Patent application title: FOUR STROKE INTERNAL COMBUSTION ENGINE HAVING VARIABLE VALVE TIMING AND METHOD
Darryl Baldwin (Lafayette, IN, US)
Rohit Menon (Lafayette, IN, US)
Willibald Berlinger (Peoria, IL, US)
IPC8 Class: AF01L134FI
Class name: Poppet valve operating mechanism with means for varying timing camshaft or cam characteristics
Publication date: 2012-05-24
Patent application number: 20120125276
A four stroke internal combustion includes at least one cylinder and at
least one intake valve associated with the cylinder. The intake valve is
configured to open and close over a predetermined range of crankshaft
rotation in accordance with a Miller thermodynamic cycle. An electronic
controller receives at least one input signal indicative of an amount of
air and/or an amount of air/fuel mixture in cylinder. The electronic
controller is configured to provide a timing phase signal that operates
to adjust a timing of operation of the intake valve based on the input
signal(s) such that a torque output of the engine is maintained
substantially constant over a predetermined range of engine speed by
shifting the predetermined range of crankshaft rotation.
1. A four stroke internal combustion engine, comprising: at least one
cylinder having a piston reciprocable between top dead center (TDC) and
bottom dead center (BDC) positions; at least one intake valve associated
with the at least one cylinder and operable by a camshaft, the at least
one valve being configured to open and close over a predetermined range
of crankshaft rotation, wherein the intake valve operates in accordance
with a Miller thermodynamic cycle; an electronic controller disposed to
receive at least one input signal indicative of at least one of an amount
of air and an amount of air/fuel mixture in the at least one cylinder;
wherein the electronic controller is configured to provide a timing phase
signal that operates to adjust a timing of operation of the at least one
intake valve based on the at least one input signal such that a torque
output of the engine is maintained substantially constant over a
predetermined range of engine speed by shifting the predetermined range
of camshaft rotation with respect to a crankshaft position.
2. The engine of claim 1, wherein the timing phase signal causes a phase shift in the predetermined range of camshaft.
3. The engine of claim 2, further comprising a phaser associated with the camshaft and disposed to index the camshaft in response to the timing phase signal.
4. The engine of claim 1, wherein the at least one input includes at least one of engine speed, engine load, charge air oxygen concentration, altitude, and fuel quality.
5. The engine of claim 1, wherein a ratio of air to fuel in the at least one cylinder is maintained constant during engine operation to yield at least one of a rich-burn stoichiometric combustion and a lean burn stoichiometric combustion.
6. The engine of claim 5, wherein the timing phase signal operates to decrease a Miller effect of the engine when at least one of the engine speed is low, the engine load is low, the altitude is high, the fuel quality is high, and the engine is in a startup mode.
7. The engine of claim 1, wherein the engine is fueled by at least one or natural gas, propane, field gas, biogas, and producer gas.
8. The engine of claim 1, wherein at least one of the torque output and a speed of the engine is maintained within a range of +/-10% of a target torque output value.
9. The engine of claim 1, wherein the engine is configured to, at times, admit and compress an intake charge for combustion in the at least one cylinder that is within the range of between 20 and 80% of a total possible intake charge, which is defined as an intake charge that would have been admitted into the at least one cylinder had the intake valve been opened substantially at TDC and closed substantially at BDC during an intake stroke.
10. The engine of claim 9, wherein a volumetric efficiency of the engine is within the range of 45-60%.
11. The engine of claim 1, wherein the at least one cylinder is configured to operate at a compression ratio 10:1 and 15:1.
12. A method for operating a four-stroke internal combustion engine, comprising: operating the engine at a stoichiometric air to fuel ratio and at an engine valve timing in a fashion consistent with a Miller thermodynamic combustion cycle; receiving operating parameters at an electronic controller, the operating parameters being indicative of an amount of an air and fuel combustion mixture present in at least one engine cylinder; processing the operating parameters in the electronic controller to determine at least one of a desired valve timing and a timing phase variation, wherein the operating parameters include at least one of engine speed, engine load, altitude, and fuel quality; determining a valve phase signal based on at least one of the desired valve timing and the timing phase variation; and changing a valve timing based on the valve phase signal to selectively adjust the amount of the combustion mixture such that an engine torque output is maintained substantially constant over an engine speed range.
