A fuel injector is a device for actively injecting fuel into an internal combustion engine by directly forcing the fuel into the combustion chamber at an appropriate point in the combustion cycle. For piston engines, the fuel injector is an alternative to a carburetor, in which a fuel-air mixture is drawn into the combustion chamber by the downward stroke of the piston. Current fuel injectors suffer from an inability to operate at high frequencies, which limits their applicability to advanced and emerging engine designs. In addition, current injectors cannot vary the fuel delivery profile for each injection/combustion cycle, which further limits their inclusion in more sophisticated combustion configurations, particularly those operating at higher frequencies. Furthermore, current injector configurations have a response lag associated with various factors, including a stroke amplification requirement, which impedes higher frequency operation. Finally, injectors which rely on piezoelectric actuators cannot directly actuate the flow control member that allows fuel to pass through an injection orifice into a combustion chamber due to an inability to move the flow control member a sufficient distance off seat to allow sufficient fuel to flow at a desired rate. For purposes described herein, “direct” actuation is defined as the direct physical interaction of the prime actuating device with the primary flow control member which, when moved by the prime actuating device, immediately causes fuel to flow into the combustion chamber, typically through a nozzle portion. “Direct actuation” is defined herein as having a one-to-one relationship between the actuating device and the flow control member with no additional interposing elements, amplification steps, flow channels, control pressures or other such ancillary elements necessary to operate the flow control member.
Current piezoelectric stack actuator systems used in fuel injectors do not rely on direct actuation of the nozzle assembly—in particular, that portion of the nozzle that allows fuel to flow. Instead, the piezoelectric stack is typically used to simply open and close a separate valve which varies hydraulic pressure to assist in opening the nozzle. As a result, this multi-step process of indirect hydraulic actuation and amplification creates an inherent limit to the operational frequency of the injector due to the intrinsic response lag. Consequently, these dual stage piezoelectric injectors cannot support the higher frequency operations of advanced and emerging engine technologies.
In typical fuel injectors, a nozzle assembly portion is located adjacent the combustion chamber of the engine. The nozzle includes a pin, considered the primary flow control member, and an orifice through which fuel flows into the combustion chamber. When the pin seats on a sealing portion of the orifice, fuel flow is cut off. When the pin is unseated from the sealing portion of the orifice, fuel flow is enabled.
In existing injector configurations, hydraulic amplification is used to open and close the nozzle. High pressure fuel is delivered to the entire nozzle compartment. The shape of the pin results in over-balanced pressure, causing the pin to be seated on the orifice in a closed position. An upstream actuator opens a pressure relief valve associated with the fuel delivery system, reducing pressure on one side of the pin; this results in a directional net linear force and causes the pin to lift off its seat and the nozzle to open. By closing the relief valve, pressure returns to its original level and the pin reseats to close the nozzle.
When a piezoelectric stack is used in this manner, the overall system is mechanically and operationally complex. Amplification is required due to the limited displacement of the piezoelectric stack; however, amplification requires more intricate flow arrangements within the body of the injector, additional valves, and sealing elements. More importantly, hydraulic amplification introduces significant response lag due to the two-step actuation process. This unavoidable response lag prevents a hydraulically amplified injector, even those using piezoelectric actuators, from operating at higher frequencies, such as those that might be required for pulse detonation engines.
Present injector actuation methods have other inherent limitations. For example, such injectors can only operate in a binary fashion; i.e., either fully open or fully closed. It would be preferable to provide essentially analog control of the entire fuel injection profile over each injection/combustion cycle. Attempts have been made to obtain such analog control by simply opening and closing the injector valve frequently and at differing durations in each injection cycle. Unfortunately, this approach creates an even higher operational demand due to the multiplication of actuation cycles during each injection cycle.
Two primary technologies used as “actuating” means, electromagnetic actuators and piezoelectric actuators, have inherent strengths and weaknesses. First, electromagnetic actuators (also known as solenoids) can supply sufficient linear stroke (displacement) of an injector pin to support desired maximum fuel flow, but can operate only in two modes: fully open and fully closed. A solenoid valve is an electromechanical valve incorporating an electromagnetic solenoid actuator. The valve is controlled by an electric current through a solenoid. In some solenoid valves, the solenoid acts directly on the main valve. Others use a small, complete solenoid valve, known as a pilot, to actuate a larger valve. Piloted valves require much less power to control, but are noticeably slower. Piloted solenoids usually require full power at all times to open and remain open, whereas a direct acting solenoid may only require full power for a short period of time to open, and only low power to hold in a closed position. Irrespective of the type of solenoid used, the actuator will still suffer from significant response lag, which is exacerbated as operational frequencies increase. And, again, the solenoid actuated injector is only able to operate in two states: fully open and fully closed.
The second actuator type, using a piezoelectric device, can provide faster response than a solenoid actuator, but has miniscule stroke length. Generally, a standard piezoelectric stack provides maximum displacement of 1/10th of 1% of its height; stacks with single crystal piezoelectric material can provide displacement up to 1% of their height. Consequently, heretofore, this limited stroke length has forced piezoelectric actuation mechanisms in fuel injectors to be used in an amplification configuration. Necessarily, prior injector configurations relying on amplification have been unable to deliver direct actuation.
Various attempts have been made to increase or amplify the displacement of piezoelectric actuators. For example, one design includes a geometrically-constrained piezoelectric actuator device that amplifies displacement along an opposing axis using a diamond-shaped enclosure. As the piezoelectric element contracts or expands in a horizontal direction, the external diamond-shaped enclosure also changes shape, causing the vertical vertices of the enclosure to move a slightly greater distance than the horizontal vertices, which are controlled by the piezoelectric element. Unfortunately, the inclusion of this mechanical feature introduces the limitation of a mechanical spring variable that limits high frequency operation of the actuator and longevity. Additionally, this flextensional tensional approach used to increase displacement also results in a decrease in the maximum force applied, which is another increasing displacement by only a very small amount and would still require amplification if used as an actuator in a fuel injector.
Information relevant to other attempts to address these problems can be found in U.S. Pat. Nos. 7,786,652; 7,455,244; 7,406,951; 7,140,353; 6,978,770; 6,834,812; 6,585,171; and 4,803,393. However, each one of these references suffers from one or more of the following disadvantages which will tend to impede high frequency operation and the optimization of each combustion cycle to create maximum efficiency: indirect actuation, partial spring actuation; complex mechanisms with a plurality of components and parts; operation only in a fully open or fully closed position; stroke distances which would require prohibitively long piezoelectric stacks; multiple boosters required to achieve necessary forces; actuating mechanisms that are unable to accommodate sufficient stroke; the inclusion of spring elements likely to induce valve float at higher frequency operation; indirect actuation via hydraulic amplification resulting in lag and hysteresis; no analog control of valve position; and inability to provide refined prestress on the piezoelectric stack to avoid placing it in tension or adapting to differing operating parameters. Additionally, it is evident that these other attempts fail to provide an injector having a one-to-one relationship between the prime actuating force and the flow control member without interposing elements. Consequently, these other attempts do not provide direct actuation.
For example, Nakamura et al., U.S. Pat. No. 7,786,652 B2 issued Aug. 31, 2010, describes an injection apparatus using a multi-layered piezoelectric element stack. The invention disclosed by Nakamura et al. is directed to a need for a multi-layer piezoelectric element that can be operated continuously with a high electric charge without peel-off or cracking between the external electrode and the piezoelectric layer, which can lead to contact failure and device shutdown. The injector apparatus described by Nakamura et al. uses a needle valve which is sized to plug an injection hole to shut off fuel. The injector apparatus includes a spring underneath a piston valve member so that when power is removed from a piezoelectric actuator, the spring actually causes the valve to open and allow fuel injection. The stack only acts to close the valve. Furthermore, Nakamura et al. does not describe a method for prestressing the piezoelectric stack. General operation of the injector is either fully open or fully closed, with no ability to provide variable injection rates. The fuel flow rate is controlled by an orifice and is not adjustable. Additionally, it is unclear how the piezoelectric stack described by Nakamura et al. would provide sufficient stroke or contraction to move the needle sufficiently to unplug the injection hole, even with the inclusion of a supplementary spring. For the operational requirements associated with pulse detonation engines, the injector described by Nakamura et al. would neither enable sufficient flow nor operate at a sufficiently high frequency. Thus, the injector described by Nakamura does not have a one-to-one relationship between the prime actuating force and the flow control member without interposing elements and is therefore not directly actuated.
