Tomographic imaging of the liquid and vapour fuel distributions in a single-cylinder direct-injection gasoline engine

This article reports the application of optical tomography and chemical species tomography to the characterisation of the in-cylinder mixture preparation process in a gasoline, direct-injection, single-cylinder, motored research engine. An array of 32 near-infrared beams is transmitted in a horizontal plane across the cylinder bore near the top of the cylinder, through a circular quartz annulus. A novel approach to enable the optical alignment of the transmitting and receiving optics is utilised. The engine is operated at a stoichiometric condition at 1200 r/min, with negative valve overlap timing. Two tomographic measurement schemes (optical attenuation and chemically specific absorption) were used to acquire data on the spatial and temporal distribution of fuel throughout the engine cycle. Optimised data pre-processing methods are described for maximal beam count and data reliability. The presence of fuel during the intake stroke was detected by the optical beam attenuation due to scattering from the liquid gasoline droplets. Optical tomographic reconstruction of the spatial distribution of these droplets was achieved at an imaging rate of 7200 frames per second, revealing rapid intra-cycle spatial variations that were consistent between consecutive cycles. During the compression stroke, chemical species tomography images of fuel vapour were reconstructed from data acquired using chemically selective spectral absorption by the hydrocarbon molecules, at an imaging rate of 2400 frames per second. Later in the compression stroke, the temporal evolution of the fuel vapour distribution in the plane of observation is relatively slow and displays inhomogeneities that are consistent between consecutive cycles. This is the first report of the use of tomography to image, within individual engine cycles, the in-cylinder evolution of both fuel spray droplet distribution and fuel vapour distribution.