Abstract: Fire flashover tests were conducted with different diameters of ethanol under different ventilation conditions by using ethanol as the main fuel and diesel as the second fuel. The average temperature in the upper layer of the hot smoke， the radiation heat flux at the floor and the oxygen concentration were measured to analyze the critical ignition conditions of the diesel. The results illustrated that the diesel can be ignited when the average temperate in the upper layer of the hot smoke is over 364℃， the radiation heat flux at the floor is higher than 4.38kW/m² and the oxygen concentration reaches 20.4% under the experiment state. If the oxygen concentration is reduced to 18.5%， the diesel can be ignited when the critical average temperate in the upper layer of the hot smoke is over 390℃and the radiant heat flux at the floor is higher than 6.65kW/m². It is concluded that in confined space， the critical parameters for the ignition of disel， i.e. the oxygen concentration， the average temperature in the upper layer of the hot smoke and the radiant heat flux at the floor are mutually interinhibitive.

Key words: ventilation, hot smoke layer temperature, flashover, radiantion heat flux, ignition