13. The method of claim 12, wherein the processing of the operating parameters involves determining the at least one of desired valve timing and timing phase variation based on a then-present engine speed and engine load.
14. The method of claim 13, wherein the processing of the operating parameters involves compensating the at least one of desired valve timing and timing phase variation based on altitude.
15. The method of claim 13, wherein the processing of the operating parameters involves compensating the at least one of desired valve timing and timing phase variation based on fuel quality.
16. The method of claim 12, wherein changing the valve timing is accomplished by indexing a camshaft in response to the valve phase signal.
17. The method of claim 16, wherein the valve phase signal is provided to a phaser device that operates to adjust the indexing of the camshaft in response to the valve phase signal.
18. The method of claim 12, wherein operating the engine consistent with the Miller thermodynamic combustion cycle is accomplished by at least one of: maintaining at least one intake valve associated with a cylinder of the engine open beyond a BDC position of a piston such that an intake stroke is generally prolonged and a compression stroke is generally abridged under a late inlet closing (LIC) type of engine operation, and closing the at least one intake valve before the BDC position of the piston such that the intake stroke is generally abridged and the compression stroke is generally prolonged under an early inlet closing (EIC) type of engine operation.
19. The method of claim 18, wherein determining the at least one of desired valve timing and timing phase variation is consistent with: quickening the closing of the at least one intake valve when the engine is operating under a LIC type of operation, or delaying the closing of the at least one intake valve when the engine is operating under an EIC type of operation, such that an effect of the Miller cycle is decreased when the operating parameters are indicative of at least one of a low engine speed, a low engine load, a high altitude, or a high fuel quality, or when the engine is in a startup mode.
20. The method of claim 18, wherein determining the at least one of desired valve timing and timing phase variation is consistent with: delaying the closing of the at least one intake valve when the engine is operating under a LIC type of operation, or quickening the closing of the at least one intake valve when the engine is operating under an EIC type of operation, such that an effect of the Miller cycle is increased when the operating parameters are indicative of at least one of a high engine speed, a high engine load, or a low fuel quality.
 This patent disclosure relates generally to spark ignition internal combustion engines and, more particularly to engines having variable valve timing capability.
 The Miller cycle is a combustion process used in a type of four-stroke internal combustion engine. A traditional Otto cycle engine uses four "strokes," two of which have the greatest impact on the engine's power output--the compression stroke, during which power is used to compress a fuel and air mixture, and the power stroke, during which the mixture combusts to produce power. An appreciable portion of an engine's power output depends on the amount of power internally consumed by the engine during the compression cycles of the various engine cylinders.
 In an engine operating under a Miller cycle, one or more intake valves of a combustion cylinder are left open for a longer period after bottom dead center (BDC) or are closed sooner, i.e. before BDC, when compared to a corresponding engine operation in an Otto cycle. In this way, a smaller amount of charge air is compressed in the cylinder than what would otherwise have been admitted into the cylinder had the intake valve timing been performed in the standard Otto cycle operation.
 When operating in a late intake valve closing fashion (LIC), the charge is partially expelled back out the still-open intake valve as the piston initially moves upwards in what is traditionally the compression stroke. Similarly, when the intake valve is closed early (EIC), the charge admitted in the cylinder is expanded as the piston approaches BDC before being compressed. Typically this loss of charge air would result in a loss of power. However, in the Miller cycle, the reduction in charge air is compensated for by the use of a supercharger or a turbocharger. One aspect of the Miller cycle is that the compression stroke actually starts in LIC engines only after the piston has pushed out this "extra" charge and the intake valve closes, or in EIC engines after the piston has passed BDC and begins to compress the charge in the cylinder.
 One example of a known engine operation mode can be seen in U.S. Pat. No. 7,178,492, which issued on Feb. 20, 2007. Here, a mixture of pressurized air and recirculated exhaust gas is fed from intake manifold to an intake port of a combustion chamber. An air intake valve can be selectively operated to open the port to allow the mixture of pressurized air and recirculated exhaust gas to flow between the combustion chamber and the intake manifold during a major portion of the compression stroke of the piston. A fuel supply system is controlled to inject fuel into the combustion chamber after the intake valve is closed. In this engine application, a variable intake valve closing mechanism selectively interrupts the closing timing of the intake valve such that the intake valve is held open for a desired period. Fuel, in this case diesel, is injected into the combustion chamber when the intake valve is closed, and the engine has a compression ratio of about 4/1.
