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Abstract:

This paper has two parts. The first compares the measured burned gas temperature using Coherent Anti-Stokes Raman Scattering (CARS) with the predictions of a multiple zone computer simulation of combustion. The second part describes a system that is capable of determining the heat flux into the combustion chamber by means of measuring the chamber surface temperature. It is shown that the multi-zone computer simulation can accurately predict the burned gas temperature once the fuel burn rate has been analyzed and the model tuned correctly. The effect of different fuels (methane and iso-octane) on the burned gas temperature is reported. A high burn rate or more advanced ignition timing gave a lower burned gas temperature towards the end of the engine cycle. The surface heat flux was deduced from measurements of the surface temperature by using a finite difference method. From the experimental results, it was found that there are significant cycle-by-cycle variations in the surface heat flux in both the magnitude and phasing. Therefore, a cycle averaged heat flux has significantly different characteristics from a single cycle. These cycle-by-cycle variations in the heat flux were associated with corresponding variations of the propagation of the flame through the combustion chamber. This in turn is due to the variations in combustion. The effects of ignition timing and air-fuel mixture on the surface heat flux are reported. Comparisons between experimental surface heat flux measurements and established heat transfer models show large discrepancies. Copyright © 2000 Society of Automotive Engineers, Inc.

Abstract:

Self-ignition of hydrocarbon fuels, which causes engine knock, initiates at localized sites called hot-spots. The occurrence of such hot-spots was studied using double-pulse planar laser induced fluorescence of unburned hydrocarbons in the end-gas of an optically accessible internal combustion (IC) engine. The IC engine was fuelled with a mixture of iso-octane (90%) and n-heptane (10%) and equipped with quartz windows allowing optical access into the combustion chamber. The pulsed radiation of two XeCl Excimer lasers at 308 nm was formed into planar sheets, which are subsequently overlapped and directed into the combustion chamber of the operating engine. An intensified CCD camera was used per laser to image the resulting fluorescence. An image pair was obtained to allow the temporal development of hot-spots to be studied. A large number (around 20,000) of image pairs was observed. The boundary curves of the hot-spots and the velocity of hot-spot expansion from image pairs were extracted and stored in a database. Probability density functions (pdfs) of such measures as hot-spot spatial distribution, boundary curvature, and expansion velocity, can be derived from this database. Preliminary pdfs for hot-spot spatial distribution, boundary curvature, and expansion velocity are presented and compared with numerical one-dimensional simulations of self-ignition in hydrogen-oxygen mixtures, where the interaction of two closely neighboring hot-spots was modelled using a detailed chemical mechanism. Original is an abstract.