This paper summarizes the first results from isolated droplet combustion experiments performed on the International Space Station (ISS). The long durations of microgravity provided in the ISS enable the measurement of droplet and flame histories over an unprecedented range of conditions. The first experiments were with heptane and methanol as fuels, initial droplet droplet diameters between 1.5 and 5.0 m m, ambient oxygen mole fractions between 0.1 and 0.4, ambient pressures between 0.7 and 3.0 a t m and ambient environments containing oxygen and nitrogen diluted with both carbon dioxide and helium. The experiments show both radiative and diffusive extinction. For both fuels, the flames exhibited pre-extinction flame oscillations during radiative extinction with a frequency of approximately 1 H z. The results revealed that as the ambient oxygen mole fraction was reduced, the diffusive-extinction droplet diameter increased and the radiative-extinction droplet diameter decreased. In between these two limiting extinction conditions, quasi-steady combustion was observed. Another important measurement that is related to spacecraft fire safety is the limiting oxygen index (LOI), the oxygen concentration below which quasi-steady combustion cannot be supported. This is also the ambient oxygen mole fraction for which the radiative and diffusive extinction diameters become equal. For oxygen/nitrogen mixtures, the LOI is 0.12 and 0.15 for methanol and heptane, respectively. The LOI increases to approximately 0.14 (0.14 O ₂/0.56 N ₂/0.30 C O ₂) and 0.17 (0.17 O ₂/0.63 N ₂/0.20 C O ₂) for methanol and heptane, respectively, for ambient environments that simulated dispersing an inert-gas suppressant (carbon dioxide) into a nominally air (1.0 a t m) ambient environment. The LOI is approximately 0.14 and 0.15 for methanol and heptane, respectively, when helium is dispersed into air at 1 atm. The experiments also showed unique burning behavior for large heptane droplets. After the visible hot flame radiatively extinguished around a large heptane droplet, the droplet continued to burn with a cool flame. This phenomena was observed repeatably over a wide range of ambient conditions. These cool flames were invisible to the experiment imaging system but their behavior was inferred by the sustained quasi-steady burning after visible flame extinction. Verification of this new burning regime was established by both theoretical and numerical analysis of the experimental results. These innovative experiments have provided a wealth of new data for improving the understanding of droplet combustion and related aspects of fire safety, as well as offering important measurements that can be used to test sophisticated evolving computational models and theories of droplet combustion.

An investigation of twin-pilot-injection strategies was performed in a low-compression-ratio engine in order to improve the combustion performance and to reduce the exhaust emissions. All tests were conducted on a modified four-cylinder test engine running on only one cylinder with a reduced compression ratio (15.3:1), and analyses of the combustion pressure, the heat release rate and the net work output in the cylinder were performed. At the same time, the reduction in the exhaust emissions such as carbon monoxide, hydrocarbons and soot under different twin-pilot-injection strategies was compared with other combustion cases (single-injection strategy and one-pilot-injection strategy). Higher combustion pressures, lower heat release rates, shorter ignition delay times, higher indicated mean effective pressures (about 2.1%) and lower coefficients of variation in the indicated mean effective pressure (approximately 29.4%) in all multiple-injection (one-pilot-injection or twin-pilot-injection) cases, which indicated improved operation of the low-compression-ratio engine, were observed. Retarded injection timing ( t_p,SOE = 20–10° crank angle before top dead centre) in the twin-pilot-injection case proved to the best timing for reduction in the carbon monoxide emissions because the relatively short ignition delay with the high in-cylinder temperature enhanced the carbon monoxide oxidation rate. Twin-pilot-injection strategies formed higher hydrocarbon emissions because of the rich fuel mixture during the ignition delay time than one-pilot-injection combustion did. As more multiple injections were applied, lower nitrogen oxide emissions (up to 45.7%) were observed because the early termination of the first injection event and the restart of injection created a time interval between injections in which freshly charged air could be entrained into the flame.

This study aims to confirm the effect of changing the various factors with the directed relation graph error propagation–based methods and to adopt a new approach of the mechanism evaluation in the reduction of biodiesel mechanism. The factors considered in this study were a threshold value, target species, ambient conditions, and the evaluation formula consists of the reduction rates and the maximum error rate of ignition delay to objectively compare the skeletal mechanisms generated under different conditions. For a threshold value, the automatic mechanism reduction process was used to select the appropriate threshold value by applying the relative tolerance and absolute tolerance; so relative tolerance and absolute tolerance represent the factor of the threshold value. Also, the seven steps of mechanism reduction process consist of directed relation graph error propagation, directed relation graph error propagation with sensitivity analysis, peak concentration analysis, full species sensitivity analysis, and A-factor modification. As a result of the mechanism reduction, different relative tolerance and absolute tolerance values should be applied to each step to select the appropriate threshold value. For target species, considering polycyclic aromatic hydrocarbon species as target species shows higher efficiency of mechanism reduction. Also, considering the negative temperature coefficient region as ambient conditions helps the mechanism be reduced efficiently than a wide range of ambient conditions. Finally, the reduced mechanism which had 247 species and 1129 reactions was generated, and the maximum error rate of ignition delay was about 30%. For the applicability of three-dimensional computational fluid dynamics and verification of the reduced mechanism, the compression ignition engine simulation was performed. As a result of three-dimensional computational fluid dynamics, the predicted cylinder pressure, rate of heat release, indicated mean effective pressure, and power were similar to the experimental results. However, the results of carbon monoxide and nitrogen oxide emissions did not match the experimental results.