System and method for determining knock margin for multi-cylinder engines

A method includes receiving a signal indicative of a change in an air-fuel ratio (AFR) for a mixture of air and fuel entering a first combustion chamber of a combustion engine, advancing firing timing of the first combustion chamber, receiving, from a knock sensor, a knock signal indicating that the combustion engine has begun to knock, determining a knock margin of the first combustion chamber based on when the combustion engine begins to knock, and storing the knock margin as associated with the knock timing and the AFR.

BACKGROUND

The subject matter disclosed herein relates to knock sensors, and more specifically, to utilizing knock sensors mounted to large, multi-cylinder reciprocating devices (e.g., combustion engine, compressors, etc.) in conjunction with standard quality control techniques to improve knock margin detection and efficiency of the firing timing of the reciprocating devices.

Combustion engines typically combust a carbonaceous fuel, such as natural gas, gasoline, diesel, and the like, and use the corresponding expansion of high temperature and pressure gases to apply a force to certain components of the engine, e.g., piston disposed in a cylinder, to move the components over a distance. Each cylinder may include one or more valves that open and close correlative with combustion of the carbonaceous fuel. For example, an intake valve may direct an oxidizer such as air into the cylinder, which is then mixed with fuel and combusted. Combustion fluids, e.g., hot gases, may then be directed to exit the cylinder via an exhaust valve. Accordingly, the carbonaceous fuel is transformed into mechanical motion, useful in driving a load. For example, the load may be a generator that produces electric power. During use, combustion engines may experience various noises, mechanical faults, or changes in conditions that may be difficult to detect and/or predict.

BRIEF DESCRIPTION

In accordance with a first embodiment, a method includes receiving a signal indicative of a change in an air-fuel ratio (AFR) for a mixture of air and fuel entering a first combustion chamber of a combustion engine, advancing firing timing of the first combustion chamber, receiving, from a knock sensor, a knock signal indicating that the combustion engine has begun to knock, determining a knock margin of the first combustion chamber based on when the combustion engine begins to knock, and storing the knock margin as associated with the knock timing and the AFR.

In accordance with a second embodiment, a method includes receiving a signal indicative of an air-fuel ratio (AFR) for a fuel entering a first combustion chamber of a combustion engine, advancing firing timing of a first combustion chamber at a first advancing rate from an operating timing to a predetermined safe timing, advancing firing timing of the first combustion chamber at a second advancing rate from the safe timing, wherein the second advancing rate is slower than the first advancing rate, receiving, from a knock sensor, a knock signal indicating that the combustion engine has begun to knock, determining a knock margin of the first combustion chamber based on when the combustion engine begins to knock, storing the knock margin as associated with the knock timing and the AFR.

In accordance with a third embodiment, a system includes a controller programmed to receive a signal indicative of a change in an air-fuel ratio (AFR) for a mixture of air and fuel entering a first combustion chamber of a combustion engine, advance firing timing of the first combustion chamber, receive, from a knock sensor, a knock signal indicating that the combustion engine has begun to knock, determine a knock margin of the first combustion chamber based on when the combustion engine begins to knock, and store the knock margin as a relationship between knock timing and the AFR.

DETAILED DESCRIPTION

During use, combustion engines (or other reciprocating devices such as reciprocating compressors) operate at a firing timing wherein the reciprocating components of the engine complete a cycle in a given time. The firing timing may be affected by a number of conditions within the engine, and in turn the firing timing may affect the power output of the engine. Generally, reciprocating engines are able to produce higher torque and thus more power when the rotational speed is faster. Thus, a higher firing timing generally is desirable. Unfortunately, a high firing timing may result in engine conditions that are undesirable. For example, a high firing timing may result in engine knock, which can contribute to wearing in the engine and/or decrease in efficiency of the engine. Firing timing may be chosen to prevent engine knock, but several factors may contribute to the specific firing timing at which knock will occur. As described in further detail below, systems and methods are provided for determining a knock margin whenever a change in air-fuel ratio creates a potential for a change in the firing timing that may cause engine knock.

