A method of operating an internal combustion engine having a combustion
chamber with a piston and a spark plug, comprising during a first mode,
bringing the temperature of the combustion chamber to auto-ignition
temperature by adjusting engine operating conditions and producing
auto-ignition in said combustion chamber without requiring spark from
said spark plug; and during a second mode, bringing the temperature of
the combustion chamber close to auto-ignition temperature by adjusting
engine operating conditions, forming a small cloud of stratified air-fuel
mixture near said spark plug, igniting said fuel cloud by a spark form
said spark plug, and then causing cylinder pressure to rise, thereby
producing auto-ignition at other sites in said combustion chamber wherein
said first mode is implemented in a first operating range and said second
mode is implemented only in a second operating range where engine speed
and load are lower than said first operating range.