 The disclosure describes, in one aspect, a four stroke internal combustion engine. The engine includes at least one cylinder and at least one intake valve associated with the cylinder. The intake valve is configured to open and close over a predetermined range of crankshaft rotation in accordance with a Miller thermodynamic cycle. An electronic controller receives at least one input signal indicative of an amount of air and/or an amount of air/fuel mixture in cylinder. The electronic controller is configured to provide a timing phase signal that operates to adjust a timing of operation of the intake valve based on the input signal(s) such that a torque output of the engine is maintained substantially constant over a predetermined range of engine speed by shifting the predetermined range of crankshaft rotation.
 In another aspect, the disclosure describes a method for operating a four-stroke internal combustion engine. The method includes operating the engine at a stoichiometric air to fuel ratio and at an engine valve timing in a fashion consistent with a Miller thermodynamic combustion cycle. Operating parameters are received at an electronic controller. The operating parameters are indicative of an amount of an air and fuel combustion mixture present in at least one engine cylinder. The operating parameters are processed in the electronic controller to determine a desired valve timing and/or a timing phase variation. The operating parameters include engine speed, engine load, altitude, and/or fuel quality. A valve phase signal is determined based on the desired valve timing and/or the timing phase variation. A valve timing of the engine is changed based on the valve phase signal to selectively adjust the amount of the combustion mixture such that an engine torque output is maintained substantially constant over an engine speed range.
BRIEF DESCRIPTION OF THE DRAWINGS
 FIG. 1 is a block diagram of an engine in accordance with the disclosure.
 FIG. 2 is a partial cross section of a combustion cylinder of an engine in accordance with the disclosure.
 FIGS. 3 and 4 are perspective and section views of a phaser in accordance with the disclosure.
 FIG. 5 is a block diagram for an engine controller in accordance with the disclosure.
 FIG. 6 is a block diagram of a phaser controller in accordance with the disclosure.
 FIG. 7 is a qualitative chart illustrating various engine operating conditions in accordance with the disclosure.
 FIG. 8 is a flowchart for a method in accordance with the disclosure.
 FIG. 9 is a valve timing diagram in accordance with the disclosure.
 This disclosure relates to internal combustion engines having variable valve and fuel injection and/or spark timing capability. Although the disclosed embodiment describes a spark ignition engine operating on gaseous hydrocarbon fuel, such as natural gas, compression ignition engines or engines operating on gasoline or any other hydrocarbon fuel are contemplated and are well suited for the devices and methods disclosed herein.
 In various engine applications such as those operating at various speeds while maintaining a constant torque output, as speed is reduced, the effective "amount" of Miller changes depending on the LIC or EIC type of Miller mode of operation of the engine. Typically, the air charge of the engine is maintained constant by appropriate compensation of intake manifold pressure or boost and intake valve timing. In most engine applications, such additional boost is not readily attainable, which results in those engines being unable to maintain their constant torque output when operating at lower speeds. Having identified this problem, the present disclosure proposes a method and system for controlling the amount of Miller that an engine uses dynamically based on engine operating parameters, which include engine speed as well as other parameters. These parameters are processed in a controller which, in one embodiment, is configured to control the operation of a cam phaser. The cam phaser operates to change the valve timing and thus adjust the amount of Miller depending on engine speed, which permits the engine to operate at lower speeds than the speeds that were previously possible.
 FIG. 1 shows a block diagram for an internal combustion engine 100. The internal combustion engine 100 includes a crankcase 102 forming a plurality of cylinders 104. Each cylinder 104 is operably associated with an injector 106, an intake runner 108, and an exhaust runner 110. During operation of the engine 100, air enters each cylinder 104 via its respective intake runner 108. While in the cylinder 104, the air mixes with fuel injected from the injector 106 to form a combustible mixture. In an alternative embodiment, the fuel is mixed with intake air before it enters the engine cylinders to yield a combustible mixture. In either engine configuration, the combustible mixture is compressed via a piston (not shown) and is ignited by a spark producing power. Exhaust gas remaining in the cylinder 104 is evacuated via the respective exhaust runner 110 and the process is repeated. Air entering each cylinder via its respective intake runner 108 is supplied to the intake runners 108 through an intake manifold 112. Similarly, exhaust gas from the cylinders 104 is collected in an exhaust manifold 114. The fuel supplied to each injector 106 is compressed by a fuel pump 116, which supplies compressed fuel to a common rail 118 that is in fluid communication with each of the injectors 106. Alternatively, the fuel is provided to a mixing valve (not shown) that mixes fuel with incoming engine air in a predetermined ratio.
