Effects of H2 Addition to Compressed Natural Gas Blends on Cycle to Cycle and Cylinder to Cylinder Combustion Variation in a Spark Ignition Engine

Abstract

An experimental investigation and a burningrate analysis have been performed on a production 1.4 liter compressed natural gas (CNG) engine fueled with methanehydrogen blends. The engine features a pentroof combustion chamber, four valves per cylinder, and a centrally located spark plug. The experimental tests have been carried out in order to quantify the cycletocycle and the cylindertocylinder combustion variation. Therefore, the engine has been equipped with four dedicated piezoelectric pressure transducers placed on each cylinder and located by the spark plug. At each test point, incylinder pressure, fuel consumption, induced air mass flow rate, pressure, and temperature at different locations on the engine intake and exhaust systems as well as “engineoutâ€‌ pollutant emissions have been measured. The signals related to engine operation have been acquired by means of a National Instruments PXIDAQ system and software developed in house. The acquired data have then been processed through a combustion diagnostic tool resulting from the integration of an original multizone thermodynamic model with a computeraided design (CAD) procedure for the evaluation of the burnedgas front geometry. The diagnostic tool allows the burning velocities to be computed. The tests have been performed over a wide range of engine speeds, loads, and relative airfuel ratios (up to the lean operation limit (LOL)). For stoichiometric operation, the addition of hydrogen to CNG has produced a brakespecific fuel combustion (bsfc) reduction ranging between 2% and 7% and a brakespecific total unburned hydrocarbons (bsTHCs) decrease up to 40%. These benefits have appeared to be even higher for lean mixtures. Hydrogen has shown to significantly enhance the combustion process, thus leading to a sensibly lower cycletocycle variability. Hydrogen addition has generally resulted in extended operation up to relative airtofuel ratio (RAFR) = 1.8. Still, the LOL consistently varies depending on the considered cylinder.