a b s t r a c tThe increasing awareness of the depletion of fossil fuel resources and the environmental benefits motivates the use of vegetable oils, however there is little known information about ignition and combustion characteristics of vegetable oil based fuels under off diesel engine conditions. These conditions are normally reached either during starting or when the engine is sufficiently worn out. A fuel was prepared by co-solvent blending of coconut oil with 20% butyl alcohol and was analysed. An experimental study of the measurement of ignition delay (ID) characteristics of conical fuel sprays impinging on hot surface in cylindrical combustion chamber was carried out. The objective of the study was to investigate the effect of hot surface temperatures on ignition delays of microemulsion of coconut oil at various ambient air pressures and temperatures which would have reached under off diesel engine conditions. An experimental set-up was designed and developed for a maximum air pressure of 200 bar and a maximum temperature of 800 C with the emphasis on optical method for the measurement of ignition delay. Hot surface temperature range chosen was 300e450 C and ambient air pressure (inside the combustion chamber) range chosen was 10e25 bar. Present study shows that at fixed injection pressure and fixed ambient (hot surface) temperature, at higher ambient air pressure (25 bar) inside the combustion chamber, ignition delay of diesel and microemulsion of coconut oil are comparable and therefore are having matching combustion characteristics. Although a pressure of 25 bar is much less than the precombustion pressure of most diesel engines but again conclusively establish that combustion characteristics are same despite lower air pressure, temperature and lower injection pressure. At higher injection pressure ignition delay of microemulsion of coconut oil and pure diesel attains the lower value at the same ambient air pressure inside the combustion chamber. It can also be seen that at fixed injection pressure and higher ambient temperature conditions, ignition delay variations of microemulsion of coconut oil with chamber pressure show approximate same trend as that of conventional diesel fuel.