there are many more turns of wire in the secondary,
the secondary voltage is many times that of the 12 volt battery,
usually 20,000 volts before the air/fuel mixture at the spark plug gap
ionizes and the spark plug gap is jumped. The simple version is points
close, coil saturates, points open, plug fires. The amount of time the
points remain closed for coil saturation is called the dwell time. The
condenser acts as an AC path to ground as the points open, diverting
the current flow away from the points, avoiding burning them from
drawing an arc as they open. The coils primary inductance and the
condenser act as an oscillator when the points are finally open. The
result is that the primary winding/condenser circuit "rings",
increasing the primary circuit voltage above the 12v battery
potential, perhaps to several hundred volts.

The Energy Transfer (ET) System

This system is comprised of a different looking coil, more accurately
defined as a transformer, a set of points, a uniquely wound
alternator, and a condenser. No battery is used because the alternator
consists of a set of windings dedicated to providing AC energy to the
ignition coils. The ET system operates in a completely different
manner from the battery coil system. When the points are closed the
generator current is routed directly to ground. At the correct firing
point the points open and direct the generators current into the coil.
As the current flows through the coils the change from no current to
current induces a voltage in the secondary windings. When that voltage
ionizes the air/fuel mixture at the spark plug gap and gets to about
20,000 volts the spark plug gap is jumped. The condenser acts to
protect the points from the drawing of an arc as they open and to work
with the coil as an oscillator to increase the coil's primary voltage
through "ringing". The short explanation is the points open, current
rushes into the coil, plug fires.

The Differences

In the battery/coil system current flows for a relatively long time in the coils during dwell time, while in the ET system the coils current flow is only during the relatively short period until the plug fires and shortly thereafter. Since no dwell time is necessary in an ET system the points cam can be ground to minimize the time current flows through the coil and Lucas did just that. The different look of the coils is because the battery/coil system coil can get hot due to the long dwell time and is therefore oil coiled.
The ET system coils require only air cooling and can be physically
much smaller. In the case of the Japanese replacement coils they are
actually encapsulated in epoxy. As an aside, the Japanese coils are
"rescued" coils from dual sport and enduro "magneto" bikes from the
late 1960's and early 1970's. They are marked as 6 volt AC coils and
were mounted under the tank or at least away from the actual alternator.

Advantages/Disadvantages

On the surface it would seem that the ET system could be superior but
the low quality of the Lucas components and the mere fact that most of
the parts are at least 30 years old limits the systems reliability.
Add to that, the included lighting coils are 6 volt and anemic in
power output. On top of that, the generating source is an alternator
winding whose peak output is a function of where the magnetic rotor is
timed in reference to the piston's position in its' stroke. The
ignition must be timed to the alternator's peak output point. That
peak is relatively narrow, making the timing more critical than the
battery/coil system and limits the amount of available ignition
advance. The Japanese made the system work for their enduro bikes but
they used superior components and lighting coils that would illuminate
a 24 volt landing light at any RPM greater than about 2000 RPM. As
with anything British, they had the idea correct but failed to
continue development. The Japanese eventually developed a
transistorized/capacitive discharge version of the ET system that is
basically still in use today on off road bikes. The latest Japanese
system eliminated most of the ET's limits including the critical
timing and limited advance characteristics. To be fair other countries
actually preceded the Japanese in these CDI systems with units from
Femsatronic and Motoplat.

Solutions

My solution was to replace the ET system with a new two wire alternator, an electronic rectifier/regulator, a battery (actually optional if you choose the right electronic rectifier/regulator or you use a "capacitor"), new coils, and a Boyer Analog Electronic Ignition. Expensive, probably. Reliable, you bet, mostly because it is all new. But that's another story, isn't it!