Abstract:
A follow-on circuit for extending the duration of the spark provided by a spark plug. The circuit is connected between the coil secondary and the spark gap, and has a capacitor that discharges to the point of connection through a resistor and inductor. The resistor may be made variable to control the amount and duration of the follow-on current, and hence the energy and duration of the spark event. The circuit may also be used with other ignitors and non-coil ignition circuits.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/684,839 filed on May 26, 2005, which is incorporated herein by reference in its entirety. 

   TECHNICAL FIELD OF THE INVENTION 
   This invention relates to engine ignition systems, and more particularly to an ignition circuit for providing an extended duration charge to an igniter. 
   BACKGROUND OF THE INVENTION 
   Recent research has shown that increased levels of exhaust gas recirculation (EGR) in spark ignition engines can enable operation at higher compression ratios and loads than were previously possible, due primarily to a reduction in knock tendency. Increasing the amount of dilution by increasing the air/fuel ratio has also been shown to have similar effects. 
   Implementation of these features gives rise to the problem of ignition and flame propagation at these increased dilution levels. Several companies now sell enhanced ignition circuits and new types of igniters to improve ignitability and promote faster burn rates in the engine. Two examples are “plasma jet” and “railplug” igniters. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic of one embodiment of the extended duration ignition circuit in accordance with the invention. 
       FIGS. 2 and 3  compare the spark duration of a conventional coil ignition circuit ( FIG. 2 ) to that of an extended duration coil ignition circuit in accordance with the invention ( FIG. 3 ). 
       FIG. 4  illustrates the effect of changing the capacitance of the circuit of  FIG. 1  on the spark duration and current levels. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a schematic of one embodiment of an extended duration spark circuit  100  in accordance with the invention. Circuit  100  is typical of an automobile ignition system, whose switching may be performed using a mechanical system (distributor) or a solid state electronic system. Circuit  100  has a low voltage primary ignition coil  10   a , which will induce a high voltage in the secondary coil  10 , which is then directed to a spark plug represented by spark gap  12 . 
   Advances in automotive ignition systems have led to a variety of ignition coil alternatives. Rather than using a single coil and switching the spark voltage to a number of spark plugs, some engines use a “coil on plug” (COP) design, in which each spark plug has its own coil mounted on top. In a typical COP ignition system, a crankshaft position sensor generates a basic timing signal by reading notches on the crankshaft, flywheel or harmonic balancer. The crank sensor signal goes to the a control module, where it is used to determine firing order and turn the individual ignition coils on and off. 
   Regardless of the placement of, or number of coils, the ignition system operation is essentially the same. Coil  10  is a conventional ignition coil. Coil  10  has a low primary resistance, and steps up the primary system voltage from 12 volts to as much as 40,000 volts to produce a spark for the spark plug. 
   For purposes of this description, circuit  100  is described in terms of use with a spark plug, represented schematically in  FIG. 1  by spark gap  12 , used in an automotive (internal combustion engine) ignition system. However, the same concepts could apply to extending the duration of electrical energy applied to ignition circuits other than those having a coil as the energy source and to igniters other than spark igniters. In other ignition circuits, the battery and/or coil could be replaced by another type of energy source, and the add-on current circuit of the present invention could be placed between the energy source and the igniter. 
   Experience has shown that for very dilute mixtures, a longer duration spark leads to better ignitability, by increasing the probability of an ignitable mixture moving through the spark gap. To this end, coil  10  is complemented with a follow-on circuit  100 , which is imposed between the secondary ignition coil  10   b  and the spark plug gap  12 . A significant feature of the circuit  100  is a resistor that allows an ignition control unit  180  to vary the duration of the spark event. 
   More specifically, ignition coil  10  is connected to a blocking capacitor  120 , which prevents the low frequency discharge of a follow-on capacitor  130  from flowing into the secondary coil  10   b . The follow-on capacitor  130  is charged from a voltage source  140  through a resistor  150 . Voltage source  140  can be a variable voltage source. Resistor  150  defines the charge time of the capacitor  130  and limits the spark duration. 
   Capacitor  130  discharges to Node A, which is interposed between coil  10  and the spark gap  12 . Discharge is through an inductor  160  and a resistor  170 . Inductor  160  helps limit the peak current, whereas resistor  170  acts primarily to set the time constant of the circuit  100 . In the example of  FIG. 1 , resistor  170  is a variable resistor. 
     FIGS. 2 and 3  compare the spark duration of a conventional coil ignition circuit ( FIG. 2 ) to that of a coil ignition circuit having an extended duration (follow-on current) circuit, such as circuit  100 , in accordance with the invention ( FIG. 3 ). 
   Specifically,  FIGS. 2 and 3  show the effect of adding the follow-on circuit  100  to the stock coil  10 . Circuit  100  provides a follow-on current, which is capable of extending the spark event by at least a factor of 6 at the longest duration. The values for capacitor  130  and inductor  160  are 330 μF and 250 μH, respectively. 
     FIG. 3  further illustrates the effect of varying the value of the variable resistor  170 . Generally, a higher resistance results in higher current, but with shorter duration. In a production engine application, the charging voltage and resistance of circuit  100  can be a part of an engine control algorithm, both to ensure optimum ignitability and to extend plug life, by only using the maximum energy when it is absolutely necessary. 
     FIG. 4  illustrates the effect of changing the value of capacitor  130  on the spark duration and current levels. The values for resistor  170  and inductor  160  are 8 ohms and 250 μH, respectively. Generally, a higher capacitance results in higher current without substantial reduction in spark duration. 
   It is important to note from a durability standpoint, that the most desirable result is a combination of capacitance, inductance and resistance that results in the longest spark at the lowest current levels possible. Circuit  100  can be used to provide improved ignition and burn rates, and is especially useful in dilute air/fuel conditions. 
   Control unit  180  is used to control the input parameters for resistor  170  and voltage source  140 . As explained above, control unit  180  may receive a variety of signals for controlling ignition timing as well as spark duration. It executes the ignition control algorithm in accordance with the guidelines discussed above, and is implemented with appropriate processing and memory devices. It may be a stand-alone unit or integrated with other engine control processing devices and systems.