Simulation and experimental investigation of performance and emissions of a turbocharged lean-burn natural gas engine considering thermal boundary layer

Emission characteristics of pure natural gas is fairly known, however more experimental investigation of natural gas combustion is necessary. In this research, emission and performance of a turbocharged natural gas SI engine are investigated experimentally at WOT and lean burn conditions. A computer code is also developed in MATLAB environment for predicting engine performance and NO emission and 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, wastegate opening and increase of friction losses at high engine speed resulted in better torque back-up. Boost pressure, controlled by compressor outlet pressure, remained nearly constant at speeds higher than 1450 rpm. NO emission reduced with engine speed increase due to shorter time that burned gases remain at high temperatures, although they have higher temperature at higher speeds. It however 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, while it decreased substantially with increase of spark timing retard. Brake specic UHC and CO2 emissions were min at mid speed range and WOT.