EP4065822A1

EP4065822A1 - A valve control system and methods of operation thereof

Info

Publication number: EP4065822A1
Application number: EP20816572.0A
Authority: EP
Inventors: Ben NEAVES; Gavin BARTLEY
Original assignee: Camcon Auto Ltd
Current assignee: Camcon Auto Ltd
Priority date: 2019-11-25
Filing date: 2020-11-23
Publication date: 2022-10-05
Also published as: GB2590764A; GB202015590D0; WO2021105654A1; GB201917118D0

Abstract

A valve control system (20) for an internal combustion engine, wherein the valve control system is configured to output control signals to an exhaust valve actuator (32) relating to a demanded opening and closing cycle of the exhaust valve (14). The control signals define a target actuator velocity profile (90) that extends at least between a point (EVO) at which the valve opens and a point (MOP) of peak valve lift, and comprises a raised portion (98), a base portion (102), wherein the raised portion is before the base portion and raised above the base portion, and a transition portion (100) after the raised portion and before the base portion, wherein the transition portion has a negative average gradient which is less than the average gradient of the raised portion. This profile serves to counteract high pressure in the cylinder which acts on the valve as the valve opens.

Description

Title: A Valve Control System and Methods of Operation thereof

Field of the invention The present invention relates to a valve control system for an internal combustion engine. More particularly, it relates to a valve control system for controlling an actuator for an exhaust valve of an engine.

Background of the invention

It is well known to operate inlet and exhaust valves of an internal combustion engine by means of a rotating camshaft. Usually, the camshaft is rotated together with the crankshaft of an engine and it is not possible to alter the valve movement profile and/or timing in relation to the engine speed or loading.

In order to give greater control over valve actuation, it has been proposed to operate the valves using electromagnetic solenoid actuators governed by a computer- controlled engine management system. An alternative approach is described in W02004/097184. This relates to an electromagnetic actuator having a driven rotor which is coupled to the valve by a suitable linkage.

Summary of the invention

The present invention provides a valve control system for an internal combustion engine, the engine having a cylinder with an exhaust valve and a piston, and an actuator for actuating the exhaust valve, wherein the valve control system is configured to output control signals to the actuator relating to a demanded opening and closing cycle of the exhaust valve, the control signals define a target actuator velocity profile that extends at least between a point at which the valve opens and a point of peak valve lift, and the target actuator velocity profile comprises a raised portion and a base portion, wherein the raised portion is before the base portion and raised above the base portion, and a transition portion after the raised portion and before the base portion, wherein the transition portion has a negative average gradient which is less than the average gradient of the raised portion.

The target actuator velocity profile may correspond to the velocities at which the actuator is commanded to move the exhaust valve during an opening and closing cycle.

When an exhaust valve is opened against a high pressure within a cylinder generated by a combustion event, this can cause the valve to decelerate rapidly, such that it deviates substantially from the demanded opening and closing cycle.

In order to address this, the target actuator velocity profile may be modified to increase the momentum of the valve as it opens. A higher actuator velocity may be targeted at the point at which the valve opens. This may serve to decrease the overall deviation of the actual velocity profile of the actuator from the target actuator velocity profile (and therefore reduce deviation from the demanded opening and closing cycle). The target actuator velocity profile may be adjusted such that the deceleration of the valve caused by high in-cylinder pressures brings the actual valve velocity profile closer to that required by the demanded opening and closing cycle.

Inclusion of a raised portion in the target actuator profile may increase the target actuator velocity at the point at which the valve opens by a predetermined extent above that defined by an unmodified profile which does not include a raised portion. The raised portion may commence at or before the point at which the valve opens. The base portion may end at the point of peak valve lift.

The valve control system may output control signals which define a target actuator velocity profile which takes into account the pressure expected within the associated cylinder to bring at least a portion of the actual actuator velocity profile closer to that defined by a demanded opening and closing cycle of the exhaust valve. The raised portion may be raised above the base portion in that the magnitude of the velocities targeted during the raised portion is greater than those targeted during the base portion. The velocity profile may represent a plot of a parameter related to the velocity of the actuator against a parameter related to time. For example, the profile may be represented as a plot of a coefficient dependent on the velocity of the actuator against the rotational angle of a crankshaft of the engine.

The raised portion of the target actuator velocity profile may lie above a reference line formed by projecting the base portion back to the point at which the valve opens. The reference line may be a straight line of best fit to the base portion.

