Typically, internal combustion engines include spark plugs along with spark-generating ignition circuitry to ignite an air-fuel mixture in the cylinder of the engine. Some engines employ permanent magnets attached to a rotating flywheel to generate a voltage on a charge coil. In a typical capacitive discharge system, electrical energy from a low voltage battery is fed into a power supply that steps it up to a higher voltage on a capacitor, which provides the voltage necessary to cause an electrical spark across the spark gap of a spark plug. The capacitor transfers its energy into the primary winding of an ignition coil and into the magnetic core of the ignition coil. Energy is extracted from the ignition coil secondary winding until the capacitor and magnetic core are absent of sufficient energy. In an inductive system, energy is pulled from a low-voltage battery in the primary of the coil. When the current is interrupted in the coil primary winding, a flyback occurs which initiates breakdown on the secondary winding and energy from the ignition coil core is extracted via the secondary winding. In both capacitive discharge and inductive ignition systems, energy is transferred to the magnetic core of the ignition coil through current flow in the primary winding of the ignition coil at a time T1. At a later time T2, the ignition coil secondary voltage and current are produced from the energy stored in the magnetic core. The ability to change secondary coil characteristics of open circuit voltage (OCV), current amplitude (CA), and spark duration (SD) are all related to changing the energy stored in the magnetic core of the coil. However, once energy has been placed in the magnetic core, the secondary coil characteristics are for the most part predetermined to be whatever the secondary load allows and cannot be changed until the next firing.
For a given inductive or capacitive discharge coil design, OCV, CA, and SD are directly proportional to stored energy. As the energy stored in the magnetic core is increased, all three of these values increase. The biggest constraint in these systems is open circuit voltage. This parameter always has to be large enough to reliably initiate a spark. So there is some minimum energy that is required to be applied to the coil so that there is reliable spark generation. For typical inductive and capacitive discharge ignition systems, the OCV is on the order of 25-40 kV. This limits the amount of adjustability in CA and SD that is available through adjusting energy application. Further, CA and SD must both increase or both decrease. In conventional inductive or capacitive discharge coil designs, these parameters cannot be adjusted independently. To modify the overall response of the ignition system, it is generally necessary to modify the coil design. And, typically, for a given coil design, the relationship between the OCV, CA, and SD cannot be optimized for different engine operating conditions.
As an alternative to capacitive discharge and inductive ignition systems, some engine systems employ alternating current ignition (AC) systems. In an AC ignition system, the alternating current is typically developed by a DC-to-AC inverter. There are several types of inverters that may be used in such a system. For example, an exemplary AC ignition system includes a transformer with a center-tapped primary coil and a secondary coil connected to a spark plug. An arc may be initiated at the spark plug by discharging a capacitor to one of the windings of the center-tapped primary coil. Both of the primary coil terminals are connected to a switch or transistor. The switches can be alternated between on and off to reverse the direction of current flow in the primary coil and, therefore, in the secondary coil. Control of these switches may be effected in a manner that facilitates adjustment of the CA or SD period.
However, AC ignition systems generally use more power semiconductors, such as switches and diodes, than capacitive discharge and inductive systems. Or, alternatively, the AC ignition requires ignition coils with more than two windings, such as a center tapped coil primary arrangement. Generally, as coil complexity decreases, the use of power semiconductors increases and vice versa. This makes AC ignition systems more costly to build and potentially less reliable as the additional components and increased complexity provide more points of possible failure. Further, many AC ignition systems do not permit precise real-time control of the secondary coil current, which determines the characteristics of the spark discharge.
It would therefore be desirable to have an alternating current ignition system that can be built less expensively using fewer components than conventional alternating current ignition systems and be able to fire a simple two-winding ignition coil. It would also be desirable to have an ignition system that allows for a greater degree of precise real-time control of the SD and CA than typically found in conventional inductive, capacitive discharge, or alternating current ignition systems.
Embodiments of the invention provide such an alternating current ignition system. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.