Patent Publication Number: US-6705259-B1

Title: 3-step cam-profile-switching roller finger follower

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Serial No. 60/432,198, filed Dec. 10, 2002. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to roller finger followers used in internal combustion engines. More particularly, the present invention relates to a roller finger follower rocker arm device that accomplishes cam profile switching in an internal combustion engine. 
     BACKGROUND OF THE INVENTION 
     Historically, the efficiency, emissions, and performance of internal combustion engines have been adversely limited by fixed valve lift profiles, i.e., valve lift profiles wherein the timing of the opening and closing of the valves is fixed relative to the angular position of the engine crankshaft and the amount of lift imparted to the valves is also fixed. By fixing the valve lift profiles of the engine, inherent compromises were made between low-speed operation (idle) and high-speed operation for peak power. Importantly, engines having fixed valve lift profiles must incorporate a throttle device to control the airflow (and output) of the engine. Throttle devices introduce large throttling losses (pumping work) and greatly reduce the efficiency of the engine, and also negatively impact emissions of oxides of nitrogen (NOx) and hydrocarbons (HC). 
     In contrast, modern internal combustion engines may utilize one of several methods and/or devices to vary the valve lift profile to, at least in part, control the flow of gas and/or air into and/or out of the engine cylinders. One such method is two-step cam-profile switching, wherein the engine valves, usually the intake valves, are actuated by a selected one of two valve lift profiles. Typically, the two valve lift profiles consist of a high-lift long-duration lift profile designed to provide high power output at high engine operating speeds, and a low-lift short-duration lift profile that is designed for high efficiency and low NOx emissions at low operating speeds. 
     Selection of, or switching between, the valve lift profiles is accomplished by a cam-profile-switching device, such as, for example, a mode-switching or two-step roller finger follower (RFF). Generally, a two-step RFF includes a body and a central roller that is selectively coupled to and decoupled from the RFF body by a shaft. The central roller is engaged by a first cam lobe of the engine camshaft. When the shaft carrying the central roller is coupled to the RFF body, engagement of the first cam lobe with the central roller causes the RFF body to pivot thereby actuating an associated engine valve according to the lift profile of the first cam lobe. When the shaft is decoupled from the RFF body, engagement of the central roller by the first cam lobe does not cause the RFF body to pivot. Rather, the shaft and central roller reciprocate relative to the RFF body thereby absorbing the motion of the first cam lobe. The body of the RFF, or a pair of outer rollers affixed to opposite sides of the RFF body, is engaged by a corresponding pair of second cam lobes. When the shaft carrying the central roller is decoupled from the RFF body, the RFF body is pivoted according to the lift profile of the pair of second cam lobes. Typically, the first cam lobe is the higher lift cam, and the pair of second cam lobes are zero-lift or low-lift cam lobes. Such a two-step RFF is more fully described in commonly-assigned U.S. Pat. No. 6,467,445, which issued Oct. 22, 2002. 
     Two-step cam profile switching systems are relatively simple and are operable over a relatively wide range of engine operating speeds. Further such systems are relatively easy to package on new and even existing engines. By operating the two-step cam-profile-switching mechanism in conjunction with a cam phaser a wide range of variation in the valve lift characteristic is obtained. Although such two-step variable valve actuation (VVA) systems achieve a relatively wide range of variation in the valve lift profile, they nonetheless represent a tradeoff between mechanical simplicity and less than continuous variation they provide relative to the mechanical complexity yet full variation that a continuously-variable VVA system provides. Two-Step VVA systems also require cam phasers having a wide range of authority and high or fast response rates in order to achieve the full benefit of these systems. 
     Therefore, what is needed in the art is a cam-profile-switching system that enables an increased and relatively wide range of variation in the valve lift profiles, and yet is relatively simple. 
     Furthermore, what is needed in the art is a cam-profile-switching system that provides an increased and relatively wide range of variation in the valve lift profiles over a relatively wide range of engine operating speeds. 
     Moreover, what is needed is a method of cam-profile-switching that achieves an increased and relatively wide range of variation in the valve lift profiles, and does so with conventional cam phasers having conventional cam phaser rates. 
     SUMMARY OF THE INVENTION 
     The present invention provides a three-step cam-profile-switching roller finger follower. 
     The present invention comprises, in one form thereof, a body with low-lift, high-lift and medium-lift body sections. Low, high and medium cam followers are carried by the low-lift, high-lift and medium-lift body sections, respectively. At least one locking assembly selectively couples together and decouples the low and high-lift body sections, and selectively couples together and decouples the low and medium-lift body sections. 
     An advantage of the present invention is that an increased range of variation in the valve lift profile is achieved with relative mechanical simplicity. 
     Another advantage of the present invention is that an increased range of variation in the valve lift profile is achieved across an increased range of engine operating speeds. 