Turning to the drawings,FIG. 1illustrates a block diagram of an embodiment of a portion of an engine driven power generation system8. As described in detail below, the system8includes an engine10(e.g., a reciprocating internal combustion engine) having one or more combustion chambers12(e.g., 1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 14, 16, 18, 20, or more combustion chambers12). An air supply14is configured to provide a pressurized oxidant16, such as air, oxygen, oxygen-enriched air, oxygen-reduced air, or any combination thereof, to each combustion chamber12. The combustion chamber12is also configured to receive a fuel18(e.g., a liquid and/or gaseous fuel) from a fuel supply19, and an air-fuel mixture ignites and combusts within each combustion chamber12. The air-fuel mixture mixes at an air-fuel ratio (AFR) that may depend on the composition of the fuel and/or other environmental conditions. The AFR is the mass ratio of air to fuel. For example, fuel of a first type may mix with the pressurized oxidant16at a first AFR due to having a first temperature, composition, viscosity, octane, etc. On the other hand, a second fuel type may mix at a different AFR due to having different temperature, composition, viscosity, octane, etc. The air-fuel mixture combusts within the combustion chamber12and the hot pressurized combustion gases cause a piston20adjacent to each combustion chamber12to move linearly within a cylinder26and convert pressure exerted by the gases into a rotating motion, which causes a shaft22to rotate. Further, the shaft22may be coupled to a load24, which is powered via rotation of the shaft22. For example, the load24may be any suitable device that may generate power via the rotational output of the system10, such as an electrical generator. Additionally, although the following discussion refers to air as the oxidant16, any suitable oxidant may be used with the disclosed embodiments. Similarly, the fuel18may be any suitable gaseous fuel, such as natural gas, associated petroleum gas, propane, biogas, sewage gas, landfill gas, coal mine gas, for example.

The system8disclosed herein may be adapted for use in stationary applications (e.g., in industrial power generating engines) or in mobile applications (e.g., in cars or aircraft). The engine10may be a two-stroke engine, three-stroke engine, four-stroke engine, five-stroke engine, or six-stroke engine. The engine10may also include any number of combustion chambers12, pistons20, and associated cylinders (e.g., 1-24). For example, in certain embodiments, the system8may include a large-scale industrial reciprocating engine having 4, 6, 8, 10, 16, 24 or more pistons20reciprocating in cylinders. In some such cases, the cylinders and/or the pistons20may have a diameter of between approximately 13.5-34 centimeters (cm). In some embodiments, the cylinders and/or the pistons20may have a diameter of between approximately 10-40 cm, 15-25 cm, or about 15 cm. The system10may generate power ranging from 10 kW to 10 MW. In some embodiments, the engine10may operate at less than approximately 1800 revolutions per minute (RPM). In some embodiments, the engine10may operate at less than approximately 2000 RPM, 1900 RPM, 1700 RPM, 1600 RPM, 1500 RPM, 1400 RPM, 1300 RPM, 1200 RPM, 1000 RPM, 900 RPM, or 750 RPM. In some embodiments, the engine10may operate between approximately 750-2000 RPM, 900-1800 RPM, or 1000-1600 RPM. In some embodiments, the engine10may operate at approximately 1800 RPM, 1500 RPM, 1200 RPM, 1000 RPM, or 900 RPM. Exemplary engines10may include General Electric Company's Jenbacher Engines (e.g., Jenbacher Type 2, Type 3, Type 4, Type 6 or J920 FleXtra) or Waukesha Engines (e.g., Waukesha VGF, VHP, APG, 275GL), for example.

The engine driven power generation system8may include one or more knock sensors23suitable for detecting engine “knock.” The knock sensor23may sense vibrations caused by the engine, such as vibration due to detonation, pre-ignition, and or pinging. In addition, the engine driven power generation system may include other sensors27(e.g., one or more temperature transducers) to detect other operating conditions (e.g., temperature (e.g., global temperature and/or temperature gradient) of a medium (e.g., cast iron) that the one or more knock sensors23are coupled to). The knock sensor23is shown communicatively coupled to an engine control unit (ECU)25. During operations, signals from the knock sensor23are communicated to the ECU25to determine if knocking conditions (e.g., pinging) exist. The ECU25may then adjust certain engine10parameters to ameliorate or eliminate the knocking conditions. For example, the ECU25may adjust ignition timing and/or adjust boost pressure to eliminate the knocking. As further described herein, the ECU25may receive signals from the air supply14and/or the fuel supply19that are indicative of an AFR or an equivalence ratio or lambda (λ) (i.e., ratio of actual AFR to stoichiometric AFR). In certain embodiments, sensors within the cylinder26may directly detect an amount of air16and/or fuel18that is injected into the cavity30of the cylinder26to determine the AFR/λ. The ECU25may use the AFR or λ in combination with signals from the knock sensor23to determine a knock margin at which the engine10may operate without knocking. Although the following techniques are discussed in terms of a combustion engine, the same techniques may be applied to other reciprocating devices such as compressors.

