2d infrared imaging at different wavelenghts to characterized cycles-to-cycle variation of the combustion process in a diesel engine

The internal combustion engine is the most common motor used for civil and commercial transportation even if it is a technology more than one century old. In order to update the engine, the most advanced diagnostics have been applied to investigate directly within the engine the processes that regulate its operation. Among these diagnostics, infrared cameras have taken a big technological leap in combustion research. While the typical techniques used for the visualization of liquid/vapor fuel, flames and burned gases are characterized by complex optical setup and the use of laser, the infrared diagnostic can be performed easily and can be effective as well. In this work, both a visible and an infrared cycle resolved cameras have been used to visualize the phenomena that take place inside the cylinder of a compression ignition research engine fuelled with diesel fuel. The engine head is placed on an elongated cylinder and an optical access in the piston head provides the bottom view of the combustion chamber. The engine runs a multi-injection strategy typical of an operating point of the New European Driving Cycle. In order to interpret the different processes that occur at the same instant, three kinds of images have been recorded in the infrared spectrum. The first band analyzed is from 1.5 to 5 ?m that is the camera range. A band pass filter at 4.2 ?m has been used to impress the radiation of the CO2 molecules. In addition, a filter at 3.9 ?m has been applied to visualize the hydrocarbons distribution during the injection process. The comparative analyses of the images recoded in the visible and infrared range, with and without band pass filters, has allowed to identify the presence of different processes at the same time. For example, during the main injection the fuel is delivered in a mix of fresh air, recirculated gas and reacting spray from the previous injection. Moreover, the availability of a cycle resolved infrared camera has allowed to investigate the engine cycle-to-cycle variation in terms of infrared emitted radiation and to compare it to the results of visible imaging. The data dispersion has been evaluated both in temporal and spatial location. It is the first time that this analysis is performed inside the combustion chamber of an engine. It denoted that the infrared imaging is suitable for this kind of applications. The better knowledge of in-cylinder processes is a powerful tool to control the combustion and to make the internal combustion engine more efficient