AN INVESTIGATION OF LEAN COMBUSTION IN A NATURAL GAS-FUELED SPARK-IGNITED ENGINE

Natural gas has been used extensively as art engine fuel in gas pipeli ne transmission applications and, more recently, as a fuel for transpo rtation applications including both light-duty and heavy-duty vehicles . The objective of this work was to investigate the performance and em ission characteristics of natural gas in an original equipment manufac turer (OEM), light-duty, spark-ignited engine being operated in the le an fueling regime and compare the operation with gasoline fueling case s. Data were acquired for several operating conditions of speed, throt tle position air-fuel equivalence ratio, and spark timing for both fue ls. Results showed that for stoichiometric fueling with a naturally as pirated engine, a power loss of 10 to 15 percent can be expected for n atural gas over gasoline fueling. For lean operation, however, power i ncreases can be expected for equivalence ratios below about phi = 0.80 with natural gas fueling as compared to gasoline. Higher brake therma l efficiencies can also be expected with natural gas fueling with maxi mum brake torque (MET) timings over the range of equivalence ratios in vestigated in this work. Coefficient of variation (COV) data based on the indicated mean effective pressure (IMEP) demonstrated that the eng ine is much less sensitive to equivalence ratio leaning for natural ga s fueling as compared to gasoline cases. The lean limit for a COV of 1 0 percent was about phi = 0.72 for gasoline and phi = 0.63 for natural gas. Lean fueling resulted in significantly reduced NOx levels where a lower plateau for NOx concentrations was reached at phi near or belo w 0.70, which corresponded to about 220 ppm. For natural gas fueling, this corresponded to about 1.21 gm/(kW-h). Finally, with MBT timings, relatively short heat release durations were obtained for lean fueling with natural gas compared to gasoline.