The target actuator velocity profile may define an increased actuator velocity at the point at which the valve opens which is greater than a demanded actuator velocity at the point at which the valve opens according to the demanded opening and closing cycle of the exhaust valve. The demanded opening and closing cycle of the exhaust valve may define a demanded actuator velocity at valve opening and this may be increased by the valve control system in the target actuator velocity profile to compensate for resistive forces acting on the exhaust valve as it opens due to gas pressure in the cylinder. Accordingly, the difference between the increased actuator velocity at valve opening and the demanded actuator velocity at valve opening may vary with the pressure within the cylinder at the point at which the valve opens. This pressure may be an estimated pressure dependent on the current operating point of the engine. The valve control system may receive signals related to or indicative of the current operating point of the engine. These signals may be received from an engine control unit for example.

The valve control system may be arranged to receive an engine operation signal which is responsive to an operating point, band or state of the engine and to determine the increased actuator velocity at valve opening in response to this engine operation signal.

The increased actuator velocity at valve opening may be responsive to the extent to which at least a portion of a preceding opening and closing cycle of the exhaust valve differed from a corresponding portion of a respective demanded opening and closing cycle of the exhaust valve. Thus, the extent to which the actuator velocity at valve opening is increased may be adjusted having regard to the extent to which at least a portion of a preceding valve event deviated from the corresponding portion of a demanded valve event. In this way, the system may modify the target actuator velocity profile having regard to one or more previous valve events in order to reduce deviation of the valve opening and closing cycle from a demanded opening and closing cycle. This adjustment having regard to a previous valve lift event (or events) may be made having regard to a previous valve lift event (or events) at substantially the same engine operating point, band or state.

The average magnitude of the velocity profile during the raised portion may be a raise factor of up to 2 times (and more preferably in the range of 1.5 to 2 times) greater than the average magnitude of the profile during the base portion. The magnitude of the raise factor may vary with the pressure within the cylinder at the point at which the valve opens. It may also depend on the capabilities of the actuator. The valve control system may be arranged to receive an engine operating band signal which is responsive to an operating band of the engine and to determine the raise factor in response to the engine operating band signal. The raise factor may be responsive to the extent to which at least a portion of a preceding opening and closing cycle of the exhaust valve differed from a corresponding portion of a respective demanded opening and closing cycle of the exhaust valve.

Preferably, the duration of the raised portion is in the range of 5 to 15%, and more preferably, about 10% of the duration of the time between the point at which the valve opens and the point of peak valve lift. Preferably, the duration of the base portion is in the range of 70 to 90%, and more preferably about 80% of the duration of the time between the point at which the valve opens and the point of peak valve lift.

Preferably, the change in the magnitude of the profile over the transition portion may be up to 2 times (and more preferably in the range of 1.5 to 2 times) the magnitude of the profile at the end of the transition portion. The target actuator velocity profile that extends at least between the point at which the valve opens and the point of peak valve lift may consist essentially or only of a raised portion, a transition portion and a base portion. Preferably, the duration of the transition portion is in the range of 5 to 15%, and more preferably, about 10% of the duration of the time between the point at which the valve opens and the point of peak valve lift. It is desirable to keep the duration of the transition portion as short as practicable to enable the actuator to be controlled to settle onto the base portion ahead of peak valve lift at MOP.

The output control signals may be generated with reference to a predetermined control signal profile, independently of the resulting actuator positions, until a predetermined switching threshold is reached, and the valve control system may be configured to then adjust the output control signals with reference to a feedback signal responsive to the resulting actuator positions.

The present disclosure also provides a valve control system for an internal combustion engine, the engine having a cylinder with an exhaust valve and a piston, and an actuator for actuating the exhaust valve, wherein the valve control system is configured to output control signals to the actuator relating to a demanded opening and closing cycle of the exhaust valve, the output control signals are generated with reference to a predetermined control signal profile, independently of the resulting actuator positions, until a predetermined switching threshold is reached, and thereafter the valve control system is configured to adjust the output control signals with reference to a feedback signal responsive to the resulting actuator positions.

It has been realised that the power efficiency of the valve control system may be increased by controlling the valve actuator during an initial portion of the opening and closing cycle of the exhaust valve independently of the resulting actuator positions (that is, in an “open loop” manner), before switching partway through the opening and closing cycle to control with reference to the actuator position (that is, in a “closed loop” manner). Such a control strategy has been found to reduce energy consumption by the valve control system relative to a configuration using closed loop control throughout the valve’s opening and closing cycle.

This control strategy may be advantageous for an exhaust valve opening against a high load due to high in-cylinder pressure, as greater energy savings may be achievable (during at least part of an opening phase of the lift event) by using open loop rather than closed loop control. The energy savings may outweigh the potential reduction in accuracy of control of the actuator (and therefore the valve) position resulting from open, rather than closed, loop control over the relevant phase of the opening and closing cycle.

