<div class="htmlview paragraph">Homogeneous charge compression ignition (HCCI) combustion enables higher thermal efficiency and lower NOx emission to be achieved in internal combustion engines compared with conventional combustion systems. Adjusting the proportion oftwo fuels with different ignition properties is an effective technique for controlling ignition timing in HCCI combustion.</div> <div class="htmlview paragraph">The authors have proposed a new HCCI combustion engine system fueled with dimethyl ether (DME) with a high cetane number and methanol-reformed gas (MRG) with a low cetane number in previous research. In the system, both DME and MRG are to be produced from methanol by onboard reformers utilizing exhaust heat from the engine. The research has shown high thermal efficiency of the system over a wide operable range of equivalence ratio. MRG effectively controls the timing of the second stage heat release by the high temperature reactions in HCCI of DME to expand operable range of equivalence ratio and engine load.</div> <div class="htmlview paragraph">This research newly proposes an HCCI combustion engine system fueled with DME and DME-reformed gas (DRG). In the system, just DME is stored in a fuel tank, and the DRG is to be produced by an onboard reformer utilizing exhaust heat from the engine. The system has an advantage of using the less toxic fuel over using methanol, though the heat recovery effect is lower than the previous system with MRG and DME produced from methanol. This research analyzes characteristics of HCCI combustion of DME and two types of DME-reformed gases. One type is a reformed gas by partial oxidation, and the other is by steam reforming. Both reformed gases contain hydrogen and carbon monoxide with low cetane numbers. The experiments are conducted by varying the proportion of the fuels and equivalence ratio. The overall thermal efficiency based on DME was also analyzed for ideal reforming conditions.</div>