Optical measurements inside the combustion chamber of internal combustion engines have been carried out for many decades. Methods for optical access include glass or quartz windows inserted as replacement for one of the valves as well as endoscopic imaging systems. Often a scheme is utilized based upon an elongated piston design, often referred to as the Bowditch design after its inventor. The principle is visualised in Fig. 1.
The ordinary steel piston is extended with a hollow cylinder on top of which a quartz window is mounted giving optical access to the combustion chamber from below. Two oblong openings on opposite sides of the extension enable both mounting of a mirror on the engine block, and optical access from outside to the interior of the piston extension. The cylinder liner also has to be extended, and equipped with an opening for optical access. The Bowditch design enables optical access from below and is mostly used for signal detection. One drawback of the hollow cylinder construction is that the view is limited near the cylinder wall. Optical access from the sides can be achieved using quartz parts inserted as part of the cylinder liner. Often a quartz ring is used, enabling access from all directions not obstructed by the four support beams (See Fig. 1). For laser-based measurements this ring is most frequently used for enabling the laser beam to propagate through the engine.
The optical parts have a substantial impact on the heat transfer characteristics of the engine and, due to the fragility of the material, also on the compression ratio and load for which the engine can be operated. For this reason a number of engine parameters like inlet gas temperature, start of injection and spark timing are adjusted in order to simulate the conditions in real engines.