An open loop control strategy may be implemented by calculating and storing a range of predetermined control signal profiles. Each profile may be calculated for a given set of engine conditions (or engine operating points) for a particular engine. The engine conditions used to choose a predetermined control signal profile may be selected from: engine speed, engine load, and engine crank angle at which to open the valve, for example. Each predetermined control signal profile may be calculated so as to counteract the effects of the expected in-cylinder pressure in order for the exhaust valve to execute a demanded opening and closing cycle.

The valve control system may be arranged to detect when a predetermined switching threshold is reached and then switch from open to closed loop control. This may correspond to a point at which the initial high load experienced when opening the exhaust valve against in-cylinder pressure has largely been overcome.

The predetermined switching threshold may correspond to a predefined engine crank angle. The valve control system may be configured to receive a signal responsive to the engine crank angle and determine when the predetermined switching threshold is reached with reference thereto.

In some embodiments, the predetermined switching threshold is when the difference between the actual actuator position and a demanded actuator position corresponding to the demanded opening and closing cycle of the exhaust valve at the current engine crank angle is within a predetermined tolerance. Accordingly, the valve control system may switch from open to closed loop control when it detects that the actuator (and therefore the valve) position is sufficiently close to a desired opening and closing cycle profile.

In preferred embodiments, the predetermined switching threshold is reached when the earlier of the following occurs: (a) when the difference between the actual actuator position and the demanded actuator position at the current engine crank angle is within a predetermined tolerance; and (b) a predefined engine crank angle is reached.

In this manner, a default for the predetermined switching threshold is when a predefined engine crank angle is reached to ensure that switching takes place if the demanded actuator position for the current engine crank angle is not reached. The predefined engine crank angle corresponding to the predetermined switching threshold may be different at two or more different engine speeds. The predefined engine crank angle may be dependent on the current engine speed.

The valve control system may be configured to generate a modified control signal profile by modifying a predetermined control signal profile with reference to a signal responsive to actual actuator positions resulting from outputting the predetermined control signal profile to the actuator during a preceding opening and closing cycle of the exhaust valve. Accordingly, a predetermined control signal profile may be adapted or modified in response to a resulting opening and closing cycle or cycles of the exhaust valve with a view to reducing deviation of the actuator cycle from a demanded profile in a subsequent cycle using the modified control signal profile. In this way, the accuracy with which a demanded valve event is followed may be monitored and a corresponding predetermined control signal profile adjusted if appropriate.

The valve control system may be configured to determine from the signal responsive to actual actuator positions at least one of the following parameters: an engine crank angle at which the exhaust valve opened, an exhaust valve velocity at an engine crank angle, an engine crank angle when the exhaust valve closed, and an integral of the exhaust valve lift over at least part of an opening and closing cycle, in order to generate the modified control signal profile.

The present invention further provides an internal combustion engine including a valve control system as described herein. In addition, the present invention may also provide a vehicle including a valve control system or engine as described herein.

The present invention also provides a method of controlling an actuator for actuating an exhaust valve of an internal combustion engine cylinder, the method comprising the step of outputting control signals to the actuator relating to a demanded opening and closing cycle of the exhaust valve, wherein the control signals define a target actuator velocity profile that extends at least between a point at which the valve opens and a point of peak valve lift, and the target actuator velocity profile comprises a raised portion and a base portion, wherein the raised portion is before the base portion and raised above the base portion, and a transition portion after the raised portion and before the base portion, wherein the transition portion has a negative average gradient which is less than the average gradient of the raised portion. The control signals may be outputted by a valve control system as described herein.

In a preferred implementation, a transition start control signal triggering the start of the transition portion is outputted to the actuator in response to detection of the point at which the valve opens. For example, the valve control system may receive a feedback signal from the actuator which indicates that the valve has opened and send the transition start control signal in response to the feedback signal.

The present invention further provides a method of controlling an actuator for actuating an exhaust valve of an internal combustion engine cylinder, the method comprising the steps of: outputting control signals to the actuator relating to a demanded opening and closing cycle of the exhaust valve, wherein the output control signals are generated with reference to a predetermined control signal profile, independently of the resulting actuator positions; detecting when a predetermined switching threshold is reached, and thereafter adjusting the output control signals with reference to a feedback signal responsive to the resulting actuator positions.

