Ion Current Measurement in Diesel Engines 2004-01-2922
Contemporary diesel engines are high-tech power plants that provide high torques at very good levels of efficiency. By means of modern injecting-systems such as Common-Rail Injection, combustion noise and emissions could be influenced positively as well. Diesel engine are therefore used increasingly in top-range and sports cars.
Today's production ECUs have no or only very low feedback regarding the process in the combustion chamber. As long as this data is missing, the design of the maps in the ECU can only be a compromise, since production tolerances and aging processes have to be considered in advance. Disturbances in the combustion process may not be detected at all.
If more knowledge about the course of combustion is provided, especially the start of combustion (SOC), various operating parameters, such as the pilot injection quantity or the beginning of current feed to the injector, could be adjusted more precisely and individually for every cylinder. For the HCCI, which is currently in research, knowledge of the SOC is even of decisive importance.
The ion current measurement technique lends itself to acquiring this information. Compared to cylinder pressure sensors or optical probes, it is much easier to integrate into existing constructions. It promises great reliability and provides meaningful data for monitoring multiple control variables simultaneously.
In the paper at hand, ion current measurements were carried out on a 2.2 litre diesel engine with common-rail injection. It first deals with the fundamentals and the signal progression of the ion current measurement. It was shown that the ion current signal is suitable as an analysis signal for detecting the correct PI quantity, a disturbance of the intake port deactivation system and the EGR-rate.
Different methods are described of detecting the start of combustion using the ion current signal in steady-state and transient mode. Using this SOC it is possible to control the PI quantity to an optimum where combustion noise and emissions are concerned.
This work thus contributes to making modern diesel engines even more powerful and eco-friendly.