 A detail section view of a cylinder 104 of the engine 100 is shown in FIG. 2. In the description that follows, elements and features that are the same or similar to corresponding elements and features previously described are denoted with the same reference numerals as previously used for simplicity. Accordingly, the cylinder 104 includes a piston 202 reciprocally mounted therewithin and eccentrically connected to a rotating crankshaft (not shown) via a connecting rod 204 (partially shown). A cylinder head 206 forms portions of the intake runner 108 and the exhaust runner 110. An intake valve 208 is reciprocally mounted in the cylinder head 206 and disposed to selectively fluidly block air entering the cylinder 104 from the intake runner 108. Similarly, an exhaust valve 210 selectively fluidly blocks exhaust gas present in the cylinder 104 after a power stroke of the engine from entering the exhaust runner 110. Although single intake and exhaust valves are shown for simplicity, the engine 100 may include multiple valves per cylinder 104.
 The opening and closing of the intake and exhaust valves 208 and 210 in the illustrated embodiment is accomplished by two overhead cams, but other configurations may be used. Moreover, although dedicated intake and exhaust cams are shown, alternate engine configurations may include a single cam operating both the intake and exhaust valves of the engine. In the illustrated embodiment, an intake valve cam 212 includes a plurality of intake lobes 214 that form eccentric features configured to push the intake valve 210 open through a corresponding intake valve bridge 216 as the intake cam 212 rotates. Similarly, an exhaust valve cam 218 includes exhaust lobes 220 that push the exhaust valve 210 open through a corresponding exhaust valve bridge 222. Although the illustration in FIG. 2 is simplified, similar structures operating valves for cylinders arranged in any inline, V, or any other configuration are contemplated.
 The engine 100 is a four stroke engine, which means that four strokes of the piston 202 are successively performed to produce power. In the illustrated embodiment, the engine 100 is operating under a Miller thermodynamic cycle, in which the intake valve 208 is kept open after the piston 202 has passed its BDC position longer or shorter than what a typical engine running an Otto or Diesel cycle would have. More specifically, a qualitative valve timing chart 300 is shown in FIG. 9. Although typical valve timing charts are configured based on the particular structures of each engine, the chart 300 is shown simplified and without valve lead, lag, or overlap effects for simplicity.
 The chart 300 represents various intake and exhaust valve opening events with respect to the rotation of the engine's crankshaft, which is viewed from the front as it rotates in the direction of the arrow, R. Accordingly, TDC is shown at the top of the chart 300 and represents the crankshaft position (0 degrees) at which the piston 202 is at the topmost position in the cylinder 104 as shown in FIG. 2. Similarly, BDC is shown at the bottom of the chart 300 and represents the position at which the piston 202 is at the bottommost position in the cylinder 104 (180 degrees). In the chart, an intake stroke 302 extends from TDC, at which the intake valve 208 is assumed to instantaneously open for purposes of the present disclosure, to an angle belonging in the range of about 1 to 100 degrees before or after BDC over an angle, α (alpha), which is generically illustrated. The compression stroke 304 begins when the intake valve has closed, which in the present discussion is assumed to occur instantaneously, and extends up to TDC. A combustion or power stroke 306 immediately follows until about BDC, and is followed by an exhaust stroke 308. The initiation of the power stroke 306 can be selectively advanced or retarded by either providing a spark in a spark ignition engine or by permitting auto-ignition to occur in a compression ignition engine by creating appropriate conditions within the combustion cylinder.
