Engine Systems Laboratory, Kyushu University
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Enhancement of Ignition and Burning Rate of Lean Mixtures by Pulsed Flame Jet (PFJ)
In internal combustion engines, lean-burn is particularly attractive for minimizing pollutant emissions, in particular NOx, with a concomitant improvement in fuel economy. For combustion in lean fuel-air mixtures, achievement of adequate reliability of ignition and sufficiently high burning rate requires special devices. The most effective among them is the injection of active radicals by means of PFJ (Pulsed Flame Jet) ignition system. The PFJ igniter is of the same size and shape as the conventional spark plug, and it contains a small cavity and an orifice as shown in Fig. 1. The fuel injector is connected to the cavity via a check valve, an insulator, and a hollow electrode (1.5 mm inner diam.). The latter is sealed at the end and the mixture is admitted into the cavity by two inlet ports (each 1.0 mm in diam.). Rich fuel-air mixture, the equivalence ratio of which is around 1.5, is introduced into the cavity by the fuel injector at an overpressure of 0.1 MPa and ignited in the cavity by spark discharge from a conventional automotive ignition circuit between the hollow electrode and the orifice plate. Thus a jet of incomplete combustion products is issued from the orifice to form a turbulent jet plume consisting of large scale vortex structures where the process of combustion in the combustion chamber is initiated (Figs. 2, 3, and 4).

Normal spark ignition

By PFJ, Vp = 170 mm3, d = 2.5 mm

By PFJ, Vp = 170 mm3, d = 4.0 mm

Fig. 3 Schlieren records of combustion in methane-air mixture, f = 0.8
from left: 1, 3, 5, 7, 9 ms after spark discharge

By PFJ, Vp = 170 mm3, d = 2.5 mm

By PFJ, Vp = 340 mm3, d = 2.5 mm

Fig. 4 PLIF images of OH, viewed region: 36mm * 44 mm, methane-air mixture, f = 0.8
from left: 0.4, 0.6, 0.8, 1.0, 1.5, 3.0, 4.5 ms after spark discharge