Patent Application
20090308340
This disclosure relates to cam-driven, variable valve timing, valvetrains for internal combustion engines.
Variable valve actuation timing seeks to adjustably control valve lift and timing during cam rotation in an internal combustion engine. At low engine speeds, it may be desirable to reduce cam lift and/or delay valve-open timing to minimize the amount of air drawn into the cylinder to increase efficiency and improve torque.
A valve operating system for an internal combustion engine is provided, including a cam and a roller follower configured to follow the oscillatory motion of the cam. An input piston is spring-biased to follow motion of the roller follower. A hydraulic chamber filled with fluid selectively transfers motion from the input piston to an output piston operatively engaged with a poppet valve for opening the poppet valve. A variable-bleed valve is in fluid communication with the hydraulic chamber and an outlet channel. The variable-bleed valve is configured to selectively vary a bleed rate between the hydraulic chamber and outlet channel, which allows volumetric and pressure control over the fluid in the hydraulic chamber. The input piston, output piston and bleed valve are all in fluid communication with the hydraulic chamber, such that displacement of the input piston into the hydraulic chamber causes movement of the output piston, depending on selection of the bleed rate.
A pushrod may be interposed between the roller follower and the input piston, such that motion of the cam is transferred through the pushrod to the input piston. A valve spring may bias the output piston toward a closed position, and a hydraulic damping shoulder may be interposed between the output piston and hydraulic chamber and configured to restrict the closing velocity of the output piston. Additionally, the valve operating system may include a pressure accumulator in fluid communication with the outlet channel, and may be further characterized by an absence of a rocker arm.
In another embodiment, an auxiliary piston is in fluid communication with the hydraulic chamber and movable with respect to an adjustable stop member. The auxiliary piston replaces the variable-bleed orifice in the lost-motion hydraulic linkage. The input piston, output piston and auxiliary piston are in fluid communication with the fluid in the hydraulic chamber, such that displacement of the input piston into the hydraulic chamber causes movement of the output piston and/or auxiliary piston, depending on the adjust position of the adjustable stop member, while the volume of fluid in the control chamber remains substantially constant.
The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes and other embodiments for carrying out the invention when taken in connection with the accompanying drawings.
Referring to the drawings, wherein like reference numbers correspond to like or similar components throughout the several figures, there is shown in
A cam 12 drives a follower 14 creating an oscillatory motion. The follower 14 is an end-pivoted roller finger follower. A pivot end 16 (on the right, as viewed in
Lash compensation closes any gaps in fixed cam-follower-to-valve connections. These gaps are designed expansion joints and are normally closed by thermal expansion as the engine heats up. Without some form of lash compensation, there may be gaps between the moving elements of the valvetrain, especially while the engine is cold, which may result in increased noise or wear. Furthermore, valvetrain component wear may cause gaps over time. A hydraulic lash adjuster is a mechanism which closes gaps during both cold and hot operating conditions by using pressurized fluid to move the lash compensator into contact with the follower element, regardless of engine and valvetrain temperature.
The mechanical lash adjuster 18 is a support element that provides mechanical lash compensation. An initial adjustment, usually at the time of assembly of the valvetrain 10, is made to contact the mechanical lash adjuster 18 to the pivot end 16. Due to the hydraulic characteristics (described in detail below) of the valvetrain 10, it does not require a separate hydraulic lash-compensating support element.
At the center of the follower 14 is a roller 20, through which the oscillatory motion of cam 12 is transferred to follower 14. An alternative embodiment (not shown) the cam 14 would act directly on a flat surface on top of (as viewed in
Roller 20 maintains contact with cam 12 without relative motion between the contact surfaces. In the valvetrain 10 shown, therefore, the use of the roller 20 on the finger follower 14 yields lower valvetrain friction losses due to the elimination of cam-to-follower contact friction. In other embodiments of a cam-driven hydraulic lost-motion valvetrain, the cam 12 acts directly on a flat surface that is integral with the input piston 24, such that there is no follower 14.
An input end 22 is on the opposing end of the follower 14 from the pivot end 16. The input end 22 has a defined radius of curvature and contacts the tip of an input piston 24. As cam 12 rotates, the input piston 24 is driven in a reciprocating linear motion by the follower 14 against a biasing spring 26, which maintains contact between the input piston 24 and input end 22 (and, indirectly, the cam 12). With each rotation or event of the cam 12, the roller 20 moves from riding on the base circle portion, during which no axial displacement is transferred to the input piston 24, to riding on the lift profile portion, during which the follower 14 causes the input piston 24 to rise and fall.
