WO1992007174A1

WO1992007174A1 - Valve control apparatus

Info

Publication number: WO1992007174A1
Application number: PCT/GB1991/001804
Authority: WO
Inventors: David Burke; Giles Muddell
Original assignee: Lotus Cars Ltd.
Priority date: 1990-10-16
Filing date: 1991-10-16
Publication date: 1992-04-30

Abstract

The invention relates to valve control apparatus for controlling the opening and closing of an inlet and/or exhaust valve of an engine or compressor. With reference to Figure 2, the apparatus comprises a piston and cylinder arrangement (40) having at least one chamber (44) and a piston (42) connected to the inlet or exhaust valve (21, 22) such that motion of the piston (42) within the cylinder (41) causes motion of the inlet or exhaust valve (21, 22). The apparatus also comprises means (11) for connection to a source of pressurised fluid (12) and means (13) for connection to an exhaust for pressurised fluid (14). The apparatus further comprises an electrical or electronic control device (23) and a valve means (100) controlled by the control device (23). The valve means (100) has three operating conditions, a first in which the valve means (100) connects a first chamber (44) of the piston and cylinder arrangement (40) to the source of pressurised fluid (12), a second in which the valve means (100) connects the first chamber (44) to the exhaust for pressurised fluid (14) and a third in which the valve means (100) isolates the first chamber (44) to prevent flow of fluid into or out of the chamber. The control device (23) controls motion of the piston (42) by selectively controlling the valve means (100) to operate in one of the three operating conditions and by controlling the length of time that the valve means (10O) operates in the chosen operating condition.

Description

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The invention relates to valve control apparatus for controlling the opening and closing of an inlet and/or exhaust valve of an engine or compressor. The invention particularly relates to apparatus for controlling the opening and closing of a valve which comprises a hydraulic actuator.

The present invention will be discussed in an application in a reciprocating internal combustion (IC) engine. However, the invention should not be considered limited to such an application. The present invention could be used in any engine which has valves to control flow of fluid or gas or any compressor having valves to control flow of fluid or gas.

In reciprocating IC engines the opening and the closing of valves in the cylinder head of the engine are controlled by use of cam shafts. These cam shafts are machined each with a particular profile. The profile determines the reciprocal motion of the valves. The valves are generally held in contact with the cam shaft by resilient means and move into and out of engagement with their respective valve seats in a manner determined by the cam profiles of the camshafts.

The ideal valve for use as inlet or exhaust vlave would be a valve tiat was either open or closed, with no intermediate position. Having a valve partially open leads to pumping losses, since the flow of gases into or out of the combustion chamber is restricted. However, the deceleration of valves has been limited in the past by the need to maintain the integrity of mechanical linkages operating the valve. In usualy systems, only a certain limit of deceleration of the valve is possible before the valve stem becomes separated from the mechanical member acting upon it.

Systems have been proposed in the past which provide an alternative to the activation of valve gear by cam shafts. Such an alternative can be found in U.K. Patent GB 2061560B in the name of Daimler-Benz. In the systems described in GB 2061560B a valve element is connected to a working piston which can move within a cylinder. The position of the valve element is varied by the movement of the working piston. The working piston is subjected to a pre-tensioning by a spring which biases the piston to move in one direction. The movement of the piston is controlled by the supply of pressurised medium into a chamber defined by one surface of the piston and the inner surface of the cylinder. The force acting on the piston due to the presence of pressurised medium in the chamber acts against the resilient means. A system of hydraulic logistors is used to control the supply of the pressurised medium to the chamber and thereby controls the movement of the valve element.

In another example of a system of hydraulically actuated valves can be seen in in US 3963006. The valves are actuated by a mechanical control system which is powered from the engine crankshaft.

In U.S. 4162883 a system is described in which a valve is controlled by supplying pressurised fluid to one side of the piston, the force applied by the pressurised fluid acting against a biasing force supplied by the spring. The valve is controlled mechanically by selectively applying pressure to the piston, the supply of fluid being controlled mechanically by a plunger and valve system.

The applicant has developed a system of hydraulically operable inlet and exhaust valves which has been described in U.K. application 9022440.3. The system described uses a servo-valve to accurately control the position of the piston with the piston/cylinder arrangement. The system along with the other prior art systems mentioned above has practical disadvantages of complexity and power consumption.