     Yet another advantage of the present invention is that full potential of the system is achieved with conventional cam phasers having conventional cam phaser rates. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become apparent and be better understood by reference to the following description of one embodiment of the invention in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a schematic diagram of one embodiment of a three-step variable valve actuation (TSVVA) system of the present invention; 
     FIG. 2 is a plot of the un-phased intake valve lift profiles for the TSVVA of FIG. 1; 
     FIG. 3 is a plot of the exhaust and intake valve lift profiles for the TSVVA of FIG. 1 operating in the cold-start idle mode; 
     FIG. 4 is a plot of the exhaust and intake valve lift profiles for the TSVVA of FIG. 1 operating in the warm-idle mode; 
     FIG. 5 is a plot of the exhaust and intake valve lift profiles for the TSVVA of FIG. 1 operating in the light-load low-speed mode; 
     FIG. 6 is a plot of the exhaust and intake valve lift profiles for the TSVVA of FIG. 1 operating in the part-load low-to-medium speed mode; 
     FIG. 7 is a plot of the exhaust and intake valve lift profiles for the TSVVA of FIG. 1 operating in the high-load, low-to-medium speed operating mode; 
     FIG. 8 is a plot of the exhaust and intake valve lift profiles for the TSVVA of FIG. 1 operating in the high-load, medium-to-high speed operating mode; 
     FIG. 9 is a perspective view of one embodiment of a three-step switching device of the TSVVA of FIG. 1; 
     FIG. 10 is a cross-sectional view of the three-step switching device of FIG. 9 in a first mode of operation; 
     FIG. 11 is a cross-sectional view of the three-step switching device of FIG. 9 in a second mode of operation; 
     FIG. 12 is a cross-sectional view of the three-step switching device of FIG. 9 in a third mode of operation; 
     FIG. 13 is a perspective view of one embodiment of a camshaft of the TSVVA of FIG. 1; 
     FIG. 14 is a plot illustrating the ranges of brake mean effective pressure (BMEP) and engine speed that correspond to each of the above-described operating modes of TSVVA system  10 ; 
     FIG. 15 is a perspective view of one embodiment of a camshaft having low, medium and high-lift cam lobes, and having a relative offset between the low and high-lift cam lobes. 
     FIG. 16 is a plot of the intake valve lift profiles obtained with the camshaft of FIG. 15; and 
     FIG. 17 is a chart summarizing the operating modes and corresponding engine operating conditions of the TSVVA of FIG.  1 . 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, and particularly to FIG. 1, a schematic diagram of one embodiment of a three-step variable valve actuation (TSVVA) system of the present invention is shown. TSVVA system  10  includes pedal module  12 , engine control module (ECM)  14 , three-step switching devices  16  (only one shown), intake cam phaser  18 , exhaust cam phaser  20 , electronic throttle control module (ETC)  22 , and mass air flow (MAF) sensor  24 . 
     Pedal module  12  converts the position of gas pedal  26  of motor vehicle  30  to a desired load command  32 , such as, for example, a pulse-width modulated electrical signal. Desired load command  32  is indicative of the current position, direction of movement, and rate of movement of gas pedal  26 , and determines at least in part the load operating conditions of engine  40 . Pedal module  12  is electrically connected to ECM  14 , as will be described more particularly hereinafter, such that ECM  14  receives desired load command  32 . 
     ECM  14  is a conventional engine control module, including, for example, a microprocessor (not shown) interconnected with various interface circuitry, read only memory  14   a  and random access memory  14   b . ECM  14  further includes a plurality of inputs and outputs through which ECM  14  transmits and receives data to and from the devices connected thereto. More particularly, ECM  14  includes inputs  44   a - 44   g  and outputs  46   a-d , the functions and interconnections of which will be described in greater detail hereinafter. Pedal module  12  is electrically connected with pedal input  44   a , and provides desired load command  32  to ECM  14 . 
     Three-step switching devices  16 , such as, for example, three-step rocker arm assemblies or three-step roller finger followers to be described more particularly hereinafter, are switchable between a first/low-lift position, a second/medium-lift position, and a third/high-lift position. When a three-step switching device  16  is in the first/low-lift position, the associated engine valve (not shown) is actuated according to a low-lift cam of a camshaft (FIGS. 13 and 15, described more particularly hereinafter) of engine  40 . The low-lift cam of the camshaft engages the three-step switching device  16 , and pivots the three-step switching device  16  to thereby actuate the associated valve in a manner that is generally similar to valve actuation via a conventional rocker arm or roller finger follower. The low-lift cam imparts a relatively low amount of lift L LOW , such as, for example, from approximately 3.0 millimeters (mm) to approximately 5.5 mm, to the valve. The low-lift valve profile has a total duration D LOW , such as, for example, from approximately 100 crank angle degrees (CAD) to approximately 160 CAD. This low-lift valve lift profile is plotted as lift profile LLP in FIG.  2 . 
     Similarly, with three-step switching device  16  in the second or medium-lift position the associated engine valve is actuated/lifted according to a medium-lift cam of the camshaft to thereby impart a medium amount of lift L MED , such as, for example, from approximately 7.0 mm to approximately 9.0 mm, to the valve. The medium-lift valve profile has a total duration D MED , such as, for example, from approximately 180 CAD to approximately 230 CAD. This medium-lift valve lift profile is plotted as lift profile MLP in FIG.  2 . 
     Likewise, with three-step switching device  16  in the third or high-lift position, the associated engine valve is actuated/lifted according to a high-lift cam of the camshaft to thereby impart a relatively high amount of lift L HIGH , such as, for example, from approximately 11.0 mm to approximately 13.0 mm, to the valve. The high-lift valve profile has a total duration D HIGH , such as, for example, from approximately 280 crank angle degrees (CAD) to approximately 320 CAD. This high-lift valve lift profile is plotted as lift profile HLP in FIG.  2 . 
     The heights or amounts of lift for each of lifts L LOW , L MED , and L HIGH , and the total durations D LOW , D MED , D HIGH  of each of the LLP, MLP and HLP lift profiles, are fixed by the lift profile of the corresponding or actuating cam lobe as ground on the engine camshaft. 
