Cycle by Cycle Variation in HD Natural Gas Fuelled Engine

The cyclic variability of a Heavy Duty, turbocharged, Compressed Natural Gas (CNG) engine has been investigated, by analysing a large number of consecutive pressure cycles, 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 lower flammability limit with acceptable burning stability has been demonstrated.