High-speed chemical species tomography in a multi-cylinder automotive engine

Abstract We report here the first application of chemical species tomography (CST) in a multi-cylinder automotive engine. This technique offers high-speed continuous imaging of hydrocarbon fuel distribution and mixing within the combustion chamber and is therefore of interest to both engine designers and combustion scientists. Many of the methods described are equally applicable to chemically selective imaging of other highly dynamic mixing and reaction processes. A measurement grid consisting of 27 dual-wavelength optical paths has been implemented in one cylinder of an otherwise standard four-cylinder port-injected gasoline engine, using a unique OPtical Access Layer (OPAL) carrying embedded optical fibres and collimators. The OPAL provided adequate performance on many beams for more than 2 h of fired engine operation. To improve sensitivity and to cope with fuel spray injection directly into the cylinder (in other engine types), a low-noise opto-electronic system has been developed, offering laser intensity modulation at frequencies up to 1 MHz. Dual-wavelength measurements are recorded on each channel at 100 kSPS, prior to off-line processing that typically reduces the effective frame rate to 3000–4000 frames/s, dependent upon engine speed. The performance of the system is assessed, using running conditions chosen to provide a qualitatively known (homogeneous) fuel distribution for validation purposes. Examples of measured data and processing schemes are discussed. Sample tomographic images, obtained using a novel quality-based approach to data selection, are presented.