Diode Laser Wavelength Control for High­speed Chemical Species Tomography

Solon Karagiannopoulos, Ed Cheadle, Paul Wright and Hugh McCann

School of Electrical & Electronic Engineering, University of Manchester, UK

ABSTRACT

High­speed continuous imaging of hydrocarbon fuel distribution and mixing within the combustion cylinder of spark­ignition (SI) automotive engines has previously been demonstrated by Chemical Species Tomography (CST), with a system based on fixed­wavelength diode lasers. Engine applications of CST are now envisaged for a variety of other target species important for combustion and emissions control. One such application is the imaging of water as a marker for the distribution of in­cylinder residual gas to study Homogeneous Charge Compression Ignition (HCCI) engines. As discussed elsewhere in these proceedings, the Near­IR spectroscopy of water is characterised by strong, but narrow absorption lines (relative to long­chain hydrocarbons). A practical opto­electronic scheme demands the use of at least three wavelengths. In this paper, we address the engineering design of a practical opto­electronic system to implement the necessary measurement strategy. It is necessary to operate at least two diode lasers in wavelength­scanning mode at scan rates in the kHz region. Choices between different wavelength multiplexing schemes are discussed, and Time­Division Multiplexing (TDM) appears to be the favoured option. Real­time wavelength measurement is considered, for accurate spectroscopic control. The choice between analogue and digital detection electronics is discussed.

Keywords Tunable­diode laser, Absorption spectroscopy, Combustion, In­cylinder, Multiplexing