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A trembler coil, buzz coil or vibrator coil is a type of high-voltage ignition coil used in the ignition system of early automobiles, most notably the Benz Patent-Motorwagen and the Ford Model T. Its distinguishing feature is a vibrating magnetically-activated contact called a trembler or interrupter,
(2025). 9781465517715, Library of Alexandria. ISBN 9781465517715 which breaks the primary current, generating multiple sparks during each cylinder's power stroke. Trembler coils were first used on the 1886 Benz automobile, and were used on the Model T until 1927.
The switch contacts are on a springy iron arm, which holds them closed. The arm is mounted near the iron core. When battery power is applied, the coil acts as an electromagnet; the magnetic field from the core pulls the springy iron arm, opening the switch contacts, interrupting the primary current. The magnetic field of the core is switched off, allowing the arm to spring back, closing the contacts again. Then the primary current is turned on again, and the magnetic field opens the contacts again. This cycle repeats many times per second, while power is applied to the coil. A similar mechanism is used in the electric bell.
As the circuit opens each time, the energy stored in the solenoid's magnetic field is released and by electromagnetic induction produces a pulse of high voltage in the secondary coil winding. This voltage is sufficient to fire a spark plug located in the engine's cylinder, igniting the petrol mixture.
The difference between a trembler coil and a modern ignition coil is that in a modern coil the primary current is broken only once by the contact breaker for each cycle of the piston, creating a single spark which is precisely timed to ignite the fuel at the correct point in the cycle. In the trembler coil, in contrast, the vibrating interrupter contact breaks the current multiple times during each cycle, creating multiple pulses of high voltage and multiple sparks.
The trembler coil operates equally well from AC or direct current electricity. In the Model T, a battery was used for starting, but once the engine started the supply was switched to the magneto. The Model T was unusual in being fitted with an AC alternator (a permanent magnet magneto) rather than a dynamo. In those early days, this was not rectified and so the alternator's output remained as AC. This was entirely adequate to operate the ignition system, and after 1915 to power electric headlights, although it could not be used for battery charging.
A simpler device, the low tension coil, was already in use for stationary engines and spark-ignition . This was a simple iron-core inductor, used with a battery and a switch called a contact breaker. It had a single winding and so was not a transformer like the trembler coil. When the piston was at the right point, the contact breaker opened, breaking the battery current. Self-induction due to the collapsing magnetic field generated a high voltage pulse in the coil, which was applied to the spark plug. The drawback of the low tension coil was that the high ignition voltage was generated in the same circuit that the low voltage battery current flowed in.
For correct operation, each cylinder must be fired in turn, and at the right time. Both of these tasks were conducted by the 'timer' or low-voltage distributor, a rotary switch. In a four-stroke engine, the timer is driven at half-crankshaft speed, like the camshaft. The timer switched the primary current to each coil in turn and also started the coils at an appropriate time in the cylinder's cycle. As was common for engines of this period, the ignition timing was also controlled by a manual advance and retard control. Starting a cold engine could require dexterous manipulation of the control to get the engine to start. Modern engines control such timing even more carefully; this is now automatic and not obvious to the driver.
A significant difference from modern ignitions is in the strength and number of sparks produced. A modern system produces one, large, spark at exactly the time commanded. The trembler coil systems cannot produce such a high energy spark, but it does produce a continuous stream of sparks for as long as the timer circuit is closed. Early engines, like the Model T, ran at slow speeds with large cylinders filled with weakly burning mixtures of low octane ratings. These were both less sensitive to the accuracy of ignition timing and their mixtures also benefited from having a sustained ignition source.
The Model T was also available in versions tuned for kerosene or ethanol fuels. Trembler ignitions were particularly suitable for igniting these mixtures. Tremblers remained popular for kerosene and TVO tractor engines long after they were obsolete for petrol.
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