A recording medium or carrier storing computer interpretable instructions for causing a valve control system to perform the methods disclosed herein is also provided. Brief description of the drawings

Embodiments of the invention will now be described by way of example and with reference to the accompanying schematic drawings, wherein: Figure 1 is a block diagram of an engine control system including a valve control system according to an embodiment of the invention;

Figure 2 is a graph illustrating operation of a conventional valve camshaft; Figures 3 and 4 are graphs illustrating actuator velocity profiles of an unmodified form and a form modified according to an embodiment of the invention, respectively;

Figures 5 to 8 show further velocity profiles according to some embodiments of the invention;

Figure 9 is a graph showing a plot of simulated values of a velocity coefficient against cylinder pressure;

Figure 10 is a schematic graph of velocity coefficients at different engine operating points;

Figure 11 illustrates adaptive modification of target velocity coefficients at different engine operating points; Figure 12 shows bar charts illustrating portions of two predetermined control signal profiles for an actuator connected to an exhaust valve; Figure 13 is a graph illustrating an opening and closing cycle of an exhaust valve; and

Figure 14 is a graph illustrating an opening and closing cycle of an exhaust valve and acceptable ranges for different parameters. Detailed description of the drawings

Figure 1 shows a block diagram of an engine control system including a valve control system embodying the invention, together with part of an associated engine cylinder block. A piston 2 is arranged to reciprocate up and down within a cylinder block 4. The flow of charge air (or an air and fuel mixture, depending on the engine configuration) from an inlet port 6 is controlled using inlet poppet valve 12. An exhaust poppet valve 14 allows exhaust gases to escape from the combustion chamber 10 after combustion has taken place, with the exhaust gases being carried away by exhaust port 16.

An actuator 30 is provided to operate the inlet valve 12 and an actuator 32 operates the exhaust valve 14. A valve lift event is caused by the associated actuator executing an actuation cycle in which forces are transmitted from the actuator to the valve stem to move it away from a closed position and then return it to that position. The actuator may be linear or rotary. In the case of a rotary actuator, as described for example in WO 2004/097184, a lift event comprises a part rotation of the rotor, or a complete rotation of the rotor, away from and back to its initial rest position.

The overall operation of the engine is governed by an engine control unit 34. It controls the fuel injection and ignition of a spark ignited engine or the fuel injection of a compression ignition engine. The engine control unit is responsive to signals 36 from various transducers monitoring operating conditions of the engine. For example, they may monitor the crankshaft position, the coolant temperature, the oil temperature, the engine speed, the engines cranking mode, and so on.

A bi-directional communication link 38 is provided between the engine control unit 34 and a valve control unit 40. Valve control unit 40, together with an actuator power electronics module 42, forms a valve control system 20 controlling the operation of the inlet and exhaust valves 12, 14 via the respective actuators 30 and 32. In practice, control units 34 and 40 and module 42 may be physically separate units or integrated into a single controller.

The signals received by the valve control unit from the engine control unit via link 38 may define a demanded opening and closing cycle or lift event of an exhaust valve. It will be appreciated that processing steps to determine dynamically control signals to send to the actuators having regard to current operating conditions and requirements may be distributed between the parts of an engine control system in different ways.

Having regard to the signals received via link 38 from the engine control unit, the valve control unit 40 in turn generates inlet actuator and exhaust actuator drive signals 44, 46 which are sent to the actuator power electronics module 42. In response to these input signals, module 42 generates inlet actuator and exhaust actuator drive currents along respective conductive lines 48 and 50.

Feedback signals 52 and 54 are communicated to the valve control unit 40 from the inlet valve and exhaust valve actuators, respectively. These feedback signals may provide information relating to one or more operating conditions of the respective valve actuator, such as its position, the temperature of electromagnetic windings, current flow in the windings, and the like. The information conveyed by these signals may of course vary depending on the type of actuation employed, whether electromagnetic, hydraulic or pneumatic, for example.

Link 38 may carry signals providing information relating to one or more operating conditions of the engine. For example, this information may be indicative of a current operating point or band of the engine. The engine control unit 34 and valve control unit 40 may be implemented in the form of one or more control units or computational devices having one or more electronic processors. They may include a set of instructions which, when executed, cause the unit(s) to implement the methods described herein. The present disclosure encompasses computer programs comprising such instructions provided on or in a carrier. The program may be in a form of source code, object code, a code intermediate between source and object codes such as in partially compiled form, or in any of the forms suitable for use in the implementation of the processes described herein. The carrier may be any entity or device capable of carrying a program.

For example, the carrier may comprise a storage medium, such as ROM, for example a CD ROM or a semiconductor ROM, or a magnetic recording medium, for example a floppy disk or hard disk. Furthermore, the carrier may be a transmissible carrier such as an electrical or optical signal which may be conveyed by electrical or optical cable or by radio or other means. When the program is embodied in a signal which may be conveyed directly by a cable or other device or means, the carrier may be constituted by such cable or other device or means. Alternatively, the carrier may be an integrated circuit in which the program is embedded, the integrated circuit being adapted for performing or for use in the performance of the relevant processes.