The present invention provides valve control apparatus for opening and closing an inlet or exhaust valve of an engine or compressor comprising; a piston and cylinder arrangement comprising at least one chamber and a piston connected to the inlet or exhaust valve such that motion of the piston within the cylinder causes motion of the inlet or exhaust valve; means for connection to a source of pressurised fluid; means for connection to an exhaust for pressurised fluid; an electrical or electronic control device; and a valve means controlled by the control device, wherein the valve means has three operating conditions; a first operating condition in which the valve means connects a first chamber of the piston and cylinder arrangement to the source of pressurised fluid, a second operating condition in which the valve means connects said first chamber to the exhaust for pressurised fluid, and a third operating condition in which the valve means isolates the said first chamber to prevent fluid flowing into or out of the chamber, and wherein the control device controls motion of the piston within the cylinder by selectively controlling the valve means to operate in one of the three operating conditions and by controlling the length of time that the valve means operates in a chosen operating condition.

In a first preferred embodiment the valve means operates in the first operating condition to connect a second chamber to the exhaust for pressurised fluid and in the second operating condition to connect the second chamber to the source of pressurised fluid.

In a second preferred embodiment a resilient means is provided in a second chamber to bias the piston in one direction such that when the first chamber is connected to the source of pressurised fluid by the valve means the flow of fluid into the first chamber causes the piston to move against the biasing force provided by the resilient means.

Preferably the control device is programmed to control the valve means to cause the inlet or exhaust valve to open and close when the engine or compressor is at selected stroke positions.

Preferably the control device is also programmed to control the valve means to cause the inlet or exhaust valve to open and close at different stroke positions of the engine for different operational speeds of the engine or compressor. When the apparatus is used in an engine the control device is preferably further programmed to control the valve means to cause the inlet or exhaust valve to open and close at different stroke positions for different displacements of the throttle and or accelerater pedal controlling the engine.

In the first preferred embodiment the valve means preferably comprises a four port valve with two ports connected one each to the two chambers, one port connected to a source of pressurised fluid and one port connected to an exhaust for pressurised fluid.

In the first preferred embodiment the control device preferably comprises a processor for controlling the four port valve by electrical control signals, which processor can generate three different control signals each of which instructs the four port valve to operate in one of the three operating conditions.

In the second preferred embodiment the valve means preferably comprises a three port valve with one port connected to the said first chamber, one port connected to the source of pressurised fluid and one port connected to the exhaust for pressurised fluid.

In the second preferred embodiment the control device preferably comprises a processor for controlling the three port valve by electrical control signals, which processor can generate three different control signals each of which controls the three port valve to move to one of the three operating conditions.

The processor of the first or second preferred embodiments preferably receives a signal indicative of the stroke position of the engine or compressor and adjusts the control signal it generates according to the stroke position. The processor then preferably differentiates the signal indicative of the stroke position to obtain a signal indicative of the operational speed of the engine or compressor and the processor adjusts the generated control signal according to the operational speed. When the apparatus is used in an engine the processor preferably also receives a signal indicative of the displacement of the throttle and/or accelerater pedal controlling the engine from a fixed reference and adjusts the control signal according to the throttle and/or accelerator pedal displacement.

The present invention also provides a method of throttling an internal combustion engine using an inlet valve controlled by engine valve control apparatus as hereinbefore described.

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings in which;

Figure 1 is a schematic representation of one embodiment of the engine valve control apparatus.

Figure 2 is a schematic representation of a second embodiment of the engine valve control apparatus.

Figure 3 shows a flow diagram which outlines the mode of operation of the control processor of the engine valve control apparatus.

Figures 4 to 8 show the relationship between the control signals of the system and the actual motion of the controlled inlet or exhaust valve.

Referring to Figure 1 the system can be seen to comprise a valve means 10 which is connected by a line 11 to a source of pressurised hydraulic fluid 12 which in the preferred embodiment is a pump powered by the engine. The valve means 10 is also connected via a line 13 to an exhaust for pressurised fluid 14, which in the preferred embodiment is a sump. The valve means 10 is further connected via a line 15 to the top chamber 16 of a cylinder 17 which is divided in two by a piston 18. The valve means 10 is also connected via a line 19 to the bottom chamber 20 of the cylinder 17. The piston 18 is connected via a rod 21 to a valve 22.

