488 F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 Fig. 48. Valve size limitation for optional injector and spark plug locations . this configuration avoids spark plug wetting during early injection by taking advantage of the intake charge motion. However, there was no mention as to how the higher thermal loading on the exhaust side could negatively affect the injector deposit buildup. Another configuration was investigated that had the injector and spark plug mounted longitudinally, namely in line with the crankshaft in order to avoid the adverse effect of air motion on transporting the fuel to the spark plug in a wall-guided system. An air-guided system was also discussed for this geometry, which was reported to require an injector location that was offset towards the exhaust side. A central piston bowl was used for optimizing the wall-guided system, whereas a piston bowl offset toward the exhaust side was used for optimizing the air-guided system. It was reported that the central injector engine, in a wall-guided configuration, produces the highest UBHC emissions among all the concepts evaluated; and that the MBT combustion phasing is far more advanced. Lake et al.  reported that the central spark plug and injector configuration is not common, but that it does provide a long spray path before wall impingement. It was found to yield excellent homogeneous operation with good EGR tolerance at part load and good air utilization at full load. It was claimed that this configuration can be successfully packaged Fig. 49. Combustion chamber layout of the close-spaced fuel injector and spark plug with a conventional tumble flow for creating a stratified charge .
F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 489 Fig. 50. Schematic of a close-spaced, three-valve, GDI combustion chamber for a small-bore engine . in a bore size as smaller as 82 mm. Compared to other GDI layouts, this configuration requires the fewest changes from a conventional PFI design. A swirl-flow configuration using a side-mounted injector was reported to provide the best unthrottled stratified results in the tests; however, it was noted that more development efforts to achieve satisfactory WOT operation were required. To relieve the packaging difficulty associated with applying GDI technology to small-bore engines, Leduc et al.  recommended that the narrow-spacing concept is more suitable. For GDI engines with a bore diameter of less than 75 mm, a three-valve per cylinder layout with the spark plug close to the injector and a piston cavity that confines the fuel spray to the vicinity of the spark gas was proposed. Fig. 50 shows a schematic of this proposed combustion chamber. To allow a large intake valve diameter, the intake valve stems are vertical. The exhaust valve is located in the pentroof of the combustion chamber. This single exhaust valve was also considered an advantage in providing a more rapid catalyst light-off than is achieved with a two-valve combustion chamber. An eccentric location of the spark plug was chosen for packaging considerations. It was reported that due to the smaller bore size, such a location did not result in a large degradation regarding heat loss and knock resistance. The confinement of the fuel spray in the vicinity of the spark plug was made possible by a welloptimized bowl in the piston. The inclination angle of the injector and the spray characteristics of cone angle and penetration were considered to be the key parameters that must be optimized to minimize any possible fuel impingement on the cylinder wall and piston crown. Two important ways to achieve a stable stratified mixture are to decrease the time interval between injection and ignition and to decrease the distance between the injector tip and the spark gap . However, decreasing the distance between the injector and the spark gap also decreases the time available for mixture preparation, which generally has a negative effect on UBHC emissions and soot formation. Improved fuel–air mixture formation can be obtained with the loss of some combustion stability by increasing the separation between the spark gap and the injector tip, or by increasing the time delay between the fuel injection and ignition. For open chamber designs in which the stratification is supported mainly by charge motion, a stable stratification can be obtained with directed spray impingement on the piston crown or bowl cavity, with subsequent transport of the fuel vapor towards the spark plug by the charge motion . With the use of a specially contoured piston and an optimized balance of tumble and swirl, the transport of the fuel that impinges on the piston can be controlled to obtain stratified combustion. For the current, widely used, four-valve, PFI engine, the possible injector locations for increasing the separation between the spark gap and the injector tip are illustrated in Fig. 51. One is to locate the injector on the intake valve side of the combustion chamber, and the other is to locate the injector at the cylinder periphery between the intake and exhaust valves. Miok et al.  analyzed the mixture formation process that is associated with an injector located at the periphery of the cylinder between the intake and exhaust valves. For this study, the spray was directed towards the center of the combustion