The present invention relates to fuel injectors, and more particularly, to use of carbon nanotubes in a fuel injector.
There are many current and future military applications where small (˜5 hp) spark ignition engines are desirable, such as UAV (unmanned aerial vehicle) propulsion. Typically, these engines operate on gasoline fuel, but the DOD (Department of Defense) has directed that all land-based ground and air forces use a single fuel with JP-8 as the leading candidate (see MIL-DTL-83133E, “Detailed Specification—Turbine Fuels, Aviation, Kerosene Type, NATO F-34 (JP-8), NATO F-35, and JP-8+100,” Apr. 1, 1999). JP-8 is a high flash point fuel that requires atomization to a small droplet size in order to burn properly in an internal combustion engine. As will be discussed below, this can be achieved with high pressure atomizers, but these are not practical for small engine applications.
Electrostatic atomization techniques have demonstrated fine droplet atomization of fuels. Another advantage is that electrostatic injection can be driven in a pulsed mode to synchronize with the engine operating cycle. The present invention uses carbon nanotubes (CNTs) as a charge injector to assist in atomizing fuel (e.g., JP-8) for engine applications.
This technology also has significant dual use applications. Future aircraft and automobiles need highly improved propulsive power plants to achieve their performance goals; high-efficiency engines with low-level exhaust emissions are strongly demanded. The fuel atomization as part of the fuel injection process is a critical factor influencing engine efficiency and pollutant emission. A finer fuel mist allows a more efficient burn of the fuel, resulting in less harmful emission. This is attributed to the fact that combustion starts from the interface between the fuel and air. By reducing the size of the fuel droplet, the surface area is increased at the start of combustion, boosting the combustion efficiency and improving emission quality. See R. Tao. “Electric-Field Assisted Fuel Atomization”; and http://www.stwa.com/images/E-Spray.pdf. In parallel, in order to further improve the engine performance, pulsed control techniques such as pulsed detonation and pulsed injection have been developed. Correspondingly, an atomizer should not only make fine droplets but also have a pulsed operation capability. Specifically, the atomizers need to split fuel into tiny droplets within a short period of time and be compatible with on/off operation. It has been reported that droplets as small as 3 μm would be required for pulsed detonation engines. C. M. Brophy et al. 36th A1AA/ASME/SEA/ASSEE Joint Propulsion Conference, 17-19, Jul. 2000, Huntsville, Ala., A1AA paper 2000-3591.
The characteristics of the fuel atomizers with respect to small scale engines are:
1. Strong atomization ability to split fuel into fine droplets (3 μm or less).
2. Low fuel pressure operation (less than 3 bar).
3. Capable of pulsed operation.
4. Low parasitic power consumption (high efficiency).
5. Simple and low-cost.
An electrostatic atomization technique may be used for achieving a fine fuel mist. See J. S. Shrimpton and A. R. H. Right, Atomization and Spray, Vol. 16, pp. 421-424 (2006); W. Lehr and W. Hiller, Journal of Electrostatics, Vol. 30, pp. 44-440 (1993); and J. S. Shrimpton and Y. Laoonual, Intl. J. for Numerical Methods in Engineering, Vol. 67, pp. 1063-1081. One method atomized hydrocarbon fluid by applying a high voltage on a metal needle with a sharp point located at the injector outlet (A. J. Yule et al., Fuel, Vol. 74, pp. 1094-1103-(1995)). The fluid was atomized immediately on exiting the charging unit. But, this single sharp point charging unit had significant alignment and flow-rate limitation problems. To overcome these problems, a planar type charging unit was developed with multiple exit orifices as illustrated in
The basic process of electrostatic atomization is to charge the liquid with a strong electric field. When the repulsive forces between the like charges on the liquid surface exceed the surface tension, the liquid will split into droplets, so-called Rayleigh fission. From a quantum mechanical basis, the amount of charge on each small droplet that is separated from others by an electrical force can be roughly estimated by dividing the droplet diameter by the radius of Bohr's quantum mechanical model of the hydrogen atom (0.53 Å). A. J. Kelly, R&D Innovator, Vol. 3, No. 8, Article #113 (1994). Consequently, the modeling results indicate that if the droplet diameter is as small as 3 μm, the electron charge delivered from the charging unit is expected to have a density of over 1016 electrons/cm3.
