GB2588855A

GB2588855A - Internal combustion engines including independently controllable valve actuators and methods of operation thereof

Info

Publication number: GB2588855A
Application number: GB2016294.7A
Authority: GB
Inventors: Derrick Stone Roger
Original assignee: Camcon Auto Ltd
Current assignee: Camcon Auto Ltd
Priority date: 2019-10-17
Filing date: 2020-10-14
Publication date: 2021-05-12
Also published as: GB202016294D0; GB201915030D0; WO2021074618A1

Abstract

A method of operating an i.c. engine comprises suspending fuel injection during a piston stroke from top dead centre (TDC) to bottom dead centre (BDC) and during the following stroke from BDC back to TDC, opening an inlet valve during the stroke of the piston from TDC to BDC to introduce an air charge into the cylinder, and opening an exhaust valve during the stroke of the piston from BDC to TDC so that the exhaust valve releases air from the cylinder, eg an additional exhaust event A may be scheduled towards the end of the compression stroke for compression release retarding. An optional exhaust event B may occur just before the inlet valve closes to recover air back into the cylinder from the exhaust system when exhaust pressure is high enough to increase the compression energy consumed. The valves may be independently controllable by electromagnetic, hydraulic or pneumatic actuators. A valve opening cam (fig.7) may rotated anti-clockwise for a normal lift sector (78) or clockwise for a dwell sector (86) for the exhaust event A. Two-stroke retarding schemes are disclosed (fig. 4,5).

Description

Title: Internal Combustion Engines including Independently Controllable Valve Actuators and Methods of Operation thereof

Field of the Disclosure

The present disclosure relates to methods of operating internal combustion engines which include cylinders having independently controllable valve actuators.

Background to the Disclosure

A system for operation of internal combustion engine poppet valves using electromagnetic rotary actuators has been developed by the present applicant. This system allows engine valve operation that is independent of the engine crankshaft motion.

The electromagnetic actuators operate the valves through a cam and desmodromic linkage. The actuator is controllable to carry out a variety of valve events, enabling emissions, fuel economy and engine torque output to be optimised on a cycle-by-cycle basis. Aspects of this technology are disclosed in WO 2004/097184 and WO 2011/061528, for example.

Summary of the Disclosure

The present disclosure relates to a method of operating an internal combustion engine so as to provide an engine-based retarder function. It provides a method of operating an internal combustion engine including at least one cylinder having a piston, at least one inlet valve and at least one exhaust valve, with each valve having a respective independently controllable valve actuator for actuating the valve, wherein the method comprises the steps of (a) suspending injection of fuel into the at least one cylinder during a first stroke of the piston from top dead centre (TDC) to bottom dead centre (BDC) and the following first stroke from BDC back to TDC; (b) actuating the valve actuator of the at least one inlet valve to carry out an inlet valve lift event so that the inlet valve is open during the first stroke of the piston from TDC to BDC to introduce an air charge into the cylinder; and (c) actuating the valve actuator of the at least one exhaust valve to carry out an exhaust valve lift event so that the exhaust valve is open during the first stroke of the piston from BDC to TDC and the exhaust valve releases air from the air charge out of the cylinder.

The inlet valve may open in such a way that work is done in overcoming pumping it) losses during an induction stroke. The exhaust valve may release air from a compressed air charge to avoid energy being recovered after a compression stroke In seeking to develop further ways in which the versatility of independent valve actuators may be employed to beneficial effect, the inventor realised that this technology may be used to provide an engine-based vehicle retarder function.

Engine-based pneumatic control technology is employed in heavy duty diesel engines to provide a vehicle retarding effect. Two basic operating methods have been adopted. A valve in the exhaust system can be used to restrict flow so that the engine works against pressure created in the exhaust downpipe. Alternatively, engine valve events may be modified to provide a retarding torque by managing the airflow into and out of engine cylinders under vehicle deceleration conditions, to provide an integrated pneumatic retarder.

In the latter approach, the retarding torque is generated during the compression stroke of the engine. An additional mechanism is provided between the engine camshaft and valves in order to execute an alternative valve event. The exhaust valve may be opened around the end of the compression stroke to prevent the energy used in compression from being returned to the crankshaft in the expansion stroke In this way, the compression stroke provides a net retarding effect on the crankshaft and therefore the whole vehicle This is frequently referred to as "compression release braking".

