Ion current (IC) is a novel and feasible method for the diagnosis of in-cylinder combustion. The improvement of IC signal quality and reliability is always a challenge before it is applied as a sensor of engine combustion. In this paper, the characteristics of homogeneous charge compression ignition (HCCI) and IC signal with ethanol/gasoline dual fuel have been presented. The results show that increased ethanol mass fraction XEtoll retards and weakens both combustion and IC signal, with extending the phase difference (AOlc) between start of ion current (SOIC) and CA(50) (crank angle of 50% heat release). The delayed start of gasoline injection (GSOI) enhances combustion and IC signal, but the effect is declined when XEtoll increases. This result provides a balance achieving both better quality and wider misfiring diagnosis window of IC signal. The CFD simulation results present the differences between in-cylinder local and global ion concentration with different XEtoll and GSOls. According to the spatial distribution of electrons during the combustion, delayed GSOI causes more concentrated ionization and smaller difference between local and global start of electron appearance (SOEA). (C) 2015 Elsevier Ltd. All rights reserved.