The valve 22 is a valve of an internal combustion engine which controls the opening and closing of an aperture in the cylinder head of the engine. The valve could be the inlet valve controlling the flow of fuel/air mixture into a cylinder or could be an exhaust valve controlling the flow of exhausted gases from the cylinder. In the multi-valve engines of current cars, several such valves could open and close several apertures opening onto a cylinder of an engine.

The valve means 10 is controlled by an elctrical or electronic control device 23 via line 24. The control device 23 controls the control valve 10 by electrical control signals.

The control device 23 comprises in the preferred embodiment a digital processor which uses pre-programmed control algorithms to derive the control signals it generates. The algorithms use information provided by two input signals provided by lines 25 and 26. Line 25 conveys to the control device 23 a signal representative of the instantaneous position of a piston of the engine with respect to its encompassing working cylinder, that is to say the stroke position of the engine. This signal may be produced by a measurement means which measures the rotational position of the crankshaft. Line 26 conveys to the processing means 23 a signal representative of the position of the throttle and/or accelerator pedal controlling the engine. The latter signal can be obtained by for instance measuring the displacement of a vehicle accelerator pedal from an arbitrary fixed reference.

The processing means 23 calculates a "demanded opening period" signal. Examples of such "demanded opening period" signals can be see in Figures 4 to 8. The demanded opening period signals are square edge signals of a pulse width T. The control device varies the pulse width T in response to varying input signals and in accordance with preprogrammed instructions.

A flow-chart showing the method of operation of the described preferred embodiment of the engine valve control apparatus can be seen in Figure 3.

The piston position signal is fed into the control device 23 via the line 25, whilst the throttle position and/or acclerator pedal position is fed into the control device 23 by the line 26.

The control device 23 in the preferred embodiment operates in a digital manner repeating an algorithm at clocked intervals. The control device 23 firstly differentiates the engine stroke position signal provided by line 25 in box 32. By differentiating the engine stroke position signal, the algorithm calculates the revolutionary speed of the engine in revolutions per minute (RPM) . The RPM signal is provided by line 27 to the next stage of the control device, the valve timing look up table 28.

Three signals are input into the valve timing look up table; the stroke position signal, the revolutionary speed signal and, in the preferred embodiment, the throttle position signal (or accelerator pedal position) .

By referencing the three signals onto pre-programmed information, the table 28 is able to provide a control signal for control of the valve means 10. The processor 23 can determine from the two signals input by the lines 25 and 26 whether the controlled engine inlet or exhaust valve should be opened or closed and also can determine the duration for which the engine inlet or exhaust valve should be opened or closed.

After processing the input signals, the valve timing look up table is able to provide a signal at 29 which indicates the desired engine valve position. The box 30 considers the signal provided by the line 29 to decide whether the engine valve should be opened, closed or maintained in its present position. The corresponding "open", "maintain" or "close" position signals are then output via line 24 to the valve means 10.

When the engine valve control apparatus shown in Figure 3 is used in the embodiment of Figure 1, the "open" position corresponds to the position in_; which the engine valve 22 is being opened. The source of pressurised fluid 12 is connected by the valve means 10 to the line 15 and therefore to the chamber 16. At the same time, the valve means 10 connects the chamber 20 to the exhaust for pressurised fluid 14.

Referring again to Figures 1 and 3, when a "maintain" position signal is provided by the line 24, the engine valve 22 is kept stationary, either fully open or fully closed. This is achieved by controlling the valve means 10 to connect the connecting lines 15 and 19 to null.

Once again referring to Figures 1 and 3, when the "close" position signal is provided by line 24, the valve means 10 connects the chamber 20 via the line 19 to the source of pressurised fluid 12. The valve means 10 simultaneously connects the chamber 16 via line 15 to the exhaust 14. The valve means 10 is therefore controlled to close the engine valve 22.

The valve means 10 of the invention is a simple valve means, needing only three operating conditions. The valve means 10 need not meter the flow of fluid to and from the piston and cylinder arrangement to control the velocity of the piston. In the embodiment of Fig 1 the valve means 10 is a four port valve which may be switched between three operating conditions.

The "demanded opening period" signal output from the processor via line 24 is a square edged signal. The signal corresponds to ideal performance for a valve. Automotive designers would like valves to have two positions, open and closed, with no intermediate positions. Having such "full lift" or "full closure" prevents losses that are usually incurred in drawing air or exhausting gases over a smaller area during the intermediate positions between full opening and full closure.

