Patent Grant
7240651
There are many advantages to variable valve timing, including improved efficiency, power and emissions. In cam-based engines, variable valve timing is commonly achieved by varying the relative angle between the crankshaft and camshaft. Hydraulically-actuated cam phasers may be used to provide the variation in the relative angle.
In such cam phaser devices, a plurality of advance chambers and retard chambers are defined between a rotor and a stator. The rotor is coupled to the camshaft, and the stator is coupled to the crankshaft via a timing belt or chain. A hydraulic valve system is employed to control relative hydraulic pressure between the advance and retard chambers. To advance cam timing, hydraulic pressure is increased in the advance chambers relative to the retard chambers, thereby producing a relative rotation between the rotor and stator. Conversely, timing is retarded by increasing pressure in the retard chambers relative to the advance chambers. A given timing is maintained by keeping the pressures within the advance and retard chambers substantially equal.
While providing effective variable valve operation, many cam timing phasers produce significant noise. For example, when maximum retarded or maximum advanced timing is commanded, the hydraulic forces can cause the rotor to impact the stator at a significant velocity. In addition, when the rotor is close to the stator (e.g., nearly fully advanced or retarded), cam torsional effects can cause the rotor to forcefully impact the stator. This can result in noise, vibration and harshness (NVH) levels high enough to cause operator dissatisfaction.
Accordingly, the present description provides for variable cam timing phaser having a stator and a rotor. The stator has a plurality of inwardly-extending stator lobes, and the rotor has a plurality of outwardly-extending rotor lobes. The rotor is rotatably disposed within the stator so that the rotor lobes interleave with the stator lobes to form a first timing chamber and a second timing chamber between each of the stator lobes.
According to one example, the phaser further includes a valve, and where upon operation of the valve to selectively couple the second timing chambers to a hydraulic fluid supply and the first timing chambers to a hydraulic fluid sink, the rotor is caused to rotate toward a terminal position, in which at least one of the first timing chambers is at least partially sealed off from the hydraulic fluid sink, thereby producing a tendency toward pressure equalization between the first timing chambers and the second timing chambers.
According to another example, the phaser further has a plurality of hydraulic fluid orifices. One such orifice is associated with each of the first timing chambers for permitting hydraulic fluid to fill and drain from each of the first timing chambers. The orifices are positioned so that when the stator and rotor are in a first relative rotational position, each of the orifices is fluidly coupled with its associated first timing chamber. When the stator and rotor are in a second relative rotational position, at least one of the orifices is sealed off from its associated first timing chamber.
In certain settings, the exemplary embodiments described herein provide the advantages of variable cam timing, while minimizing or eliminating the undesirable NVH levels produced by prior variable cam timing systems.
Referring first to
Intake manifold 44 is shown communicating with throttle body 58 via throttle plate 62. In this particular example, throttle plate 62 is coupled to electric motor 94 so that the position of throttle plate 62 is controlled by controller 12 via electric motor 94. This configuration is commonly referred to as electronic throttle control (ETC), which is also utilized during idle speed control. In an alternative embodiment (not shown), which is well known to those skilled in the art, a bypass air passageway is arranged in parallel with throttle plate 62 to control inducted airflow during idle speed control via a throttle control valve positioned within the air passageway.
Exhaust gas sensor 76 is shown coupled to exhaust manifold 48 upstream of catalytic converter 70. Note that sensor 76 may correspond to various different sensors and sensor types, depending on the particular exhaust configuration. Sensor 76 may be any of many known sensors for providing an indication of exhaust gas air/fuel ratio such as a linear oxygen sensor, a UEGO, a two-state oxygen sensor, an EGO, a HEGO, or an HC or CO sensor. In this particular example, sensor 76 is a two-state oxygen sensor that provides signal EGO to controller 12 which converts signal EGO into two-state signal EGOS. A high voltage state of signal EGOS indicates exhaust gases are rich of stoichiometry and a low voltage state of signal EGOS indicates exhaust gases are lean of stoichiometry. Signal EGOS is used to advantage during feedback air/fuel control in a conventional manner to maintain average air/fuel at stoichiometry during the stoichiometric homogeneous mode of operation.
