Internal combustion engines operate by igniting fuel in order to apply a force to a mechanical system. For example, in a piston engine the piston provides a movable surface which forms a combustion chamber. Fuel enters this chamber as the piston retracts and is then compressed as the piston advances to reduce the volume of the combustion chamber. When the fuel ignites, the chamber volume is forced to expand, thus pushing the piston and applying force to a drive system. The momentum of this drive system causes the piston to advance which expels the expended fuel and the process then repeats. The performance of such engines depends highly on the relationship between the piston position and the timing of the fuel ignition. For example, if the fuel is ignited before the piston reaches its top position (known in the art as “top dead center” or TDC) the force of the burning fuel will apply force to the drive system in the wrong direction. However, if the piston has moved well past TDC when the fuel ignites the energy applied to the drive system is reduced representing a loss of output power. This dependency on ignition timing is common to most internal combustion engines, including two-cycle and rotary engines. As such, nearly all such engines include a method of timing the fuel ignition with the state of the combustion chamber.
In small engines, such as those used in gasoline-powered lawn equipment, the ignition timing system comprises a permanent magnet embedded in a flywheel synchronized with the piston, a primary coil of wire and a secondary coil of wire. Both coils are wrapped about a ferrous armature which is located in close proximity to the flywheel. Each pass of the magnet generates a set of voltage pulse across the coils. Pulses generated in the primary coil are captured in a capacitor in the ignition timing circuit. Pulses generated by the secondary coil are used to initiate the transfer of this captured energy to a fuel igniter which serves to ignite the fuel. The position of the secondary coil determines the timing between the piston reaching TDC and the ignition of the fuel.
In small engines the desirable timing between TDC and fuel ignition varies with the operating conditions. For example, when first starting the engine, it is often desirable to delay the fuel ignition until the piston has passed TDC by several radial degrees. This allows for easier start-up, particularly for pull-rope engines. However, when the engine is operating at high revolutions per minute (RPM), better performance in terms of output power and reduced emissions is obtained by advancing the fuel ignition to just before the piston reaches TDC. Unfortunately, the timing of ignition in small engines is often controlled largely by the physical position of the secondary coil armature with respect to the piston position. Given this, such engines typically have a constant ignition delay measured in radial degrees. As such, this delay is often a compromise between ease-of-start and high-speed performance. Prior art attempts to provide variable ignition timing, such as those disclosed in U.S. Pat. Nos. 5,931,137, 6,408,820, 6,932,064, 6,973,911, and 7,069,921 implement multiple secondary coils and/or complicated circuitry which increase the complexity and cost of the engine's construction. As such, these solutions are poorly adapted to the low-cost engines typical of small hand-held equipment.
U.S. Pat. No. 5,931,137 describes an ignition apparatus for internal combustion engines which provides spark advance. The spark advance is generated by application of a trigger coil and an auxiliary coil wound about different permeable cores in close proximity to the engine's flywheel. The coils are connected to triggering circuitry. When the flywheel rotational speed crosses a design threshold, pulses from the auxiliary coil cancels pulses from the trigger coil, thereby providing spark advance at higher engine RPMs. This approach increases the cost and weight of an engine by requiring multiple coils and permeable cores. It can also provide only two spark advance settings, one for low RPM values and one for higher RPM values. The amount of spark advance is also limited by the physical location of the coils. An additional disadvantage is that this approach requires a complete redesign of the flywheel and armature system, thus increasing the cost of incorporating this invention or the cost of retrofitting an existing engine design.
U.S. Pat. No. 6,408,820 discloses an ignition system with automatic ignition advance.
This invention utilizes both a charging coil and a trigger coil connected to a circuit which produces a spark advance as a function of the amplitude of pulses generated by the trigger coil. As in the '137 patent, this approach increases the cost and weight of an engine by requiring multiple coils (and additional permeable core mass). Also, since this invention's circuitry requires multiple connections to these coils, its application would require considerable reworking on existing engine designs and can not be made pin-compatible with existing silicon controlled rectifiers. U.S. Pat. Nos. 6,932,064 and 7,069,921 disclose related inventions with similar limitations.
U.S. Pat. No. 6,973,911 discloses a device for controlling ignition timing for internal combustion engines which utilizes a single coil to both power the device and provide ignition timing as a function of engine speed. A coil produces pulses which are synchronous with the engine's operation. The device uses a comparator to detect these pulses and uses a microcontroller to measure the time between pulses in order to calculate the rotational speed of the engine. This calculation is used to determine an appropriate delay between each pulse and initiation of a spark. While this invention avoids the expense of additional coils and cores, it requires a microcontroller, thus adding considerable cost. This microcontroller also makes the device poorly suited for retrofitting of existing engine designs. This device also has limited performance since it calculates engine speed by measuring the delay between pulses over a complete rotation of the engine's flywheel, resulting in a slower response to changes in the engine's speed.