The input piston 24 acts through a hydraulic linkage 28 (described in detail below) on an output piston 30. Depending upon the settings of the hydraulic linkage 28, the output piston 30 is driven in a reciprocating linear motion by the hydraulic linkage 28.
As shown in
Although the cylinder valve 32 in this embodiment is shown to be substantially aligned with the input piston 24, the two do not have to be aligned. For example, the axis of cylinder valve 32 could be perpendicular to the axis of the input piston 24. Furthermore, the centerlines of input piston 24 and cylinder valve 32 do not have to be on the same plane. One input piston 24 could, for example, actuate two cylinder valves 32 of the same cylinder, simultaneously. Those having ordinary skill in the art will recognize that the tip geometry of the input piston 24 may be comparable to the tip of a cylinder valve used in conventional engines not having a hydraulic linkage.
The input and output pistons 24 and 30 hydraulically communicate with each other through the hydraulic linkage 28. A hydraulic fluid 42 having low compressibility, such as engine oil, fills a high-pressure chamber 44, which is in fluid communication with both the input piston 24 and output piston 30. The chamber 44 is located within a housing 45. Per rotation of cam 12, a fixed volume of fluid 42 is displaced, every cycle, by the input piston 24.
Displacement by input piston 24 results in hydraulic pressure generation in the chamber 44. If the chamber 44 is otherwise closed—such that the volume of fluid 42 within the chamber 44 remains essentially constant without substantial leakage—and the hydraulic fluid 42 is substantially incompressible, the output piston 30 will be displaced by an equal volume. If the input and output pistons 24 and 30 have substantially equal diameter (a hydraulic diameter ratio of 1:1) the axial displacement of the input piston 24 results in a substantially equal axial displacement of the output piston 30, thereby displacing the cylinder valve 32 by the same distance.
A checked supply line 46 connects the chamber 44 to a hydraulic pressure source, such as the oil pump (not shown), and permits flow into the hydraulic linkage 28 when the pressure inside the hydraulic linkage 28 falls below the supply pressure. A check valve 48 located on the supply line 46 prevents backflow towards the pressure source when the pressure inside of hydraulic linkage 28 is above the supply pressure.
Also shown in
The flow control valve 52 shown in
Those having ordinary skill in the art will recognize that neither the variable-bleed orifice 50 nor the drain 56 have to be located in the either the block 35 or housing 45. The variable-bleed orifice 50 need only be in fluid communication with chamber 44 and the drain 56. Furthermore, the drain 56 may feed the pressurized fluid 42 escaping chamber 44 into an accumulator which supplies downstream components with pressurized fluid instead of repressurizing fluid for those components directly with a pump. Furthermore, the accumulator could also return the bled volume of fluid 42 back to the hydraulic chamber 44 during the base-circle event (during which the chamber 44 is repressurized).
The proportion of the displaced volume of fluid 42 converted into motion of the output piston 30 is dependent on the bleed rate through the variable-bleed orifice 50 to the drain port. The principle of volume continuity requires that the sum of the bled volume and the volume swept by the output piston 30 equals the volume displaced by the input piston 24—neglecting any leakage and fluid compressibility effects.
In one extreme, where the flow control valve 52 is set to provide the largest variable-bleed orifice 50 opening, the displaced input volume could equal the bled volume. At this operating condition, all motion of the input piston 24 is lost in the hydraulic linkage 28, and the output piston 30 and the cylinder valve 32 remain stationary. This total lost-motion condition may be used to completely deactivate the cylinder, or may be used to limit intake or exhaust flow by deactivating one of multiple intake or exhaust valves.
In the other extreme, the variable-bleed orifice 50 could completely seal the drain port 54, enabling the transfer of the entire input motion through the hydraulic linkage 28 to the output piston 30 and the cylinder valve 32. This zero lost-motion condition directly transfers lift of the cam 12 to the cylinder valve 32 as if the hydraulic linkage 28 were a mechanical linkage. For any intermediate setting of the variable-bleed orifice 50 by the flow control valve 52, displacement is proportionally transferred from the input piston 28 to the output piston 30, and different cylinder valve 32 lift profiles are achieved, from no lift (valve deactivation due to total lost-motion) to full lift (relative to the lift profile of the cam 12).
