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Modern imaging and laser technologies form the basis of many diagnostic techniques used to better understand the in-cylinder process governing engine performance and emissions in modern internal combustion engines. In order to perform many of these diagnostic techniques, a suitable apparatus that has optical access and that simulates the process from real engines is required. While much useful information can be acquired from a relatively simple closed combustion bomb-type apparatus equipped with a window, the most useful information can sometimes only be acquired from an operating engine.
The challenge with using these techniques on engines is providing suitable optical access. Optical access to some engine designs, such as older L-head designs that have their valves mounted in the cylinder block, can be readily achieved through the cylinder head. However, these engine designs are relatively archaic and the information obtained would be of little use to understanding processes in modern engines. For modern engines, the most common approach to obtaining optical access is through the piston—an elongated piston is used with a window in the crown that reciprocates over a stationary mirror. This approach was pioneered in the 1960’s by Fred Bowditch at General Motors and the piston design is often referred to as a “Bowditch Piston” .
Extensive engine modifications are necessary to provide the optical access. Such extensive modifications to an engine means that it is no longer identical to the production engine it was derived from. There may also be limitations on engine operation and the engine may no longer be operated in exactly the same way as a production engine. While these differences are important, proper design can help ensure that the impact on the processes of interest is minimal. Some of the measures that may be required to accommodate optical access include: