Patent Application
20120227397
The present disclosure relates generally to a gaseous fuel-powered engine system and, more particularly, to a gaseous fuel-powered engine system having turbo-compounding.
Engines combust a mixture of fuel and air to generate a mechanical power output and a flow of exhaust. The amount of mechanical power produced by the engine through the combustion process is directly related to an amount of air and fuel that can be provided into the engine. For this reason, engines are often equipped with one or more turbochargers that are driven by exhaust to compress combustion air entering the engine. By forcing air into the engine, more air becomes available for combustion than could otherwise be drawn into the engine by motion of the engine's pistons. This increased supply of air allows for increased fueling, resulting in an increased amount of mechanical power produced by the engine. A turbocharged engine typically produces more mechanical power than the same engine without turbocharging.
Unfortunately, turbochargers do not remove all of the energy contained within an engine's exhaust prior to the exhaust being discharged to the atmosphere. Thus, upon discharge to the atmosphere, some amount of energy may still be wasted in the form of heat and/or pressure. If this energy could be recuperated, efficiency of the engine could be improved.
One attempt to recuperate exhaust energy is disclosed in U.S. Patent Publication No. 2010/0148518 (the '518 publication) of Algrain that published on Jun. 17, 2010. Specifically, the '518 publication discloses an engine, for example a gaseous fuel-powered engine, that together with a main generator forms a part of a generator set that functions to generate electricity directed to an external load. The engine includes at least one main turbocharger having a compressor connected to and driven by a turbine. The turbine is oversized relative to the compressor and provides a greater mechanical power output than consumed by the compressor to pressurize combustion air. The extra mechanical power output from the turbine is used to drive an auxiliary generator that generates additional electricity directed to the external load. During operation of the generator set, electrical synchronizing and transforming is performed to produce a common electrical power supply. In this manner, the gas engine utilizes turbo-compounding to improve an efficiency of the gas engine.
Although the '518 publication describes utilizing turbo-compounding to improve an engine's efficiency, the configuration may be problematic when applied to a conventional gaseous fuel-powered engine. Specifically, turbo-compounding increases the backpressure of an engine and, when applied to a conventional gaseous fuel-powered engine, the increased backpressure can cause detonation and associated instabilities within the engine.
The engine system of the present disclosure addresses one or more of the problems set forth above and/or other problems of the prior art.
In one aspect, the present disclosure is directed toward an engine system. The engine system may include an engine configured to receive air and a gaseous fuel, and combust a mixture of the air and gaseous fuel to generate a power output and a flow of exhaust. The engine system may also include at least one power turbine driven by the flow of exhaust to compound the power output of the engine. The engine may employ the Miller Cycle during compounding by the at least one power turbine.
In another aspect, the present disclosure is directed toward a method of generating power. The method may include directing a mixture of gaseous fuel and air into an engine, and combusting the mixture to generate a power output and a flow of exhaust. The method may also include drawing energy from the flow of exhaust to compound the power output, and employing the Miller Cycle when compounding the power output.
Engine 12 may include an engine block 16 that at least partially defines one or more cylinders 18, and a piston 20 disposed within each cylinder 18 to form a main combustion chamber 22. It is contemplated that engine system 10 may include any number of combustion chambers 22 and that combustion chambers 22 may be disposed in an “in-line” configuration, a “V” configuration, or in any other conventional configuration. It is also contemplated that, in some embodiments, engine 12 may include a pre-combustion chamber (not shown) in communication with each main combustion chamber 22, if desired, to facilitate ignition during some lean burn operations.
Each piston 20 may be configured to reciprocate between a bottom-dead-center (BDC) or lower-most position within cylinder 18, and a top-dead-center (TDC) or upper-most position within cylinder 18. In particular, piston 20 may be pivotally coupled to a throw of a crankshaft 24 by way of a connecting rod (not shown). Crankshaft 24 of engine 12 may be journaled within engine block 16 and each piston 20 coupled to crankshaft 24 such that a sliding motion of each piston 20 within each cylinder 18 results in a rotation of crankshaft 24. Similarly, a rotation of crankshaft 24 may result in a reciprocating motion of piston 20. As crankshaft 24 rotates through about 180 degrees, piston 20 may move through one full stroke between BDC and TDC. As shown in
Engine 12 may also include a plurality of gas exchange valves associated with each cylinder 18 and configured to meter air and fuel into and exhaust out of combustion chambers 22. Specifically, engine 12 may include at least one intake valve 26 and at least one exhaust valve 28 associated with each cylinder 18.
Each of intake and exhaust valves 26, 28 may be actuated in any conventional way to move or “lift” and thereby open the respective port 30, 32 in a cyclical manner. For example, intake and exhaust valves 26, 28 may be normally lifted by way of an engine cam (not shown) that is rotatingly driven by crankshaft 24, by way of a hydraulic actuator (not shown), by way of an electronic actuator (not shown), or in any other manner. During normal operation of engine 12, intake and exhaust valves 26, 28 may be lifted in a predefined cycle related to the motion of the associated piston 20 and rotation of crankshaft 24. It is contemplated, however, that a variable valve actuator (not shown) may additionally or alternatively be associated with intake and/or exhaust valves 26, 28 to selectively interrupt the cyclical movements described above (e.g., to adjust an opening time, a closing time, and/or a lift height) and thereby implement particular temporary operations of engine 12.
As shown by the solid curve in
Engine 12 may include multiple different subsystems that cooperate to facilitate combustion within cylinders 18. The subsystems of engine 12 may include, among others, an air induction system 34, and an exhaust system 36 (referring back to
Air induction system 34 may include multiple components that cooperate to condition and introduce compressed air and fuel into combustion chambers 22. For example, air induction system 34 may include an air cooler 38 located downstream of one or more compressors 40. Air cooler 38 may be connected to compressors 40 by way of a passage 42 and to intake ports 30 by way of a passage 44. Compressors 40 may be configured to pressurize a mixture of air and gaseous fuel, for example, natural gas, propane, or methane, that is directed through cooler 38 and into engine 12 via passages 42, 44 and intake ports 30. In the disclosed embodiment, the mixture of air and fuel supplied to compressors 40 may be lean (i.e., have an actual air-to-fuel ratio greater than a stoichiometric air-to-fuel ratio) for a majority of an operational time of engine 12 to help lower an amount of NOX emitted to the atmosphere. It is contemplated that air induction system 34 may include different or additional components than described above such as, for example, a throttle valve, filtering components, compressor bypass components, and other components known in the art.
Exhaust system 36 may include multiple components that condition and direct exhaust from combustion chambers 22 to the atmosphere. For example, exhaust system 36 may include an exhaust passage 46, one or more exhaust turbines 48 driven by exhaust flowing through passage 46, and a power turbine 50 located downstream of exhaust turbine 48 and connected to exhaust turbine 48 by way of a passage 52. Exhaust passage 46 may fluidly connect exhaust ports 32 associated with combustion chambers 22 to exhaust turbine 48. In some embodiments, one or more aftertreatment components 54, for example oxidation catalysts, filters, traps, adsorbers, absorbers, reduction catalysts, scrubbers, exhaust gas recirculation circuits, etc., may be disposed within or connected to passage 52 at a location where pressures and/or temperatures are within a desired activation and/or efficiency range of the components (e.g., between exhaust turbine 48 and power turbine 50). It is contemplated that exhaust system 36 may include different or additional components than described above such as, for example, bypass components, an exhaust compression or restriction brake, an attenuation device, and other known components, if desired.
