Emissions and Performance of a Small L-Head Utility Engine Fueled with Homogeneous Propane/Air and Propane/Air/Nitrogen Mixture 932444

The objective of this study was to observe and attempt to understand the effects of equivalence ratio and simulated exhaust gas recirculation (EGR) on the exhaust emissions and performance of a L-head single cylinder utility engine. In order to isolate these effects and limit the confounding influences caused by poor fuel mixture preparation and/or vaporization produced by the carburetor/intake port combination, the engine was operated on a premixed propane/air mixture. To simulate the effects of EGR, a homogeneous mixture of propane, air, and nitrogen was used. Engine measurements were obtained at the operating conditions specified by the California Air Resources Board (CARB) Raw Gas Method Test Procedure.

Measurements included exhaust emissions levels of HC, CO, and NOx, and engine pressure data. Analysis of the pressure data included IMEP determination, heat-release analysis, and determination of the characteristics of the cycle-to-cycle variations and interactions, for all operating modes of the CARB RGM test cycle for small utility engines.

Results indicate that the performance and emissions of the engine were strongly influenced by both air-fuel ratio and amount of N₂ dilution. The HC emissions reached a minimum at approximately an equivalence ratio of 0.8, CO emissions began to increase at an equivalence ratio, Φ, of 0.95, and NOx peaked just lean of stoichiometric. Peak burn rates were observed at slightly rich of stoichiometric. Addition of simulated EGR reduced NOx emissions approximately 50 percent at a given equivalence ratio. However, fixed ignition timing limited the engine performance under lean conditions and with simulated EGR. Cyclic variation in the burn rate increased at some operating conditions with the lean equivalence ratios, and two burn modes were observed. However, prior cycle effects were not observed, indicating these variations are probably a result of same-cycle-processes.