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This paper describes the technique of in-cylinder ionization sensing in a common rail diesel engine. The technology detects in real time, the start of combustion for both pilot and main combustion enabling the fuel control strategy to change from open to closed loop, thus, maintaining the desired start of combustion for all speeds and loads. Additionally, the ionization sensing enables the ECM to truly correct for changes in ignition delays caused by as an example a change in fuel cetane number or in air, fuel and engine temperature. The conclusions are that ionization sensing improves the ability to control a diesel engine and is a feasible technology for production vehicles.

Prototypes of gas sensitive Schottky diodes with a platinum gate electrode have been fabricated on monocrystalline 6H-SiC substrates and tested on petrol engines. The sensors are mounted in the housing and on the heater of HEGO sensors. The air/fuel ratio of each cylinder is individually controlled by UHEGO sensors. Three gas sensitive SiC diodes together with another UHEGO sensor, serving as the reference sensor, are mounted into the exhaust pipe. At the operating temperature used, around 600-800°C, the SiC sensors show a large variation of the output signal for small variations of the air/fuel ratio around the stoichiometric ratio. The response time of the sensor is small enough to detect the air/fuel ratio of individual cylinders. We show how the sensor can be used to detect misfire and/or individual cylinders that deviate from stoichiometry. The SiC sensor thus shows promising potential for cylinder selective and more effective combustion control.

A model based on an ionization equilibrium analysis, that can relate the ion current to the state of the gas inside the combustion volume, has been presented earlier. This paper introduces several additional models, that together with the previous model have the purpose of improving the pressure predictions. One of the models is a chemistry model that enables us to realistically consider the current contribution from the most relevant species. A second model can predict the crank angle of the peak pressure and thereby substantially increase the accuracy of the pressure predictions. Several other additions and improvements have been introduced, including support for part load engine conditions.

The use of a spark plug as an ionization sensor in an engine, and its physical and chemical explanation has been investigated. By applying a small constant DC voltage across the electrodes of the spark plug and measuring the current through the electrode gap, the state of the gas can be probed. An analytical expression for the current as a function of temperature is derived, and an inverse relation, where the pressure is a function of the current, is also presented. It is also found that a relatively minor species, NO, seems to be the major agent responsible for the conductivity of the hot gas in the spark gap.