     Generally, the method of the present invention includes selecting between six primary operating modes, i.e., cold start idle, warm idle, light-load low speed, part-load low-to-medium speed, high-load low-to-medium speed, and high-load medium-to-high speed operating modes, dependent at least in part upon engine operating conditions and parameters. More particularly, albeit still generally, the method of the present invention conjunctively controls three-step switching devices  16  and input and output cam phasers  18  and  20 , respectively, dependent at least in part upon engine operating parameters and conditions to select a particular operating mode in order to increase fuel efficiency, decrease undesirable NOx and HC emissions, increase low-to-medium speed torque, and increase performance. The method of the present invention, by selecting a particular operating mode suited to the particular engine operating conditions and parameters, achieves a level of improvement in fuel economy, an increase in torque and performance, and a reduction in emissions that approach the level of improvement achieved in an engine incorporating a more complex continuously variable valve actuating mechanism(s), and yet does so across a wider range of engine operating speeds and with reduced cost and complexity relative thereto. 
     The first primary operating mode, i.e., cold start idle, controls three-step switching devices  16  and intake and exhaust cam phasers  18  and  20 , respectively, to place TSVVA system  10 , and thereby engine  40 , into a late intake valve opening (LIVO) operating mode or strategy wherein the intake valve opening is fully retarded and the exhaust valve opening is substantially fully advanced. These are the respective default positions for the phasers. First or cold start idle operating mode is invoked for BMEPs of from approximately 200 to approximately 300 kilopascals (kPa) and at engine speeds of from approximately 1,000 to approximately 1,400 rpm. The exhaust and intake valve lift profiles, designated EV COLDSTART  and IV COLDSTART , respectively, for the cold-start idle operating mode are shown in FIG.  3 . 
     Lift profile EV COLDSTART  shows that in the cold start idle operating mode exhaust cam phaser  20  is controlled to implement an exhaust valve opening (EVO) that occurs from approximately 95 to approximately 110 crank angle degrees, and an exhaust valve closing (EVC) that occurs at approximately 375 to approximate 390 crank angle degrees. The lift of the exhaust valves is fixed at a relatively high lift, such as, for example, from approximately 10 mm to approximately 12 mm, by the associated actuating cam lobes. It should be noted, however, that actual exhaust valve lift will depend at least in part upon engine size. 
     Lift profile IV COLDSTART  shows that in the cold start idle operating mode intake cam phaser  18  is controlled to implement an intake valve opening (IVO) that occurs from approximately 380 to approximately 400 crank angle degrees, and an intake valve closing (IVC) that occurs at approximately 535 to approximately 555 crank angle degrees. The three-step switching devices  16  that actuate the intake valves are placed into the low-lift position or mode, and are thus engaged by corresponding low-lift cams of the camshaft of engine  40  which impart low lift L LOW  to the corresponding intake valves. 
     The cold start idle operating mode, as described above, achieves a reduction of from approximately 30 to approximately 50 percent in the level of undesirable hydrocarbon emissions relative to a conventional fixed-valve-timing engine in the critical first twenty seconds of engine operation when the exhaust catalytic converter is not operating. 
     The second primary operating mode, i.e., warm idle, controls three-step switching devices  16  and intake and exhaust cam phasers  18  and  20 , respectively, to place TSVVA system  10 , and thereby engine  40 , into an early intake valve closing (EIVC) operating mode wherein exhaust valve opening is fully advanced and intake valve opening is optimized for improved efficiency. Second or warm idle operating mode is invoked for BMEPs of less than approximately 100 to approximately 200 kPa and at engine speeds of from approximately 600 to approximately 800 rpm. The exhaust and intake valve lift profiles, designated EV WARM IDLE  and IV WARM IDLE , respectively, for the warm idle operating mode are shown in FIG.  4 . 
     Lift profile EV WARM IDLE  shows that in the warm idle operating mode exhaust cam phaser  18  is controlled to implement an EVO that occurs from approximately 95 to approximately 110 CAD, and an EVC that occurs at approximately 375 to approximately 390 CAD. The lift of the exhaust valves is fixed as described above. 
     Lift profile IV WARM IDLE  shows that in the warm idle operating mode intake cam phaser  20  is controlled to implement an early or advanced IVO that occurs from approximately 300 to approximately 340 CAD, and an IVC that occurs at approximately 455 to approximate 495 crank angle degrees. The three-step switching devices  16  that actuate the intake valves are placed into the low-lift position or mode, and are thus engaged by corresponding low-lift cams of the camshaft of engine  40  which impart low lift L LOW  to the corresponding intake valves. 
     The warm idle operating mode, as described above, achieves an increased level of efficiency in the operation of engine  40  by reducing pumping losses, advancing EVO to reduce residuals, and improves combustion stability at engine idle thereby potentially enabling a reduction in engine idle speed. 
     The third primary operating mode, i.e., light-load low speed (LLLS), controls three-step switching devices  16  and intake and exhaust cam phasers  18  and  20 , respectively, to place TSVVA system  10 , and thereby engine  40 , into an early intake valve closing (EIVC) operating mode wherein intake and exhaust valve openings are timed for achieving peak efficiency and minimizing NOx emissions. More particularly, the exhaust cam phaser is retarded somewhat to minimize blowdown losses, and the intake cam phaser is moderately advanced to increase the early intake valve closing (EIVC) effect, and to control internal residuals at or near the combustion dilution limit. Third or LLLS operating mode is invoked for BMEPs of less than approximately 500 kPa and for engine speeds from approximately 600 to approximately 4,500 rpm. The exhaust and intake valve lift profiles, designated EV LLLS  and IV LLLS , respectively, for the LLLS operating mode are shown in FIG.  5 . 