Further, Boecking, U.S. Pat. No. 7,455,244 B2 issued Nov. 25, 2008, describes a piezoelectric fuel injector for injecting fuel into a combustion chamber of an internal combustion engine, wherein the injector includes a first and second booster piston, and the first booster piston is actuated using a piezoelectric stack to actuate the second booster piston which then moves a pin off seat to open the injection opening. The injector described by Boecking is directed to a need for a fuel injector of especially compact structure. Multiple springs within the injector body are used to generate closing forces. The system described by Boecking is a complex mechanism with minimal stroke displacement to move the pin sufficiently to support high volume fuel delivery. Due to the inclusion of spring-loaded elements, the described injector will suffer float at higher frequency operation. Additionally, Boecking's injector relies on the movement of a small needle valve, which will inhibit the ability to deliver flow at higher rates. Further, Boecking's injector does not have a one-to-one relationship between the prime actuating force and the flow control member without interposing elements and is therefore not directly actuated.
Stoecklein, U.S. Pat. No. 7,406,951 issued Aug. 5, 2008, describes a piezoelectric fuel injector for injecting fuel into an internal combustion engine wherein the fuel injector has an injection valve member that is indirectly actuated by a piezoelectric actuator. Stoecklein suggests that the injection valve member is “directly” actuated by the piezoelectric stack, but the description confirms that hydraulic amplification is used between the actuator and the injection valve. Hence, as defined herein, the injector of Stoecklein is not directly actuated. Additionally, the valve member relies on a spring element to move into a closed position. Stoecklein's invention also attempts to solve the problem in prior piezoelectric fuel injectors whereby intermediate positions of the valve between fully open and fully closed are unstable and cannot be maintained. Stoecklein describes a solution involving multistage hydraulic boosting of the actuator stroke to achieve stable intermediate stop positions. To overcome system pressure and open the valve member, an initial force is applied by reducing the current supply to the piezoelectric actuator. The shrinking length causes a pressure decrease in a hydraulic coupling chamber and, in turn, the control chamber. After a critical pressure has been reached, the valve opens to an intermediate stroke position. In order to achieve a complete opening of the valve member, the boosting is changed once the piezoelectric actuator has traveled a certain amount of its stroke distance. However, Stoecklein's approach does not address issues of response lag nor adaptation to operate at high frequencies. Furthermore, although limited two-stage control is described, highly granular, essentially analog control is not supported by Stoecklein's injector system. As with the prior referenced designs, the injector includes springs which can cause valve float at higher operational frequencies. Stoecklein also confirms that a stroke of several hundred micrometers would be required to deliver desired flow rates, whereas the stroke available from reasonably sized stacks is on the order of 20 to 40 microns. Additionally, the injector of Stoecklein must rely on a two-stage boost to achieve sufficient opening. As in the other referenced designs, Stoecklein's injector also does not have a one-to-one relationship between prime actuating force and the flow control member without interposing elements and is therefore not directly actuated.
Rauznitz et al., U.S. Pat. No. 7,140,353 B1 issued Nov. 28, 2006, describes a piezoelectric injector containing a nozzle valve element, a control volume, and an injection control valve for controlling fuel flow wherein a preload chamber is used to apply a preload force to the piezoelectric stack elements. Rauznitz et al. emphasizes the necessity of the hydraulic preload to adequately prestress the piezoelectric stack to ensure reliable operation. However, as described, the injector of Rauznitz et al. only operates in fully closed and fully open positions. Hence, even though the injector may improve firing for opening and closing to address flow profile, it fails to provide analog control of the valve position to deliver highly granular control of the flow profile throughout each combustion/injection cycle. Additionally, opening and closing of the valve requires amplification with actuation of multiple components. Thus, the injector of Rauznitz et al. fails to provide direct actuation of the valve control member, limiting application in high frequency injection scenarios, and, fails to provide highly granular control of the fuel flow profile, limiting use, for example, in pulse detonation engines. Finally, the injector is designed to accommodate only smaller injector needles and would not support large injector sizes to accommodate increased fuel flow. Thus, this Rauznitz et al. injector does not have a one-to-one relationship between the prime actuating force and the flow control member without interposing elements and is therefore not directly actuated.