By way of background, Figure 2 shows a velocity profile 60 for a valve camshaft of a conventional engine valvetrain. The resultant valve lift profile 62 is plotted against a right-hand valve lift y-axis.

This event profile begins at the point at which the exhaust valve opens (marked “EVO” on the x-axis). The profile rises to a point of peak valve lift. This is denoted “MOP” in Figure 2 (“maximum opening point”). As the crank angle increases, the profile then returns to a point at which the exhaust valve closes (marked “EVC”).

The camshaft maintains a continuous rotational speed as it is directly connected to the crankshaft of the engine. Plots 64, 66 and 68 illustrate resistive forces exerted on the valve by the pressure within the cylinder as it moves away from its closed position. The magnitude of these forces varies with the operating point of the engine and this is shown in the Figure by the plots increasing in height from plot 64 to 66. The camshaft-based drive system is able to transfer significant force instantly to the valve to overcome this resistance and so the resultant valve lift profile is not materially affected by higher resistive forces.

Figure 3 illustrates a target exhaust valve actuator velocity profile 78 for the rotor of an electromagnetic valve actuator. This profile is not modified in accordance with the present disclosure. The engine crankshaft angle increases along the x-axis. For the purposes of illustration, a desired valve lift event profile 70 is plotted against a right- hand valve lift y-axis.

In order to cause the exhaust valve to carry out this lift event, the valve control system sends control signals to the exhaust valve actuator related to this lift event (or opening and closing cycle). These control signals define a target actuator velocity profile 78 comprising three points 72, 74 and 76. These points represent desired actuator velocities at a sequence of crank angles.

In the diagram of Figure 3, these velocities are in terms of a velocity coefficient, plotted along the left-hand y-axis. This coefficient is calculated as the ratio of the instantaneous angular velocity of the actuator rotor to the instantaneous angular velocity of the crankshaft. It is therefore a dimensionless coefficient, which does not need to be recalculated as and when the crankshaft speed changes. In the example illustrated in Figure 3, the MOP of the valve lift event is shifted slightly closer to EVC than EVO. As a result, the velocity profile defined by points 72, 74 and 76 is a straight line inclined with a positive gradient between points 72 and 76. Thus, the velocity coefficient 74 at MOP is slightly greater than that at EVO (72), whilst the velocity coefficient 76 at EVC is slightly greater than that at MOP. It will be appreciated that shifting MOP closer to EVO than EVC would result in this line being inclined with a negative gradient instead. The inventors have found that a velocity profile 78 of the form shown in Figure 3 tends, in practice, to cause the actuator to follow an actual velocity profile 80. It can be seen that the desired velocity coefficient is not reached at the point of EVO and then the profile 80 quickly drops away from the desired target actuator velocity profile 78. This was found to be due to the high in-cylinder pressures experienced at the valve opening as a result of the preceding combustion event causing the actuator to decelerate rapidly due to a drastic increase in the resistive forces experienced by the exhaust valve. To illustrate these forces, a schematic plot 82 of the resistive forces experienced by the exhaust valve is shown in Figure 3. As the actuator has a relatively low inertia to optimise the actuation system efficiency, it is decelerated significantly by the resistive forces.

Examples of modified target actuator velocity profiles 90 devised according to embodiments of the invention in order to address the issue discussed above are shown in Figures 4 and 5. The modified velocity profiles 90 retain points 74 and 76. However, they replace the opening velocity coefficient 72 with three additional points 92, 94 and 96.

