Simulation and Experimental Investigation of Performance and Emissions of a Turbocharged Lean-Burn NG Engine Considering Thermal Boundary Layer

Abstract

Natural gas is considered to be the most promising alternative fuel considering economy and clean burning. At the present, emission regulations for NG are restricted with Euro IV, V and even for Euro VI and these regulations are expected to become more restricted in future. Emission generation characteristics of pure natural gas is fairly known, however more experimental investigation of natural gas combustion is necessary. In this research, a turbocharged natural gas SI engine is investigated experimentally. Emission and performance characteristics of the engine at WOT and lean burn conditions are studied and validated experimentally and theoretically. A computer code is also developed in MATLAB environment for predicting engine performance and NO emissionsand the results are validated with experimental data. Thermal boundary layer, as a novel approach, is considered in the simulation and a better agreement of predicted mass fraction burned was achieved at end part of combustion process when compared with experimental results. Experimental results have revealed that turbocharger match at mid engine speed, waste-gate opening and increase of friction losses at high engine speed would decrease the torque and resulted in better torque back-up. Boost pressure, controlled by opening range of waste gate based on compressor outlet pressure, remained nearly constant at speeds higher than 1450 rpm. NOx emission reduced with engine speed increase due to shorter timethat burned gases remain at high temperatures, although they have higher temperature at high engine speeds.NO emission increased with the increase of excess air ratio until it reached to its peak value at about 1.1 from stoichiometric mixture and then decreased. NO emission decreased substantially with increase of spark timing retard. Brake specific UHC and CO2 emissions were min at mid speed range and WOT.