FIG. 2is a side cross-sectional view of an embodiment of a piston assembly25having a piston20disposed within a cylinder26(e.g., an engine cylinder) of the reciprocating engine10. The cylinder26has an inner annular wall28defining a cylindrical cavity30(e.g., bore). The piston20may be defined by an axial axis or direction34, a radial axis or direction36, and a circumferential axis or direction38. The piston20includes a top portion40(e.g., a top land). The top portion40generally blocks the fuel18and the air16, or a air-fuel mixture32, from escaping from the combustion chamber12during reciprocating motion of the piston20.

As shown, the piston20is attached to a crankshaft54via a connecting rod56and a pin58. The crankshaft54translates the reciprocating linear motion of the piston24into a rotating motion. As the piston20moves, the crankshaft54rotates to power the load24(shown inFIG. 1), as discussed above. As shown, the combustion chamber12is positioned adjacent to the top land40of the piston24. A fuel injector60provides the fuel18to the combustion chamber12, and an intake valve62controls the delivery of air16to the combustion chamber12. An exhaust valve64controls discharge of exhaust from the engine10. However, it should be understood that any suitable elements and/or techniques for providing fuel18and air16to the combustion chamber12and/or for discharging exhaust may be utilized, and in some embodiments, no fuel injection is used. In operation, combustion of the fuel18with the air16in the combustion chamber12cause the piston20to move in a reciprocating manner (e.g., back and forth) in the axial direction34within the cavity30of the cylinder26.

During operations, when the piston20is at the highest point in the cylinder26it is in a position called top dead center (TDC). When the piston20is at its lowest point in the cylinder26, it is in a position called bottom dead center (BDC). As the piston20moves from top to bottom or from bottom to top, the crankshaft54rotates one half of a revolution. Each movement of the piston20from top to bottom or from bottom to top is called a stroke, and engine10embodiments may include two-stroke engines, three-stroke engines, four-stroke engines, five-stroke engine, six-stroke engines, or more.

During engine10operations, a sequence including an intake process, a compression process, a power process, and an exhaust process typically occurs. The intake process enables a combustible mixture, such as fuel and air, to be pulled into the cylinder26, thus the intake valve62is open and the exhaust valve64is closed. The compression process compresses the combustible mixture into a smaller space, so both the intake valve62and the exhaust valve64are closed. The power process ignites the compressed air-fuel mixture, which may include a spark ignition through a spark plug system, and/or a compression ignition through compression heat. The resulting pressure from combustion then forces the piston20to BDC. The exhaust process typically returns the piston20to TDC while keeping the exhaust valve64open. Under certain conditions, the air-fuel mixture32may combust prematurely before the piston20returns to TDC, or after the piston20has passed TDC. These conditions may be called “knock” or “pinging” and may be detected by the knock sensor23. The knock may be affected by many conditions including environmental conditions, engine health, load on the engine10, air flow, fuel flow, or composition of the fuel. As a specific example, a change from one fuel source to another fuel source may include a change in fuel composition and an accompanying change in the AFR of the air-fuel mixture32. After combustion, the exhaust process concludes by expelling the spent air-fuel mixture through the exhaust valve64. It is to be noted that more than one intake valve62and exhaust valve64may be used per cylinder26.

The depicted engine10also includes a crankshaft sensor66, the knock sensor23, and the ECU25, which includes a processor72and memory74. The processor72may include one or more “general-purpose” microprocessors, one or more special-purpose microprocessors, and/or one or more application specific integrated circuits (ASICS), system-on-chip (SoC) device, or some other processor configuration. For example, the processor72may include one or more reduced instruction set (RISC) processors or complex instruction set (CISC) processors. The processor72may execute instructions to carry out the operation of the engine10. These instructions may be encoded in programs, or code stored in a tangible non-transitory computer-readable medium (e.g., an optical disc, solid state device, chip, firmware, etc.) such as the memory74. In certain embodiments, the memory74may be wholly or partially removable from the ECU25. The memory74may store a number of operating parameters that may be used by the ECU25to adjust the operation of the engine10. The crankshaft sensor66senses the position and/or rotational speed of the crankshaft54. Accordingly, a crank angle or crank timing information may be derived. That is, when monitoring combustion engines, timing is frequently expressed in terms of crankshaft54angle. For example, a full cycle of a four stroke engine10may be measured as a 720° cycle. The ECU25is thus able to track the timing of the combustion event within the cylinder26for determining specifically when knock occurs. The knock sensor23may be a Piezo-electric accelerometer, a microelectromechanical system (MEMS) sensor, a Hall effect sensor, a magnetorestrictive sensor, and/or any other sensor designed to sense vibration, acceleration, sound, and/or movement. In other embodiments, sensor23may not be a knock sensor, but any sensor that may sense vibration, pressure, acceleration, deflection, or movement.

