The 2-stroke engine has great potential for aggressive engine downsizing due to its double firing frequency which allows lower indicated mean effective pressure (IMEP) and peak in-cylinder pressure with the same output toque compared to the 4-stroke engine. With the aid of new engine technologies, e.g. direct injection, boost and variable valve trains, the drawbacks of traditional 2-stroke engine, e.g. low durability and high emissions, can be resolved in a Boosted Uniflow Scavenged Direct Injection Gasoline (BUSDIG) engine. Compared to the loop-flow or cross-flow engines, the BUSDIG engine, where intake ports are integrated to the cylinder liner and controlled by the movement of piston top while exhaust valves are placed in the cylinder head, can achieve excellent scavenging performance and be operated with high boost. In order to fulfil the potential of the BUSDIG engine, various scavenge ports were designed with different scavenge port number (SPN), Axis Inclination Angle (AIA) and Swirl Orientation Angle (SOA), and their effects were evaluated by three dimensional (3D) computational fluid dynamics (CFD) under different intake pressures and engine speeds. The scavenging process was analyzed by its delivery ratio (DR), trapping efficiency (TE), scavenging efficiency (SE) and charging efficiency (CE). In addition, the in-cylinder flow motions, which play important roles in controlling the charge mixing and combustion process, were studied for different scavenge port designs. Finally, the vertical position of scavenge ports, which determines the scavenge port opening (SPO) timing, the scavenge port height (SPH), and the exhaust valve opening (EVO) timings were varied to investigate their impacts on the scavenging performance and in-cylinder flow motions.
a b s t r a c tIn this research, the stratified flame ignition (SFI) hybrid combustion process was proposed to enhance the control of SI-CAI hybrid combustion and moderate the maximum pressure rise rate (PRR max ) by the combination of port fuel injection (PFI) and direct injection (DI). The effect of the stratified flame formed by different piston shapes, start of direct injection (SOI) timings and direct injection ratios (r DI ) on the stoichiometric SFI hybrid combustion and heat release process was studied using the three-dimensional computational fluid dynamics (3-D CFD) simulations. The spark ignited flame propagation near the spark plug and the auto-ignition heat release process of the diluted mixture were modelled in the framework of 3-Zones Extended Coherent Flame Model (ECFM3Z) by the extended coherent flame model and tabulated auto-ignition chemistry of a 4-component gasoline surrogate, respectively. The operating load of indicated mean effective pressure (IMEP) 3.6 bar was selected to represent a typical part-load operation. The sweep of the spark timing (ST) was performed for different pistons, SOI timings and direct injection ratios. The SFI hybrid combustion process with the same combustion phasing was investigated in details. The optimal stratified mixture pattern, characterized with the central rich mixture around spark plug and stratified lean mixture at the peripheral region, formed by the newly designed Piston A and B effectively lowers the PRR max with a slight deterioration of IMEP. The later SOI timing advances the crank angle of 50% total heat release (CA50) and significantly reduces the PRR max with a little deterioration of IMEP. As the direct injection ratio is increased, both the PRR max and IMEP decrease. During the SFI hybrid heat release process, spark timing is effective to control CA50, IMEP and PRR max regardless the piston shapes, SOI timings and direct injection ratios. However, the sensitivity of SFI hybrid combustion to the stratified mixture varies with the spark timing. The reduction of the PRR max caused by the stratified flame enables the advance of spark timing to achieve maximum IMEP.
INTRODUCTIONControlled Auto-ignition (CAI) combustion has been shown to effectively reduce the NOx emissions and increase fuel efficiency [1] and it has been subject to extensive studies in the past few decades. However, the lack of direct control on the auto-ignition process and relatively narrow operation range inhibit the practical application of this low temperature high efficient combustion mode [2]. In order to enhance the control of auto-ignition and extend the operation range of CAI combustion, spark ignition (SI) was used to assist CAI combustion. In this case, the early flame propagation induced by spark discharge could effectively control the later autoignition [3,4]. In addition, this hybrid combustion mode could be used to bridge pure SI mode and CAI mode during mode transition when applying to the full load operations in the practical engine [2,5,6,7,8]. However, the recent study on the SI-CAI hybrid combustion indicated the existence of significant cycle-to-cycle variation (CCV) during mode transition [8, 9, 10, 11].In the SI-CAI hybrid combustion, the flame propagation is mainly controlled by the transport of heat and active species in the flame front and distorted by the in-cylinder turbulence [12]. The variation of the early flame propagation would in turn affect the later auto-ignition process [13]. The premise of controlling and stabling SI-CAI hybrid combustion is to effectively manage the early flame propagation process. The local fuel/air equivalence ratio around the spark plug significantly affects the spark kernel formation and flame propagation process in spark ignition engine. Persson et al. [14] used high speed fuel Planar Laser-Induced Fluorescence (PLIF) to study the effect of fuel stratification on spark assisted compression ignition (SACI) with ethanol as fuel. They found the lower fuel concentration in the vicinity of the spark plug decreased the flame expansion speed with the overall lean mixture (lambda 1.4). Middleton et al. [15] investigated the propagation of premixed laminar reaction fronts with different fuel/air equivalence ratios using transient one-dimensional flame simulations and indicated the increased flame burning velocity with the isooctane/air equivalence ratio increasing from 0.1 to 1.0. The experimental Numerical Study of the Effect of Piston Shapes and Fuel Injection Strategies on In-Cylinder Conditions in a PFI/GDI Gasoline EngineXinyan Wang and Hua Zhao Tianjin Univ., Brunel Univ.Hui Xie and Bang-Quan He Tianjin Univ.ABSTRACT SI-CAI hybrid combustion, also known as spark-assisted compression ignition (SACI), is a promising concept to extend the operating range of CAI (Controlled Auto-Ignition) and achieve the smooth transition between spark ignition (SI) and CAI in the gasoline engine. In order to stabilize the hybrid combustion process, the port fuel injection (PFI) combined with gasoline direct injection (GDI) strategy is proposed in this study to form the in-cylinder fuel stratification to enhance the early flame propagation process and control the auto-...
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