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Natural gas is a promising alternative fuel for internal combustion engine application due to its low carbon content and high knock resistance. Performance of natural gas engines is further improved if direct injection, high turbocharger boost level, and variable valve actuation (VVA) are adopted. Also, relevant efficiency benefits can be obtained through downsizing. However, mixture quality resulting from direct gas injection has proven to be problematic. This work aims at developing a mono-fuel small-displacement turbocharged compressed natural gas engine with side-mounted direct injector and advanced VVA system. An injector configuration was designed in order to enhance the overall engine tumble and thus overcome low penetration. Gas injection, interaction thereof with charge motion and geometrical bounding walls, and the resultant mixture formation process was investigated in detail by the combination of planar laserinduced fluorescence (LIF) in an optical engine and computational fluid dynamics (CFD) analysis with moving injector model to verify the design of the injector and combustion chamber. Then a prototype engine was tested to compare the rated torque against target performance. The planar LIF investigation underlined the influence of the Coandǎ effect whereby the gas jet was deflected to the adjacent injector niche and then to the combustion chamber roof. Such effect was inhibited at early injection timings due to strong intake air flow. CFD analysis confirmed this behavior and pointed out that the mixing process is dominated by the gas jet during injection and flow patterns promoted by it. It was concluded that the principal mixing mechanism is the jet-promoted tumble and elliptical swirl motion, and the mixing rate is thereby scaled with absolute time, rather than crank angle degree, and mainly determined by the strength of these two motion patterns. It was in addition found that the injection contributes to combustion-relevant turbulence mainly by intensifying the large-scale charge motion. Overall high mixing capacity was observed, and the injector and combustion chamber design deemed efficacious. The engine design has been successfully accomplished and the prototype multi-cylinder engine (MCE) is ready for extensive performance and emission analysis on the test rig.

Hydrogen is regarded as a future energy carrier. Within the H2BVplus research project sponsored by the Austrian Federal Ministry for Transport, Innovation and Technology (BMVIT), a high-efficiency combustion system has been developed for passenger car hydrogen engines by the Institute of Internal Combustion Engines and Thermodynamics of Graz University of Technology in collaboration with BMW Group Research and Technology. The conventional diesel combustion process served as a template.

The present paper is the result of a cooperation between Politecnico di Torino and AVL List Gmbh within a recent collaborative research project funded by the EC. The research work was focused on the experimental and numerical characterization of mixture formation, combustion, and emissions in direct-injection NG engines, to draw useful indication for the design of innovative, high-performance engine concepts. As a matter of fact, direct-injection IC engines running on NG are believed to be a competitive transition solution towards a sustainable mobility scenario, given their maturity, technological readiness, and flexibility with respect to the fuel quality. Moreover, gaseous-fuel engines can further decrease their carbon footprint if blending of natural gas with hydrogen is considered. Provided that mixture formation represents a key aspect for the design of direct-injection engines, the activity presented in this paper is focused on the characterization of NG injection and on the mixing process, as well as the impact these latter hold on the combustion process as well as on engine-out emissions. The mixture formation process was analyzed by means of combined CFD and optical investigations. Furthermore, a full version of the engine was tested on a dynamic test rig, providing quantitative information on the engine performance and emission characteristics. The CFD results highlighted the correlation between the mixture homogeneity and the combustion stability and hinted at a relevant impact of the jet characteristics on the air charge tumble and turbulence characteristics.

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