An experimental study on the impact of hydrogen and carbon dioxide addition to methane on a HCCI engine performance and emissions

Biogas is produced from organic materials, typically containing methane (CH 4 ) with content varying between 50% and 70%, and carbon dioxide (CO 2 ) ranging from 30% to 50%. It may also include hydrogen (H 2 ) with appropriate source material. Biogas is commonly utilized for power or heat generation in combined heat and power systems. The Homogeneous Charge Compression Ignition (HCCI) engine is well-suited for this purpose due to its efficient and rapid combustion process, with a lean air-fuel mixture. However, to limit the high pressure rise rate resulting from simultaneous premixed combustion in the engine cylinder, the combustion process must be controlled. The dilution inherent to this combustion mode helps in limiting the maximum cylinder temperature, and so the nitrogen oxides emissions. Moreover, particulate emissions can be lower than with diesel engine due to the homogeneous charge. Nonetheless, hydrocarbon (HC) and carbon monoxide (CO) levels are greater than those observed with spark ignition (SI) combustion. Additionally, high intake temperatures are needed to operate HCCI engines, depending on compression ratio, intake pressure, fuel type, and equivalence ratio. This paper focuses on an experimental study involving a modified diesel engine initially built for passenger vehicles, adapted to run in HCCI mode. Different fuel mixtures containing methane, hydrogen, and carbon dioxide with a fixed equivalence ratio of 0.4 were tested to reproduce an innovative biogas composition naturally containing hydrogen. The intake temperature and pressure were set according to the fuel mixtures to properly phase the combustion onset. A combustion analysis has been conducted to determine the effect of fuel composition on the combustion process. For each fuel type, the optimal intake pressure and temperature for achieving best combustion timing were identified, resulting in a peak indicated efficiency of 40% and specific NOx emissions as low as 0.1 g/kWh. Results show the effect of both CH 4 /H 2 ratios and CO 2 content as well as intake conditions on the load (IMEP), efficiencies, engine stress (i.e. pressure gradient) and emissions. • Hydrogen reduces intake temperature and pressure requirements for HCCI combustion. • Maximum in-cylinder pressure is highly sensitive to intake temperature and pressure. • Findings indicated excellent combustion stability and Indicated efficiency of 40%. • Fuels containing CO 2 showed very low NOx emissions.