Numerical and Experimental Analysis of Injection and Mixture Formation in High-Performance CNG Engines

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Mixture preparation of gaseous fuels for internal combustion engines using optical diagnostics

The flow and mixing behaviour of high pressure directly injected fuel is important in the study of spark-ignition engines owing to its strong influence on the combustion process. This in turn governs emissions and power generation, which are important considerations in contemporary vehicle design. Whilst liquid fuel delivery has and continues to be a topic of detailed research, there is a deficit in the understanding of gaseous fuel delivery. Three topics remain largely neglected for high pressure CNG fuel injection in spark-ignition engines: i. the targeting and dispersion behaviour of the gaseous fuel jet, ii. the transient jet behaviour near the nozzle exit region (“nearfield”) and iii. the effects of the jets on the in-cylinder flow and turbulence. Using optical diagnostics that include schlieren high-speed imaging and particle image velocimetry (PIV), this work addresses the knowledge deficit. The investigation primarily covers jets issued from a direct injector for gaseous fuels that was constructed in-house. A constant volume chamber was employed to replicate engine-like conditions quiescently, allowing isolation of the injection delivery phenomena. For topic i, jet targeting behaviour was characterised by axial and radial penetration, spread angle and projected area. The targeting profile of a freestream jet is in good agreement with previously established density normalised incompressible jet relationhips. Additionally, empirical correlation is provided between the jet dispersion and pressure ratio (8.3 < PR < 400) for conditions when the jet impinges on the cylinder boundaries. For topic ii, the injector needle lift profile was found to be a dominant factor in controlling nearfield compressible and incompressible flow structures. The presence of the needle was shown to reduce the Mach disc location downstream of the nozzle by ~45%, at steady-state conditions. Moreover, in transient conditions the Mach disc location and diameter are shown to correlate strongly with the needle lift profile. The high-resolution characterisation of the compressible is important for stratified engine operation where a slightly mistimed jet may result in misfire due to the large velocities across the spark-plug electrode In relation to topic iii, PIV was used to capture the flow velocity in two key regions: the air in the nearfield and the ignition zone where a spark plug would be conventionally placed. The rate of air entrainment into the jet is shown to be proportional to the fuel delivery rate and to steadily increase with increasing delivery time aSOI. Spatially, the nearfield entrainment coefficient, K’2, is shown to remain constant at a mean value of K’2 = 0.123. Moreover, the displacement of air caused from the impinging induced jet vortices is shown to feed air into the nearfield entrainment region. Consequently, for the highest PR experiments (PR320 and 400) the nearfield entrainment coefficient, K’2, is shown to increase ~65%, relative to the lower PR experiments. Ignition region turbulent kinetic energy levels induced by the impinging jet are conducive to good flame propagation where mean values (0.5-19 m2/s2) are similar to those created by typical air induction bulk-flow. Mean flow velocities are also within an acceptable spark plug ignition range (1.8-21.0 m/s). Both flow properties are shown to be heavily influenced by the proximity of the jet boundary. As a result of the ignition and nearfield region measurements, a clear understanding of the transient nearfield processes has been developed to help one design appropriate fuel delivery and combustion strategies