The use of independently controllable valve actuators to implement engine braking enables it to be implemented more flexibly and precisely with a greater degree of variability than existing systems. The valve lift profile can be selected on a cycle-bycycle basis as required. The duration, start point, end point, peak lift and/or lift profile shape may be varied.

Furthermore, when using independently controllable valve actuators, no supplementary mechanisms such as lost motion devices between a conventional camshaft and the valves are required in order to implement engine braking. All valve io events for managing the provision of driving or retarding torque are provided by the same actuators and the respective valve linkages. The valve event request signals generated by a valve control system are changeable on an event-by-event basis.

Independent valve actuators may provide engine braking in response to actuation of a brake pedal of the vehicle by its driver. The use of the cylinder valves to provide engine braking may be integrated into the whole vehicle braking system alongside the vehicle foundation brakes, and other systems such as traction motor regenerative braking in hybrid applications, for example.

The use of independently controllable valve actuators to implement engine braking functions provides significantly increased versatility relative to known approaches. As known approaches rely on a mechanism linking the engine camshaft and the valve, the braking function is either on or off Furthermore, the braking power is a function of engine speed. However, when using independently controllable valve actuators, the engine braking effort may be controlled as desired at any engine speed. For example, progressive braking power may be achieved.

For example, the braking power may be modified by adjusting the timing of the inlet and/or exhaust valve braking events. Furthermore, the braking gas flow can be adjusted by varying the timing, duration and/or lift of the exhaust and/or inlet brake events. Thus, the inlet valve(s) can allow varying amounts of air into the cylinders in the braking mode. This allows inlet stroke pumping loss energy and exhaust stroke compression loss energy to be optimised for the most appropriate braking power. The engine braking variables and parameters may be determined as appropriate to provide the desired braking effort. This may also facilitate smooth transitions in and out of retarding modes In examples of the present disclosure, independently controllable valve actuators may be controlled to provide engine braking using either inlet or exhaust valves or both. One or more exhaust valves may be controlled to begin lifting at an appropriate time before top dead centre (TDC) during a piston stroke from bottom dead centre (BDC) to TDC. The exhaust valve may then be held open long enough to exhaust a desired to amount of compressed gas and therefore expel as much energy as required. No fuel is injected.

A retarding torque may be generated by modifying the lift profile of one or more inlet valves during an induction stroke. Optimising the event timing and limiting the valve Is lift will produce a pumping loss across the valve(s) which requires energy from the vehicle via the crankshaft to overcome. In this operating mode, the inlet valve would start lifting around or after TDC and for long enough to permit induction of sufficient air to support the desired compression event for the compression braking event.

In one implementation, steps (a) to (c) are repeated after a predetermined number of subsequent piston strokes, that is, without repeating them during the predetermined number of subsequent piston strokes.

In another implementation, steps (a) to (c) are repeated over the next two piston strokes.

Since braking effort can be applied using any engine cycle, progressive braking performance can be achieved by using every crankshaft revolution, every second revolution or every third crankshaft revolution, and so on, as required. Those crankshaft revolutions for which braking mode is not deployed could be operated with conventional valve events. Alternatively, the valve events could be omitted altogether, which would have the effect of minimising the mass flow of cool gas through the after treatment system.

Braking events may be deployed in a cylinder in association with every crankshaft revolution. This mode potentially provides maximum engine braking effort by means of a '2 stroke' mode where high pumping loss induction, compression and charge release phases are all accomplished in one crankshaft revolution.

The at least one exhaust valve may be opened at the start of the exhaust valve lift event before the at least one inlet valve has closed at the end of the inlet valve lift event.

Cylinder filling may be enhanced early in a compression stroke when exhaust back pressure is high by opening the exhaust valve towards the end of the inlet valve event. This allows airflow from the exhaust to be recovered into the cylinder due to high exhaust back pressure. This enhanced cylinder filling leads to increased compression energy consumption and thus increased overall retarding torque applied to the crankshaft following compression release The at least one exhaust valve may be opened at the start of the exhaust valve lift event, and then close and open again during the first stroke of the piston from BDC to TDC. Accordingly, the exhaust valve may be controlled to not only release cylinder pressure during a later portion of the first stroke of the piston from BDC to TDC, but may also be opened during an earlier portion. The exhaust valve lift event during the earlier portion of the stroke may overlap with the inlet valve lift event, to provide increased compression energy consumption as described above.

