Access Type

Open Access Dissertation

Date of Award

January 2013

Degree Type


Degree Name



Mechanical Engineering

First Advisor

Naeim A. Henein


Diesel engines have been known for their high thermal efficiency and specific power output, but there is concern about engine-out NOx and particulate matter emissions. To meet the current emission standards, advanced diesel engines are fitted with electronically controlled fuel injection systems and sophisticated and expensive after-treatment devices. Further improvements are still needed to meet future goals in better fuel economy and the more stringent emission standards. In order to meet these goals, there is a need for the control of the combustion process to reduce engine-out emissions in real-time and reduce the demand on the after-treatment devices. This requires a signal indicative of the in-cylinder conditions to be fed in the ECU (Engine Control Unit). The most promising sensors in internal combustion engines are the cylinder gas pressure transducer and the combustion produced ion current sensor. Ion current probes have many advantages over pressure transducers because they are less expensive, more rugged, and are sensitive to the in cylinder gas temperature, and the composition of the combustion products.

The ion current technique has been used in some SI engines, based on an understanding of the ionization produced from the combustion of a homogeneous charge. This is not the case in diesel engines where different types of flames are produced from the combustion of the heterogeneous mixture. This study investigates in details the characteristics of the ion current signal in diesel engines and its use for combustion diagnostics and feedback control of the engine. Experimental investigations and CFD simulation models are used to understand the characteristics of the ion current signal under different operating conditions. The investigations proved that the ion current signal carry basic information about combustion. 3-D mathematical models developed gave more insight into the distribution of the ionized species in the combustion chamber and enhanced the development of feedback control of the combustion process and enable the engine to autonomously operate properly on fuels of a wide range of physical and chemical properties. In addition, algorithms have been developed to use the signal for on-board diagnostics of different combustion, performance and engine-out emissions parameters.