Design of an Endoscopic Fully Optically Accessible High-Speed Large-Bore Engine

Rising engine efficiency and exhaust limitations demand a deeper knowledge of the combustion process. State-of-the-art investigations use laser optical test equipment that relies on optical accesses into the engine. In this article, a new endoscopic, fully optical access for a high-speed large-bore engine is demonstrated. The successfully realized concept consists of two individually usable accesses: The lateral access with a vertical field of view is realized via a ring, which is inserted between the cylinder liner and cylinder head. The ring offers several screw-in positions for an ultraviolet (UV)-enhanced endoscope. This access can be used independently for full-load investigations of the mixture formation and combustion processes taking place inside the cylinder; The second access from above is realized by inserting a self-developed fisheye endoscope. The endoscope replaces one of the exhaust valves and supplements the lateral access with a horizontal field of view covering nearly the whole combustion chamber. The development of the optical design of the fisheye endoscope using ray tracing is detailed for this type of all-optical engine. The materials and design changes made for the second access reduce the convertible engine load. Thus finite element method (FEM), conjugate heat transfer (CHT), and multibody simulations validate strength and temperature behavior under engine conditions. A similar concept was already implemented and successfully tested at a dual-fuel engine with 40 l displacement. With the current investigations, it is shown that this concept represents a reasonable and feasible solution for in situ engine investigations for smaller engines as well, here with a bore of 170 mm and a stroke of 210 mm resulting in a 4.8 l displacement. For this reason, the design shown here approximately halves the diameter of the fisheye optics compared to the design already implemented. The main focus is thus on reducing the installation space of the optical accesses while at the same time optimizing the optical imaging properties and the field of view. The significantly reduced concept shown here, in turn, has the possibility of being integrated more efficiently into an engine with a larger displacement class through a reduced modification effort, thus simplifying the in-situ investigations and enhancing the likeness to the original engine.