Please check the manuscript for details of any other licences that may have been applied and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. You may not engage in further distribution of the material for any profitmaking activities or any commercial gain. You may freely distribute both the url (http://uhra.herts.ac.uk/) and the content of this paper for research or private study, educational, or not-for-profit purposes without prior permission or charge. Take down policy If you believe that this document breaches copyright please contact us providing details, any such items will be temporarily removed from the repository pending investigation.

Experiments were conducted in a constant-volume combustion chamber to investigate the spray and combustion characteristics of soybean biodiesel (B100) and diesel (B0). The in-cylinder density was kept at 15 kg/m3 and the ambient oxygen concentrations of 21, 18, and 15 per cent were maintained to simulate no exhaust gas recirculation (EGR), medium EGR, and heavy EGR in diesel engines respectively. The ambient gas temperature was varied from 800 K to 1200 K. The in-cylinder pressure and heat release rate were measured and the liquid penetration, natural flame emission, and soot formation characteristics were studied via new optical diagnostics. The results show that the peak pressure decreases with increasing ambient gas temperature and decreasing oxygen concentration for B0, except that at the oxygen concentration of 15 per cent. For B100, a higher peak pressure is obtained at the oxygen concentration of 15 per cent as opposed to that of 18 per cent. A lower heat release rate is found for B100 at the oxygen concentrations of 21 per cent and 18 per cent compared to B0, while a higher value is shown at the oxygen concentration of 15 per cent. The liquid penetration increases with decreasing oxygen concentration and ambient gas temperature. As the oxygen concentration increases for B0 at the ambient gas temperature from 800 K to 1000 K, the integrated natural flame luminosity (INFL) increases, while at 1200 K, the INFL at the oxygen concentration of 15 per cent has the highest value. For B100, with increasing oxygen concentration, the INFL increases at 800 K and 900 K, but decreases at 1000 K and 1200 K. The total soot mass increases with decreasing oxygen concentration and the soot formation duration is longer at a lower oxygen concentration. B100 has a shorter soot formation duration and soot is dramatically reduced by burning B100. The peak soot mass for B100 at 1200 K is comparable to that for B0 at 800 K.