     Lift profile EV LLLS  shows that in the light-load low-speed operating mode exhaust cam phaser  20  is controlled to implement an EVO that occurs from approximately 100 to approximately 125 CAD, and an EVC that occurs from approximately 380 to approximately 405 CAD. The lift of the exhaust valves is fixed as described above. 
     Lift profile IV LLLS  shows that in the LLLS operating mode intake cam phaser  18  is controlled to implement an early or advanced IVO that occurs from approximately 290 to approximately 330 CAD, and an IVC that occurs at approximately 445 to approximate 485 crank angle degrees. The three-step switching devices that actuate the intake valves are placed into the low-lift position or mode, and are thus engaged by corresponding low-lift cams of the camshaft of engine  40  which impart low lift L LOW  to the corresponding intake valves. 
     The LLLS operating mode, as described above, significantly improves fuel efficiency. In this mode, the timing of the IVC is advanced and valve overlap is regulated to achieve high levels of internal residuals for operation at or near the combustion dilute limit. This increases manifold pressure and thereby reduces pumping losses (i.e., the EIVC effect). Blow down losses are also minimized by retarding the exhaust phaser from the default value. Indicated thermal efficiency is increased due to the high levels of internal residuals, which improves the ratio of specific heats of the gases, and NOx emissions are also substantially reduced due to reduced flame temperatures. 
     The fourth primary operating mode, i.e., part-load low-to-medium speed (PLLMS), controls three-step switching devices  16  and intake and exhaust cam phasers  18  and  20 , respectively, to place TSVVA system  10 , and thereby engine  40 , into an early intake valve closing (EIVC) operating mode wherein intake and exhaust valve openings are timed for increased charge dilution to reduce pumping losses, improve efficiency and reduce NOx emissions. Fourth or PLLMS operating mode is invoked for BMEPs of from approximately 500 to approximately 1,100 kPa and for engine speeds of from approximately 600 to approximately 6,000 rpm. The exhaust and intake valve lift profiles, designated EV PLLMS  and IV PLLMS , respectively, for the PLLMS operating mode are shown in FIG.  6 . 
     Lift profile EV PLLMS  shows that in the PLLMS operating mode exhaust cam phaser  20  is controlled to implement an EVO that occurs from approximately 110 to approximately 135 CAD, and an EVC that occurs from approximately 380 to approximately 415 CAD. The lift of the exhaust valves is fixed as described above. 
     Lift profile IV PLLMS  shows that in the PLLMS operating mode intake cam phaser  18  is controlled to implement an even earlier or further advanced IVO relative to the LLLS operating mode, and that occurs from approximately 270 to approximately 310 CAD, and an IVC that occurs at approximately 495 to approximate 535 crank angle degrees. The three-step switching devices that actuate the intake valves are placed into the medium-lift position or mode, and are thus engaged by corresponding medium-lift cams of the camshaft of engine  40  which impart medium-level lift L MED  to the corresponding intake valves. 
     The PLLMS operating mode, as described above, advances the timing of the IVC to reduce pumping losses, increases charge dilution to improve efficiency, and substantially reduces emissions of NOx during warm operating conditions relative to an engine having conventional valve actuation and/or relative to an engine with two-step VVA. The reduction in NOx emissions and the improvement in fuel economy that are achieved by TSVVA system  10  are approximately equal to the benefits achieved therein by a continuously variable valve actuation mechanism, yet TSVVA  10  is operable over a substantially wider range of engine operating speeds than are conventional continuously variable valve actuation mechanisms. 
     The fifth primary operating mode, i.e., high-load low-to-medium speed (HLLMS), controls three-step switching devices  16  and intake and exhaust cam phasers  18  and  20 , respectively, to place TSVVA system  10 , and thereby engine  40 , into an operating mode wherein the lift, timing and duration of the intake and exhaust valves are optimized to achieve high volumetric efficiency for a low-to-medium engine operating speed range. Fifth or HLLMS operating mode is invoked, for example, for BMEPs of from approximately 900 to approximately 1,100 kPa and for engine speeds of from approximately 600 to approximately 2,500 rpm. The exhaust and intake valve lift profiles, designated EV HLLMS  and IV HLLMS , respectively, for the HLLMS operating mode are shown in FIG.  7 . 
     Lift profile EV HLLMS  shows that in the HLLMS operating mode exhaust cam phaser  20  is controlled to implement an EVO that occurs from approximately 100 to approximately 120 CAD, and an EVC that occurs from approximately 380 to approximately 405 CAD. The EVO and EVC are increasingly advanced as engine speed increases. The lift of the exhaust valves is fixed as described above. 
     Lift profile IV HLLMS  shows that in the HLLMS operating mode intake cam phaser  18  is controlled to implement an IVO that occurs from approximately 320 to approximately 360 CAD, and an IVC that occurs at approximately 545 to approximate 585 crank angle degrees. The IVO and IVC are increasingly delayed relative to crank angle as engine operating speed increases. The three-step switching devices that actuate the intake valves are placed into the medium-lift position or mode, and are thus engaged by corresponding medium-lift cams of the camshaft of engine  40  which impart a medium-level lift L MED  to the corresponding intake valves. 
     The HLLMS operating mode, as described above, provides an increase in volumetric efficiency of approximately ten percent relative to a conventional engine. The magnitude of this improvement depends on engine application. 
