Cycle by cycle variation in HD natural gas fuelled engine

The cyclic variability of a Heavy Duty, turbocharged Compressed Natural Gas (CNG) has been investigated by analyzing a large number of consecutive pressure cycle measured with respect to steady engine operating conditions. Cycle-by-cycle variation has been characterized by means of both pressure and combustion related parameters so allowing a "measure" of this kind of instability. The engine has been run under stoichiometric as well as lean and ultra lean burning conditions. As far as the first conditions are concerned, some appreciable fluctuations have been observed also for this kind of engine for which cyclic variability causes are minimized. For highly diluted burning conditions the importance of this instability becomes more evident and specific tuning of the engine control parameters as spark advance needs to be operated. However the engine capability to run under mixture conditions very close to the flammability limits with acceptable burning stability has been demonstrated.