Patent Application
20050145715
Drop ejectors are known devices used in ink jet printers to eject discrete drops of liquid ink onto a medium, such as paper, adapted to receive the liquid ink. An exemplary drop ejector for ejecting discrete drops of liquid ink is described in U.S. Pat. No. 6,162,589 to Chen.
As described in U.S. patent application Ser. No. 10/086,002 commonly assigned to the owner of this application, it has been suggested to use liquid ink drop ejectors as a central component in an improved fuel injector for ejecting discrete drops of liquid fuel to create a combustible vapor for an internal combustion engine. The use of a drop ejector allows more precise control of the air/fuel mixture provided to the internal combustion engine as compared to conventional fuel injectors.
However, liquid fuel, such as gasoline or diesel fuel, has different physical properties, including a lower viscosity, than the liquid ink for which drop ejectors have historically been used. As a result, various problems exist in attempting to use known liquid ink drop ejectors to dispense liquid fuel, as well as other lower viscosity liquids. For example, the inventors have recognized that when known drop ejectors are used to dispense relatively low viscosity liquids, problems exist with “puddling” and “bubble trapping”, which are all described in more detail hereinafter.
The embodiments described in this application were developed in light of and to address these and other problems associated with using drop ejectors to eject discrete drops of relatively low viscosity liquids.
An embodiment of an improved drop ejector for ejecting discrete drops of liquid, such as liquid fuel, is described. The improved drop ejector is described, in one exemplary embodiment, as used in a fuel injector that generates a combustible vapor from the discrete drops of fuel ejected from the drop ejector. The drop ejector includes a plurality of firing chambers from which the liquid drops are ejected. Liquid is delivered to each of the firing chambers through a plurality of fluid feed slots, wherein each firing chamber is associated with at least one feed slot. A constricted inlet is located between each firing chamber and the corresponding feed slot. Liquid fuel is drawn into each firing chamber from its corresponding feed slot through its constricted inlet. Each inlet is narrower than the fluid feed slot and the firing chamber (i.e., the inlet is “constricted”) so as to provide improved control of the liquid fuel being delivered to the firing chamber and the fuel drops being ejected from the firing chambers. The constricted inlet prevents or reduces “puddling” and “bubble trapping” problems, which are described in more detail hereinafter, as well as others.
Referring to
In response to control signals received from the control circuit 22 (
In some embodiments, the protruding points 52(a) and 52(b) are formed by converging surfaces 54(a), 56(a) and 54(b), 56(b), respectively. Surfaces 54(a) and 54(b), located on the feed slot side of the protruding points 52(a) and 52(b), are “flat” in the sense that they are substantially perpendicular to the flow of liquid through the feed slot 42. On the other hand, surfaces 56(a) and 56(b), located on the firing chamber side of the protruding points 52(a) and 52(b), are “angled” in the sense that they create an acute angle a with the respective flat surfaces 54(a) and 54(b), thereby providing an expanded lateral area in the firing chamber for the liquid to fill after it passes through the constricted inlet 50.
In operation (with reference to all of the drawings), the fuel injector 12 creates a combustible vapor 16 by passing an air stream (provided through air filter 18) through a plurality of drops of liquid fuel. The liquid fuel drops are generated by the drop ejector 30 in response to control signals received from control circuit 22. The drop ejector 30 generates and ejects the fuel drops by selectively (in response to the control signals) energizing the energy dissipation devices 46, which causes the liquid fuel in the corresponding firing chambers 44 to bubble in the firing chamber 44. Because of the constricted inlet 50, the bubble also expands through the inlet 50 and at least partially into the feed slot 42. Inside the firing chamber 44, the expanding bubble causes a drop of liquid fuel to be ejected from the outlet orifice 48 of the firing chamber 44. Once the energy dissipation device 46 is de-energized, the expanding bubble collapses. As the bubble collapses, liquid fuel is drawn into the firing chamber 44 from feed slot 42 (due to the surface tension of the fuel) to fill the void left by the collapsing bubble, effectively “re-loading” the firing chamber for the next fuel drop to be ejected.