     The sixth primary operating mode, i.e., high-load medium-to-high speed (HLMHS), controls three-step switching devices  16  and intake and exhaust cam phasers  18  and  20 , respectively, to place TSVVA system  10 , and thereby engine  40 , into an operating mode wherein the lift, timing and duration of the intake and exhaust valves are optimized to achieve high volumetric efficiency for medium-to-high engine operating speeds, such as, for example, from approximately 2,000 to approximately 8,000 rpm and greater. Sixth or HLMHS operating mode is invoked for BMEPs of from approximately 1,000 to approximately 1,200 kPa and for engine speeds of greater than approximately 2,500 rpm. The exhaust and intake valve lift profiles, designated EV HLMHS  and IV HLMHS , respectively, for the HLMHS operating mode are shown in FIG.  8 . 
     Lift profile EV HLMHS  shows that in the HLMHS operating mode exhaust cam phaser  20  is controlled to implement an EVO that occurs from approximately 95 to approximately 110 CAD, and an EVC that occurs from approximately 375 to approximately 390 CAD. The EVO and EVC are generally advanced as much as possible over this speed range to maximize volumetric efficiency. The lift of the exhaust valves is fixed as described above. 
     Lift profile IV HLMHS  shows that in the HLMHS operating mode intake cam phaser  18  is controlled to implement an IVO that occurs from approximately 300 to approximately 360 CAD, and an IVC that occurs at approximately 580 to approximately 640 crank angle degrees. The IVO and IVC are increasingly delayed relative to crank angle as engine operating speed increases. The three-step switching devices that actuate the intake valves are placed into the high-lift position or mode, and are thus engaged by corresponding high-lift cams of the camshaft of engine  40  which impart a high-level lift L HIGH  to the corresponding intake valves. 
     The HLMHS operating mode, as described above, provides increases in torque and peak power relative to a conventional engine due to improved optimization of valve lift and duration over the speed range. If peak engine speed is increased, peak power can also be increased. For example, at an engine operating speed of 8,000 rpm a peak power improvement of engine  40  of approximately 10 to 20 percent is achieved. 
     It should be noted that the intake and exhaust valve lift profiles for each of the above-described operating modes are substantially continuously adjustable by the associated cam phasers within and over the respective and indicated ranges for IVO, IVC, EVO and EVC. This adjustability is indicated by the arrows associated with each of the valve lift profiles in FIGS. 3-8. 
     It should further be noted that the low, medium and high-lift cams, in addition to having different maximum lift amounts, are generally configured with lift profiles having different lift durations configured to, for example, increase fuel economy and/or improve torque and/or increase peak power. More particularly, as shown in FIG.  2  and as described above, the HLP lift profile has a relatively high maximum lift L HIGH , as described above, that occurs relatively late (i.e., at a relatively high crank angle), whereas the MLP lift profile has a medium value of maximum lift L MED  that occurs relatively early (i.e., at a relatively low crank angle). Similarly, the LLP profile has a relatively low maximum lift L LOW  that occurs relatively early (i.e., at a relatively low crank angle). Intake and exhaust cam phasers  18  and  20 , respectively, phase the crank angle at which these lift profiles occur dependent at least in part upon engine operating conditions and parameters. 
     Referring again to FIG. 1, one actuating device  48 , such as, for example, an electronically controlled fluid control valve, is associated with all three-step switching devices  16  in a cylinder head. If a second cylinder head exists on the engine, a second actuating device is used. Actuating device  48 , as is more particularly described hereinafter, controls the flow of a pressurized fluid to three-step switching device  16  thereby switching the device between the above-described low, medium and high-lift operating positions. As stated above, only one three-step switching device and only one actuating device are shown for the sake of clarity. Each three-step actuating device  48  is electrically interconnected with a respective switching output  46   b  (only one shown) of ECM  14 . However, it is to be understood that alternate switching methods and modes may be implemented, such as, for example, one switching or control device to switch multiple three-step devices between low-lift mode to medium-lift mode, and another switching or control device to switch multiple three-step devices between medium-lift mode to high-lift mode. 
     Intake cam phaser  18  is a conventional cam phaser as described in commonly-assigned U.S. Pat. No. 6,276,321, the disclosure of which is incorporated herein by reference. Intake cam phaser  18  enables phasing of the intake cam relative to the engine crankshaft, i.e., the angular position of the camshaft relative to the crankshaft (not shown) of engine  40 . Intake cam phaser  18  thus enables the opening and/or closing of the intake valves of engine  40  to be phased relative to the rotational or angular position of the crank, thereby phasing the opening and/or closing of the valves relative to piston position. Preferably, intake cam phaser  18  has an average or moderate range of authority. Associated with intake cam phaser  18  is intake cam phaser actuating device  50  and intake cam position sensor  52 . 
     Intake phaser actuating device  50 , such as, for example, a fluid control valve or electric motor, is associated with and actuates intake cam phaser  18 . Intake phaser actuating device  50  is electrically interconnected with intake phaser control output  46   c  of ECM  14 . Intake cam position sensor  52 , such as, for example, a conventional electrical, optical or electromechanical cam position sensor, is associated with intake cam phaser  18 . Intake cam position sensor  52  is electrically connected to intake cam position input  44   d  of ECM  14 . 
     Similarly, exhaust cam phaser  20  is a conventional cam phaser that enables the phasing of the opening and/or closing of the exhaust valves of engine  40  relative to the rotational or angular position of the crankshaft. Preferably, exhaust cam phaser  20  also has an average or moderate range of authority. Associated with exhaust cam phaser  20  is exhaust cam phaser actuating device  53  and exhaust cam position sensor  54 . 