Electrostatic atomization offers a number of advantages over more conventional methods. Electrostatic atomization usually needs very little power to operate; power consumption is typically less than 1 W (kilovoltages, micro-ampere currents). On the other hand, charged droplets are naturally self-dispersive thereby avoiding droplet agglomeration that can occur in a conventional uncharged spray. See H. Okuda and A. J. Kelly, Phys. Plasmas, Vol. 3, pp 2191-2196 (1996). Furthermore, the direction of a highly charged stream can be controlled or adjusted by subsequent electromagnetic forces, allowing one to directly pull the fuel stream into a combustion chamber for a Direct Injection Spark Ignition (DISI) cycle without using high pressure air to achieve penetration. Uncharged droplets do not allow for this option.
In the electrostatic charging process, the electrical field plays a key role, both to inject the charge into the fuel and to split the fuel into smaller droplets, as explained above. The electrical field used to inject charge into the fuel should be so strong that electrons and ions can be expelled from the droplet surface. Droplet size is inversely proportional to the square of electrical field. Id. To improve electrostatic atomization, one should first find a way to efficiently enhance the charge injection electrical field.
Other configurations of the assembly shown in
CNTs have strong electrical field enhancement due to their high aspect ratio, as shown in
In summary, the CNT-based electrostatic charging unit has the following merits:
CNT films can be deposited on metal wire mesh substrates with various opening sizes and opening ratios. These parts are obtainable from small parts catalogs.
There are many ways to prepare the CNT films. Two approaches are:
a) Printing a CNT-based paste made with a low-outgassing, inorganic binding.
b) Spraying on a CNT-based ink.
The formulations for these materials have been presented in previous patent applications, such U.S. application Ser. No. 11/124,332 and U.S. Pat. No. 7,452,735, which are hereby incorporated by reference herein. These formulations are used to deposit CNT materials onto the metal mesh substrates, using a low out-gassing binder to anchor the nanotubes to the substrate. The next step is to activate the CNT film using techniques such as disclosed in U.S. Pat. No. 7,452,735 and U.S. application Ser. No. 11/688,746, which are hereby incorporated by reference herein. Activation will re-arrange CNTs in such a way that they are more effective as electron emitters. One way this is achieved is by reducing electrical screening between the CNT emitters and by freeing the CNT fibers enough so that they align with the applied electrical field.
After depositing CNT films on metal mesh substrates, a charging unit is fabricated by using one CNT coated mesh (shown in 9a)), one conducting or metal (e.g., stainless steel) plate with orifices (shown in (b)), and isolating spacers, as shown in
The basic atomization process overcomes the surface tension forces, making the surface of the liquid unstable and allowing it to form into ligaments and then droplets. See A. H. Lefebvre, Atomization and Spray, Taylor and Francis, ISBN 0-89116-603-3, 1989. For electrostatic atomization, this disruption is achieved by a repulsive force acting between like charges on the surface of the liquid (see Jeffrey Allen and Paul Ravenhill, SEA 2005, Paper 2005-32-0090). For droplet sizes larger than one micron, the size of the droplet is dependent on the amount of charge in the liquid forming the droplet (A. J. Kelly, R&D innovator, Vol. 3, No. 8, Article #113 (1994)).
The device is operated by placing an electrical bias between the two electrodes leading to the CNT-coated mesh and metal plate with holes. The electrical bias may be continuous in one direction (+ on one electrode and − on the other electrode) or in a pulsed mode or in an AC mode (polarity switching from one electrode to the other). In continuous mode, the bias polarity may be with the CNT-coated mesh biased negative (−) with respect to the conducting plate.
Other configurations of the assembly shown in
This application claims priority from U.S. Provisional Patent Application Ser. No. 61/046,859.
Number | Date | Country | |
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61046859 | Apr 2008 | US |