Understanding the effect of modifying the properties of the engine charge on the cyclic variations of ignition timing is one essential aspect of being able to predict and control the ignition timing in Homogeneous Charge Compression Ignition (HCCI) engines. This paper investigates cyclic variability of HCCI ignition timing using the experimental data from two different engines at over 300 operating points for five different blends of iso-octane and n-heptane. Experimental results indicate that the cyclic variations of HCCI auto-ignition timing decrease with an increase in the intake manifold temperature and mixture richness, but it increases with an increase in the EGR rate.

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