Wang, Bo
(2017)
Investigation of deposits and flashing effect on spray characteristics in gasoline direct injection engines.
PhD thesis, University of Nottingham.
Abstract
In recent development of gasoline combustion engines, direct injection (DI) technology has been widely used to improve fuel economy and reduce exhaust emissions. Precise control of spray characteristics is one of the keys realizing the benefits of gasoline direct injection (GDI) engines. However, it is challenging as the thermal conditions vary greatly inside the combustion chamber. Flash boiling occurs when a superheated fuel spray is exposed to sub-saturation pressure. Spray structures will be altered and better atomization can be obtained. Injector deposit is another concern for the GDI engines as it could distort carefully designed spray patterns and lead to deterioration in combustion and emissions. Situation is worse in GDI engines than in traditional port fuel injection (PFI) engines, due to their injectors being directly exposed to harsh in-cylinder conditions.
The first part of this thesis focuses on the effect of injector deposits on spray and air/fuel mixing processes. Both experimental and numerical studies are presented. An optical-based experimental study was performed first and it is found that the injector deposits can significantly alter the spray and droplet behaviors, leading to loss of mass flow rate, longer spray penetration and larger droplet sizes. To figure out the effect of deposits on the in-nozzle flow, CFD simulations were then carried out in the Euler-Euler framework with cavitation taken into account by a homogeneous relaxation model (HRM). High-resolution X-ray microtomographic scans were performed to obtain the morphologies and topology of the nozzle and deposits. Results highlight that rough deposit surfaces lead to additional cavitation inception in the counterbore and restrict the flow area, causing mass flow rate losses. Deposits inside the counterbore act as an extension to the inner orifice and constrain air recirculation, thus a lower turbulent level. Finally, after careful calibrating the spray model, in-cylinder simulations were carried out to study the deposit effect on the air/fuel mixing process in an optical research GDI engine. Results have shown that injector deposits can lead to stronger fuel impingement on the piston and cylinder walls, as well as a lower mean equivalence ratio during late injection events. The distorted spray pattern leads to higher fuel stratification. In late injection cases, injector deposits results in a very lean mixture near the spark plug, which can be the source of unstable combustion; while rich regions at cylinder sides can result in higher emissions. Comparison of simulation results with PLIF (planar laser induced fluorescence) images provides a satisfactory validation for the results.
In the second part of this work, flash boiling effect on near-nozzle spray characteristics is investigated. Firstly, the near-nozzle spray behaviors of hexane and isooctane were studied both experimentally and numerically under different temperatures and back pressures. High-speed imaging was performed using a long-distance microscope coupled with an ultra-high speed camera (1 million fps). Simulations were conducted with OpenFOAM and the HRM model was implanted to account for phase change. Results have shown that flashing could significantly change the near-nozzle spray behavior. Then the near-field spray patterns of 2-methylfuran (MF), ethanol (ETH) and isooctane (ISO) under different saturation ratios were investigated in an optically accessible constant volume chamber. Results have shown that among the three fuels, MF shows the most intense flash boiling behavior due to its highest vapor pressure. Effects of different non-dimensional numbers are also studied. Saturation ratio and cavitation number are found to be two main governing factors for the near-nozzle spray behaviors under flash boiling conditions. During end of injection process, low effective pressure leads to poorly atomized spray with a compact liquid column and large ligaments; this can result in poor air/fuel mixing and thus HC and particle emissions. Significant improvements are observed at saturation ratio equaled to 0.2 where flash boiling greatly promotes the spray atomization, even with low fuel velocity. This also suggests a possible way to alleviate deposits formation on the injector tip due to reduced fuel residue.
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