The heat release process in a free volume combustion chamber and in porous reactors has been analyzed under Diesel engine-like conditions. The process has been investigated in a wide range of initial pressures and temperatures simulating engine conditions at the moment when fuel injection starts. The resulting pressure history in both porous reactors and in free volumes significantly depends on the initial pressure and temperature. At lower initial temperatures, the process in porous reactors is accelerated. Combustion in a porous reactor is characterized by heat accumulation in the solid phase of the porous structure and results in reduced pressure peaks and lowered combustion temperature. This depends on reactor heat capacity, pore density, specific surface area, pore structure, and heat transport properties. Characteristic modes of a heat release process in a two-dimensional field of initial pressure and temperature have been selected. There are three characteristic regions represented by a single- and multistep oxidation process (with two or three slopes in the reaction curve) and characteristic delay time distribution has been selected in five characteristic ranges. There is a clear qualitative similarity of characteristic modes of the heat release process in a free volume and in porous reactors. A quantitative influence of porous reactor features (heat capacity, pore density, pore structure, specific surface area, and fuel distribution in the reactor volume) has been clearly indicated. 1. Introduction Future internal combustion engines are to feature a clean combustion process. Clean process means a homogeneous combustion requiring simultaneous (volumetric) ignition of a homogeneous (preferably premixed) charge. Such a process results in simultaneous heat release characterized by a homogeneous temperature field in the combustion chamber, and the process is flameless. In the literature, such a process in a free volume combustion chamber is often called HCCI. There are a number of challenges in realizing homogeneous combustion in an engine operating under variable load and speed conditions. Especially critical are control of ignition timing, combustion duration, heat release rate, and corresponding pressure gradient and pressure peak, control of combustion temperature for nearly zero- -emissions, and completeness of the process for low CO and HC emissions. There is no system known to the authors that can satisfy all conditions selected above, at least if variable load conditions are considered. Two of them receive special attention from the point of
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