Conventional distributorless ignition system 88 provides ignition spark to combustion chamber 30 via spark plug 92 in response to spark advance signal SA from controller 12. Though spark ignition components are shown, engine 10 (or a portion of the cylinders thereof) may be operated in a compression ignition mode, with or without spark assist.
Controller 12 may be configured to cause combustion chamber 30 to operate in either a homogeneous air/fuel mode or a stratified air/fuel mode by controlling injection timing. In the stratified mode, controller 12 activates fuel injector 66A during the engine compression stroke so that fuel is sprayed directly into the bowl of piston 36. Stratified air/fuel layers are thereby formed. The strata closest to the spark plug contain a stoichiometric mixture or a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures. During the homogeneous spark-ignition mode, controller 12 activates fuel injector 66A during the intake stroke so that a substantially homogeneous air/fuel mixture is formed when ignition power is supplied to spark plug 92 by ignition system 88. Controller 12 controls the amount of fuel delivered by fuel injector 66A so that the homogeneous air/fuel mixture in chamber 30 can be selected to be at stoichiometry, a value rich of stoichiometry, or a value lean of stoichiometry. The stratified air/fuel mixture will always be at a value lean of stoichiometry, the exact air/fuel ratio being a function of the amount of fuel delivered to combustion chamber 30. An additional split mode of operation wherein additional fuel is injected during the exhaust stroke while operating in the stratified mode, is also possible.
Nitrogen oxide (NOx) adsorbent or trap 72 is shown positioned downstream of catalytic converter 70. NOx trap 72 is a three-way catalyst that adsorbs NOx when engine 10 is operating lean of stoichiometry. The adsorbed NOx is subsequently reacted with HC and CO and catalyzed when controller 12 causes engine 10 to operate in either a rich homogeneous mode or a near stoichiometric homogeneous mode such operation occurs during a NOx purge cycle when it is desired to purge stored NOx from NOx trap 72, or during a vapor purge cycle to recover fuel vapors from the fuel tank. For example, fuel system 164 is also shown in schematic form delivering vapors to intake manifold 44. Various fuel systems and fuel vapor purge systems may be used in accordance with the engine embodiments of the present description.
Controller 12 is shown in 2 as a conventional microcomputer, including microprocessor unit 102, input/output ports 104, an electronic storage medium for executable programs and calibration values shown as read only memory chip 106 in this particular example, random access memory 108, keep alive memory 110, and a conventional data bus. Controller 12 is shown receiving various signals from sensors coupled to engine 10, in addition to those signals previously discussed, including measurement of inducted mass air flow (MAP) from mass air flow sensor 100 coupled to throttle body 58; engine coolant temperature (ECT) from temperature sensor 112 coupled to cooling sleeve 114; profile ignition pickup signal (PIP) from Hall effect sensor 118 coupled to crankshaft 40; throttle position TP from throttle position sensor 120; absolute Manifold Pressure Signal MAP from sensor 122; indication of knock from knock sensor 182; and indication of absolute or relative ambient humidity from sensor 180. Engine speed signal RPM is generated by controller 12 from signal PIP in a conventional manner and manifold pressure signal MAP from a manifold pressure sensor provides an indication of vacuum, or pressure, in the intake manifold. During stoichiometric operation, this sensor can give an indication of engine load. Further, this sensor, along with engine speed, can provide an estimate of charge (including air) inducted into the cylinder. In a one example, sensor 118, which is also used as an engine speed sensor, produces a predetermined number of equally spaced pulses every revolution of the crankshaft.
In this particular example, temperature Tcat1 of catalytic converter 70 and temperature Tcat2 of emission control device 72 (which can be a NOx trap) are inferred from engine operation as disclosed in U.S. Pat. No. 5,414,994, the specification of which is incorporated herein by reference. In an alternate embodiment, temperature Tcat1 is provided by temperature sensor 124 and temperature Tcat2 is provided by temperature sensor 126.
Continuing with
Teeth 138, being coupled to housing 136 and camshaft 130, allow for measurement of relative cam position via cam timing sensor 150 providing signal VCT to controller 12. Teeth 1, 2, 3, and 4 are preferably used for measurement of cam timing and are equally spaced (for example, in a V-8 dual bank engine, spaced 90 degrees apart from one another) while tooth 5 is preferably used for cylinder identification, as described later herein. In addition, controller 12 sends control signals (LACT, RACT) to conventional solenoid valves (not shown) to control the flow of hydraulic fluid either into advance chamber 142, retard chamber 144, or neither.