 As shown by the shaded area 310 in the chart 300, the opening and closing of the intake valve prolongs the intake stroke 302 past the BDC position, which delays the compression stroke 304 in a fashion that is characteristic of one type of the Miller cycle (LIC). It should be appreciated that in an EIC type of Miller cycle, the valve timing chart would be different. The actuation of the intake valve is advantageously variable based on other engine operating and environmental conditions such that engine operation may be optimized under most operating conditions. The point of ignition or initiation of the power stroke 306 can also be selectively controlled in the engine 100 of the present disclosure. The duration of the intake stroke and/or the initiation of the combustion stroke are two parameters that can be actively controlled in the engine 100. Such control is effective in improving fuel economy, extending the constant torque engine speed operating range of the engine, adjusting for altitude effects, compensating for different fuel types, and generally providing other advantages to the operation of the engine 100 as is described in further detail in the paragraphs that follow.
 One embodiment of a component configuration used to selectively adjust the timing of the opening and closing of the intake and exhaust valves of the engine 100 is shown in FIGS. 3 and 4. Accordingly, a perspective view of the intake cam 212 is shown in FIG. 3. The intake cam shown includes twelve lobes 214 but other configurations may be used.
 The intake cam 212 includes a timing wheel 224 that forms a plurality of notches 226, and a phaser 228. In the illustrated embodiment, and exemplary phaser 228 includes a housing 230 forming four internal chambers 231, but other phaser configurations having fewer or more chambers. In general, any appropriate type of rotary actuator may be used. The exemplary phaser 228 further includes a cam rotor 232. The cam rotor 232 forms four arms 234, one corresponding to each chamber 231, sealably disposed to rotate within the chambers 231. The cam rotor 232 further includes a network of fluid passages 236 configured to provide pressurized fluid between the walls of the chambers 231 and the arms 234 such that the relative angular position of the cam rotor 232 relative to the housing 230 may be adjusted.
 The angular adjustment of the cam rotor 232 relative to the housing 230 during operation of the engine 100 creates an offset or phase difference in the rotational position of the lobes 214, which in turn yields a phase shift in the opening and closing of the intake valves relative to the rotational position of the crankshaft and thus the position of the pistons within their respective engine cylinders. This phase shift may be accomplished by appropriately adjusting the pressure of fluid in the fluid passages 236.
 A block diagram of the PCM 254 is shown in FIG. 5. As shown, the PCM 254 is disposed to receive various inputs indicative of engine operating parameters and other parameters. Specifically, the PCM 254 receives an engine speed signal (RPM) 402, an engine load signal (LOAD) 404, which may be expressed as a torque applied to the engine, an actual intake valve phase or intake cam timing signal (I_TIM) 406, an exhaust cam timing signal (E_TIM) 408, a fuel methane or octane rating signal (METH) 410, an altitude signal (ALT) 412, and other parameters that are not shown here, such as intake manifold pressure, exhaust pressure, engine oil or coolant temperature, ignition timing (IGN 444 as shown in FIG. 6) and the like. Of the illustrated signals, the RPM 402 may be provided as an engine speed value in revolutions per minute, or it may alternatively be provided as a raw series of pulses from the crankshaft position sensor, which are then used to derive the engine speed. The LOAD 404 may be provided directly by a load sensor (not shown), or it may alternatively be calculated indirectly from other parameters, such as the current and voltage output of a generator or alternator connected to the engine (not shown), a pressure and flow of hydraulic fluid provided by a fluid pump connected to the engine (not shown), or any other appropriate parameters indicative of the load applied to the engine during operation. The I-TIM and E-TIM 406 and 408 respectively may be provided from cam position sensors associated with the intake and exhaust valves of the engine. The ALT 412 may be provided by a barometric pressure sensor (not shown), while the METH 410 may be provided automatically by a fuel quality sensor (not shown) and/or provided by a manually selectable mechanical or electronic switch, which can be set by an operator if the fuel quality provided to the engine is known.
 The PCM 254 includes various sub-modules as shown and described here, but it should be appreciated that the functionality of the modules illustrated is not exhaustive. Accordingly, fewer or more functions than those shown may be integrated with the PCM 254. Moreover, the PCM 254 shown here is an electronic control device or, stated differently, an electronic controller. As used herein, the term electronic controller may refer to a single controller or may include more than one controller disposed to control various functions and/or features of the engine. For example, a master controller, used to control systems associated with the engine, such as a generator or alternator, may be cooperatively implemented with a motor or engine controller, used to control the engine 100. In this embodiment, the term "controller" is meant to include one, two, or more controllers that may be associated with one another and that may cooperate in controlling various functions and operations of the engine 100. The functionality of the controller, while shown conceptually in FIGS. 5-6 to include various discrete functions for illustrative purposes only, may be implemented in hardware and/or software without regard to the discrete functionality shown. Accordingly, various interfaces of the controller are described relative to components of the engine. Such interfaces are not intended to limit the type and number of components that are connected, nor the number of controllers that are described.
