Abstract
This paper evaluates strategies for reducing the intake temperature requirement for igniting biogas in HCCI engines. HCCI combustion is a promising technology for stationary power generation using renewable fuels in combustion engines. Combustion of biogas in HCCI engines allows high thermal efficiency similar to Diesel engines, with low net CO2 and low NO emissions. However, in order to ensure the occurrence of autoignition in purely biogas fueled HCCI engines, a high inlet temperature is needed. This paper presents experimental and numerical results. First, experimental analysis on a 4 cylinder, 1.9 L Volkswagen TDI Diesel engine running with biogas in HCCI mode shows high gross indicated mean effective pressure (close to 8 bar), high gross indicated efficiency (close to 45%) and NO emissions below the 2010 US limit (0.27g/kWh). Stable HCCI operation is experimentally demonstrated with a biogas composition of 60% CH4 and 40% CO2 on a volumetric basis, inlet pressures of 2-2.2 bar (absolute) and inlet temperatures of 200-210 degrees C for equivalence ratios between 0.19 - 0.29. At lower equivalence ratios, slight changes in inlet pressure and temperature caused large changes in cycle-to-cycle variations while at higher equivalence ratios these same small pressure and temperature variations caused large changes to ringing intensity. Second, numerical simulations have been carried out to evaluate the effectiveness of high boost pressures and high compression ratios for reducing the inlet temperature requirements while attaining safe operation and high power output. The one zone model in Chemkin was used to evaluate the ignition timing and peak cylinder pressures with variations in temperatures at IVC from 373 to 473 K. In-cylinder temperature profiles between IVC and ignition were computed using Fluent 6.3 and fed into the multi-zone model in Chemkin to study combustion parameters. According to the numerical results, the use of both higher boost pressures and higher compression ratios permit lower inlet temperatures within the safe limits experimentally observed and allow higher power output. However, the range of inlet temperatures allowing safe and efficient operation using these strategies is very narrow, and precise inlet temperature control is needed to ensure the best results.