Point 92 is located at EVO in Figure 4, like point 72, but has a significantly higher velocity coefficient value. The next point along the velocity profile, point 94 occurs after point 92 and has the same velocity coefficient value. A raised profile portion 98 is defined between points 92 and 94. Continuing along the profile with increasing crank angle from point 94, the modified profile has a negative gradient and the velocity coefficient decreases until point 96 is reached. Here, the profile meets the unmodified profile 78 (shown as a dotted line in Figure 4). A transition portion 100 is defined between points 94 and 96. The gradient of the transition portion 100 is negative and less than the average gradient of the raised portion 98. The average gradient of the raised portion may in some implementations be a negative value, in which case, the gradient of the transition portion 100 is less than the average gradient of the raised portion 98, that is, it is more negative. Then, continuing with increasing crank angle beyond point 96, the unmodified profile 78 is followed until point 74 is reached at the point of peak valve lift, MOP. A base portion 102 of the modified velocity profile as defined between points 96 and 74. By adopting an increased velocity coefficient at EVO, the actuator and valve have increased momentum at this point of high in-cylinder pressure. This serves to compensate for the increased resistive forces experienced such that the overall deviation of the actual velocity profile 104 from the desired profile 78 is significantly reduced. The velocity of the actuator at EVO is increased such that the deceleration experienced due to the rapid onset of high resistive forces brings the actual velocity profile 102 close to the desired profile 78. In some preferred implementations, a proportion or the majority of the raised portion 98 may occur prior to EVO. Thus, rather than point 92 coinciding with EVO as shown in Figure 4, it may be reached just before EVO. In this way, the raised portion acts to pre-empt the resistance to opening of the valve. The change from the raised portion 98 to the transition portion 100 may occur in response to detection of the valve beginning to open. Valve opening may be detected by reference to feedback signals received from the actuator. Thus the raised portion may end very shortly after EVO. The resistance to the valve opening further drops of significantly as soon as it has started to open (as illustrated by plot 82 in Figure 4), and so it may be desirable to end the raised portion soon after EVO, and transition quickly to the base portion, to avoid excessive overshoot of the desired velocity profile 78. Once the resistance to motion of the valve has substantially reduced, the transition portion ends and the base portion 102 commences. The duration of the base portion (labelled “a”) in Figure 5 may be about 10% of the time between EVO and peak valve lift. As noted above, the base portion may commence before EVO. The duration of transition portion (labelled “b”) in Figure 5 may also be about 10% of the time between EVO and peak valve lift. The height of the raised portion above the base portion (labelled “c” in Figure 5) may be up to 2 times the height of the base portion (labelled “d” in Figure 5), for example. Figure 6 illustrates variation of the actuator velocity at EVO (point 92) to counteract increasing resistive pressures represented by plots 64, 66 and 68. Increasing the velocity reached by the actuator at EVO increases the rotational inertia of the actuator rotor at this point, when the resistive forces are at their highest.

Figure 7 shows a three velocity profiles (90’, 90” and 90’”) plotted against time (instead of rotor angle as used in Figure 6) to illustrate how increasing the velocity achieved by the actuator at EVO requires a steeper acceleration of the rotor prior to EVO and takes a reduced amount of time. As the rotor needs to spin faster to travel between its stationary starting position and EVO to acquire a higher velocity at EVO, the time taken decreases significantly.

The profiles shown in Figures 4 to 7 relate to valve lift events which may be implemented by a rotation of a rotary actuator in one rotational direction. Such an actuator may also be employed to implement a valve lift event by rotating the actuator in one direction to open a valve and then reversing the direction of rotation of the actuator to close the valve. Such an event may be referred to as a “bounce” valve event. A part of a modified velocity profile 102’ according to an embodiment of the invention for implementing a bounce event is shown in Figure 8. It can be seen that the velocity coefficient reduces to zero at MOP as the actuator changes its direction of rotation at this point.

Figure 9 is a graph showing data generated using a simulation of a valve actuation system in software. “Delta P” is plotted on the x-axis, which is the pressure difference across the exhaust valve head just before EVO. At a range of pressures, a velocity coefficient was calculated with a view to minimising deviation from the desired actuator velocity profile whilst seeking to maintain the desired critical timings for EVO and MOP. Preferably, the velocity coefficient is calculated so that the actuator reaches EVO at the desired time and with the desired velocity. This represents the velocity coefficient of point 92 in the modified velocity profiles of Figures 4 to 8. Dashed line 110 represents a best fit line to these points. In one implementation of the invention, an operating range of the engine is divided into a plurality of bands. In the illustration of Figure 10, five engine operating bands are defined. In Figure 10, each band may cover a range of four bar of the cylinder gas pressure at EVO. The operating range may extend across the range of cylinder gas pressures which may be generated. For example, in Figure 10, a baseline curve 112 is drawn which is derived from the results shown in Figure 9. In each of the five bands, a respective engine operating point is defined on this line which has an associated velocity coefficient (points 114 to 118). These points may be stored by the valve control system and retrieved in order to modify the target actuator velocity profile at point 92 on the profile according to the current operating band of the engine.