The at least one exhaust valve may be opened during the first stroke of the piston from BDC to TDC, after the at least one inlet valve has closed at the end of the inlet valve lift event This may provide compression energy release, by opening the exhaust valve after the inlet valve has closed In some implementations, the at least one exhaust valve is then kept open until it is closed after the end of the first stroke of the piston from BDC to TDC. The at least one exhaust valve may be kept open throughout the next stroke of the piston from TDC to BDC. Furthermore, the at least one exhaust valve may be kept open until it is closed after the end of the next stroke of the piston from BDC to TDC occurring after the first stroke of the piston from BDC to TDC. The duration of the exhaust valve lift event may be varied to meet different requirements. For example, it may be held open beyond the end of the stroke of the piston from BDC to TDC which immediately follows an opening event of the inlet valve, so as to cause pumping losses during the next stroke from TDC to BDC. In a further variation, the exhaust valve may continue to be open during the following stroke from BDC to TDC to release air from the cylinder again within the same valve opening event.

The at least one cylinder may have a set of inlet valves, and steps (a) to (c) may only be carried out by a subset of the set of inlet valves. Steps (a) to (c) may then be carried out over the next two piston strokes by a different subset of the set of inlet valves.

The at least one cylinder may have a set of exhaust valves, and steps (a) to (c) may only be carried out by a subset of the set of exhaust valves. Steps (a) to (c) may then be carried out over the next two piston strokes by a different subset of the set of exhaust valves.

One or multiple inlet valves may be used on a cylinder to provide braking events. Only one or a plurality of exhaust valves on a cylinder may be actuated during engine braking.

The internal combustion engine may include a set of cylinders, and steps (a) to (c) may only be carried out by a subset of the set of cylinders. Steps (a) to (c) may be carried out over the next two piston strokes by a different subset of the set of cylinders Where subsets of a set are referred to above, the subsets may consist of different members to each other, or may share some members.

Engine braking may only be carried out in selected engine cylinders, whilst the others operate under normal "over-run" conditions. The over-nn and retarding cylinders may be cycled during a longer braking event.

In some examples of the present disclosure, the method may include the steps of: generating a pressure signal responsive to the pressure inside the cylinder; inputting the pressure into a valve actuator controller; and controlling the valve actuators with the valve actuator controller having regard to the pressure signal Preferably, the cylinder pressure at the point of braking inlet valve opening should not be higher than the inlet manifold pressure. The system may include in-cylinder pressure as an input to the valve operating system controller. The in-cylinder pressure can be directly measured using a pressure sensor or estimated by a control system of the engine with regard to the current operating point of the engine.

The present disclosure also provides a valve control system for an internal combustion engine, wherein the system is arranged to control the valve actuators so as to carry out a method as described herein.

When using an electromagnetic rotary actuator to operate a valve, valve lift events can be provided either by full rotation of the actuator or by means of "bounce" events. In a bounce event, the actuator rotates to open the associated valve to a desired lift, before reversing the direction of rotation of the actuator rotor to return it to is park position and thereby close the valve. Introducing an exhaust valve event (either by full rotation or using a bounce event) at other points in the cycle which would not normally be used, for example at the end of a compression stroke of the cylinder, results in the compression stroke energy having a retarding effect on the crankshaft and vehicle The valve control system may include at least one valve actuator which is a rotary actuator, the rotary actuator having a driven rotor which carries a cam that defines a cam surface, and a linkage for coupling the cam surface to a valve stem so as to translate lift of the cam surface into opening motion of the valve, and wherein the cam surface has a base portion which corresponds to a valve closed position, and a first raised portion of the cam surface which corresponds to a valve open position and has a constant radius In some implementations, the cam surface may have a second raised portion which corresponds to a valve open position, and the cam surface rises from the base portion to the first and second raised portions in opposite circumferential directions.

