Ion sense biasing circuit

Details Ion sense biasing circuit Ion sense biasing circuit

1999-08-02

2001-02-13

Yuen, Henry C. (Department: 3747)

Internal-combustion engines

High tension ignition system

Additional spark energy supply

C073S035080, C123S406260

Reexamination Certificate

active

06186129

ABSTRACT:

TECHNICAL FIELD
The present invention relates generally to a system for detecting a combustion condition in an internal combustion engine, and more particularly, to a system configured to develop a bias voltage for ion current measurement within a combustion chamber of the engine.
BACKGROUND OF THE INVENTION
It is desirable to detect a misfire condition (or a knock condition) during operation of an internal combustion engine. One approach taken in the art to detect misfire involves assessing the degree of engine speed (RPM) variation. Although such an approach does not require additional circuit components, it does require the availability of a microprocessor or the like and relatively extensive software to perform the evaluation, and, further, is incapable of producing accurate and reliable results over the entire engine speed and load range. In addition, one approach taken in the art for detecting knock involves the use of vibration knock sensors. Such an approach, however, involves, additional circuitry, including the sensors, which may be undesirable in certain circumstances.
In addition, so-called ion sense systems for detecting a combustion condition (e.g., misfire) are known. The combustion of an air/fuel mixture in an engine results in molecules in the cylinder being ionized. Applying a relatively high voltage across, for example, the electrodes of a spark plug just after the ignition operation is known to produce a current across the electrodes. Such current is known as ion current. The ion current that flows is proportional to the number of combustion ions present in the area of, for example, the spark plug gap referred to above, and is consequently indicative of the ionization throughout the entire cylinder as combustion occurs. The level or amount of ion current is indicative of a quality of the combustion event, or whether in fact combustion has occurred at all (e.g., a misfire condition). The level of an AC component of the ion current (e.g., at particular frequencies) may be used to determine knock.
Known ion current sensing systems generally include, in addition to an ignition coil, a capacitor or the like configured to store a voltage. The stored voltage is thereafter used as a “bias” voltage which is applied to the spark plug to generate the ion current. It is desirable to apply the bias voltage to the plug from the low voltage side of the secondary winding, rather than the high voltage side, to reduce the usage of high-voltage rated components. Accordingly, such biasing circuits are known that apply the biasing voltage to the low voltage side of a secondary winding of the ignition coil. However, known biasing circuits of this type incur substantial losses in spark energy. In particular, the bias voltage should be approximately 80-200 volts, and in one approach, the storage capacitor is disposed in parallel with a zener diode (having a reverse breakdown voltage in that range) between the low voltage end of the secondary and ground. Thus, during spark, the zener dissipates a relatively large amount of the spark energy (e.g., V
ZENER
*I
SPARK
) Compare this with the energy delivered by the plug (e.g., V
SPARK
*I
SPARK
where V
SPARK
may be as low as 200 volts), and it may be seen that as much as ⅓ to ½ of the spark energy is lost.
There is therefore a need to provide an apparatus for detecting a combustion condition such as a misfire condition or a knock condition, particularly a biasing circuit for use in an ion sense system, that minimizes or eliminates one or more of the shortcomings as set forth above.
SUMMARY OF THE INVENTION
This invention provides for accurate and reliable detection of a combustion condition, such as a misfire condition (or a knock condition) at a relatively reduced cost (e.g., non-microprocessor based), as well as without incurring substantial losses in spark energy.
An apparatus is provided, in accordance with the present invention, suitable for use in detecting a combustion event in a cylinder of an internal combustion engine of the type having a spark plug proximate the cylinder. The apparatus includes an ignition coil, a switch, and a biasing circuit. The ignition coil includes a primary winding and a secondary winding. The primary winding has a first end coupled to a power supply and a second end coupled to the switch for selective connection to ground. The secondary winding has a first end, namely a high voltage end, coupled to the spark plug. The secondary winding further has a second end, namely a low voltage end, coupled to the biasing circuit. The switch is configured to cause a primary current to flow through the primary winding.
In function, the biasing circuit includes structure configured to bias the spark plug with a bias voltage for producing an ion current. The ion current is indicative of the combustion event. The biasing circuit, preferably, includes a silicon-controlled rectifier (SCR) with a gate terminal thereof floating. The SCR further has an anode terminal coupled to the low voltage end of the secondary winding. The SCR is configured to conduct a spark current from the secondary winding to ground when an ignition voltage at the anode terminal of the SCR reaches a predetermined level.
Advantageously, during conduction of the spark current, the voltage drop across the SCR is at a relatively low level. Accordingly, very little spark energy is lost in the biasing circuit. In the preferred embodiment, the biasing circuit also includes a capacitor which is initially charged to a predetermined voltage level from the low voltage side of the primary winding. This predetermined voltage level constitutes the bias voltage, which is applied through the low voltage side end of the secondary winding. While connection of the biasing circuit, in particular the capacitor, to the low voltage end of the secondary has the advantage of not requiring high-voltage rated components, it should be appreciated that the SCR provides a path to ground which, unless properly dealt with, might inadvertently allow discharge of the capacitor during the spark event itself. Therefore, in another aspect of the invention, a resistor is disposed between the capacitor and the low voltage end of the secondary that is valued such that the current sourced from the capacitor is limited to a predetermined level. This level is such so as to not keep the SCR in a conductive state. Therefore, during operation, the spark energy is allowed to be “rung out” until the spark current falls below a sustaining level of the SCR. The SCR then shuts off. With the SCR off, ion current flows from the capacitor/resistor through the secondary winding across the spark plug gap to ground. In a further embodiment, the magnitude of the ion current is sensed by a sensing resistor, which generates an ion sense signal. The sensing resistor is disposed between ground and the capacitor to form a complete circuit. The ion sense signal can thereafter be processed in a number of known ways to detect combustion condition (i.e., lack of a misfire condition) and a knock condition.


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