Those having ordinary skill in the art will recognize that even where the variable-bleed orifice 50 has completely closed the drain port 54 for the zero lost-motion condition, the linear displacement of the input and output pistons 24 and 30 may not be exactly equal. Leakage of fluid 42 from the chamber 44 will reduce the displaced volume transferred to outlet piston 30, and compression of the (non-ideal) fluid 42 may also reduce the displacement of outlet piston 30.
Those having ordinary skill in the art will recognize that—even in a perfectly sealed chamber 44 filled with an incompressible fluid 42—displacement of the output piston 30 is dependent upon the hydraulic diameter ratio of the input and output pistons 24 and 30. Matching linear displacement of the valve 32 (through the output piston 30) to the axial displacement of the cam 12 (through linear displacement of the input piston 24), dictates a 1:1 ratio of hydraulic diameters. The linear displacement ratio of the input piston 24 over the output piston 30 is equal to the ratio of the area of output piston 30 over the area of input piston 24 (if there is no lost motion).
Where the input and output pistons 24 and 30 are not equal in diameter, the linear displacement ratio is inversely related to the hydraulic diameter ratio. For example, where the hydraulic diameter ratio (input:output diameter) is 2:1, the output piston 30 will have four times the linear displacement of the input piston 24 (the input:output linear displacement ratio will be 1:4). A configuration having a smaller output piston 30 allows a relatively smaller cam 12, because displacement of the input piston 24 is multiplied through the hydraulic linkage 28 to result in larger displacement of the cylinder valve 32.
In operation, the volume displaced by the input piston 24 equates approximately to the summation of the volumes displaced by the output piston 30 and the volume lost through the variable-bleed orifice 50. A small amount of volumetric loss results from fluid compressibility and leakage through piston-to-wall clearances and other sealing surfaces. The input motion from the cam 12 has a fixed displacement-time characteristic determined from the cam profile. However, the output motion of the cylinder valve 32 can be varied by controlling the variable-bleed orifice 50.
Coming off of the base circle of the cam 12, at the start of the down stroke (into the chamber 44) of the input piston 24, the fluid 42 in chamber 44 is pressurized, and the check valve 48 is sealed. The rate of pressure increase depends on the bleed rate through variable-bleed orifice to the drain 56 because it takes time for the fluid 42 to flow out of the chamber 44.
The pressure gradually rises to a level just enough to overcome the preload of the valve biasing spring 36, and the cylinder valve 32 lifts off. Subsequently, pressure in chamber 44 continues to increase while input piston 24 continues to be forced into chamber 44, further compressing the spring 36 and lifting the cylinder valve 32. Simultaneous with the valve motion, the bleed off of fluid 42 to the drain 56 continues.
As long as the pressure in chamber 44 applies a force to output piston 30 greater than the spring force of valve biasing spring 36, the cylinder valve 32 will continue to lift. Those having ordinary skill in the art will recognize that the spring force of valve biasing spring 36 increases with displacement of spring, and that the force on output piston 30 is equal to the pressure in chamber 44 divided by the area of output piston 30. When the force applied to output piston 30 is less than the spring force of valve biasing spring 36, the cylinder valve 32 begins to close under the force of valve biasing spring 36.
Partial lifts of cylinder valve 32 correspond to rotations of the cam 12 in which the cylinder valve 32 is displaced a shorter distance than, or stays open for less time then, the lift profile of the cam 12 would directly provide. The partial lifts occur whenever the variable-bleed orifice 50 allows fluid 42 to bleed off of the hydraulic linkage 28 during the lift profile of cam 12.
Maximum lift of cylinder valve 32 is achieved when the force on output piston 30 reaches a level corresponding to the spring force of valve biasing spring 36 at that displacement. The maximum pressure in chamber 44 is determined by the variable-bleed orifice 50 setting and the rate of input piston (24) motion. Maximum lift of cylinder valve 32 always occurs at or before the time of maximum lift of the cam 12. Also, the time of cylinder valve 32 liftoff is always at or before maximum velocity of the profile of cam 12.
The cylinder valve 32 will not continue to open if the input piston 24 is already into its return motion (upward in
The closing motion (upward in
With larger bleed rates to drain 56, the cylinder valve 32 could close completely while the input piston 24 is still in the down-stroke motion. In this case, the bled volume to drain 56 accounts for the sum of the volumes swept by the input piston 24 in down stroke and the output piston 30 in return.