to The rotary actuator may have a cam mounted on its rotor which has a different cam profile depending on which way the rotor is rotated from its park position. For example, a first cam surface may be configured to provide valve events appropriate for normal engine running conditions, whilst a second cam surface may be intended for use in opening the associated valve to create a retarding torque lift event. Thus, Is the second cam profile may be selected for the particular purpose of providing a retarding torque event. For example, the profile may include a "dwell" period at a selected valve lift, with a dwell period corresponding to a section of the cam surface of constant radius. With no cam eccentricity over a dwell period, the very high pressure difference, for instance across an exhaust valve when cylinder pressure is high but exhaust manifold pressure low cannot produce a resultant torque which would try to close an exhaust valve. The valve is effectively mechanically locked in position. This minimises the actuator power requirement and the actuator size, and most importantly ensures operation of the function as intended. The cam profile could include a detent detail in the dwell region, or an additional mechanical arrangement may be provided, as an additional safeguard to ensure reliable selection of the dwell region. The additional mechanical arrangement may for example include a latch or pawl which is arranged to engage with a surface defined by a component carried by the rotor, so as to promote reliable selection of a predetermined rotational position of the rotor which corresponds to the dwell region.

The second cam profile may have a lower maximum height (corresponding to a lower valve lift) than that of the first cam surface. The dwell section of the second cam profile may have a lower height than the maximum height of the first cam surface.

A cam surface profile used for normal inlet valve events may be used for inlet valve braking events, as valve-to-piston contact is not an issue and the pressure drop across the inlet valve(s) is relatively small In other implementations, a dedicated cam surface profile may be used for the inlet valve braking events. The inlet valve cam surface may be configured to carry out bounce events only, or may be suitable for both bounce and full rotation events.

Although examples are described herein in which the independently controllable io valve actuators are in the form of electromagnetic rotary actuators, it will be appreciated that the valve actuators may take the form of other electric machines, or hydraulic or pneumatic actuators could be employed.

The present disclosure also provides an internal combustion engine including a valve control system as described herein. A vehicle comprising such a control system or engine is also provided. According to a further aspect, a recording medium is provided which stores computer interpretable instructions for causing a processor of a valve control system to perform a method disclosed herein.

Brief Description of the Drawings

A known engine control system and timing, and implementations of the present disclosure, will now be described by way of example and with reference to the accompanying schematic drawings, wherein.

Figure 1 is a block diagram of a known engine control system including a valve control system; Figure 2 shows timing diagrams illustrating conventional engine valve events; Figures 3 to 6 illustrate valve events according to examples of the disclosure; Figure 7 represents a valve actuator cam profile according to an example of the disclosure; and Figure 8 shows valve actuator opening and closing cam profiles according to examples of the present disclosure.

Detailed Description of the Drawings

Figure 1 shows an engine control system including a valve control system in combination with a cylinder of a well-known internal combustion engine configuration. 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 within cylinder head 8 into the combustion chamber is controlled using inlet poppet valve 12. Exhaust poppet valve 14 allows exhaust gases to escape from the combustion chamber after combustion has taken place, with the exhaust gases being carried away via exhaust port 16.

Both the inlet valve and the exhaust valve are individually electronically controllable, independently of the rotation of the engine crankshaft. An actuator 30 is provided to operate the inlet valve and actuator 32 operates the exhaust valve.

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. This is responsive to signals from various transducers monitoring the operating conditions of the engine. For example, they may monitor the crankshaft position, the coolant temperature, the oil temperature, the engine speed, the engine's 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. In practice, control units 34 and 40 may be physically separate units or integrated into a single controller. Valve control unit 40, together with an actuator power electronics module 42 and the actuators 30 and 32, are part of a valve control system controlling the operation of the inlet and exhaust valves 12, 14.

Having regard to control signals from the engine control unit, the valve control unit 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.

Figure 2 shows timing diagrams to illustrate examples of typical exhaust and inlet events. It can be seen that the exhaust valve opens shortly before BDC and the start of to the exhaust stroke and then closes shortly after TDC at the end of the exhaust stroke The inlet valve opens shortly before TDC at the end of the exhaust stroke and start of the induction stroke and then closes shortly after BDC at the end of the induction stroke.