The valve control system may be configured to adjust the magnitude of a velocity coefficient associated with a particular engine operating band or point having regard to the behaviour of the exhaust valve during one or more preceding valve events. The system may be configured to calculate a measure of the quality of a preceding lift event. This may be a measure of the cumulative valve or rotor position errors over a defined period. For example, such period may extend from 10 crank degrees before EVO to 50 crank degrees after EVO. The system may be able to analyse data corresponding to the value of this metric for a preceding valve event or a sample of preceding valve events at an (or each) engine operating point or band. In response to this analysis, the valve control system may adjust the value of the velocity coefficient associated with that point or band with a view to achieving an improved value for the quality metric in a subsequent valve event. As illustrated in Figure 11, the system will aim to reduce the cumulative position errors at each engine operating point by adjusting the value of the velocity coefficient (“VC”) stored for each point. As a result, the baseline curve 112 shown in Figure 10 may over time be modified to form an adapted curve 120.

Figure 12 shows graphs by way of example to illustrate portions of two predetermined control signal profiles for use under different engine operating conditions. They illustrate actuator current magnitudes at corresponding engine crank angles. This represents a pre-stored schedule of actuator currents to be referred to by the valve control system in order to carry out the demanded opening and closing cycle of the valve at a given engine operating point. The valve control system may determine the required actuator current at predetermined intervals, for example, every 100ms. If the crank angle (at the instant when an actuator current is to be determined) is between two angles in a pre-stored schedule, linear interpolation may be used to calculate the actuator current required at that crank angle from the current magnitudes associated with the adjacent crank angles in the schedule.

Figure 13 shows a plot 130 of valve lift against engine crank angle for a typical valve event. The dashed lines identify parameters which may be used to define boundaries of an “acceptable” valve event. These parameters may include one or more of: a range 132 of engine crank angles at the point of valve opening, a range 134 for the gradient of the valve lift profile during an initial portion of the lift event, a range 136 of engine crank angles at the point of valve closing, a range 138 for the gradient of the valve lift during a final portion of the lift event, and the area 140 under the resulting valve event profile 142 (corresponding to an integral of the valve lift over the lift event).

Figure 14 illustrates an example of acceptable ranges for different parameters of the valve event. The extent of the acceptable ranges is indicated by dashed lines. Should a resulting valve event stray outside these acceptable event windows, for example by entering a “no go zone” 144, the corresponding predetermined control signal profile may be modified with a view to it resulting in an acceptable valve event when used in a subsequent engine cycle. This feedback may be implemented by providing an additional proportional integral controller to modify the predetermined control signal profile if the resulting valve event exceeds the allowed windows.

In embodiments described herein, “about” may mean plus or minus 5% of the stated value.

Claims

1. A valve control system for an internal combustion engine, the engine having a cylinder with an exhaust valve and a piston, and an actuator for actuating the exhaust valve, wherein the valve control system is configured to output control signals to the actuator relating to a demanded opening and closing cycle of the exhaust valve, the control signals define a target actuator velocity profile that extends at least between a point at which the valve opens and a point of peak valve lift, and the target actuator velocity profile comprises: a raised portion; a base portion, wherein the raised portion is before the base portion and raised above the base portion; and a transition portion after the raised portion and before the base portion, wherein the transition portion has a negative average gradient which is less than the average gradient of the raised portion.

2. A system of claim 1, wherein the raised portion lies above a reference line formed by projecting the base portion back to the point at which the valve opens.

3. A system of claim 1 or claim 2, wherein the target actuator velocity profile defines an increased valve opening velocity at the point at which the valve opens which is greater than a demanded valve opening velocity at the point at which the valve opens according to the demanded opening and closing cycle of the exhaust valve.

4. A system of claim 3, wherein the difference between the increased valve opening velocity and the demanded valve opening velocity varies with the pressure within the cylinder at the point at which the valve opens.

5. A system of claim 3 or claim 4, wherein the valve control system is arranged to receive an engine operating band signal which is responsive to an operating band of the engine and to determine the increased valve opening velocity in response to the engine operating band signal.

6. A system of any of claims 3 to 5, wherein the increased valve opening velocity is responsive to the extent to which at least a portion of a preceding opening and closing cycle of the exhaust valve differed from a corresponding portion of a respective demanded opening and closing cycle of the exhaust valve.

7. A system of any preceding claim, wherein the average magnitude of the profile during the raised portion is a raise factor of up to 2 times greater than the average magnitude of the profile during the base portion.

8. A system of claim 7, wherein the magnitude of the raise factor varies with the pressure within the cylinder at the point at which the valve opens.

9. A system of claim 7 or claim 8, wherein the valve control system is arranged to receive an engine operating band signal which is responsive to an operating band of the engine and to determine the raise factor in response to the engine operating band signal.

10. A system of any of claims 7 to 9, wherein the raise factor is responsive to the extent to which at least a portion of a preceding opening and closing cycle of the exhaust valve differed from a corresponding portion of a respective demanded opening and closing cycle of the exhaust valve.

