Development of an Optical Investigation Method for Diesel and Oxymethylene Ether Spray in a Large-Bore Dual-Fuel Engine Using a Fisheye Optical System

Optical combustion phenomena investigation is a common tool for passenger car and automotive engines. Large-bore engines for stationary and mobile applications, on the other hand, have a lower optical examination density. This is mainly due to the technically more complex design of the optical accesses that have to provide a larger field of view and withstand high mechanical and thermal loads. Nevertheless, an optical investigation of in-cylinder phenomena in large-bore engines is essential to optimize efficient and environmentally friendly combustion processes using new sustainable e-fuels. To realize a simple optical access with maximum observability of the combustion chamber, a fisheye optic for the direct integration into internal combustion engines was developed and used for in-cylinder Mie-scattering investigations of diesel and Oxymethylene Ether (OME_3-5) pilot fuel spray of natural gas dual-fuel combustion processes in a MAN 35/44DF single-cylinder research engine. As this special application of a fisheye lens poses some technical challenges, a special image processing procedure is necessary for result evaluation. This innovative postprocessing of the fisheye images comprises a calibration of the fisheye optic and a virtual three-dimensional (3D) re-projection method. Investigations prove the accuracy of the method to be within 2.1 mm. To prove the advantage of the method, optical spray investigations of two different fuels using Mie-scattering in the skipped-fire optical accessible medium-speed large-bore engine are carried out under realistic engine conditions. With the newly developed post-processing procedure, it was possible to derive the mean liquid penetration depth of the in situ investigations. Further, the postprocessing includes a rectification of the fisheye images to improve the observability of the pilot fuel spray in the fired combustion engine. The analysis reveals a more compact and dense spray for OME_3-5 compared to marine diesel fuel (DMA) as well as about 39% reduced liquid penetration length.