Exhaust turbine 48 may be configured to receive exhaust discharged from combustion chambers 22, and may be connected to one or more compressors 40 of air induction system 34 by way of a common shaft 56 to form a turbocharger. As the hot exhaust gases exiting engine 12 move through exhaust turbine 48 and expand against vanes (not shown) thereof, exhaust turbine 48 may draw heat and pressure energy from the exhaust and use the energy to rotate and drive the connected compressor 40 to pressurize the mixture of inlet air and gaseous fuel. Exhaust turbine 48 may have any number of inlet volutes, embody a fixed or variable geometry turbine, or include a combination of fixed and variable geometry technology.
Power turbine 50 may be configured to receive exhaust discharged from exhaust turbine 48, and may be connected to compound a power output of engine 12. For the purposes of this disclosure, the compounding performed by power turbine 50 may be defined as the direct adding of mechanical or electrical power by power turbine 50 to the main output of engine 12. In other words, during compounding, power turbine 50 acts as a mechanical or electrical power producing device that functions in parallel with the main output of engine 12 to add to the main output. In the disclosed embodiment, power turbine 50 may be mechanically connected to an end of crankshaft 24 opposite load 14, for example by way of a gear reduction box 58, a chain 60, a belt (not shown), a hydraulic circuit (not shown), a combination of these technologies, or in another suitable manner. As the hot exhaust gases exiting exhaust turbine 48 move through power turbine 50 and expand against vanes (not shown) thereof, power turbine 50 may draw heat and pressure energy from the exhaust and use the energy to rotate and drive crankshaft 24, thereby compounding the output of engine 12. It is contemplated that power turbine 50 may alternatively be configured to drive an auxiliary generator, if desired, and compound the output of engine 12 by producing electrical power that supplements a mechanical and/or electrical power output of engine 12 and/or load 14. Power turbine 50 may have any number of inlet volutes, embody a fixed or variable geometry turbine, or include a combination of fixed and variable geometry technology. In one embodiment, the power output of power turbine 50 may account for about 5-25% of a total output of engine system 10.
The disclosed engine system may have application in any stationary or mobile platform where efficiency and exhaust emissions may be concerns. The disclosed engine system may improve efficiency and lower exhaust emissions by implementing turbo-compounding of a lean-burn, gaseous-fueled engine. Operation of engine system 10 will now be explained.
Referring to
Historically, turbo-compounding of a spark-ignited, gaseous fuel-powered engine was not possible, as the turbo-compounding resulted in excessive exhaust backpressures. These high exhaust backpressures caused a significant amount of high-temperature exhaust and unburned hydrocarbons to remain within combustion chambers 22 following an exhaust stroke. Although the trapped hydrocarbons may help to improve fuel efficiency through additional combustion during a subsequent cycle and thereby also lower emissions of the engine (e.g., lower NOX and hydrocarbon emission), the residual heat and hydrocarbons also elevate pressures and temperatures within combustion chambers 22 to levels sufficient to cause detonation of a newly received air/fuel mixture during a subsequent combustion cycle. In the disclosed embodiment, however, engine system 10 may employ the Miller Cycle (late or early) during the turbo-compounding. It is contemplated that implementation of turbo-compounding and/or the Miller Cycle may be continuous throughout the operation of engine system 10 or, alternatively, only selectively implemented as desired via control of VGT features and/or variable valve actuation.
As described above, by employing the Miller Cycle, pre-combustion temperatures and pressures within combustion chambers 22 may be lowered to below detonation-inducing levels of the lean air/fuel mixture, even with the increase in temperature, pressure, and residual hydrocarbons caused by turbo-compounding. Accordingly, the disclosed engine system may benefit from improved efficiencies and reduced NOX associated with the Miller Cycle, as well as with improved efficiency and reduced levels of unburned hydrocarbons associated with turbo-compounding. Emissions can be improved even further through the use of lean burn strategies.
It will be apparent to those skilled in the art that various modifications and variations can be made in the disclosed engine system without departing from the scope of the disclosure. Other embodiments of the disclosed engine system will be apparent to those skilled in the art from consideration of the specification and practice of the engine system disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents.
