Magneto-based ignition systems are well known and are often used with internal combustion engines in applications where batteries are not practical. Magnetos are robust devices that are typically highly reliable. Because of these qualities, magneto-based ignition systems have been historically used with internal combustion engines in aircraft applications. Magneto-based ignition systems are also used in applications employing small internal combustion engines with reciprocating pistons--e.g., lawn mowers, small power tools and the like.
Historically, a single mechanical breaker in the magneto rides an engine-driven cam and controls the timing for discharging energy from the magneto to a mechanical distributor, which distributes the energy to the spark plugs. More recently, electronic ignitions have replaced the mechanical breaker and distributor. In these electronic ignition systems, the spark timing can be dynamically adjusted in order to provide best performance and, specifically, to provide the best timing for starting the engine. However, for magnetos having a mechanical breaker, the advance of the spark timing is mechanically fixed and is typically set to provide full power from the magneto to the spark plugs when the engine is running. When the engine is started, the cranking speeds of the crankshaft of the engine are between 50 and 125 revolutions per minute (RPM). These speeds are much less then the normal operating speeds of the engine and, therefore, the mechanical advance of the spark timing is much more than needed or desired and is even a hindrance to easy starting of the engine.
For many small engine applications, the low energy and poor setting of the timing advance in the magneto during cranking of the engine is merely tolerated. For example, before electronic ignitions became Commonplace in lawn mowers powered by internal combustion engines, their magneto-based ignition systems were difficult to start in large part because of the low power and poor timing described above.
For large internal combustion engines or for applications where it is cost justified, a magneto-based ignition system having mechanical timing have in the past often included a sophisticated mechanical system to retard the spark and provide the necessary electrical energy to fire the spark plugs of the engine when it is started. The two most common types of these mechanical systems are retard breakers which use induction vibrators and impulse couplings.
In an induction vibrator, current from a battery flows through the vibrator coil and contacts and then to the primary coil of the magneto, completing its return circuit through a ground return. The vibrator is a type of buzzer that chops the battery voltage into about 200 sparking pulses per second, which are delivered to the plugs as a continuous stream or "shower" of sparks.
To get a retarded or late spark for starting using an inductor vibrator, the magneto has two breakers in it. One breaker defines the timing of a periodic discharge of energy from the primary coil of the magneto to one of the spark plugs during normal engine operation. The second breaker retards the normal timing for delivering energy from the coil to one of the spark plugs and is employed during engine starting in order to enhance the ability of the magneto to reliably start the engine. The breaker used during engine starting is often called the retard breaker.
When the engine starts, an ignition switch is released from a momentary position, which returns the switch to a position in which the magneto operates on the normal breaker and delivers sparks to each of the plugs timed for normal running operation of the engine. When the starter switch is released from its momentary position, the second or retard breaker is disconnected, which also disconnects the vibrator from the magneto.
In an impulse coupled magneto, a sophisticated mechanical arrangement retards the normal timing of the spark and boosts the energy output of the magneto without employing a second "retard breaker" or requiring a battery powered induction vibrator. An impulse coupling magneto relies solely on the rapid spring assisted rotation of the magnet rotor shaft to generate spark energy. In an impulse coupling, the magneto is snapped through its firing position at a fast angular velocity in order to couple the necessary energy from the magnet of the rotor to the coil of the magneto that delivers energy to the spark plugs. This snapping of the rotor is necessary in order to multiply the slow cranking speed of the engine. As soon as the engine starts, a mechanism responsive to the centrifugal force of the rotating crankshaft of the engine disengages the impulse coupling. In a magneto employing an impulse coupling mechanism, a single spark is delivered to each piston for each stroke cycle.
In French Patent No. 2,287,595, an induction vibrator is described that employs a capacitive discharge circuit for concentrating energy from a battery source and delivering the concentrated energy to the magneto having a "retard breaker." Because of the low frequency operation of the capacitive discharge circuit, the transformer used to pump energy into the storage capacitor and thereby concentrate energy from the battery is relatively large, heavy and inefficient--e.g., a volume of 125 cubic centimeters and a mass of 0.35 kilograms. Because of its considerable size and weight, an induction vibrator of the type disclosed in this French patent is impractical for applications in which size or weight is an important design consideration. For example, in an aircraft application, both size and weight are important design considerations that make impractical the use of the induction vibrator of the French patent. In portable power tools, such as small electrical generators, lawn mowers, pumps, etc. the relatively large size and weight of such an induction vibrator would also be considered impractical.
Although both a vibrator and an impulse coupling are reliable under conditions favorable for ignition, the energy they deliver to the plug is marginal in adverse conditions such as severe weather or poor operating conditions. For example, if an engine is re-started before it has cooled, the coil in the magneto is probably hot and, therefore, has a relatively high resistance. In such situations, starting of the engine may be difficult using only an induction vibrator or an impulse coupling. Also, as part of its normal use, the contact points of a magneto can go out of adjustment or the rotating magnets can lose some magnetism. These phenomena reduce the ability of the mechanized booster for the magneto to consistently deliver sparks of sufficient energy to reliably start the piston engine. Moreover, extreme weather conditions may cause even a magneto with a mechanical booster to not easily start the engine. It is also not uncommon for the spark plugs to be fouled by moisture, lead or fuel, particularly after the engine has sat idle and exposed to the ambient weather conditions for extended periods. In these situations, even a healthy magneto can have difficulty starting the engine.