Redesign and Characterization of a Single-Cylinder Optical Research Engine to Allow Full Optical Access and Fast Cleaning during Combustion Studies

This work describes the update of an optical engine design with the aim of increasing its capabilities when used for combustion studies. The criteria followed to perform the optical engine redesign were: maximize the optical accessibility to the combustion chamber, minimize the time consumed to clean the optical parts, and minimize the adaptation costs. To meet these requirements, a modular design using window-holders to fit the windows in the optical flange, was proposed. This novel solution allows optical access near the cylinder-head plane while maintaining high operating flexibility (i.e. fast transition between optical and metal engine, and very fast cleaning procedure). The new engine design has three additional optical accesses to the combustion chamber and resulted in more efficient operation compared to the original design, reducing the time consumed to clean the optical parts from 40 down to 10 min. Two main parameters of the new engine were characterized, the effective compression ratio and the rotatory flow field velocity (swirl). The characterization process revealed very similar values between the effective and geometric compression ratios (14.7:1 vs 14.2:1), which confirms the use of appropriate dimensional tolerances during the machining process and low amount of blow-by. Finally, the swirl ratio was characterized through particle image velocimetry measurements for different crank timings at 1200 rpm and motored conditions, using the optical piston with a cylindrical bowl. This method revealed swirl ratios varying from 1 to 1.7 depending on the timing considered, with increasing trend as the piston moves towards the top dead center.

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