Methane/hydrogen blends in controlled auto ignition engines with EGR: Evaluation of NOx emissions

Gaseous fuel utilisation reduces pollutant emissions and makes energy systems less dependent from oil. Their carbon content is low, which means a reduction in CO2 emissions per unit of energy in comparison with liquid fossil fuels. The use of biogases (BG) and/or natural gas (NG) in internal combustion engines (ICE) is very attractive. To increase the efficiency of NG engines hydrogen can be added to natural gas, and innovative BG containing hydrogen can be conveniently used. Hydrogen has a high combustion speed, providing a reduction of the combustion duration when mixed with methane. Consequently higher temperatures are attained in the combustion chamber and thus engines fuelled with NG/H2 blends increase the NOx emissions. NG and BG are generally adopted as fuel in positive ignition ICEs; however they are also suitable for fuelling Controlled Auto Ignition (CAI) engines. The authors investigated combustion in CAI ICE by means of numerical simulations. Starting with pure CH4, the fuel was subsequently enriched with H2, comparing engine performance and exhaust emissions. CAI systems are indirectly controlled with varying air-fuel ratio, boost pressure and charge temperature. Charge temperature depends on the residual gas content, which has an impact on the combustion process as the burning mixture composition consequently changes. The authors investigated the effect of exhaust gas recirculation on the in-cylinder gas temperature and reaction mechanism, with main focus on NOx emissions. Hydrogen addition reduces the intake temperature requirement for the autoignition, which means less EGR. Hydrogen enriched fuels also require lower boost pressure than CH4 to deliver the reference imep while engine efficiency does not change significantly between tested fuels. Hydrogen also helps in reducing NOx emissions due to lower boost pressure and EGR rates.