INVESTIGATING GM'S COMPRESSION SENSE IGNITION

Motor,  Nov 2004  by Garrido, Jim

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The CSI Sensor

The ECOTEC engine's CSI sensor detects the two secondary ignition event characteristics of breakover: polarity and order. This is accomplished by creating virtual capacitors between the secondaiy coils and the EI module electronics (Fig. 3, page 46). These capacitors act as a capacitive probe of the ignition secondary circuits and bring the secondary event characteristics into the ignition module to be processed.

One side of these capacitor plates is connected to the ignition secondary outputs. The other side is connected to one end of a sensing resistor network inside the EI module. The other end of this sensing resistor network is connected to ground. As current flows on the capacitor plates, a voltage is created on this sensing resistor. The voltage pattern measured across this resistor is reflected in the information contained in the CSI signal.

This measuring resistor, since it's combined with a capacitor, also serves to create a first-order high-pass filter network. This high-pass filter tends to reject the signal created by the rise of the secondary firing voltages and allows only the faster falling edge of each plug gap breakover voltage to pass through it for measurement.

The ignition firing voltages are shown in Fig. 4 on page 48, magnified by a time factor of 50 to get a closer look at the firing line characteristics as they're reflected in the CSI signal. When the 1/4 coil fires as cylinder 1 is up on its compression stroke, the No. 4 spark plug will fire first on its respective waste stroke. This cylinder 4 waste firing event will create a high-speed transient breakover voltage change from the positive peak breakdown firing voltage back toward zero volts. This breakover transient voltage is passed across the CSI measuring resistor in die form of a rapid voltage change in a negative direction. Then, a few microseconds later, as cylinder 1 fires on compression, there will be a high-speed transient breakover voltage change from the negative peak breakdown firing voltage back toward zero volts. This breakover transient voltage is passed across the CSI measuring resistor in the form of a rapid voltage change in a positive direction.

Conversely, as cylinder 4 (Fig. 5) is up on its compression stroke, die No. 1 spark plug will fire first on its respective waste stroke. This cylinder 1 waste firing event will create a high-speed transient breakover voltage change from the negative peak breakdown firing voltage back toward zero volts. This breakover transient voltage is passed across the CSI measuring resistor in the form of a rapid voltage change in a positive direction. Then, a few microseconds later, as cylinder 4 fires on compression, there will be a high-speed transient breakover voltage change from the positive peak firing breakdown voltage back toward zero volts. This breakover transient voltage is passed across the CSI measuring resistor in the form of a rapid voltage change in a negative direction.

The 2/3 coil firing event characteristics are also picked up by the CSI sensor in the same manner as above. As shown in Figs. 4 and 5, the firing voltage for cylinder 3, like cylinder 1, rises in a negative direction, and then breaks over in a positive-going direction. The firing voltage for cylinder 2, like cylinder 4, rises in a positive direction, then breaks over in a negative-going direction. The CSI signal reflects the polarity and timing (and thus the amplitude) of each cylinders spark plug breakover voltage event.