Relative cam timing is measured using the method described in U.S. Pat. No. 5,548,995, which is incorporated herein by reference. In general terms, the time, or rotation angle between the rising edge of the PIP signal and receiving a signal from one of the plurality of teeth 138 on housing 136 gives a measure of the relative cam timing. For the particular example of a V-8 engine, with two cylinder banks and a five-toothed wheel, a measure of cam timing for a particular bank is received four times per revolution, with the extra signal used for cylinder identification.
Other examples of variable cam timing systems are disclosed in U.S. Pat. Nos. 5,386,807; 6,053,138; 6,085,708; 5,002,023; 5,107,804; 5,172,659; 5,184,578; 5,361,735 and 5,497,738, the disclosures of which are hereby incorporated by this reference, in their entireties and for all purposes.
Sensor 160 may also provide an indication of oxygen concentration in the exhaust gas via signal 162, which provides controller 12 a voltage indicative of the O2 concentration. For example, sensor 160 can be a HEGO, UEGO, EGO, or other type of exhaust gas sensor. Also note that, as described above with regard to sensor 76, sensor 160 can correspond to various different sensors.
As described above,
Also, in the example embodiments described herein, the engine is coupled to a starter motor (not shown) for starting the engine. The starter motor is powered when the driver turns a key in the ignition switch on the steering column, for example. The starter is disengaged after engine start as evidence, for example, by engine 10 reaching a predetermined speed after a predetermined time. Further, in the disclosed embodiments, an exhaust gas recirculation (EGR) system routes a desired portion of exhaust gas from exhaust manifold 48 to intake manifold 44 via an EGR valve (not shown). Alternatively, a portion of combustion gases may be retained in the combustion chambers by controlling exhaust valve timing.
The engine 10 operates in various modes, including lean operation, rich operation, and “near stoichiometric” operation. “Near stoichiometric” operation refers to oscillatory operation around the stoichiometric air fuel ratio. Typically, this oscillatory operation is governed by feedback from exhaust gas oxygen sensors. In this near stoichiometric operating mode, the engine is operated within approximately one air-fuel ratio of the stoichiometric air-fuel ratio. This oscillatory operation is typically on the order of 1 Hz, but can vary faster and slower than 1 Hz. Further, the amplitude of the oscillations are typically within 1 a/f ratio of stoichiometry, but can be greater than 1 a/f ratio under various operating conditions. Note that this oscillation does not have to be symmetrical in amplitude or time. Further note that an air-fuel bias can be included, where the bias is adjusted slightly lean, or rich, of stoichiometry (e.g., within 1 a/f ratio of stoichiometry). Also note that this bias and the lean and rich oscillations can be governed by an estimate of the amount of oxygen stored in upstream and/or downstream three way catalysts.
Feedback air-fuel ratio control may be used for providing the near stoichiometric operation. Further, feedback from exhaust gas oxygen sensors can be used for controlling air-fuel ratio during lean and during rich operation. In particular, a switching type, heated exhaust gas oxygen sensor (HEGO) can be used for stoichiometric air-fuel ratio control by controlling fuel injected (or additional air via throttle or VCT) based on feedback from the HEGO sensor and the desired air-fuel ratio. Further, a UEGO sensor (which provides a substantially linear output versus exhaust air-fuel ratio) can be used for controlling air-fuel ratio during lean, rich, and stoichiometric operation. In this case, fuel injection (or additional air via throttle or VCT) is adjusted based on a desired air-fuel ratio and the air-fuel ratio from the sensor. Further still, individual cylinder air-fuel ratio control could be used, if desired.