The "demanded opening period" signal generated by the control algorithms of the control device 23 controls the valve means 10. The valve means 10 is simple in construction, having only three operating conditions. The valve means 10 can connect the chamber 16 of the cylinder 17 to the source of pressurised fluid 12 or can connect the chamber 16 to the exhaust for pressurised fluid 14 or connect the pressure source 12 to exhaust 14. The valve means 10 acts in a similar manner in relation to the chamber 20 of the cylinder 17.

A relief valve 31 is included in the engine valve control apparatus to limit the pressure of the fluid supplied by the sump 12 to the valve means 10.

The response of the engine valve control apparatus to the "demanded opening period" signal shall now be be discussed.

Referring to Figure 4 the Y axis corresponds to

"valve lift" and the X axis is a time axis. At a time

T_Q1 the "demanded opening period" signal calculated by the processing means 23 flips from a zero value to an Xmax value. The zero value corresponds to a position of the valve wherein the valve is in contact with its associated valve seat. The X I_uc_Λ_X_r position corresponds to the maximum opening position of the valve. The "demanded opening period" signal is plotted as line 34 in Figure 4.

At T₀₁ the piston 18 will be at its uppermost position with respect to the cylinder 17. At time T. the control device 23 will instruct the valve means 10 to connect the line 15 to the source of pressurised hydraulic fluid 12. Simultaneously the processing means 23 instructs the valve means 10 to connect the line 19 to the exhaust 14. In this way pressurised hydraulic fluid is applied to the side 16 of the piston 18, whilst the fluid on the side 20 of the piston 18 is allowed to flow out of the cylinder through the line 19 to the exhaust 14. The pressure difference causes the piston to move downwardly with respect to the cylinder 17. The plot of actual valve movement can be seen as line 35 in Figure 4.

It can be seen from Figure 4 that the actual motion of the piston 18 does not correspond to the "demanded opening period" calculated by the control algorithms of processing means 23. The practical system is not a perfect system and therefore a time lag is introduced. Immediate response is not possible with a physical system. The valve does not achieve a fully opened position X___χ until time T_Q2, with a delay from the time _Q1.

At time T_. the "demanded opening period" signal flips from the X__aχ position to the zero position. The control device 23 at this time instructs the valve means 10 to supply pressurised hydraulic fluid via line 19 to the chamber 20. Simultaneously the processing means 23 instructs the valve means 10 to connect the chamber 16 in the cylinder 17 to the exhaust 14. In this way a difference in pressure is established across the piston 18, which pressure difference causes the piston 18 to move upwardly with respect to the cylinder 17 and thereby to bring the valve 22 back into engagement with its associated valve seat.

As mentioned previously the physical system is not a perfect system and therefore the response of the valve to the control signal does not correspond to the "demanded opening period" signal. The actual response can be seen on line 35 in Figure 4. The actual time at which the valve comes into contact with valve seat T_C2 is delayed with respect to the time T_c_ (T-. being the time when the "demanded opening period" signal reverts to zero) .

The control signal and response shown in Figure 4 corresponds to a situation of low engine speed. The valve at this speed is kept open for a long time.

With increasing engine speed the time period of the "demanded opening period" pulse decreases, as can be seen in Figures 5 to 8. If the valve controlled is an inlet valve then the area under the plot of valve motion is a function of the amount of fuel/air mixture flowing into a cylinder.

As the pulse width of the "demanded opening period" signal shortens the actual physical response of the valve falls well behind the "demanded opening period" signal. For the pulse period T6 shown in Figure 6 the valve only just reaches its X__aχ position before the processing means 23 instructs the valve means 10 to bring the valve back into contact with its valve seat. For the short pulses T7 and T8 the valve never reaches its fully opened position.

In one embodiment of the invention the characteristics shown in Figures 4, 5, 6, 7 and 8 are used to provide an alternative to throttling of the air/fuel mixture entering a cylinder as a means of controlling engine power output. Since the area under the curve of actual valve position is a function of the amount of fuel/air mixture flowing into a cylinder provided that valve is the inlet valve, the amount of fuel/air mixture entering a cylinder can be controlled by the processing means 23, rather than by using a conventional "butterfly" throttle. For this system an additional input signal is provided to the processor means, which signal is indicative of the position of a throttle controlled by the operator of the engine.

The throttle signal can be seen defined by line 26 in Fig 1.