The described embodiment of the firing chamber 44—and particularly the constricted inlet 50 to the firing chamber 44 defined by the two protruding points 52(a) and 52(b)—tends to prevent “puddling” and “bubble trapping” problems that could otherwise occur as a result of the relatively low surface tension (relatively low viscosity) of liquid fuels. “Puddling” occurs when excess fuel adheres to and around the outlet orifice 48, thereby causing subsequent fuel drops to have to be ejected through the excess “puddling” fuel. The “puddling” affects the trajectory of subsequent fuel drops, and, sometimes, prevents subsequent drops of fuel from being ejected at all. The constricted inlet 50 tends to eliminate “puddling” because the constricted inlet reduces the momentum of fuel rushing into the firing chamber 44 by restricting the fluid flow therethrough. Further, the constricted inlet 50 limits the degree to which an expanding liquid bubble (due to an activated energy dissipation device 46) can expand into the feed slot 42. In embodiments having flat surfaces 54(a) and 54(b), the flat surfaces provide resistance to the liquid flow, and, as a result, assist in limiting the expanding liquid bubble from expanding into the feed slot 42 more than a desired amount. Therefore, most of the bubble expansion in the firing chamber 44 occurs toward the outlet orifice 48, thereby maintaining a relatively higher drop speed (the speed at which a drop is ejected from the drop ejector). Maintaining an adequately high drop speed from the drop ejector helps to prevent or limit “puddling.”
“Bubble trapping” occurs when an insufficient amount of liquid fuel “refills” the firing chamber 44 quickly enough after a drop of fuel is ejected from the firing chamber. The in-rushing fuel cools the energy dissipation device 46 after being energized to eject a drop of fuel. If the in-rushing fuel does not sufficiently cool the energy dissipation device 46 quickly enough, the energy dissipation device 46 may cause a bubble to form in the feed slot 42, which may block the inlet 50 and prevent additional fuel from being drawn into the firing chamber 44. Without fuel in the firing chamber, the material (normally silicon) surrounding the blocked firing chamber may overheat, causing short circuits in the drop ejector. The constricted inlet 50 prevents “bubble trapping” problems by ensuring that sufficient liquid fuel “refills” the firing chamber 44 quickly enough after a drop of fuel is ejected. The constricted inlet 50 causes the bubble that forms in the firing chamber 44 to extend through the constricted inlet 50 and into the feed slot 42 so as to draw sufficient fuel from the feed slot 42 into the firing chamber 44 when the bubble collapses. In embodiments having angled surfaces 56(a) and 56(b), the angled surfaces help to increase the velocity of the liquid filling the firing chamber by reducing the resistance to the liquid flow. Thus, the angled surfaces 56(a) and 56(b) increase the speed with which the firing chamber 44 can be refilled for a given opening size of the inlet 50.
“Puddling” and “bubble trapping” problems can be limited by controlling the refill speed of the firing chamber 44 and the drop speed of the liquid fuel being ejected from the drop ejector. The refill speed and the drop speed can be effectively controlled by adjusting the size of the inlet 50 and the size of the angle a between the angled surfaces 56(a) and 56(b) and the respective flat surfaces 54(a) and 54(b).
While the present invention has been particularly shown and described with reference to the foregoing preferred and alternative embodiments, those skilled in the art will understand that many variations may be made therein without departing from the spirit and scope of the invention as defined in the following claims. For example, an embodiment of a drop ejector having a constricted inlet was described above in connection with a fuel injector apparatus. However, one skilled in the art will recognize, in light of this disclosure, that the described drop ejector may be used in a variety of settings where liquids of relatively low viscosity are to be dispensed in discrete drops in a digital fashion. This description of the invention should be understood to include all novel and non-obvious combinations of elements described herein, and claims may be presented in this or a later application to any novel and non-obvious combination of these elements. The foregoing embodiments are illustrative, and no single feature or element is essential to all possible combinations that may be claimed in this or a later application. Where the claims recite “a” or “a first” element of the equivalent thereof, such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Further, the use of the words “first”, “second”, and the like do not alone imply any temporal order to the elements identified. The invention is limited by the following claims.
