SCOOTER SERVICE SHEET
1052
Printed March 1960
250 c.c.
O.H.V. TWIN CYLINDER SCOOTER
TWIN COIL IGNITION SYSTEMS AND EMERGENCY STARTING
A description of the working principles of twin-coil ignition systems.
GENERAL
The 250 c.c. scooters have engines of the vertical twin-cylinder, four-stroke pattern. They are fitted with a
Lucas alternator and coil ignition system incorporating two ignition coils and double contact-breaker units. The
contact-breaker runs at half engine speed.
Under normal running conditions, lighting switch in the " IGN " position, a twin-coil ignition system
functions in the same way as does the single
-
coil ignition system in the " IGN " position, i.e., current from the
battery feeds the primary of the ignition. coils, producing the H.T. spark in the conventional manner. It is
only under " emergency starting " conditions that the working principles of the twin-coil ignition set differ slightly
from those of the single coil set. The system does, of course, work on the energy transfer principle which is now
standard practice on all machines fitted with a Lucas alternator and coil ignition set.
N.B
.-Before reading the following description of the working principles of the " EMG " start circuits, it should be
remembered that they, are being considered only in relation to true emergency starting conditions, i.e., the
batteries in
a
healthy non-sulphated but fully discharged condition. For the sake of clarity, reference to
mechanical considerations has been kept to a minimum, the functional descriptions being confined mainly
to the electrical characteristics.
FUNCTIONAL DESCRIPTION OF TWIN COIL IGNITION SYSTEM WHEN
EMERGENCY STARTING
The emergency ignition circuit on the twin coil system is so designed as to enable the engine to be started on
one cylinder only, utilising one of the ignition coils and its associated contact breaker.
In the combination used (the L/H cylinder) the coil has two cables (each coloured black with white tracer)
connected to its " CB " or "
+
" terminal to enable it to work on the energy transfer principle. One cable
connects to the No. 1 contact breaker (lower pair of contacts for L/H cylinder) and the other to terminal " 15 "
on the ignition switch.
The illustration shows the circuit used for emergency starting. With this circuit the No. 1 contact
breaker is arranged to open when the alternating current in the windings reaches a maximum in the direction
shown by the large arrow. The circuit functions as follows :
With the contacts closed the main return circuit to the alternator is then via one arm (element) of the
rectifier bridge and the closed contacts. In effect the four output control windings have been short-circuited allowing
a heavy current to build up and circulate through them.
At the instant of contact separation this built-up energy quickly discharges through an alternative circuit
provided by the battery and primary winding of the No. 1 or " EMG " ignition coil. The rapid transfer of current
from alternator to ignition coil primary results in H.T. being induced in the secondary winding and an efficient
spark, at the plug.
The efficiency of the energy transfer is quite high because the alternative circuit through the battery,
when the contacts are opened, is virtually a short-circuit path owing to the fact that the " flat " battery has
little or no potential difference across it. Therefore very little energy is lost at this point.
However, due to the fact that the current surges do pass through the battery, and the fact that the two
permanently connected charging coils are also in circuit, the battery begins to build up a potential difference
across its terminals until, after several current pulses, assuming the engine has fired and is running on one
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