Biogas is a gas resulting from biomass, with a volumetric content of methane (CH4) usually ranging between 50% and 70%, and carbon dioxide (CO2) content between 30% and 50%; it can also contain hydrogen (H2) depending on the feedstock. Biogas is generally used to generate electricity or produce heat in cogeneration system. Due to its good efficiency through the rapid combustion and lean air-fuel mixture, Homogeneous Charge Compression Ignition (HCCI) engine is a good candidate for such application. However, the engine load must be kept low to contain the high-pressure gradients caused by the simultaneous premixed combustion of the entire in-cylinder charge. The homogenous charge promotes low particulate emissions, and the dilution helps in containing maximum in-cylinder temperature, hence reducing nitrogen oxide emissions. However, HC and CO levels are in general higher than in SI combustion. Moreover, HCCI engines usually require high intake temperature with values depending on compression ratio, fuel, equivalence ratio, and intake pressure.
This paper presents the results of an experimental campaign on a diesel internal combustion engine for passenger cars, modified to operate in HCCI mode. The engine was fuelled by mixtures of methane, H2, and carbon dioxide, with the aim of reproducing the composition of innovative biogas naturally containing hydrogen. The equivalence ratio was kept constant at 0.4 and the intake charge temperature and pressure have been adjusted, depending on fuel composition, to control the combustion process. For each fuel, the intake charge conditions for an optimal combustion phasing have been determined, attaining a maximum indicated engine efficiency of 40% and specific NOx emissions down to 0.1 g/kWh.