Auto-ignition and Detonation of n-butanol and Toluene Reference Fuel Blends (TRF)

The primary purpose of the paper is to report a study of the auto-ignitive and anti-knock properties, firstly of neat n-butanol in air, and secondly of its blends with a gasoline-type, Toluene Reference Fuel (TRF). This involved blending ratios of 10, 20, 40, and 85% of n-butanol by volume with a TRF. Pressures were in the range 2–6 MPa, and temperatures ranged from 678 to 916 K. Mathematical modelling of the detailed chemical kinetics for these conditions, using the CANTERA code, yielded values of the duration of the main heat release rate, and revealed the main reactions influencing them. High knock intensities were not anticipated, but checks were made of whether operational points lay within the Detonation Peninsula. This peninsula is constructed by plotting values of ξ, the ratio of acoustic to auto-ignitive velocity, against ɛ, the ratio of the transit time of an acoustic wave through a hot spot, to the heat release time (τe).

At 2 MPa and 702–855 K, the addition of n-butanol to the TRF moved these coordinates away from the peninsula towards the deflagrative regime. At blend ratios of 85%, n-butanol mixtures and pure n-butanol were found to lie close to the compression curve for methane, known to have good anti-knock properties. This suggests that the addition of n-butanol to gasoline would improve knock resistance at this operating condition. However, at 6 MPa and 916 K, the addition of n-butanol had the opposite effect, reducing the anti-knock resistance of the blend and pushing it deeper into the peninsula. Uncertainties and limitations of the ξ/ɛ methodology are also discussed.