 Accordingly, the PCM 254 includes an intake valve timing module 414, which receives at least the intake valve timing signal 406, the load 404, and the engine speed 402. The intake valve timing module 414 performs calculations to provide an intake valve phase signal 416. The intake valve phase signal 416 may be the same as or provide a basis for determination of a signal controlling the operation of the phaser device, for example, the phaser 228, as was shown and previously described. Although any suitable implementation may be used for the intake valve timing module 414, a specific implementation is shown in FIG. 6.
 In reference to FIG. 6, the intake valve timing module 414 includes a lookup table 418 that is populated by valve timing values or, in the illustrated embodiment, valve phase signals that are tabulated against engine speed 402, engine load 404 and, optionally, the ignition timing value (IGN) 444. The timing values in the table 418 are arranged to provide timing advance or retard, depending on the type of Miller operation used, for decreasing engine speed and/or for decreasing engine load to provide a lessened or decreased effect of Miller operation of the engine. In other words, when the constant torque operating range of the engine is expanded to encompass lower engine speeds, the table 418 may be arranged to provide a larger air charge in the cylinder by changing the timing of the intake valves to account for the lower intake air pressure of the engine when operating at low engine speeds. A similar strategy may be used when the engine operates at higher altitudes.
 Thus, the table 418 receives the engine speed 402 and load 404 during operation, and uses these parameters to lookup, interpolate, or otherwise determine a desired intake timing value 420. In one embodiment, the ignition timing 444 is also used to determine the intake timing value 420. The desired intake timing value 420 is compared to the actual intake timing 406 at a summing junction 422 to provide an intake timing error 423. The intake timing error 423 is provided to a control algorithm 424, which yields an intake valve timing command signal 426. The control algorithm 424 may be any suitable algorithm such as a proportional-integral-derivative (PID) controller or a variation thereof, a model based algorithm, a single or multidimensional function and the like. Moreover, the control algorithm 424 may include scheduling of various internal terms thereof, such as gains, to enhance its stability.
 Returning now to FIG. 5, the intake valve timing command signal 426 is optionally compensated by the addition of compensation terms at a junction 428. In the illustrated embodiment, the compensation terms are an altitude compensation term 430 and a fuel quality compensation term 432. These compensation terms are optional and augment the flexibility of engine operation under different environmental conditions. More specifically, the altitude compensation term 430 is a timing advance or retard value that depends on the altitude 412 of operation of the engine. The altitude signal 412 is provided to an altitude compensation module 434. In the illustrated embodiment, the altitude compensation module 434 may include a function that provides an appropriate timing advance or retard value based on the expected air density at various altitudes. In this way, the altitude compensation module 434 may provide a term tending to change intake valve timing, which results in a lessened Miller effect for higher altitudes.
 In a similar fashion, the fuel quality compensation term 432 is a timing advance or retard value that depends on the measured or provided fuel methane or octane number. In the illustrated embodiment, the fuel quality compensation term 432 is provided by a fuel quality timing module 436 based on the fuel quality signal 410. In one embodiment, the fuel quality timing module 436 may provide a compensation term tending to change the intake valve timing, thus decreasing the Miller effect of the engine for fuels having relatively high methane or octane numbers. The intake valve timing command signal 426 is thus compensated to provide the intake valve phase signal 416.
 In engines having separate intake and exhaust valve camshafts, the PCM 254 may be further configured to provide a separate exhaust valve phase signal 438. The exhaust valve phase signal 438 in the embodiment illustrated is determined in a fashion similar to that of the intake valve phase signal 416. Accordingly, the exhaust valve phase signal 438 is determined by an altitude and fuel quality compensated exhaust valve timing signal 440 that is provided by an exhaust valve timing module 442. The exhaust valve timing module 442 receives as inputs the engine speed 402 and load 404 as well as the exhaust valve timing 408. The exhaust valve timing module 442 may operate similar to the intake valve timing module 414 and include similar elements and algorithms. The exhaust valve timing signal 440 may be compensated by use of the same or different compensation terms as used for the intake valve timing command, namely the altitude compensation term 430 and the fuel quality compensation term 432. It should be appreciated that in engines having a single camshaft operating both intake and exhaust valves, a separate exhaust valve phase signal will not be required.