In an automobile the relevant throttle is the acceleration pedal. The position of the pedal can be measured by suitable measurement means which generates a signal indicative of the degree of displacement of the accelerator pedal by the driver of the vehicle. The signal is supplied to the control device which accordingly varies the control signal it supplies to control the inlet valves of the controlled engine. The longer the duration of the signal, the more fuel/air mixture is supplied. Obviously the length of the signal can only be controlled within a certain duration since the maximum possible duration is limited by other factors such as engine speed.

The throttle measurement signal is not essential to the invention since a standard butterfly throttle could be used to throttle the engine, in which case the processor 23 need only receive a signal indicative of engine stroke position.

A second preferred embodiment of the invention can be seen in Figure 2. By and large the system shown in Figure 2 corresponds with the first embodiment shown in Figure 1 and identical components will be referenced with numbers equivalent to the numbers used to describe the first embodiment. Figure 2 shows a source of pressurised fluid 12, a control device 23, an exhaust for pressurised fluid 14 and a relief valve 31. Each of these elements has been described above in relation to the first preferred embodiment.

As in the first embodiment, the control device 23 receives two signals via lines 25 and 26. The signal received via line 23 corresponds to engine stroke position and the signal received via line 26 corresponds to throttle position. The control device 23 controls the operation of the a valve means 10 via line 24.

The essential difference between the operation of the second preferred embodiment shown in Figure 2 and the first embodiment shown in Figure 1 arises due to the provision of an alternative piston and cylinder arrangement 40. The piston and cylinder arrangement 40 comprises a cylinder 41 in which there is disposed a piston 42 which separates the cylinder into two chambers 43 and 44. The piston 42 is biased in one direction by resilient means such as a spring 45.

By providing resilient means within the piston and cylinder arrangement 40 the second preferred embodiment simplifies the operation of the engine valve control apparatus, since the flow of hydraulic fluid into the piston and cylinder arrangement 40 need only be controlled via one supply line 45. The first embodiment requires two supply lines 15 and 19.

The control device 23 of the second preferred embodiment operates in the manner shown in Figure 3. However, the three possible instruction signals "open", "maintain" and "close" have a different function in the second embodiment shown in Fig 2.

The "open" position control signal instructs the valve means 100 to connect the chamber 44 to the source of pressurised fluid 12. The pressurised fluid introduced into the chamber 44 acts against the resilient means 45 to force the engine inlet or exhaust valve 22 away from its associated valve seat.

The "maintain" position signal instructs the valve means 100 to close off the chamber 44 so that no fluid can escape from the chamber 44 nor can any fluid enter the chamber 44. The engine valve 22 is therefore kept stationary.

The "close" position signal instructs the valve means 10 to connect the chamber 44 to the exhaust for fluid 14. The resilient means 45 urges the piston 42 to dispel fluid from the chamber 44 through the line 46 to the exhaust 14. The resilient means (spring 45) thereby acts to bring the engine inlet or exhaust valve back into contact with its associated valve seat.

The valve means 100 comprises in the second embodiment a three port valve having three operating conditions, as discussed above, the valve means 100 can be simple and robust in construction since it need not meter flow into and out of the chamber 44 to control the velocity of the piston 42.

Whilst the description above mentions the use of hydraulic fluid it should be appreciated that the hydraulic fluid used can be conventional motor; oil. pressurised by a conventional oil pump.

Use of the system mentioned above improves engine characteristics at low rates of engine revolution. Theoretically at 2000 revs the BMEP of an engine can be increased by as much as 60 % by using the control variable inlet valve used only (not exhaust) apparatus of the present invention. At speeds approaching the peak BMEP speed of a normal mechanical engine with a cam shaft the systems efficiency corresponds by and large with the normal mechanical efficiency. The improvement in performance therefore is obtained largely at low engine speeds.

Whilst above the invention has been discussed with reference to its use in a reciprocating internal combustion engine the invention should not be considered limited to such a use. Obviously the invention works equally well in a reciprocting compressor. The invention can be used in any engine or compressor to control valves therein.

The present invention provides a significant improvement over the systems of the prior art, since it is uncomplicated and involves a simple valve means which has just three operating conditions. These switching valves are less costly and simpler in construction than continuously variable servo-valves which were used in some of the prior art systems. The continuously variable servo-valves were required by some of the prior art systems since the engine valve control apparatus of the systems controlled the velocity of the piston with the piston and the cylinder arrangement, since the valves have just three operating conditions they can be fast acting. Also their simplicity means that they do not require large power inputs for control purposes. They can also be made small enough for practical inclusion in an engine.

