Automotive spark-ignited direct-injection gasoline engines

More documents

Recommendations

Info

470 3. Combustion chamber geometry and in-cylinder mixture dynamics 3.1. Flow structure F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 Table 1 Best practice performance of current GDI injectors for key parameters (spray cone angle for single-fluid injectors: 55–75; air and fuel pressures for air-assisted injectors: 0.5–1.0 MPa; amount of fuel per injection: 15 mg; indolene fuel at 20C; injection into 1 atm) GDI injector class Single-fluid Air-assisted Fuel rail pressure (MPa) 5.0 7.5 10.0 Pulse-pressurized SMD main spray (mm) a 16.8 15.6 14.3 17.1 DV90 main spray (mm) a 32.7 29.5 25.2 31.3 SMD sac spray (mm) b 22.4 20.4 18.7 19.6 DV90 sac spray (mm) b 36.0 31.2 29.4 33.4 SMD closing spray (mm) c 19.5 17.8 16.6 17.5 DV90 closing spray (mm) c 34.6 30.5 27.9 32.8 After injections None None None One small Main spray skew () 0.0 1.0 1.5 1.0 Sac spray skew () 1.0 2.0 2.5 1.0 Opening time (ms), 12 V design 0.29 0.31 0.33 0.26 Opening time (ms), 24–70 V design 0.18 0.20 0.22 – Closing time (ms) 0.38 0.37 0.36 0.41 a Fuel-volume-flux weighted PDA across entire spray at 50 mm from injector tip. b PDA at 50 mm from injector tip on sac spray centerline over sac arrival time. c PDA at 20 mm from injector tip on injector axis for 3 ms following actual closure. The transient in-cylinder flow field that is present during the intake and compression strokes of a GDI engine is one of the key factors in determining the operational feasibility of the system. The magnitude of the mean components of motion, as well as their resultant variations throughout the cycle, are of importance that is comparable to that of the fuel injection system. On a microscopic scale, a high level of turbulence is essential for enhancing the fuel–air mixing process; but additionally, a controlled mean or bulk flow is generally required for the stabilization of a stratified mixture plume. For SI engines it is known that the turbulence velocity fluctuations near top dead center (TDC) on compression can attain the same order of magnitude as the mean velocity, and that the turbulent diffusive transport and convective transport can be of equal influence in determining the initial state of the combustion process [97,131–133]. The integral scale of the mixture concentration fluctuation inside the combustion chamber can be as large as that of the velocity fluctuation [75]. This results in strong concentration fluctuations at a fixed position, such as the spark gap location, which can lead to difficulties in obtaining a stable flame kernel. There are four key controlling features of the in-cylinder flow field; the mean flow components, the stability of the mean flow, the temporal turbulence evolution during the Fig. 31. Spray structure of an air-assisted injector [71]: (a) schematic of the air-assisted injector; and (b) schematic representation of the hollowcone spray structure.
F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 471 Fig. 32. Poppet lift and injection volume of a poppet-type, air-assisted injector [130]. compression stroke, and the mean velocity near the spark gap at the time of ignition. For homogeneous combustion in the SI engine, the combination of high turbulence intensity and low mean velocity at the spark gap is desirable. This is generally achieved for PFI engines, and also for GDI engines that operate exclusively in the early injection mode. Therefore, a flow structure that can transform the mean-flow kinetic energy into turbulence kinetic energy late in the compression stroke is considered desirable for the homogeneous combustion case. The GDI engine using late injection, however, operates best with a flow field having an elevated mean velocity and a reduced turbulence level, which aids in obtaining a more stable stratification of the mixture. This indicates that the optimum flow field depends upon the injection strategy that is being used, which is a difficult compromise for full-feature GDI engines that operate with both strategies. For the GDI combustion system, control of the mixing rate by means of the bulk flow seems to have more potential than the scheduling of turbulence generation. This is not to imply that turbulence is not important to the combustion process. In fact, turbulence is known to be an important factor in entraining the EGR into the local combustion area [134]. Fig. 33. Comparison of the injection characteristics between the pressure atomizer and the air-assisted injector [130].
Page 1 and 2: PERGAMON Automotive spark-ignited d
Page 3 and 4: F. Zhao et al. / Progress in Energy
Page 16 and 17: 452 and investigated. A schematic r
Page 18 and 19: 454 F. Zhao et al. / Progress in En
Page 36 and 37: 472 In general, a rotating flow str
Page 58 and 59: 494 ◦ reduced valve size; ◦ pos
Page 80 and 81: 516 was achieved for the unthrottle
Page 84 and 85:
520 F. Zhao et al. / Progress in En
Page 86 and 87:
Page 88 and 89:
Page 90 and 91:
Page 92 and 93:
Page 94 and 95:
Page 96 and 97:
532 because both the exhaust flow a
Page 98 and 99:
Page 100 and 101:
536 Many new mixture-preparation an
Page 102 and 103:
Page 104 and 105:
Page 106 and 107:
Page 108 and 109:
544 valve, with some of the fuel be
Page 110 and 111:
546 A motor/generator is attached t
Page 112 and 113:
548 • vertical, near-centrally mo
Page 114 and 115:
550 pre-chamber and the combustion
Page 116 and 117:
552 6.17. Orbital combustion system
Page 118 and 119:
Page 120 and 121:
Page 122 and 123:
Page 124 and 125:
Page 126:
show all

Automotive spark-ignited direct-injection gasoline engines

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?