Efficiency and raw emission benefits from hydrogen addition to methane in a Prechamber–Equipped engine

Abstract Providing high ignition energies to low reactivity (hard-to-ignite) fuels in premixed combustion modes is a challenging task. This is especially true if diluted combustion and high compression ratios are applied which are desirable for achieving high efficiency levels. One promising technology is to decouple the ignition process from the conditions in the main chamber using a prechamber, which is able to ignite the charge in the engine's main combustion chamber using turbulent jets. In this study, experimental results of a prechamber-equipped four-cylinder engine with 2 L displacement are discussed. The engine is designed to provide high efficiency using natural gas as a fuel. The prechamber is able to be fueled externally (called active prechamber mode), e.g. the air–fuel equivalence ratio λ can be decoupled from the λ in the main combustion chamber. Besides of pure methane operation, the engine is operated with the addition of 22.5 mol% (3.5 mass%, 8.0 energy%) hydrogen to methane. The results show that it is beneficial to use auxiliary fueling of the prechamber in pure methane operation mode in order to achieve highest possible efficiency at low NOx levels at λ of around 1.7. When adding hydrogen, λ of around 1.7 still provides highest efficiency but auxiliary fueling of the prechamber is not necessary any more to enable fast and robust combustion. Compared to the active prechamber operation with methane, the 3.5 mass% (8.0 energy%) addition of hydrogen is able to reduce the engine's fuel energy demand by 1.3–3.3% in relevant engine operating points, which leads to tailpipe CO2 reductions of 9.2–11%. Additionally, hydrogen addition is able to disproportionately lower the engine's unburned hydrocarbon (HC) emissions: The addition of 3.5 mass% hydrogen lowers HC mass emissions by impressive 30–40%. Simultaneously, at the lean conditions considered, CO and NOx emissions are lowered by around 20%.