The present invention also provides a system for use in which an engine can be used to throttle an engine without the need for mechanical elements such as butterfly throttles.

Claims

CLAIMS :

l. Valve control apparatus for opening and closing an inlet or exhaust valve of an engine or compressor comprising; a piston and cylinder arrangement comprising at least one chamber and a piston connected to the inlet or exhaust valve such that motion of the piston within the cylinder causes motion of the inlet or exhaust valve; means for connection to a source of pressurised fluid; means for connection to an exhaust for pressurised fluid; an electrical or electronic control device; and a valve means controlled by the control device, wherein the valve means has three operating conditions; a first operating condition in which the valve means connects a first chamber of the piston and cylinder arrangement to the source of pressurised fluid, a second operating condition in which the valve means connects said first chamber to the exhaust for pressurised fluid, and a third operating condition in which the valve means isolates the said first chamber to prevent fluid flowing into or out of the chamber, and wherein the control device controls motion of the piston within the cylinder by selectively controlling the valve means to operate in one of the three operating conditions and by controlling the length of time that the valve means operates in a chosen operating condition.

2. Valve control apparatus as claimed in Claim 1 wherein the valve means also operates in the first operating condition to connect a second chamber to the exhaust for pressurised fluid and in the second operating condition to connect the second chamber to the source of pressurised fluid.

3. Valve control apparatus as claimed in Claim 1 wherein a resilient means is provided in a second chamber to bias the piston in one direction such that when the first chamber is connected to the source of pressurised fluid by the valve means the flow of fluid into the first chamber causes the piston to move against the biasing force provided by the resilient means.

4. Valve control apparatus as claimed in any one of the preceding claims wherein the control device is programmed to control the valve means to cause the inlet or exhaust valve to open and close when the engine or compressor is at selected stroke positions.

5. Valve control apparatus also as claimed in Claim 4 wherein the control device is also programmed to control the valve means to cause the inlet or exhaust valve to open and close at different stroke positions of the engine for different operational speeds of the engine or compressor.

6. Valve control apparatus as claimed in Claim 5 for use in an engine wherein the control device is further programmed to control the valve means to cause the inlet or exhaust valve to open and close at different stroke positions for different displacements of the throttle or accelerator pedal controlling the .engine.

7. Valve control apparatus as claimed in Claim 2 wherein the valve means comprises a four port valve with two ports connected one each to the two chambers, one port connected to a source of pressurised fluid and one port connected to an exhaust for pressurised fluid.

8. Valve control apparatus as claimed in Claim 7 wherein the control device comprises a processor for controlling the four port valve by electrical control signals, which processor can generate one of three different control signals each of which instructs the four port valve to operate in one of the three operating conditions.

9. Valve control apparatus as claimed in Claim 3 wherein the valve means comprises a three port valve with one port connected to the said first chamber, one port connected to the source of pressurised fluid and one port connected to the exhaust for pressurised fluid.

10. Valve control apparatus as claimed in Claim 9 wherein the control device comprises a processor for controlling the three port valve by electrical control signals, which processor can generate one of three different control signals each of which controls the three port valve to move to one of the three operating conditions.

11. Valve control apparatus as claimed in Claim 8 or Claim 10 wherein the processor receives a signal indicative of the stroke position of the engine or compressor and selects the control signal it generates according to the stroke position.

12. Valve control apparatus as claimed in Claim 11 wherein the processor differentiates the signal indicative of the stroke position to obtain a signal indicative of the operational speed of the engine or compressor and the processor adjusts the generated control signal according to the operational speed.

13. Valve control apparatus as claimed in Claim 11 or Claim 12 for use in an engine wherein the processor receives a signal indicative of the displacement of the throttle or accelerator pedal controlling the engine from a fixed reference and adjusts the control signal according to the throttle or accelerator pedal displacement.

14. A method of throttling an internal combustion engine using an inlet valve controlled by valve control apparatus claimed in either Claim 6 or Claim 13.

15. Valve control apparatus for controlling an inlet and/or exhaust valve of an engine or compressor substantially as hereinbefore described with reference to and as shown in the accompanying drawings.