 A qualitative graph illustrating certain advantages that can be realized by the selective adjustment of the intake and exhaust valve opening duration as disclosed herein is shown in FIG. 7. More specifically, FIG. 7 is a graph of engine operating points with and without timing compensation that are plotted for specific conditions of engine speed 502. In this way, engine speed 502 is plotted against the horizontal axis and engine volumetric efficiency 504 is plotted along the vertical axis. For internal combustion engines, volumetric efficiency typically refers to the efficiency with which the engine can move the charge into and out of the cylinders. More specifically, volumetric efficiency is a ratio, which can be expressed as a percentage, of what quantity of fuel and air actually enters the cylinder during induction over the actual capacity of the cylinder under static conditions. Relevant to the present disclosure, the ability to achieve high volumetric efficiencies for lower engine speeds is desired such that an engine speed range over which an engine can operate at constant torque conditions may be increased.
 In the chart shown in FIG. 7, a baseline engine operating point 506 is plotted to serve as a baseline for comparison of the effect on volumetric efficiency of valve phase shifting for engine operation at lower speeds. The baseline operating point 506 includes engine operation under the Miller cycle. A second engine operating point 508 is plotted for a lower engine speed, which is about 6% lower than the baseline engine speed and, which was acquired with no adjustment being performed to intake or exhaust valve timing as compared to the baseline point 506. Operation at the lower engine speed provides a reduction in the volumetric efficiency of the engine by about 0.5% at the second point 508. A third point 510 was acquired at an even lower engine speed, which was about 11.5% lower than the baseline engine speed. Unlike the second point, the intake timing at the third point 510 was advanced by fewer than 10 degrees. The volumetric efficiency at the third point 510 increased by about 3% relative to the baseline volumetric efficiency.
 The increase in volumetric efficiency by timing adjustment for lower engine speeds enables the constant torque operation of the engine over a broader range of engine speeds. This engine operating ability is advantageous for various engine applications, such as generator sets, work machines, stationary compressors, hybrid electric drive systems and the like. In general, applications that can use engines operating at nearly constant engine speeds stand to benefit from the systems and methods disclosed herein because operation at reduced engine speeds while maintaining constant or nearly constant torque as higher engine speeds presents advantages such as improved fuel consumption and reliability, reduced noise and emissions, and others
 A flowchart for a method of operating an engine is shown in FIG. 8. The engine is operated at a fuel-rich stoichiometric air mixture at 602. Such air mixture may be expressed or considered as a desired air to fuel ratio (AFR) of a mixture of charge air and fuel present in the engine cylinder when combustion is initiated, which may be further maintained substantially constant over the engine operating range. An electronic controller is disposed to receive various engine and other operating parameters at 604 that are indicative of the amount of charge air or the amount of the air and fuel mixture that is ingested in the engine's cylinders during operation. The electronic controller processes the parameters received to determine a timing phase variation at 606. The timing phase variation is configured to provide relatively high Miller effects during operation of the engine such that the amount of fuel/air mixture in the engine cylinders is sufficient to yield a desired torque output while the engine operates under fuel-rich stoichiometric combustion conditions.
 In one embodiment, the valve phase changes are variable depending on the engine load and engine speed, altitude, fuel quality, and other parameters. In general, any parameter that is indicative of the concentration of oxygen, which is required for combustion, and/or fuel, may be used as an indication of the amount of fuel/air mixture in the engine during combustion. In one embodiment, the valve timing is adjusted at 608 based on engine speed and load. Optionally, the valve timing is also adjusted or compensated at 610 based on altitude, and at 612 based on fuel quality. Such adjustments are used to provide a valve phase signal at 614 by the electronic controller. The phase signal may be any appropriate type of signal that can effect a change in the valve timing of the engine during operation. In the illustrated embodiment, for example, the valve phase signal is a PWM signal that is provided to a control valve. The control valve selectively adjusts the flow of fluid into and out from a phaser device. The phaser device is in turn configured to adjust its position to provide a timing phase shift to a camshaft operating the valves.
