The characteristics of HCCI combustion were investigated experimentally in a variable compression ratio CFR engine when operating on lean mixtures of n-heptane and n-pentane in air. The effects of changes in equivalence ratio, compression ratio, the addition of nitrogen, carbon dioxide, and methane into the intake charge on the cyclic variation in the ignition and the development of the combustion processes were investigated. The limiting conditions that produce ignition in one cycle only among many, repeated ignition in every cycle, and knock were identified. It was found that HCCI combustion and the range of its operating conditions are limited by the extent of non-homogeneity of the intake charge, both in mixture quality and temperature. The optimum values for the addition of nitrogen, carbon dioxide, and natural gas to produce higher indicated work and reduce cyclic variation and the intensity of energy release depend on the intake mixture strength and specific operational conditions.

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