Fundamental Study on Dual-fuel Engine Combustion Based on Fuel Design Concept
Technical Thermodynamics
Final Report Abstract
In this joint Sino-German research projects, dual-fuel engine experiments performed by Shanghai Jiao Tong University were correlated with experiments in a constant pressure injection and combustion chamber and a rapid compression machine. The goal of the collaboration was to understand mixture formation and combustion processes of low-carbon pilot fuels against the background of specifics of a dual fuel combustion strategy. Identified as such were e.g. very short pilot injection durations, multiple injection strategies and the influences of a diluted ambient (port fuel/air mixture). Alternative fuels like polyoxymethylene dimethyl ethers (OME) and 1-Octanol were included in the investigations as potential pilot fuels. Measurements of injection rates revealed that apart from an expectable influence of fuel density on the quantitative level of the main injection, no specific effect on the hydraulic functionality of the injectors could be observed. For very short (pre-)injections, low viscosity of the fuel leads to a noticeable effect on the opening and closing dynamics of the injector needle and lower the injection quantity at equal duration. This characteristic directly effects on the development of the liquid length (investigated with Mie scattering) and thus quantity and quality of the fuel/air mixture resulting from the pre-injection. From the investigation of the liquid phase, an effect on the main injection can not be directly observed. Investigations of the gaseous fuel-air jet beyond phase change with Schlieren methodology show that the main injections omit a different temporal and shifted spatial behaviour due to the fuel specific influence on the pre-injection. Although for single injections the mixture formation is not significantly influenced by the fuel (equal fuel/air mass ratio distributions), the combination of multiple injections can lead to different temporal behaviour and thus inflict on the mixture distribution of the main injection. The independence of the fuel/air mass ratio distribution from a single main injection of fuel specific influences could be proved by Raman spectroscopy measurements, whose results are the quantitative mixture composition within the fuel/air jet. An influence of injection duration on the mixture quality could be observed, as sprays and jets from short injections are dominated by the head vortex interaction with a steady ambient and lead to overall leaner mixtures. Combustion experiments in the constant pressure chamber based on Schlieren and OH chemical luminescence measurements show that the reactivity resulting from the combination of pilot fuel and diluted ambient is a crucial parameter for the ignition and combustion behaviour in multi-injection strategies. Investigations in a rapid compression machine confirm that this reactivity alongside with injection timing and duration are key parameters for a stable and efficient operation directly influencing also peak pressure and rate of heat release in a non-linear way. Alternative fuels like OME with a high molecular oxygen content and high reactivity seem especially suitable for dual-fuel applications. The findings from the optical experiments regarding mixture formation as well as combustion could be linked to results from engine measurements and help to explain the development of heat release, efficiency and emissions. With findings regarding leaner mixtures from short injections and fuel specific effects on the duration of short and pre-injections as well as the role of the reactivity of the mixture resulting from pilot fuel and diluted ambient, the importance of injection strategy, timing and duration for the application of different (low-carbon) fuels could be understood on a fundamental level. In addition, transfer to characteristics of engine operation was possible. Optical investigations in abstracted systems like combustion chamber and rapid compression machine could be shown to be valuable tools for the design and implementation of dual-fuel combustion strategies with alternative fuels.
Publications
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MIXTURE FORMATION ANALYSIS OF POLYOXYMETHYLENETHER INJECTION. Atomization and Sprays, 30(11), 843-859.
Peter, Andreas; Siewert, B.; Riess, S.; Strauss, L.; Pastoetter, C. & Wensing, Michael
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Phase change in fuel sprays at diesel engine ambient conditions: Modeling and experimental validation. The Journal of Supercritical Fluids, 173, 105224.
Riess, Sebastian; Rezaei, Javad; Weiss, Lukas; Peter, Andreas & Wensing, Michael
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Methane Concentration Influence on Combustion in a Rapid Compression Machine under DFICE Relevant Conditions. 10th International Conference on Modeling and Diagnostics for Advanced Engine Systems (COMO- DIA 2022).
Clemente Mallada, R., Peter, A., Riess, S., Strauss, L., Fajri, H. & Wensing, M.
