INVESTIGATING GM'S COMPRESSION SENSE IGNITION

Motor, Nov 2004 by Garrido, Jim

This combustion chamber detective uses ignition signatures to deduce engine phasing and cam position. We'll show you how it gathers the clues.

Several General Motors vehicles are equipped with a 2.2L engine called IiCOTEC. This four-cylinder, fourvalves-per-cylinder, all-aluminum power plant was first installed in some Saturn models, beginning with the 2000 L-series. Saturn VUE and ION models followed. It was the optional engine in the 2002 Pontiac Sunfire and Grand Am, Oldsmobile Alero and Chevrolet Cavalier and MaIibu, and standard equipment stalling in 2003. The ECOTEC is also used in the Saab 9.3.

The ECOTEC engine uses a speed density-based fuel injection system, along with a waste-spark electronic ignition system. The waste-spark ignition is part of a unique Compression Sense Ignition (CSI) system that allows the powertrain control module (PCM) to determine proper engine phasing (cam position) without the use of a separate camshaft position sensor mounted near a rotating engine member. This article will explain how the CSI system operates. We'll also document new misfire diagnostic techniques that are made possible by die system s unique design.

Construction

The CSI system is a modular design, with nearly all die major ignition system components housed in a single cassette (Fig. 1, page 44). The cassette is mounted directly over the spark plugs, with only connector springs and insulating boots to transfer the ignition energy to the plugs. The cassette houses two ignition coils. Each simultaneously sends ignition energy to two paired cylindersone cylinder on its exhaust stroke and the other on its compression stroke. Cylinders 1 and 4 are paired as running mates on one coil and cylinders 2 and 3 are paired on the other coil.

The cassette has an integrally mounted lead-frame and CSI signal plates, which serve to create the four CSI input signal capacitors. The CSI signal plate output node is connected through the cassette cover to the electronic ignition (EI) module by a dual female plastic connector cavity called an interconnect. The EI module houses the CSI signal processing compression sense time out (CSTO) chip, as well as the usual ignition primary drivers and current-limiting electronics.

Before explaining how the CSi signal is acquired and processed to form die CAMOUT (engine phase) signal to the PCM, its necessary to understand the operating characteristics of a typical waste spark ignition system. These operating characteristics tire important hecause they directly influence the information contained in the CSI signal.

Spark Polarities

Fig. 2 on page 44 illustrates the 1/4 ignition coil firing events. One spark plug in the pair of DIS running mates always fires from the center electrode to the side electrode, while the other fires from the side electrode to the center electrode. One cylinders firing voltage rises in a negative direction, relative to engine ground, on the way to its final spark plug gap breakdown voltage. It then breaks quickly in a positive direction, back toward ground until the spark line is established. The running mate cylinders firing voltage rises in a positive diruction, relative to engine ground. It then quickly breaks over in a negative direction, back toward engine ground until the spark line is established. The polarity characteristics of the spark breakover events are one part of the information reflected in the CSI signal.

Compression Sense?

As a DIS ignition coil releases its stored energy, an electric field is created in both secondary circuits of the paired cylinders. This growing electric field simultaneously creates a voltage potential . across both spark plug electrode sets. Over a period of 5 to 10 microseconds, this voltage level becomes high enough to reach the spark plug gap breakdown voltage level. Each spark plug gap breakdown level is determined by the impedance value of the individual spark plug gaps. At the breakdown point, the air gap between the spark plugs' electrodes will ionize and quickly conduct current, break over and establish an are across the electrodes.

One of the important characteristics of this event is the voltage level at which any spark plug gap impedance will reach its breakdown voltage level. The greater the spark plug gap impedance, the greater the voltage level that must be reached for the plug gap to break down. The greater the voltage level required in order to reach breakdown, the greater the required time to reach this voltage level.

The spark plug gap impedance is affected by several factors. The greatest variable is the pressure in the cylinder when the spark is generated by the coil. A cylinder on its exhaust stroke has less incylinder pressure than a cylinder on its compression stroke. Remember, a DIS coil releases its energy to two cylinders simultaneously-to one cylinder on its exhaust stroke, to the other on its compression stroke. However, due to the pressure-related uneven plug gap impedances, both spark plug gaps will not break down at the same instant. Under most engine operating conditions, the spark plug gap in the cylinder on the exhaust stroke will break down before the spark plug gap in the cylinder on the compression stroke. The order of the spark plug gap breakover events is reflected in the CSI signal.