As indicated above, it will often be desirable to employ variable cam timing. Advantages of variable cam timing may include improved emissions, fuel economy and power density. As discussed above, one method for providing variable cam timing includes a hydraulically-actuated rotatable coupling, which may also be referred to as a cam phaser. An exemplary variable cam timing phaser will now be described with reference to
In certain example embodiments, a spool valve 302 (
As shown in
A plurality of orifices are defined in rotor 410, to enable selective fluid coupling of hydraulic supply 304 and hydraulic sink 306 (
More particularly, in a first state, spool valve 302 couples advance chambers 402 with relatively high pressure hydraulic supply 304 (e.g., of engine oil) and retard chambers 404 with relatively low pressure hydraulic sink 306. While the spool valve is in this state, the relatively higher pressure of engine oil within the advance chambers causes a relative increase in volume of the advance chambers to the retard chambers. This produces a rotation of rotor 410 relative to stator 420 (counter-clockwise in
One way of equalizing the pressure and this fixing the rotor in place relative to the stator is to place the spool valve in a second, closed state. In this state, the fluid coupling between the advance/retard chambers and the supply/sink is sealed off. Hydraulic fluid is thus sealed into the advance and retard chambers with equalized pressure on opposing sides of the rotor vanes, which in turn maintains the phaser in a fixed angular position (e.g., to maintain a desired timing relationship between the crankshaft and camshaft).
To retard cam timing, spool valve 302 is placed in a third state, in which the spool valve couples advance chambers 402 with relatively low pressure hydraulic sink 306 and retard chambers 404 with relatively high pressure hydraulic supply 304. The resulting higher pressure within the retard chambers causes the rotor to rotate in the opposite direction, thereby delaying the relative timing between the camshaft and the crankshaft. As with the advancing direction, rotation in the retarding direction continues until forces/pressures equalize on opposing sides of the vane, e.g., until the spool valve is closed or the rotor vanes abut against the stator.
Additionally, the spool valve may be dithered, or rapidly oscillated between the above-described states, as desired. For example, it may at times be desirable to dither the valve to rapidly alternate between commanding an advance and a retard of cam timing. By rapidly dithering between retard and advance, the relative camshaft/crankshaft angle can be maintained while still supplying pressurized oil to the chambers to make up for hydraulic losses in the chambers (e.g., due to small amounts of oil escaping between sealed surfaces).
The exemplary cam phasers herein may also employ other methods to effect or maintain a desired relative angle between the rotor and stator. For example, a torsion assist device, such as a spring, may be employed to bias the cam phaser toward a particular position, and/or to provide a rotating force in the absence of sufficient oil pressure. In addition, a locking pin 428 may be employed to hold or otherwise maintain the rotor and stator in a desired relative angular position. Locking pin 428 may be used, for example, to maintain a desired start-up cam timing, or to maintain a desired timing during periods of low oil pressure.
Operation of a vane-type cam phaser such as that described herein can produce can be a source of noise. In some cases, the noise can be heard or otherwise perceived by occupants of the vehicle, and thus can be an undesirable source of noise, vibration and harshness (NVH). For example, when the timing rotor goes through a maximum retard or maximum advance shift, the vanes of the rotor can hit the stator with a large force. This resulting impulsive energy can be a source of noise. In addition, the locking pins used in certain VCT systems require a small amount of backlash, (typically 0.8 degrees), to ensure the pin will unlock. The VCT rotor can flutter within this backlash due to cam torsional effects and cause noise.
Accordingly, the variable cam timing phaser of the present description may be provided with a viscous damping capability in order to reduce or eliminate the above-described noise, and thereby eliminate a source of potential operator dissatisfaction. In a first example, viscous damping is achieved via location of the fill/drain orifices of one or more of the retard or advance chambers. Referring to
Prior to the rotor reaching the terminal position shown in
The complete or partial sealing can be employed for the fully advanced state (as shown in
Typically, less than all of the chamber orifices will be positioned so as to create the described sealing as the rotor approaches its maximum advanced or retarded position. For example, referring again to
Under certain conditions, the engine oil that is trapped to provide damping can become depleted over time. For example, when the rotor is locked in a base position (such as with a locking pin), cam torsional effects on the rotor vane can pump the trapped oil out of the chamber, thereby diminishing the desired damping effect. Accordingly, for timing chambers adapted to provide the described damping, an equalization passage or communication groove 426 may be defined. As in the example of
Accordingly, the phaser typically is configured so that, in a first position (e.g., rotor is centered), the equalization passageway is closed. In a second position, such as a terminal position when damping is desired, the equalization passageway is open. Typically, where damping is employed, the chamber orifice and equalization passageway are disposed or configured so that the partial or complete sealing of the chamber orifice (which produces the damping) and opening of the equalization passageway occur at approximately the same rotational position of the rotor, as shown in
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