Evaluation of emission characteristics of a stoichiometric natural gas engine fueled with compressed natural gas and biomethane

Highlights: • The torque and thermal efficiency decrease when biomethane was used. • The composition of biomethane results in low C2–C4 hydrocarbon emissions. • Low NOx formation leads to low ammonia formation when using biomethane. • Biomethane does not satisfy the EURO-VI standards of CH4 and ammonia emissions. In this study, the performance and emission characteristics of a stoichiometric natural gas engine were investigated using biomethane and compressed natural gas (CNG). The wide-open throttle (WOT) test and world harmonized transient cycle (WHTC) mode were conducted using a 11 L class six-cylinder natural gas engine that satisfies the emission standards of EURO-6. The biomethane contained 98.69% CH₄ content and had a low calorific value of 39.46 MJ/Nm³ owing to the absence of C2–C4 hydrocarbons. In the WOT test, the engine performance deteriorated due to the poor combustion characteristics of biomethane. In particular, the thermal efficiency decreased by 2% when biomethane was used. The high CH₄ content of biomethane increased the concentration of oxygenated hydrocarbons at the engine outlet. A total of 72%-76% of non-methane hydrocarbon (NMHC) was oxygenated in the WOT test using biomethane. In the WHTC mode, the emission standards of EURO-6 for nitrogen oxide, carbon monoxide, and NMHC were satisfied using biomethane. However, CH₄ and ammonia emissions were higher than those specified in the EURO-6 emission standards when biomethane was used. Fuel consumption with biomethane was also 39.4% higher than that of CNG during the WHTC mode.