In order to improve IC engine energy utilization efficiency, an open steam power cycle used for IC engine exhaust gas energy recovery is proposed. The bottom cycle concept is designed on a four-cylinder naturally aspirated IC engine: with three cylinders taken as ignition cylinder, the last one is used for steam expansion cylinder; IC engine exhaust pipe is coupled with a Rankine steam cycle system which uses the high temperature exhaust gas to generate steam; then, the steam is injected into steam expansion cylinder and expands in the cylinder. In this way, the Otto cycle (or diesel cycle) of traditional IC engine and the steam expansion cycle (open Rankine cycle) are coupled on IC engine. On this basis, the energy recovery potential of this bottom cycle is studied by cycle processes calculation and parameters analysis. The research results show that the recovery efficiency of exhaust gas energy is mainly limited by exhaust gas temperature. The maximum bottom cycle power can reach 19.2 kW and IC engine thermal efficiency can be improved by 6.3% at 6000 r/min. All those can prove this novel bottom cycle concept has larger potential for energy saving and emission reduction on IC engine. (c) 2012 Elsevier Ltd. All rights reserved.