Mini High Speed HCCI Engine Fueled with Ether: Load Range, Emission Characteristics and Optical Analysis

Power supply systems play a very important role in everyday life applications. There are mainly two ways of producing energy for low power generation: electrochemical batteries and small engines. In the last few years, many improvements have been carried out in order to obtain lighter batteries with longer durations but unfortunately the energy density of 1 MJ/kg seems to be an asymptotic value. An energy source constituted of an organic fuel with an energy density around 29 MJ/kg and a minimum overall efficiency of only 3.5% could surpass batteries. Nowadays, the most efficient combustion process is HCCI combustion which has the ability to combine a high energy conversion efficiency with low emission levels and a very low fuel consumption.

The present paper describes an investigation carried out on a modified model airplane engine, on how a pure HCCI combustion behaves in a small volume, Vd = 4.11 Cm₃, at very high engine speeds (up to 17,500 [rpm]). Using ether like fuel, for the first part of the experiments the behavior of the engine was characterized by studying the variation of IMEP, indicated power, main combustion features (temperature, duration, phasing, residuals, efficiency and heat release) and emissions. The characterization was carried out between 7,500 and 17,500 [rpm], and the speed was changed by using six different propellers. In the second part of the experiment, an optical study was performed; the main intent was to characterize the combustion structure and its transient behavior in a small volume at high engine speed. In order to achieve this, chemiluminescence images were acquired together with images of hydroxyl, formaldehyde and C2. Two sets of measurements were performed, i.e., at 6,500 and 14,000 [rpm], and in each test the behavior of the low and high temperature reactions, as well as that of C2 was studied together with the natural emitted light. This was done to understand the degree of homogeneity of the combustion and the boundary layer behavior. The third part of this paper describes the analysis of the behavior of the optical and the metal engine in order to provide an understanding to whether the use of the optical window affected the combustion event.