Ion sense ignition bias circuit

Ion sense ignition has been in limited use in automotive engines for several years, and has recently started to appear in more applications, driven in part by regulated on-board diagnostic requirements. Ion Sense systems measure the presence of ions or free electrons in the engine's combustion chamber by electrically biasing the gap of the spark plug with a voltage. The current flow induced by the applied bias voltage is a measure of the ion or free electron density in the cylinder. The ion or free electron density is related to the chemistry of the fuel and the combustion process itself. It has been clearly demonstrated that Ion Sense Ignition systems can be used to detect and measure combustion (or misfire) and engine knock as well as the cylinder location of peak pressure, air to fuel ratio and other combustion characteristics. Disclosed is a bias and measuring circuit that improves the ion sense measurement and ignition performance of an Ion Sense Ignition system where the ion current signal processing is implemented remote from the ignition coil. Specifically, the bias and measuring circuit of the invention reduces the effects of secondary harness capacitance on the ion current signal, minimizes the effects of harness electrical leakage and reduces the chances for "spark-on-make" (an ignition firing when the ignition coil primary is initially energized).