Factors Determining Antiknocking Properties of Gaseous Fuels in Spark-Ignition Gas Engines

The factors affecting knock resistance of fuels, including hydrogen (H₂), ethane (C₂H₆), propane (C₃H₈), normal butane (n-C₄H₁₀), and iso-butane (i-C₄H₁₀), were determined using modeling and engine operation tests with spark-ignition gas engines. The results of zero-dimensional detailed chemical kinetic computations indicated that H₂ had the longest ignition delay time of these gaseous fuels. Thus, H₂ possessed the lowest ignitability. The results of engine operation tests indicated that H₂ was the fuel most likely to result in knocking. The use of H₂ as the fuel produced a temperature profile of the unburned gas compressed by the piston and flame front that was higher than that of the other fuels due to the high-specific heat ratio and burning velocity of H₂. The relation between knock resistance and secondary fuel ratio in methane-based fuel blends also was investigated using methane (CH₄) as the primary component, and H₂, C₂H₆, C₃H₈, n-C₄H₁₀, or i-C₄H₁₀ as the secondary components. When the secondary fuel ratio was small, the CH₄/H₂ blend possessed the lowest knocking tendency. But as the secondary fuel ratio increased, the CH₄/H₂ mixture possessed a greater tendency to knock than CH₄/C₂H₆ due to the high-specific heat ratio and burning velocity of H₂. These results indicate that the knocking that can occur with gaseous fuels is not only dependent on the ignitability of the fuel but it also the specific heat ratio and burning velocity.