 Accordingly, the timing of the engine valves is changed based on the valve phase signal at 616, for example, by indexing the camshaft at the phaser device. In this way, the cam is indexed to appropriately change the Miller effect depending on the type of Miller operation of the engine based on engine speed, engine load, altitude, and other conditions. In the disclosed embodiment, the valve phase signal is provided to at least one control valve. The control valve operates to adjust the timing or phase of a set of valves by indexing a camshaft. The process may be repeated continuously to adjust or shift the timing phase of the engine valves during operation of the engine.
 By shifting the timing phase of the engine valves, an engine operating in accordance with the disclosed method is advantageously capable of extending its engine speed operating range while maintaining a nearly constant engine torque output for a controlled engine speed, such as for electric power generation applications, a controlled engine speed, such as for compressor applications, and so forth. Such engine operating capability is advantageous for certain engine applications, such as those used in hybrid electric drive systems, generator sets, compressors and the like, in which the engine speed may be adjusted in response to load changes.
 The present disclosure is applicable to internal combustion engines of any type and, more particularly, to stationary engines that operate on natural gas. The disclosed devices and methods are unique in that the timing of lean or rich burning engine is adjusted to include a substantial Miller effect in the range of about 20-80%, with most applications operating in a range of about 40-50%. The valve phasing is variable and dynamically adjusted during engine operation based on various operating parameters such as engine speed, engine load, altitude, fuel quality, and others. In this way, the timing of the intake valves is changed for decreasing engine speed and load and/or increasing altitude, and also for decreasing fuel quality such that sufficient air and fuel is present in the cylinders over a wide operating range. In the illustrated embodiments, the engine 100 operates at a compression ratio of 13/1 or greater.
 In previously proposed engines using the Miller cycle, some of which are gasoline engines for automotive applications using compression ratios of 14.7 or greater, efficiency of the engine is increased by raising the compression ratio of the cylinders. Although the compression ratio is limited in a typical gasoline engine due to self-ignition of the compressed fuel and air mixture in the cylinder, a higher overall cylinder pressure is possible due to the reduced compression stroke of a Miller cycle engine.
 When the Miller cycle is used on certain stationary engine applications, such as a generator set, the engine speed may be kept constant and the engine's ability to increase or decrease its load output quickly may be advantageously improved by the methods described herein. For other stationary engine applications, such as engines driving gas compressors, the engine may be required to run at different speeds but maintain constant torque. For these engine applications, the reduced intake air compression that occurs at low engine speeds causes the Miller effect to be more pronounced but also to be limiting to the engine's effective speed range. This is especially pronounced for engines having turbochargers because of the generally inadequate exhaust gas power available at low engine speeds to drive sufficiently high intake air pressure for engine operation. By implementing the engine structures and methods disclosed herein, the engine effective speed range may be increased at a nearly constant engine torque output by shifting the engine valve phases appropriately such that the enthalpy of reaction of a fuel and air mixture in the combustion chambers of the engine may be maintained within a desired range.
 Moreover, additional benefits may be realized by use of a engine having a phaser for selectively controlling the amount of Miller effect of the engine as described herein. For example, engine applications having residual load on the engine at startup, such as engines used to drive gas compressors in the petroleum industry, may include functionality that reduces the Miller effect for engine operation at and/or immediately following engine startup. In this way, the engine is able to overcome the residual load of the driven equipment, hence enabling the engine and associated engine components to achieve engine startup. In one embodiment, an engine in accordance with the foregoing disclosure may further include a control routine that is activated when the engine is at condition for startup, for example, when the engine ignition switch is in a mode indicating that the ignition is on but the engine is not running. At such condition, the control routing may command a predetermined phase signal to the phaser of the engine that will result in a reduced Miller effect of the engine until the engine has started as indicated, for example, by the engine speed exceeding a predetermined low engine speed threshold or any other appropriate parameter.
 It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure entirely unless otherwise indicated.
 Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context.
Patent applications by Caterpillar Inc.
Patent applications in class Camshaft or cam characteristics
Patent applications in all subclasses Camshaft or cam characteristics