     Exhaust phaser actuating device  53  is substantially identical to intake phaser actuating device  50  as described above, and is electrically interconnected with exhaust phaser control output  46   d  of ECM  14 . Exhaust cam position sensor  54  is substantially identical to electrically intake cam position sensor  52  described above, and is connected to exhaust cam position input  44   g  of ECM  14 . 
     Electronic throttle control module (ETC)  22  is a conventional electronic throttle control module, and includes ETC actuating device  56  and throttle position sensor (TPS)  57 . ETC  22  further includes a main throttle valve  58  that controls the flow of air into engine  40 . ETC actuating device  56 , such as, for example, a stepper motor, is electrically connected to throttle control output  46   a  of ECM  14 , and is operable to rotate main throttle valve  58  to a desired position. TPS sensor  57  is a conventional throttle position sensor, which senses the position of throttle valve  58  and is electronically connected throttle position input  44   c  of ECM  14 . 
     Mass air flow (MAF) sensor  24  is a conventional mass airflow sensor that measures the amount of air flowing through main throttle valve  58 . MAF sensor  24  is electrically connected to MAF sensor input  44   b  of ECM  14 . 
     Referring now to FIGS. 9-12, one embodiment of a three-step switching device for use in TSVVA  10  is shown. Generally, three step switching device  60  is configured as a rocker arm having three cam followers, each of which are associated with a corresponding one of three rocker arm sections that are selectively and pivotally coupled together and decoupled from each other to thereby switch switching device  60  into and between the low, medium and high-lift operating positions. Three step switching device  60  includes body  62 , cam followers  66 ,  68  and  70 , high-lift mode locking assembly  72  (FIG.  10 ), medium-lift mode locking assembly  74  (FIG. 10) and shaft  76 . 
     Body  62  includes three elongate arm portions, i.e., main or central arm  80 , high-lift arm  82  and medium-lift arm  84 . Each of main arm  80 , high-lift arm  82  and medium-lift arm  84  are pivotally disposed upon shaft  76 . More particularly, each of arms  80 ,  82  and  84  include respective central bores (not referenced) within which shaft  76  is received and through which shaft  76  extends. Arms  80 ,  82  and  84  are disposed on shaft  76  such that medium-lift arm  84  is disposed adjacent one side of main arm  80  and high-lift arm  82  is disposed adjacent the other side of main arm  80 . Arms  80 ,  82  and  84  are configured for pivotal movement relative to and/or about central axis A of shaft  76 . Arms  80 ,  82  and  84  are retained in a predetermined axial position on shaft  76 , and axially adjacent each other, by a retaining means  86 , such as, for example, a retaining clip or C-clip, that snaps onto and over body  62  and engages shaft  76  in such a manner as to preclude axial movement of body  62 . 
     Main or central arm  80  is an elongate arm member including a first, generally T-shaped end  88  disposed on one side of shaft  76  and configured for engaging one or more valve stems  90  (shown in FIG. 9 only) of one or more engine valves. A second end (not referenced) of main arm  80  is disposed on the opposite side of shaft  76  from T-shaped first end  88 . Main arm  80  defines first orifice  92  (FIG. 10) proximate first or T-shaped end  88 . First orifice  92  extends from the outer surface of main arm  80  that is adjacent high-lift arm  82  in a direction toward medium-lift arm  84 . Main arm  80  also defines at the second end thereof, i.e., the end opposite end  88 , a second orifice  94  (FIG. 10) that extends from the outer surface of main arm  80  that is adjacent medium-lift arm  84  in a direction toward high-lift arm  82 . 
     High-lift arm  82  is an elongate arm member having ends (not referenced) that are disposed on opposite sides of shaft  76 . High-lift arm  82  defines orifice  102  (FIG.  11 ), which extends from an outer surface (not referenced) of high-lift arm  82  that is adjacent to main arm  80 , and in a direction away from main arm  80 . Orifice  102  of high-lift arm  82  is substantially coaxial relative to first orifice  92  of main arm  80  when arms  80  and  82  are in the same angular orientation relative to shaft  76 . 
     Medium-lift arm  84  is an elongate arm member having ends (not referenced) that are disposed on opposite sides of shaft  76 . Medium-lift arm  84  defines orifice  104  (FIG.  12 ), which extends from an outer surface (not referenced) of medium-lift arm  84  that is adjacent to main arm  80 , and in a direction away from main arm  80 . Orifice  104  of medium-lift arm  84  is substantially coaxial relative to second orifice  94  of main arm  80  when arms  80  and  84  are in the same angular orientation relative to shaft  76 . 
     High-lift mode locking assembly  72 , in general, couples together and decouples high-lift arm  82  and main arm  80 . High-lift mode locking assembly  72  includes main pin  110 , high-lift pin  112  and biasing means  114 . Main pin  110  is disposed substantially entirely within first orifice  92  in main arm  80 . High-lift pin  112  is disposed at least partially within orifice  102  in high-lift arm  82 . Biasing means  114  is disposed within orifice  92  of main arm  80 , between and in engagement with main pin  110  and the inside end surface (not referenced) of first orifice  92  that is perpendicular to central axis A of shaft  76 . 
     Medium-lift mode locking assembly  74 , in general, couples together and decouples medium-lift arm  84  and main arm  80 . Medium-lift mode locking assembly  74  includes main pin  120 , medium-lift pin  122  and biasing means  124 . Main pin  120  is disposed substantially entirely within second orifice  94  in main arm  80 . Medium-lift pin  122  is disposed at least partially within orifice  104  in medium-lift arm  84 . Biasing means  124  is disposed within orifice  94  of main arm  80 , between and in engagement with main pin  120  and the inside end surface (not referenced) of second orifice  94  that is perpendicular to central axis A of shaft  76 . 
