Cycle-resolved Evaluation of Directly Injected Methane using a High-Speed Laser-induced Fluorescence Measurement System
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Abstract
The usage of alternative fuels inside internal combustion engines (ICE), is one promising approach to meet the higher requirements concerning the efficiency and emission of modern combustion systems. The injection and mixture formation of such fuels has a major impact on the subsequent combustion as well as the formation of pollutants. To rate the influence and to gain a better understanding of those new alternatives a basic understanding of these dominant processes is mandatory. The principle of laser-induced fluorescence (LIF) is a well-known method to display and evaluate fuel distributions inside combustion chambers. A suiting online calibration routine allows the visualization of air-fuel equivalence ratio (λ) two-dimensionally during different operation modes. As cyclic variations can become a more critical issue while using alternative fuels, cycle- resolved test series become more and more important. To obtain several crank-angle-resolved measurements during one cycle as well as comparisons from cycle to cycle is crucial to understand the ongoing processes inside ICE for further improvements in research and development. Therefore, this study describes the development and application of a new optical measurement system to record the λ-distribution quantitatively with high spatial and temporal resolution inside an optically accessible single cylinder research engine. Based on a BMW diesel engine, the experimental apparatus is modified to inject gaseous methane directly into the cylinder using a HDEV4-based 8-hole valve. A variation of operation parameters, such as engine speed, injection pressure and injection timing, is conducted while the fuel distribution is recorded along the spray propagation. Corrections for the light sheet shape as well as pulse to pulse energy fluctuations of the laser pulses complement the High-Speed LIF-system with repetition rates up to 3 kHz using a single Nd:YAG-laser.