Figure 3 shows timing diagrams to illustrate valve events for 'compression release' retarding events using the 4-stroke cycle. Normal inlet and exhaust events (I and E) are scheduled with an additional exhaust event shown towards the end of the compression stroke (A). This additional exhaust event allows the release of the compressed air in the cylinder, preventing it from returning energy to the crankshaft.

The diagram also shows an optional exhaust event (B) which begins just before the inlet valve closes. This exhaust event provides an opportunity to recover air back into the cylinder from the exhaust system when the pressure in the exhaust is sufficiently high to increase the compression energy consumed.

The exhaust event at A may use a dwell region of a valve opening cam surface (having a constant radius) of an implementation using a rotary valve actuator to ensure that the valve remains open as required and is not forced closed by the pressure drop across it.

Whilst an engine is running in this retarder mode, the normal exhaust valve event (E) is optional in each 4-stroke cycle, as inclusion or omission of this event may not have a material effect on the torque exerted by the cylinder onto the crankshaft.

Figure 4 shows timing diagrams to illustrate valve events for 'compression release' retarding events using a 2-stroke cycle. Inlet valve opening events (C) are scheduled during each piston down stroke so as to increase work done against pumping losses These valve events may have a reduced lift as shown at (C) Compression release exhaust valve events are scheduled towards the end of the compression strokes (A). As in Figure 2, an optional exhaust valve event is shown (B) which allows air to be recovered into the cylinder from the exhaust manifold at higher pressure in order to increase the compression stroke energy consumption.

io Figure 5 shows an alternative 2-stroke retarder operating scheme. Inlet valve events (C) are scheduled during each piston down stroke to increase work done to overcome pumping losses on every piston down stroke. These inlet events may be at full lift or at a reduced lift as shown in this example (C). Compression release exhaust valve events are scheduled during the compression stroke (A) at a lift and timing Is appropriate to the retarding torque required.

Figure 6 shows an alternative valve event scheme on a 4-stroke basis. The flexibility of the system allows the possibility of holding an exhaust valve open as shown at D The lift may be in the range 1 to 3mm for example, but the exact lift will vary to suit each application and the retarding effort required from each valve event.

The early part of the D event acts to release the compression up to TDC at 720 degrees.

Exhaust gas is recycled into the cylinder between 720 and 900 degrees and released again into the exhaust between 900 and the next TDC at 1080 degrees. A pumping loss is incurred for the exhaust reverse flow into the cylinder and for the subsequent flow from the cylinder into the exhaust. The exhaust valve can be held open for some or all of the event duration indicated at D to suit the retarding torque requirement A cam 68 providing a cam surface 70 for opening an exhaust valve using a rotary actuator is shown in Figure 7. In a desmodromic system, separate cams may be provided for opening and closing each valve.

In Figure 7, a cam follower 72 is shown in contact with the cam surface 70. When the cam is in its park position with the valve closed, the cam follower is in contact with the base circle 74. When the cam is driven anti-clockwise from this position by the valve actuator, the cam follower will move up onto the main lift sector 78 via the main lift ramp 76 to carry out a normal engine running exhaust valve event. While a main lift dwell period 80 is shown in Figure 1, the cam follower may not reach this point as only 95%-98% of the maximum lift of the cam profile may be employed during normal engine running.

If the cam is rotated from its park position in a clockwise direction, the cam follower travels up the exhaust lift ramp 82 onto the exhaust lift sector 84, and then onto the exhaust dwell sector 86. The exhaust dwell sector has a constant radius and therefore corresponds to a constant lift, so that increasing cylinder pressure does not apply a Is turning moment to the cam and the valve position is stable.

In the example shown in Figure 7, the different sectors of the cam profile may have the following relative proportions by way of example: Base circle 74: 65-70 degrees Main lift ramp 76: 20 degrees Main lift sector 78: 150-167.5 degrees Main lift dwell 80: 0-10 degrees Blend segment 88: 40-50 degrees (with a convex portion having a radius of 1-2 mm and a concave portion with a radius of -16 to 18 mm) Exhaust dwell segment 86: 10 degrees Exhaust lift sector 84: 40 degrees Exhaust lift ramp 82: 10 degrees Further cam profiles are shown by way of example in Figure 8. Example (a) is an exhaust opening cam 90, with (b) representing the corresponding closing cam 92. Figure 8(c) shows the opening and closing cams positioned coaxially as they are in practice, with their respective cam followers 94, 96. In example (c), the cams are separated by rotation of 900 to match the locations of the corresponding cam followers.