     Shaft  76  is an elongate shaft member upon which one or more three-step switching devices are pivotally disposed. Shaft hydraulic channels  132  and  134  (shown in FIG. 10 only) are defined by and within shaft  76 , and are at one end in fluid communication with a source of pressurized fluid (not shown), such as, for example, hydraulic fluid or engine oil. Corresponding high and medium-lift arm hydraulic channels  142  and  144  (shown in FIG. 10 only), respectively, are defined by high and medium-lift arms  82  and  84 , respectively, and are fluidly connected at one end to the ends of shaft channels  132  and  134 , respectively, that are opposite the source of pressurized fluid. Arm channels  142  and  144  are in fluid communication with orifices  102  and  104 , respectively. Orifice  102  of high-lift arm  82  is in fluid communication the source of pressurized fluid via arm channel  142  and shaft channel  132 . Similarly, orifice  104  of medium-lift arm  84  is in fluid communication the source of pressurized fluid via channel  144  and shaft channel  134 . As the arms undergo pivotal movement, fluid communication of arm channels  142  and  144  with corresponding shaft channels  132  and  134  is maintained by at least one pair of the channels, for example, the arm channels  142  and  144 , having elongated or flared ends (not shown) at the interface thereof with the corresponding channels. A fluid control device, such as, for example, an electrically actuated fluid control valve, controls the flow of fluid into and through shaft hydraulic channels  132  and  134  and thereby through arm hydraulic channels  142  and  144 . 
     Cam followers  66 ,  68  and  70 , such as, for example, rollers with bearings, are carried by arms  80 ,  82  and  84 , respectively. Generally, each of cam followers  66 ,  68  and  70  engage a corresponding cam of three-step camshaft  150  of TSVVA  10 . More particularly, as shown in FIG. 13, camshaft  150  includes low-lift cam  160 , high-lift cam  162  and medium-lift cam  164 . Cam follower  66  of main arm  80  engages low-lift cam  160 , cam follower  68  of high-lift arm  82  engages high-lift cam  162 , and cam follower  70  of medium-lift arm  84  engages medium-lift cam  164  of camshaft  150 . 
     In use, three-step switching device  16  is placed into a default position or mode of operation by biasing means  114  biasing pins  110  and  112  toward and into a default position wherein main pin  110  is disposed substantially entirely within orifice  92  and high-lift pin  112  is disposed substantially entirely within orifice  102  thereby decoupling main arm  80  from high-lift arm  82 , and by biasing means  124  biasing pins  120  and  122  toward and into a default position wherein main pin  120  is disposed substantially entirely within second orifice  94  and medium-lift pin  122  is disposed substantially entirely within orifice  104  thereby decoupling main arm  80  from medium-lift arm  84 . Each of cam followers  66 ,  68  and  70  follow their associated/corresponding cams  160 ,  162  and  164 , respectively, and arms  80 ,  82  and  84  are thereby pivoted relative to shaft  76 . Thus, since main/low-lift arm  80  is the only arm that engages and/or actuates the associated engine valve or valves, the valves of engine  40  are actuated according to the lift profile of low-lift cam  160 , i.e., LLP (FIG.  2 ). 
     It should be particularly noted that orifices  102  and  104  must be at a relatively low pressure, hereinafter referred to as depressurized, that does not overcome the force of corresponding biasing means  114  and  124 , respectively, in order for three-step switching device  16  to be placed into and/or remain in the default operating mode. It should also be noted that the default operating mode of three-step switching device  16  corresponds to the LLP and a low-lift mode of operation. 
     Three-step switching device  16  is placed into a high-lift mode of operation by supplying pressurized fluid into orifice  102  via arm hydraulic channel  142  and shaft channel  132  and with low-lift mode locking assembly occupying the default position (i.e., orifice  104  being depressurized). The pressurized fluid displaces high-lift mode locking assembly  72  from its default position and into a high-lift mode or position. More particularly, the pressurized fluid supplied to orifice  102  overcomes the force of biasing means  114  and displaces high-lift pin  112  in a direction toward main arm  80  and partially into first orifice  92 . The disposition of high-lift pin  112  partially within each of first orifice  92  and orifice  102  in high-lift arm  82  pivotally couples together main arm  80  and high-lift arm  82 . Thus, as low and high-lift cam followers  66  and  68 , respectively, are engaged by their corresponding low and high-lift cams  160  and  162 , respectively, high-lift arm  82  is pivoted relative to shaft  76  according to the lift profile of high-lift cam  162 , i.e., HLP (FIG.  2 ). Since high-lift arm  82  is coupled to main arm  80  by high-lift locking assembly  72 , main arm  80  is also pivoted according to the lift profile of high-lift cam  162  thereby actuating the valves of engine  40  according to the lift profile HLP (FIG.  2 ). 
     Three-step switching device  16  is placed into a medium-lift mode of operation through the supplying of pressurized fluid into orifice  104  via arm hydraulic channel  144  and shaft channel  134  and with high-lift mode locking assembly occupying the default position (i.e., orifice  102  being depressurized). The pressurized fluid displaces medium-lift mode locking assembly  74  from its default position and into a medium-lift mode or position. More particularly, the pressurized fluid supplied to orifice  104  overcomes the force of biasing means  124  and displaces medium-lift pin  122  in a direction toward main arm  80  and partially into second orifice  94 . The disposition of medium-lift  122  pin partially within each of second orifice  94  and orifice  104  in medium-lift arm  84  pivotally couples together main arm  80  and medium-lift arm  84 . Thus, as low and medium-lift cam followers  66  and  70 , respectively, are engaged by their corresponding low and medium-lift cams  160  and  164 , respectively, medium-lift arm  84  is pivoted relative to shaft  76  according to the lift profile of medium-lift cam  164 , i.e., MLP (FIG.  2 ). Since medium-lift arm  84  is coupled to main arm  80  by medium-lift locking assembly  74 , main arm  80  is also pivoted according to the lift profile of medium-lift cam  164  thereby actuating the valves of engine  40  according to the lift profile LLP (FIG.  2 ). 