Because of the percussive nature of the engine10, the knock sensor23may be capable of detecting signatures even when mounted on the exterior of the cylinder26. However, the knock sensor23may be disposed at various locations in or about the cylinder26. Additionally, in some embodiments, a single knock sensor23may be shared, for example, with one or more adjacent cylinders26. In other embodiments, each cylinder26may include one or more knock sensors23(e.g., one or more arrays of knock sensors23arranged along one or more planes through the engine10). The crankshaft sensor66and the knock sensor23are shown in electronic communication with the engine control unit (ECU)25.

A sensor70(e.g., AFR sensor) is also coupled to the ECU25. The sensor70, in certain embodiments, may include sensors within the cylinder26that directly detect an amount of air16and/or fuel18that is injected into the cavity30of the cylinder26. More generally, the AFR sensors70may include sensors that detect conditions that may be used to estimate the AFR or λ. For example, without measuring the specific AFR or λ within the cavity30, the sensors70may detect the temperature and pressure of the air16at intake, or may measure the flow rates of the air and fuel separately to estimate the AFR or λ. Furthermore, sensors70in the exhaust of the engine may measure oxygen, for example, which may indicate an accurate estimation of the AFR or λ of the air-fuel mixture32. To receive and process the signals from the sensors23,66,70, the ECU25includes the processor72and the memory74(e.g., a machine-readable medium). The memory74may store non-transitory code or computer instructions that may be executed by the processor72. The ECU25monitors and controls and operation of the engine10, for example, by adjusting combustion timing, valve62,64, timing, adjusting the delivery of fuel and oxidant (e.g., air), and so on.

FIG. 3is a flow chart illustrating an embodiment of a process76for determining and storing a knock margin for the engine10. The process76may be used while the engine10is operating and begins when the ECU25receives78a signal indicative of a change in the AFR or λ. As mentioned above, the signal may be sent from the AFR sensors70. The signal may be triggered by direct detection of the air-fuel mixture, or may be triggered by detection of a change in a temperature of the air, a pressure of the air, a flow of the air, a flow of the fuel, an oxygen level of an exhaust gas, or any combination thereof. Furthermore, the signal may be triggered by user input, for example when a user knows that the fuel source changes from one source to another of a different composition. In some embodiments, the ECU25may include stored knock margins as a relationship of AFR or λ. Thus, when a change in the AFR or λ is received, the ECU25may determine whether a knock margin has previously been determined for that AFR or λ (block80). If the AFR or λ knock margin has been stored, then the ECU25may ask for a user input as to whether to run at the stored knock margin or to determine a new knock margin (block82). If a user indicates that the engine10is to run at the stored knock margin, then the engine10operates at the new knock margin (block84). Alternatively, in certain embodiments the ECU25may not ask whether or not to run at the stored knock margin, but instead, if there is a stored knock margin for the detected AFR or λ, then the engine may operate under that knock margin.

To determine a new knock margin for the detected AFR or λ, the ECU25may advance the firing timing of the engine10(block86). The advance of the firing timing may vary in speed. That is, the firing timing may advance quickly at first until a predetermined firing timing is reached, and then the advance may slow down. The predetermined firing timing may be based, for example, on former safe timing for the previous AFR or λ. The firing timing is advanced until the knock sensor23detects that the engine is knocking and sends a signal to the ECU25(block88). The knock sensor23also detects a severity of knock and relays this to the ECU25. The ECU25then determines a knock margin based on the firing timing at which knock started to occur, and the severity at which the knock occurs (block90). For example, if the knock occurs at a specific timing but immediately experiences heavy knocking, the response would be different than if the knock was only slight when the knock began. Once the knock margin is determined, then the engine10may operate at the new knock margin (block84). Operating at the new knock margin includes operating at a firing timing that does not cause knocking within the engine10.

Technical effects of the disclosed embodiments include a reciprocating engine10or other reciprocating device (e.g., reciprocating compressor) that operates under the control of an engine control unit (e.g., ECU25). The ECU25receives signals from knock sensors23, crankshaft sensors66and AFR sensors70to monitor and control the engine10through changes in the knock margin caused by varying the AFR or λ. For example, when a fuel source changes, the AFR or λ may change and affect the knock margin. The ECU25is configured to respond to these changes so that the engine10performs at or near the highest possible firing timing, without experiencing engine knock.