At the end of each event (rotation through the lift profile portion of cam 12), when the cam 12 is on the base circle and the cylinder valve 32 seated, the check valve 48 opens and permits flow into the chamber 44 to replenish the bled volume of fluid 42.
At very high camshaft rotation speeds, if there are large drag forces acting on the piston-bore clearances, the high-pressure chamber could cavitate, causing the check valve 48 to open and allow supply oil from supply line 46 to come in while the cylinder valve 32 is still open. This accelerates valve closing as the pressure is now lower than the spring force of valve biasing spring 36, and violates the volume continuity causing a temporary “pump-up” of the system. Subsequently, on the cam 12 base circle, there should be enough time to bleed off the excess volume of fluid 42 to properly seat the cylinder valve 32 for the next cam event.
During partial lifts of cylinder valve 32, at the time of valve closing the valve seating isn't controlled by the closing ramps of cam 12. Hence, a damper mechanism 58 may be incorporated into the output piston 30 or adjacent portions of chamber 44 to trap fluid 42 in a damper chamber formed between the output piston 30 and the housing 45.
During valve closing, at a pre-designed distance from the valve seat, the cylinder valve 32 is slowed down by gradual engagement of the output piston 30 into the housing 45. The trapped fluid 42 between the output piston 30 and the adjacent walls of the housing 45 bleeds back into the chamber 44 through a constricted passage 59 in the output piston 30. Fluid viscosity restricts the ability of fluid to quickly flow through the passage 59 and creates a force on the output piston 30 opposing the valve biasing spring 36. Those having ordinary skill in the art will recognize several applicable damper designs. Designing the distance between the output valve 30 and seat at the start of the damping, and the damping rate, may require application-specific attention, because the valvetrain 10 will operate at different temperatures (fluid viscosity) and speeds.
The mode of operation of valvetrain 10 described above can be called a semi-active mode of control as different bleed rates can be set for different engine conditions, but the rates are not modulated per cam event. In this mode of operation, initial duration of pressure build up during the input piston 24 down stroke enables late intake valve opening (LIVO) strategy. The cylinder valve 32, due to lost motion, always closes at or before the closing point of the cam 12, enabling the early intake valve closing strategy (EIVC). The two attributes combined allow control over the duration of opening cylinder valve 32, which enables non-throttled engine load control.
Charge control at the cylinder valve 32 also helps to eliminate partial load engine-pumping losses. The lower valve lifts associated with shorter valve-open durations, a capability of valvetrain 10, assist in achieving acceptable high speed dynamics of valvetrain 10. In the semi-active mode of operation, with a given design, the variable-bleed orifice 50 setting is the only control parameter at a given engine speed. Maximum valve lift, opening and closing points, and hence the valve-open duration, are not independently controllable.
In an active mode of control, the bleed rate can be modulated per event. Changing the bleed rate from a higher to a lower value in the early down stroke of the input piston 24 could yield later opening without affecting maximum lift and valve-open duration. Conversely, keeping the bleed orifice sealed (or allowing only very small bleed rates) early in the input down stroke followed by a rapid increase in bleed rate would yield early opening of intake valve (for intake/exhaust overlap control) combined with early closing and shorter duration.
A roller 220 is located on one end of a follower 214 and follows motion of a cam 212. The follower 214 pivots about a center pivot 216 and transfers oscillatory motion of the cam 212 to an input piston 224 via an input end 222. As in valvetrain 10, the valvetrain 210 does not require a separate hydraulic lash adjustment element. Lash compensation occurs via pressure in a high-pressure chamber 244 and the spring force of a biasing spring 226.
The input piston transfers its linear movement through a hydraulic linkage 228 to an output piston 230 which is attached to a cylinder valve 232 operating against the force of a valve biasing spring 236. A variable-bleed orifice 250 alters the displacement of output piston 230 by selectively allowing a fluid 242 to be bled from the chamber 244 into a drain 256 in a block 235. Valvetrain 210 may also include a hydraulic damping shoulder 258 and a fluid passage 259 to restrict closing and seating velocity of the cylinder valve 232 following partial lift events.
Referring now to
In the valvetrain 310, the conventional rocker arm normally used in overhead valve configurations is replaced by a hydraulic linkage 328, effectively yielding a variable rocker ratio mechanism. For the same motion of an input piston 324, a continuously-variable displacement of an output piston 330 achievable.