     FIG. 14 shows the ranges of engine BMEP and engine speed that correspond to each of the above-described operating modes of TSVVA  10 . More particularly, FIG. 14 shows that for ranges of engine BMEP and engine speed that fall within the light-load low-speed region, i.e., the region of the curve that corresponds to BMEPs of less than approximately 500 kPa at engine speeds of less than approximately 4,500 rpm, TSVVA  10  operates with three-step switching devices  16  in the low-lift position or mode of operation and within one of the cold-start idle, warm idle and light-load low speed operating modes as described above. 
     For ranges of BMEP and engine speed that fall within the part-load low-to-medium speed line, i.e., the region of the curve corresponding to BMEPs of equal to or greater than approximately 500 kPa up to engine speeds of approximately 4,000 rpm and BMEPs of less than approximately 1,000 kPa at an engine speed of approximately 6,000 rpm, TSVVA  10  operates with three-step switching devices  16  in the medium-lift position or mode of operation and within the part-load low-to-medium speed (PLLMS) operating mode as described above. 
     Similarly, for ranges of BMEP and engine speed that fall within the high-load low-to-medium speed region, i.e., the region of the curve corresponding to a BMEP of approximately 1,100 kPa at 2.500 rpm and lower, TSVVA  10  operates with three-step switching devices  16  in the medium-lift position or mode of operation and within the highl-load low-to-medium speed (HLLMS) operating mode as described above. 
     Lastly, for ranges of BMEP and engine speed that fall within the high-load medium-to-high speed region, i.e., the region of the curve corresponding to a BMEP of approximately 1,100 kPa at 2,500 rpm and higher, TSVVA  10  operates with three-step switching devices  16  in the high-lift position or mode of operation and within the high-load medium-to-high speed (HLMHS) operating mode as described above. 
     It should be noted that the TSVVA system  10  enables the use of conventional intake and exhaust cam phasers with moderate ranges of authority, such as, for example, 70 CAD for intake and 50 CAD for exhaust, and having conventional phaser rates. This is enabled by using IVO-offsets for the LLC and MLC relative to the HLC. The IVO-offsets define the offset of the opening points of each cam lobe as ground on the camshaft, and thereby the opening points of the associated engine valves. More particularly, and as best shown in FIG. 15, low-lift cam  160  is retarded by IVO LOW-OFFSET , such as, for example, from approximately 25 to approximately 50 CAD (or 12.5 to 25 cam degrees) relative to high-lift cam  162 . In the embodiment shown, medium-lift cam  164  is offset by IVO MEDIUM-OFFSET , such as, for example, approximately zero degrees, relative to high-lift cam  162 . However, it is to be understood that different values of offset for the low and medium-lift cams relative to the high-lift cam can be beneficially applied to the TSVVA system of the present invention. 
     FIG. 16 illustrates the IVO-offset achieved by TSVVA system  10  using the above-described camshaft  150  wherein low-lift cam  160  is offset relative to high-lift cam  162 . By offsetting low-lift cam  160  relative to high-lift cam  162 , the IVO of the LLP is retarded relative to the IVO of the MLP and HLP. Thus, inherent IVO timing changes are accomplished by cam profile switching. This timing change is independent of timing changes provided by the intake cam phaser, and occurs substantially instantaneously during engine transients for which switching of modes occur. This inherent timing change substantially reduces demand on the intake phaser and the intake cam phaser rate. IVO offset of the LLC relative to the HLC also reduces the required authority of the phaser, since the additional phaser retard needed for the cold start engine operating mode is achieved at least in part by the offset of the LLC relative to the HLC. Generally, it is to be understood that IVO-offsets of the LLC and MLC relative to the HLC can be advantageously applied in the TSVVA system of the present invention. 
     FIG. 17 summarizes the operating modes and corresponding engine operating parameters and conditions of the TSVVA system of the present invention. 
     In the embodiment shown, the three step switching device  16  of the present invention includes cam followers  66 ,  68  and  70  that are configured as rollers with bearings. However, it is to be understood that the present invention can be alternately configured, such as, for example, with slider-pad type cam followers or other types of cam followers. 
     In the embodiment shown, the three step switching device  16  of the present invention has a default mode that corresponds to a low-lift mode of operation wherein the associated intake valve(s) are actuated according to a low-lift profile. However, it is to be understood that the present invention can be alternately configured, such as, for example, with the default mode corresponding to a medium-lift or high-lift mode of operation. 
     In the embodiment shown, the locking assemblies are biased by a biasing means, shown as a spring, into a default position to thereby place the three-step switching device into a default mode of operation. However, it is to be understood that the three-step switching device of the present invention can be alternately configured, such as, for example, with a different type of biasing means, such as, for example, a pressurized fluid, biasing the locking assemblies into a default position to thereby place the three-step switching device into a default mode of operation. 
     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the present invention using the general principles disclosed herein. Further, this application is intended to cover such departures from the present disclosure as come within the known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.