15-05-2014, 10:59 AM
An Experimental Study on Flash Boiling Spray using Two-Component Fuel under the Condition of Advanced Injection HCCI
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ABSTRACT
The experimental study was conducted to investigate macroscopic characteristics of the flash boiling spray
with two-component fuel. The final target is the application of this kind of spray to a homogeneous charge compression
ignition (HCCI) engine in which the fuel is injected at the early timing. Recent study reported that to maintain the
heterogeneous mixture was effective to avoid the severe knocking and the unstable combustion, which were the main factors
to restrict the operating range of HCCI combustion. However, the excessive heterogeneity causes the increase in nitrogen
oxides and particulate matters that is the main issue of a diesel engine. In this study, the authors propose to utilize the flash
boiling spray of two-component fuel as the means to control the homogeneity in the mixture for a direct-injected HCCI engine.
The spray characteristics of this kind of fuel with a flash boiling phenomenon were investigated by means of Shlieren and Mie
scattering photography. Test fuel was injected into a constant volume vessel at the ambient condition corresponding BTDC of
an actual engine. As a result, it is found that the flash boiling phenomenon greatly changes the spray structure, especially at the
condition of the relatively low density and the relatively low temperature. As a consequence, the authors conclude that it is
available for HCCI to control the mixture formation by usage of the flash boiling spray particularly at the early injection
timing.
INTRODUCTION
A CI engine has an advantage of the thermal efficiency,
however, it has the problem of much more NOx and
particulate matters in its exhaust gas than an SI engine. In a
CI engine, it is able to realize the simultaneous reduction of
NOx and soot emission by applying homogeneous charge
compression ignition (HCCI) with the early injection during
compression stroke. The self-ignition of the homogeneous
mixture produced by the advanced injection causes the
rapid-lean combustion and reduces NOx and soot emission
while it maintains its thermal efficiency as same as that of a
conventional CI engine. However, HCCI has the
disadvantage of the limited operating range against a
conventional CI engine because it shows the severe
knocking at high load condition and the unstable
combustion at low one. In HCCI, the combustion
characteristics and the exhaust emissions predominantly
depend on the mixture formation and the chemical reaction
process. Many researchers have tried to control the air-fuel
mixing process by changing the injection timing or nozzle
specification [1,2], and by controlling the chemical reaction
pathway in consideration of the fuel molecule [3,4]. Thus,
the combustion control in view of both physical and
chemical phenomena is required for the wider operating
range and the low emission of HCCI engine.
EXPERIMENTAL SYSTEM AND METHOD
Experimental Apparatus
Fig.2 illustrates the schematic diagram of constant
volume vessel which is used in this experiment. In this
vessel, high pressure and temperature atmosphere is able to
be prepared. The ambient temperature is controlled by the
cartridge heater installed in the vessel. The thermo-couple
of K type monitors the temperature in the vessel. Three
quartz windows whose visible diameter is 90[mm] are set at
three sides of the vessel for the optical access.
Optical Setup
Simultaneous imaging technique of Schlieren and Mie
scattering adopted as the method to take spray
characteristics. Fig.3 is the flow sheet of optical setup. The
beam of light (λ=488 [nm]) oscillated through an Ar+ ion
laser (2.0 [W]) is expanded to parallel light of 90 [mm] in
diameter by passing through a pin hole whose pin diameter
is 10 [mm] and two convex lenses (f=40 [mm], 1000 [mm]).
Application of Flash Boiling Spray to DI-HCCI
In this study, we apply the formation of two-phase
region by mixing the fuel of low B.P. with that of high B.P.,
to initiate flash boiling easily to a diesel spray injected
through a hole nozzle. The flash boiling phenomenon is the
phase-change process from liquid to gas, which is yielded
by the rapid growth of initiated cavitation bubbles when the
ambient pressure decreased below the saturated vapor
pressure of liquid. The bubble growth rate of cavitations is
subject to the degree of super-heat, ∆T, that is the difference
between the initial temperature of liquid and the saturated
temperature at the ambient pressure [12], as illustrated in
Fig.1. Therefore, the point is that the flash boiling spray in
DI-HCCI is able to be controlled by changing the degree of
super-heat which is varied due to the following manner.
CONCLUSION
The main conclusion of this experiment is summarized
as follows:
1) In the vicinity of the nozzle outlet, the characteristics of
spray spread depend on the atomization due to the flash
boiling phenomena. The trend is more distinguished at
lower ambient density.
2) The spray tip penetration becomes shorter and the spray
angle becomes wider as the degree of super-heat
increases. This tendency is more remarkable as the
ambient density is lower.
3) The distribution area of liquid phase is shortened toward
upstream side, and spreads wider in radial direction as
the degree of super-heat increases. In flash boiling spray,
tenuous cloud of liquid phase is distributed to wide area.
4) The flash boiling phenomenon becomes remarkable as
the decrease in the ambient density and the increase in
the degree of super-heat.