It will be appreciated that references herein to a "first" stroke are not intended to indicate a first stroke after an engine has started, but are used to provide antecedent basis for later references to the same stroke

Claims

Claims 1. A method of operating an internal combustion engine including at least one cylinder having a piston, at least one inlet valve and at least one exhaust valve, with each valve having a respective independently controllable valve actuator for actuating the valve, wherein the method comprises the steps of (a) suspending injection of fuel into the at least one cylinder during a first stroke of the piston from top dead centre (TDC) to bottom dead centre (BDC) and the following first stroke from BDC back to TDC; (b) actuating the valve actuator of the at least one inlet valve to carry out an inlet valve lift event so that the inlet valve is open during the first stroke of the piston from TDC to BDC to introduce an air charge into the cylinder, and (c) actuating the valve actuator of the at least one exhaust valve to carry out an exhaust valve lift event so that the exhaust valve is open during the first stroke of the Is piston from BDC to TDC and the exhaust valve releases air from the air charge out of the cylinder.
2. A method of claim 1, wherein steps (a) to (c) are repeated after a predetermined number of subsequent piston strokes.
3. A method of claim 1, wherein steps (a) to (c) are repeated over the next two piston strokes.
4. A method of any preceding claim, wherein the at least one exhaust valve is opened at the start of the exhaust valve lift event before the at least one inlet valve has closed at the end of the inlet valve lift event.
5. A method of claim 4, wherein the at least one exhaust valve is opened at the start of the exhaust valve lift event, and then closes and opens again during the first 30 stroke of the piston from BDC to TDC.
6. A method of any preceding claim, wherein the at least one exhaust valve is opened during the first stroke of the piston from BDC to TDC, after the at least one inlet valve has closed at the end of the inlet valve lift event.
7. A method of claim 6, wherein the at least one exhaust valve is then kept open until it is closed after the end of the first stroke of the piston from BDC to TDC.
8. A method of claim 7, wherein the at least one exhaust valve is kept open throughout the next stroke of the piston from TDC to BDC.
9. A method of claim 8, wherein the at least one exhaust valve is kept open until it is closed after the end of the next stroke of the piston from BDC to TDC occurring after the first stroke of the piston from BDC to TDC.
Is 10. A method of any preceding claim, wherein the at least one cylinder has a set of inlet valves, and steps (a) to (c) are only carried out by a subset of the set of inlet valves.
11. A method of claim 10, wherein steps (a) to (c) are carried out over the next two piston strokes by a different subset of the set of inlet valves.
12. A method of any preceding claim, wherein the at least one cylinder has a set of exhaust valves, and steps (a) to (c) are only carried out by a subset of the set of exhaust valves.
13. A method of claim 12, wherein steps (a) to (c) are carried out over the next two piston strokes by a different subset of the set of exhaust valves.
14 A method of any preceding claim, wherein the internal combustion engine includes a set of cylinders, and steps (a) to (c) are only carried out by a subset of the set of cylinders.
15. A method of claim 14, wherein steps (a) to (c) are carried out over the next two piston strokes by a different subset of the set of cylinders
16. A method of any preceding claim including the steps of: generating a pressure signal responsive to the pressure inside the cylinder; inputting the pressure into a valve actuator controller; and controlling the valve actuators with the valve actuator controller having regard to the pressure signal.
17. A valve control system for an internal combustion engine, wherein the system is arranged to control the valve actuators so as to carry out a method of any preceding claim.
18. A system of claim 17, wherein the system includes at least one valve actuator which is a rotary actuator, the rotary actuator having a driven rotor which carries a cam that defines a cam surface, and a linkage for coupling the cam surface to a valve stem so as to translate lift of the cam surface into opening motion of the valve, and wherein the cam surface has a base portion which corresponds to a valve closed position, and a first raised portion of the cam surface which corresponds to a valve open position and has a constant radius.
19. A system of claim 18, wherein the cam surface has a second raised portion which corresponds to a valve open position, and the cam surface rises from the base portion to the first and second raised portions in opposite circumferential directions.
20. An internal combustion engine including a valve control system of any of claims 17 to 19.