Numerical evaluation of internal combustion spark ignition engines performance fuelled with hydrogen – Natural gas blends

Abstract

The dependence of road transportation from fossil fuels and the related economic and environmental consequences imposes the diversification of energy sources. Hydrogen can strongly contribute to this goal because it can be produced from different renewable energy sources. In order to boost the development of hydrogen technology and reduce the dependence from conventional fossil fuels, hydrogen can be used in internal combustion engines added to natural gas (NG). Hydrogen-natural gas blends, commonly named HCNG, can be distributed using the natural gas infrastructures without significant modifications if hydrogen content is lower than 30% in volume. In this paper a numerical model has been developed to predict the performance and emissions of an internal combustion engine fuelled by natural gas and hydrogen – natural gas blends. The analysis displayed the impact of hydrogen addition on engine brake efficiency and NOx emission. Stoichiometric air-to-fuel ratio was considered for each fuel in order to assure an efficient exhaust after-treatment adopting a three-way catalyst. Exhaust gas recirculation (EGR) was investigated with the aim at improving engine efficiency and reducing NOx emissions respect to undiluted charge. In fact, HCNG blends combustion properties are particularly suitable for EGR, assuring a stable combustion also when the charge is diluted. Maximum brake torque (MBT) ignition timing has been adopted for all fuels and operating conditions investigated. Simulations were performed at conditions reproducing engine operation on a passenger car over the New European Driving Cycle (NEDC). Results were displayed in terms of fuel consumption in MJ/km and NOx emissions in g/km. The results showed that HCNG blends improved engine brake efficiency, particularly at low loads and for the highest hydrogen content, with fuel consumptions on energy basis over NEDC 2.5%, 4.7% and 5.7% lower than CNG, for HCNG 10, 20 and 30 respectively. NOx emissions increased of about 4% for HCNG 10, 11% for HCNG 20 and 20% for HCNG 30, due to the higher in-cylinder gas temperatures. Further investigations, performed adopting 10% EGR for HCNG blends, showed a large reduction of NOx emission, over 80% compared with natural gas (without EGR), with a positive effect also on engine efficiency. The decrease in fuel consumption using HCNG blends together with EGR, compared with natural gas, was 5.4%, 6.6% and 7.7% for HCNG 10, 20 and 30, respectively.