Homogeneous charge compression ignition (HCCI) engines are amenable to a large variety of fuels as long as the fuel can be fully vaporized, mixed with air, and receive sufficient heat during the compression stroke to reach the autoignition conditions. This study investigates an HCCI engine fueled with ethanol-in-water mixtures, or "wet ethanol". The motivation for using wet ethanol fuel is that significant energy is required for distillation and dehydration of fermented ethanol (from biosources, not from petroleum), thus direct use of wet ethanol could improve the associated energy balance. Recent modeling studies have predicted that an HCCI engine can operate using fuel containing as little as 35% ethanol-in-water with surprisingly good performance and emissions. With the previous modeling study suggesting feasibility of wet ethanol use in HCCI engines, this paper focuses on experimental operation of a 4-cylinder 1.9-L engine running in HCCI mode fueled with wet ethanol. This paper investigates the effect of the ethanol-water fraction on the engine's operating limits, intake temperatures, heat release rates, and exhaust emissions for the engine operating with 100%, 90%, 80%, 60%, and 40% ethanol-in-water mixtures. (C) 2009 Elsevier Ltd. All rights reserved.