11. A system of any preceding claim, wherein the duration of the raised portion is about 10% of the duration of the time between the point at which the valve opens and the point of peak valve lift.

12. A system of any preceding claim, wherein the duration of the base portion is about 80% of the duration of the time between the point at which the valve opens and the point of peak valve lift.

13. A system of any preceding claim, wherein the change in the magnitude of the profile over the transition portion is up to 2 times the magnitude of the profile at the end of the transition portion.

14. A system of any preceding claim, wherein the duration of the transition portion is about 10% of the duration of the time between the point at which the valve opens and the point of peak valve lift.

15. A system of any preceding claim, wherein the raised portion starts at the point at which the valve opens.

16. A system of any preceding claim, wherein the output control signals are generated with reference to a predetermined control signal profile, independently of the resulting actuator positions, until a predetermined switching threshold is reached, and thereafter the valve control system is configured to adjust the output control signals with reference to a feedback signal responsive to the resulting actuator positions.

17. A valve control system for an internal combustion engine, the engine having a cylinder with an exhaust valve and a piston, and an actuator for actuating the exhaust valve, wherein the valve control system is configured to output control signals to the actuator relating to a demanded opening and closing cycle of the exhaust valve, the output control signals are generated with reference to a predetermined control signal profile, independently of the resulting actuator positions, until a predetermined switching threshold is reached, and thereafter the valve control system is configured to adjust the output control signals with reference to a feedback signal responsive to the resulting actuator positions.

18. A system of claim 16 or claim 17, wherein the predetermined switching threshold corresponds to a predefined engine crank angle.

19. A system of claim 16 or 17, wherein the predetermined switching threshold is when the difference between the actual actuator position and a demanded actuator position corresponding to the demanded opening and closing cycle of the exhaust valve at the current engine crank angle is within a predetermined tolerance.

20. A system of claim 19, wherein the predetermined switching threshold is reached when the earlier of the following occurs: (a) when the difference between the actual actuator position and the demanded actuator position at the current engine crank angle is within a predetermined tolerance; and (b) a predefined engine crank angle is reached.

21. A system of claim 18 or claim 20, wherein the predefined engine crank angle is different at two or more different engine speeds.

22. A system of any of claims 16 to 21, wherein the valve control system is configured to generate a modified control signal profile by modifying a predetermined control signal profile with reference to a signal responsive to actual actuator positions resulting from outputting the predetermined control signal profile to the actuator during a preceding opening and closing cycle of the exhaust valve.

23. A system of claim 22, wherein the valve control system is configured to determine from the signal responsive to actual actuator positions at least one of the following parameters: an engine crank angle at which the exhaust valve opened, an exhaust valve velocity at an engine crank angle, an engine crank angle when the exhaust valve closed, and an integral of the exhaust valve lift over at least part of an opening and closing cycle, in order to generate the modified control signal profile.

24. An internal combustion engine including a valve control system of any preceding claim.

25. A vehicle including a valve control system of any of claims 1 to 23 or an engine of claim 24.

26. A method of controlling an actuator for actuating an exhaust valve of an internal combustion engine cylinder, the method comprising the step of outputting control signals to the actuator relating to a demanded opening and closing cycle of the exhaust valve, wherein: the control signals define a target actuator velocity profile that extends at least between a point at which the valve opens and a point of peak valve lift, and the target actuator velocity profile comprises: a raised portion; a base portion, wherein the raised portion is before the base portion and raised above the base portion; and a transition portion after the raised portion and before the base portion, wherein the transition portion has a negative average gradient which is less than the average gradient of the raised portion.

27. A method of claim 26, wherein a transition start control signal triggering the start of the transition portion is outputted to the actuator in response to detection of the point at which the valve opens.

28. A method of claim 27 when dependent on claim 20, wherein the valve control system receives a feedback signal from the actuator which indicates that the valve has opened and sends the transition start control signal in response to the feedback signal.

29. A method of controlling an actuator for actuating an exhaust valve of an internal combustion engine cylinder, the method comprising the steps of: outputting control signals to the actuator relating to a demanded opening and closing cycle of the exhaust valve, wherein the output control signals are generated with reference to a predetermined control signal profile, independently of the resulting actuator positions; detecting when a predetermined switching threshold is reached, and thereafter adjusting the output control signals with reference to a feedback signal responsive to the resulting actuator positions.

30. A method of any of claims 26 to 29, wherein the control signals are outputted by a valve control system of any of claims 1 to 23.

31. A recording medium or carrier storing computer interpretable instructions for causing a valve control system to perform the method of any of claims 26 to 30.