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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.

In multijet common rail (CR) systems, the capability to manage multiple injections with full flexibility in the choice of the dwell time (DT) between consecutive solenoid current pulses is one of the most relevant design targets. Pressure oscillations triggered by the nozzle closure after each injection event induce disturbances in the amount of fuel injected during subsequent injections. This causes a remarkable dispersion in the mass of fuel injected when DT is varied. The effects of the hydraulic circuit layout of CR systems were investigated with the objective to provide design rules for reducing the dependence of the injected fuel amount on DT. A multijet CR of the latest solenoid-type generation was experimentally analyzed at different operating conditions on a high performance test bench. The considerable influence that the injector-supplying pipe dimensions can exert on the frequency and amplitude of the injection-induced pressure oscillations was widely investigated and a physical explanation of cause-effect relationships was found by energetics considerations, starting from experimental tests. A parametric study was performed to identify the best geometrical configurations of the injector-supplying pipe so as to minimize pressure oscillations. The analysis was carried out with the aid of a previously developed simple zero-dimensional model, allowing the evaluation of pressure-wave frequencies as functions of main system geometric data. Pipes of innovative aspect ratio and capable of halving the amplitude of injected-volume fluctuations versus DT were proposed. Purposely designed orifices were introduced into the rail-pipe connectors of a commercial automotive injection system, so as to damp pressure oscillations. Their effects on multiple-injection performance were experimentally determined as being sensible. The resulting reduction in the injector fueling capacity was quantified. It increased by lowering the orifice diameter. The application of the orifice to the injector inlet-pipe with innovative aspect ratio led to a hydraulic circuit solution, which coupled active and passive damping of the pressure waves and minimized the disturbances in injected fuel volumes. Finally, the influence of the rail capacity on pressure-wave dynamics was studied and the possibility of severely reducing the rail volume (up to one-fourth) was assessed. This can lead to a system not only with reduced overall sizes but also with a prompter dynamic response during engine transients.

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