Rauznitz et al., U.S. Pat. No. 6,978,770 B2 issued Dec. 27, 2005, describes a piezoelectric fuel injection system and method of control wherein the fuel injector contains a piezoelectric element, a power source for activating the element to actuate the injector, and a controller for charging the piezoelectric element directed to control of the injection rate shape. The system disclosed by Rauznitz et al. delivers closed, intermediate and fully open control. These three positions are further supported by rapid opening and closing of a nozzle valve element to create an improved rate shape; however, precise control and analog positioning of the nozzle valve needle throughout its stroke length is not possible. Furthermore, the injector uses springs to bias the valve element into a closed position, which introduces complexity and will cause the injector to suffer float at higher frequency operation. Thus, this Rauznitz et al. injector does not have a one-to-one relationship between the prime actuating force and the flow control member without interposing elements and is therefore not directly actuated.
Neretti et al., U.S. Pat. No. 6,834,812 B2 issued Dec. 28, 2004, describes a piezoelectric fuel injector directed to providing inward displacement of the valve to avoid external soilage. The valve is contained within an injection pipe and is moveable along its axis between a closed and an open position by expansion of the piezoelectric actuator. There are only two valve positions—fully open and fully closed—without the ability for analog or variable injection. A mechanical transmission is placed between the piezoelectric actuator and the valve in order to invert the displacement produced by expansion of the piezoelectric actuator and displace the valve in an inward direction. This mechanism adds complexity to the injector assembly. Thus, the injector of Neretti et al. does not have a one-to-one relationship between prime actuating force and the flow control member without interposing elements and is therefore not directly actuated.
Boecking, U.S. Pat. No. 6,585,171 B1 issued Jul. 1, 2003, describes a fuel injector system comprising a fuel return, high pressure port, piezoelectric actuator stack, hydraulic amplifier, valve, nozzle needle, and injection orifice. The piezoelectric stack of the Boecking injector does not directly actuate the nozzle needle. Close examination reveals that the piezoelectric stack instead actuates a separate hydraulic amplifier to open the valve, which allows the nozzle needle to move off the injection orifice. The needle of the Boecking injector is not directly actuated by the piezoelectric stack. Furthermore, the Boecking injector is limited to operation in two discrete modes: on and off. Hence, Boecking's injector does not have a one-to-one relationship between prime actuating force and the flow control member without interposing elements and is therefore not directly actuated.
Takahashi, U.S. Pat. No. 4,803,393 issued Feb. 7, 1989, describes a piezoelectric actuator for moving an object member wherein the actuator includes a piezoelectric element, an envelope having a bellows, and a pressure chamber where work oil is hermetically enclosed. The invention disclosed by Takahashi is directed to the need for an improved piezoelectric actuator that can prevent the breakdown of the piezoelectric element due to slanting attachments and defective sliding. This is achieved by an envelope between the piezoelectric element and the valve or object member, the envelope containing a resilient member and hermetically containing a fluid. The inclusion of the envelope and spring mechanisms in the injector of Takahashi introduces the problem of valve float at higher operational frequencies, along with indirect actuation limitations. Additionally, the piezoelectric actuator of Takahashi is not used to directly actuate the needle which controls flow but, instead, is used to move a separate upstream control valve which then allows flow to be delivered to the injector assembly. Hence, Takahashi's injector does not have a one-to-one relationship between prime actuating force and the flow control member without interposing elements and is therefore not directly actuated.
Consequently, there exists a need for a fuel injector having the rapid response afforded by direct actuation of an injector nozzle pin (flow control member) by a piezoelectric stack without interposing elements between the prime actuating force and the flow control member. There is also a need for such an injector able to provide dynamic, controlled variable flow throughout an entire combustion/injection cycle, avoiding limitations to flow rate resulting from simplistic on/off operation and selection of orifice size. There is a further need for a fuel injector able to accommodate higher frequency cycling and higher pressure operating conditions. There is also a need for a high frequency injector having minimal latency and response lag. There is an additional need for a high frequency injector able to accommodate relatively high flow rates. There is also a need for an injector that does not require boost or amplification of the actuator mechanism to meet operational requirements.