Optical Evaluation of Directly Injected Methane Using a Newly Developed Highly Repetitive Laser Diagnostics System 2019-24-0134
New certification procedures like WLTP and RDE, as well as more stringent emission regulations in general, demand for further improvements in engine research and development. In order to meet the challenges of reducing pollutants while maintaining high performance and high efficiency many different approaches are discussed. Beside various concepts for new combustion strategies and alternative fuels, gaining detailed knowledge about the ongoing processes inside engines and combustion chambers during the different operation modes is of major importance. With their influence on combustion and emission formation, fuel injection and mixture formation are playing an important role for further improvements in modern propulsion systems. With the help of optical measurement systems based on laser induced (exciplex) fluorescence (LIF/ LIEF), an advanced understanding of these mechanisms can be obtained. However, additional challenges arise when it comes to analyzing transient engine operation and cyclic fluctuations as well as rating their contribution to the formation of emissions.
This study contains the evaluations of the injection and mixture formation inside a rapid compression machine (RCM) with directly injected methane. Equipped with several optical accesses, the RCM allows two dimensionally measurements of the air-fuel equivalence ratio (λ) using a high repetitive LIF-system. The applied principle of laser-induced fluorescence enables the visualization of the λ-distribution quantitatively under various operation conditions such as different compression ratios. Fluctuations of this distribution as well as charge flow inside the combustion chamber are evaluated through the optical flow method (OFM). Interpretations of the recorded images deliver a better understanding of direct injection and mixture processes under engine like conditions as charge motion and λ-distribution are displayed during the combustion cycle.