The finger-type follower (14 and 214) used in valvetrains 10 and 210 is replaced with a lifter 313 and a pushrod 315 (shown in
A variable-bleed orifice 350 selectively alters motion transfer through the hydraulic linkage 328 by bleeding fluid 342 to a drain 356. In the embodiment shown in
Valvetrain 310 may be adapted to existing pushrod, overhead valve engines. Removal of the rocker mechanism may allow the hydraulic linkage 328 to be packaged into the space already available on the engine head. Adding valvetrain 310 to an existing overhead valve configuration may require minimal changes to the rest of the valvetrain.
Referring now to
A high-pressure chamber 444 contains a hydraulic fluid 442. Valvetrain 410, however, does not contain a variable-bleed orifice to drain fluid 442 from the chamber 444. Instead of bleeding fluid 442 in order to decrease volume displacing the output piston 430 (and thereby causing motion loss), the valvetrain 410 contains an auxiliary piston 460 in fluid communication with chamber 444. The auxiliary piston is biased toward the chamber 444 by an auxiliary piston spring 462. A variable stop 464 extends into the auxiliary piston 460 and is configured to selectively restrict the allowable displacement (right to left, as viewed in
Accordingly, the input piston 424, output piston 430, and auxiliary piston 460 are all in continuous contact with the fluid 442 in the chamber 444 such that these pistons 424, 430, 460 are in fluid communication with each other. The trapped volume of fluid 442 inside the control chamber 444 may be replenished through a check valve 448 to compensate for leakage. By replacing the variable-bleed orifice with the auxiliary piston 460, the valvetrain 410 is able to store the lost-motion energy and repressurize the chamber 444 following the cam event.
In operation, the volume displaced by the input piston 424 equates approximately to the summation of the volumes displaced by the auxiliary piston 460 and output piston 430. A small amount of volumetric loss may result from fluid compressibility and leakage through piston-to-wall clearances. As is known in the relevant art, the input motion from the cam 412 has a fixed displacement-time characteristic determined from the cam profile. However, the output motion of the cylinder valve 432 can be varied by controlling the motion of auxiliary piston 460.
One of the two operating parameters that control the displacement of auxiliary piston 460 is the relative specific force (i.e., force per unit piston area) of the valve biasing spring 436 and auxiliary piston spring 462. Where auxiliary piston spring 462 has lower relative specific force, it will be displaced further by less pressure in the chamber 444.
The other controlled parameter is the displacement of the variable stop 464, which serves as a dead stop that limits the displacement of the auxiliary piston 460. Therefore, if the variable stop 464 is set to allow no movement of auxiliary piston 460, all volume displaced by the input piston 424 will displace only the output piston 430, and the hydraulic linkage 428 essentially mimics a mechanical linkage.
Specifying the diameters of each one of the input piston 424, auxiliary piston 460, and output piston 430 determines their individual linear displacements per fixed cam 412 displacement. Once the piston dimensions are fixed, the cylinder valve 432 timing (e.g., valve opening point) can be determined by selection of the relative values for the valve and auxiliary biasing springs 436 and 462. For a late intake valve opening (LIVO) strategy, the auxiliary piston's (460) specific preload, has to be smaller than that of the valve biasing spring 436. This will cause a delay in the opening point of the cylinder valve 432 where displaced volume of the input piston 424 approximately equals the displaced volume of the auxiliary piston 460 until the auxiliary piston 460 contacts variable stop 464.
The exact timing of the cylinder valve 432 liftoff is controlled by the dead stop function of the variable stop 464. As motion of the auxiliary piston 460 is stopped or slowed, more (or all) of the volume displaced by input piston 424 must be displaced by output piston 430. By selectively adjusting variable stop 464 to allow more displacement of the auxiliary piston 460, more motion is lost into the auxiliary piston 460 and is not available to displace the output piston 430 and cylinder valve 432.
A fifth embodiment (not shown) of a cam-driven hydraulic lost-motion valvetrain could include a hydraulic network linking the high-pressure chambers of adjacent cylinders in the engine. Selectively allowing fluid passage between the chambers may allow further variability in the lost-motion hydraulic linkages. Those having ordinary skill in the art will recognize benefits of hydraulic networks linking the high-pressure chambers of multiple cylinders, such as, without limitation: bleeding pressure to an adjacent chamber instead of the drain; repressurizing the chamber during a cam event without altering the variable-bleed orifice; or by quickly restoring pressure to the high-pressure chamber.
While the best modes and other embodiments for carrying out the claimed invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.
