Fig. 4 Mode control of injection rate for preferable heat release type injection system
For example, such the injection mode as indicated in Fig. 4 is necessary to realize the lower NOx by suppressing the rapid combustion in the beginning.
For the flexible control of the injection pressure and the injection rate, the accumulator and control valve type injection system has a high potential. The basic characteristic of that system is briefly described here based on the graphical analysis of the one dimensional unsteady flow which is often used in the calculation of the injection process. For this analysis, the graph with the fuel pressure P as Y-axis and the fuel velocity in pipe V as X-axis is used. Fig. 5 is the graph where the pressure in the accumulator is set to 150 MPa. On the same graph, the throttling curves with the nozzle throttling ratio (area of injection pipe to total area of injection holes) of 6, 10, 14, and 18, are depicted. The graph is assumed that the control valve installed between the accumulator and the injection pipe is instantaneously opened without its throttling effect. Here, the peak pressure in the beginning of the injection is expressed by, for example, points (a) and (b). As the elapse of time, the state point is converged to the intersection of the accumulator pressure and the nozzle throttling curve. As indicated in comparison with the throttling ratio of 6 with that of 10, the initial peak pressure becomes higher as the nozzle throttling ratio is increased. In this case, the state point (P, V) is determined only by the pressure in the accumulator, the nozzle throttling curve and the inclination of the characteristic curve E/a (i.e., the physical value of the fuel). Thus, the injection pressure of such system is not dependent on the running point, enabling the high-pressure injection in the low engine speed zone or low load zone which has been the weak point of the conventional injection system.
Fig. 5 Injection process of accumulator and control valve type injection system
Next, from the viewpoint of the control of the injection rate, the opening characteristic of the control valve is important. Fig. 6 shows the injection process in which the control valve between the accumulator and the injection pipe is gradually opened. In this figure, ζc the ratio of the opening area of the control valve to the area of the injection pipe is gradually changed to ζc1, ζc2, ζc3 ..., showing the condition where the control valve is gradually opened. The time intervals (ζc1 and ζc2, ζc2 and ζc3, ...) of each throttling curve correspond to the pressure wave reciprocation time between the control valve and the injection nozzle. The figure shows that the pressure on the control valve side is changed to PA1, PA2, PA3..., and the pressure on the injection nozzle side is changed to PN1, PN2, PN3... Thus, this system has the possibility to control the injection rate :and realize such a injection mode as shown in Fig. 4 by controlling the opening characteristic of the control valve. It is considered important in future to develop a new system taking advantage of such characteristics.
Fig .6 Possibility of control of injection mode by opening characteristic of control valve
