Near Infra-red absorption tomography system for measurement of gaseous hydrocarbon distribution

The spatial distribution of chemical species can be a critical determinant of the performance of chemical reactors. One such reactor is the combustion chamber of the Internal Combustion engine, in which the spatial variation of air-fuel ratio has a significant influence on both fuel efficiency and emissions performance. We report the development of a fibre-based Near Infra-Red Absorption Tomography system, in order to measure the distribution of hydrocarbons in-cylinder. The technique exploits the specific (but weak) hydrocarbon absorption of 1.7 μm radiation, which wavelength has only recently become accessible for the present application by the availability of solid-state all-optoelectronic components. A custom-specified InGaAsP/InP laser diode has been supplied, delivering 3mW at 1.700μm, with about lnm tunability. A standard telecommunications laser diode is used to provide a reference wavelength at 1.55μm, which is not absorbed by any species in the combustion environment. Along each of 32 absorption paths through the subject, both wavelengths are launched simultaneously via a single-mode optical fibre and GRIN lens. The transmitted light is collected by a large-core fibre and measured by an extended-sensitivity InGaAs photodiode. The attenuation at each individual wavelength is measured by modulating the intensity of the laser sources in a frequency-division multiplexed scheme. The logarithm of the ratio of the two measurements yields the path integral of the hydrocarbon absorption, and hence, of concentration. Single-channel characterisation shows that the technique is readily calibrated for temperature and pressure effects, over the region 70-150°C and 1-10bar. Tomographic reconstruction of different gaseous hydrocarbon flows has been achieved. Design considerations will be discussed concerning the deployment of the technique to a running engine, to achieve image rates over 10,000 per second.