Ignition Characteristics of Gaseous Fuels and Their Difference Elimination for SI and HCCI Gas Engines

Ignition characteristics of natural-gas oriented gaseous fuels, which can be simply represented by ignition delays of the fuel/oxidizer mixtures, were examined precisely by using a rapid compression machine as the first step. For the non-cool-flame generating methane, n-butane mixed with it as a supplementary fuel acts as a more intense ignition promoter than ethane or propane, even though its octane rating is similar and almost as high as the commercial gasoline. Lean fuel/air mixtures with various fuel/fuel ratios between methane and n-butane were supplied to a premixed compression-ignition engine (i.e. homogeneous charge compression ignition engine, HCCI) with or without supplementary gaseous formaldehyde induction as an ignition control additive as the second step. In the no additive case the methane and butane function as the two fuels in the high/low-octane two-fuel premixed compression-ignition operation we proposed previously as another ignition control procedure. The formaldehyde addition to the methane/butane/air mixtures has given the engine desired and stable ignition timings controllable by the amount of formaldehyde to be added, almost independent on the fuel/fuel ratios between methane and butane. The efficacy of formaldehyde has been confirmed as an ignition control medium for the piston-compression ignition of hydrocarbon/air mixtures.