An experimental study on the self-ignition and knocking characteristics for hydrogen-fueled homogeneous compression charge ignition engines

Abstract

Self-ignition and knocking characteristics for hydrogen (H2)-fueled homogeneous compression charge ignition (HCCI) engines with no carbon emissions are experimentally studied using a rapid compression expansion machine (RCEM) for various compression ratio (ε) and fuel-equivalence ratio (ϕ) conditions without preheating intake air which is generally adopted to HCCI combustion but reduces engine efficiency. Results show that H2-air premixtures self-ignite at ϕ = 0.42 and ε = 22.0, while their theoretical self-ignition ε is 27.5, due to different boundary conditions. With increasing ε, the range of ϕ for stable operation with self-ignition and no knocking tends to increase, but it is limited, e.g., ϕ ≈ 0.1–0.2 at ε = 32.0. Depending on ϕ and ε when knocking occurs, various knocking peaks (bands) are observed in the frequency domain by the fast Fourier transform (FFT): the lowest knocking frequencies around 9–13 kHz, exhibiting a tendency of gradually increasing with increasing ϕ, and additional knocking peaks for enhanced ϕ. Compared with the narrow stable operation range in ϕ, the condition where the maximum indicated mean effective pressure (IMEP) is obtained includes moderate knocking and thus the actual operating range is significantly extended, e.g., up to 88% in ϕ at ε = 28.0.