GB2429260A - Throttle valve - Google Patents

Abstract

A throttle valve for use with an internal combustion engine comprises an inlet port 2 arranged to receive an air/fuel mixture, an outlet port 3 arranged to communicate an air/fuel mixture to an engine and a closure member 6 arranged between the inlet port and outlet port. The closure member 6 is arranged in use to cooperate with the inlet port so as to define a passage between the inlet and outlet port with an adjustable cross-sectional area. The profile(s) of the of the inlet port and/or closure member is/are adapted in accordance with at least one operating characteristic of an engine. The closure member 6 is rotated by a stepper motor 8.

Description

Throttle Valve The present invention relates to a throttle valve for use

in conjunction with an internal combustion engine and particularly, but not exclusively, to a throttle valve used to supply a fuel to an internal combustion engine forming part of a combined heat and power (CHP) unit or installation Throttle valves are well known in the art for controlling the flow of an air/fuel mixture into the inlet manifold of an internal combustion engine The fuel may be any convenient fuel including petrol, gas and so forth A throttle valve of the type described above typically comprises a butterfly valve arrangement located in a passage formed between an inlet port and an outlet port of the valve The butterfly valve is rotatably mounted in the passage so as to close or restrict the passage of the air/fuel mixture through the valve The valve is normally controlled by means of a throttle cable which is in turn connected to a throttle lever or throttle pedal for example Thus, the user can operate the valve so as to control the flow of fuel and air into the inlet manifold and thereby control the power of the engine Throttle valves are most commonly used to control the power of an engine Conventionally, the power of the engine is controlled in combination with an engine speed indicator or, in the case of a moving vehicle, a vehicle speedometer. The engine can thereby he controlled to match the particular requirements of the user For particular applications it is necessary to operate an internal combustion engine to meet specific requirements. This is particularly the case where an internal combustion engine is used as a prime mover in a combined heat and power installation where a unit is controlled in response to specific power demands In a typical CHP unit an internal combustion engine is coupled to an electrical generator The electrical generator supplies electricity to the installation and the engine cooling circuit supplies heat in the form of hot water i'he demand on the engine is a function of the total power output required from the engine It will he recognised that the demand of any installation will vary continuously over time and accurate control of the engine in response to the demands is therefore required A further complication for CHP units is that the units are generally required to operate for extended periods with little or no user intervention In particular it is desirable for the units to operate fully automatically to match supply and demand by means of computer or microprocessor control Throttle valves of the type presently available in the art do not lend themselves conveniently to the type of control required in these applications Specifically, existing throttle valves do not provide any accurate way to control an engine in response to specific demands on the engine (such as those inherent in a CHP unit) without complex and expensive feedback control arrangements and so forth.

Furthermore, existing throttle valves to do lend themselves to computer or microprocessor control which is another requirement for applications such as CHP units where automatic control is a necessity There is therefore a need for a throttle valve arrangement which overcomes at least the problems identified above and which provides a throttle valve * arrangement which can for example be employed in a CHP unit Thus, viewed from a first aspect, according to an invention described herein there is provided a throttle valve for use with an internal combustion engine comprising an inlet port arranged to receive an air/fuel mixture, an outlet port arranged to communicate an air/fuel mixture to an engine and a closure member arranged between the inlet port and outlet port and arranged in use to cooperate with the inlet port to define a passage between the inlet and outlet port with an adjustable cross-sectional area, wherein the profile(s) of the inlet or outlet port and/or closure member is(are) adapted in accordance with at least one operating characteristic of an engine Thus, the cross-sectional area of the passage between the inlet port and outlet port can be adjusted by cooperation of the closure member and inlet or outlet port so as to control the flow of an air/fuel mixture though the valve The profile is preferably formed as part of the inlet port leading into the valve However, it will be recognised that the profile may equally be applied to the outlet port and the closure member may, in this arrangement, be arranged in use to cooperate with the outet port to define a passage between the inlet and outlet port with an adjustable cross-sectional area.

In a simple case, the profile may be arranged to provide a substantially linear relationship between the cross-sectional area of the passage and the position of the closure member relative to the inlet In a more complex case, where the engine operating characteristic is engine power output and where the power output of an engine is not linearly related to the cross sectional area of the inlet outlet port, the profile may be selected so as to provide a substantially linear relationship (or some other predetermined relationship) between the relative position of the closure member relative to the inlet port and the engine power output The profile may be formed as part of the inlet port alone or as part of the closure member Alternatively, the closure member and inlet port may he formed with cooperating profiles to form a suitable overall profile The closure member may he any suitable member arranged to cooperate with the inlet port so as to define a cross-section allowing an air/fuel mixture to flow from the inlet to the outlet The closure member is preferably arranged to move between a first position where the inlet port is substantially sealed i e where the open cross-sectional area of the inlet port is substantially zero and a second position where the inlet port is unrestricted The closure member is preferably arranged to move relative to a surface of the inlet port within the valve so as to increase or decrease the cross-section of the inlet port which is open i e the cross- section of the inlet port which permits air/fuel mix to flow Preferably, the closure member is arranged to seal against an inlet port surface such that an air/fuel mixture is only permitted to pass through the open portion of the inlet port defined by the closure member The closure member may be arranged to move relative to, and cooperate with, the inlet port in any suitable way.

Preferably, the closure member is rotatably mounted For example, it may be provided on a suitable actuation means such as on a shaft passing through the valve assembly which may be connected to a drive means For example, the closure member may be in the form of a suitable butterfly valve disposed proximate the inlet port and rotatably mounted on a shaft passing through the valve housing In the case where a butterfly valve is used as the closure member, the profile may he formed as part of the inlet port only In one arrangement where the profile is formed as part of the inlet port, the profile of the inlet port may be a circular shape with an inverted U' shape portion extending from a periphery towards the centre of the circle, preferably from a lower portion of the circle periphery As the closure member moves relative to the inlet port, the cross-section of the inlet port is progressively obscured until the entire cross-section is covered by the closure member and the passage is substantially sealed It will be recognised that different profiles or geometries of the inlet port and/or closure member which are configured according to the particular operating characteristic or characteristics of an engine are within the scope of the invention The profile(s) of the inlet port and/or closure member may be determined for a particular engine using any suitable means For example the required profile(s) may be determined by experimentation or on engine manufacturer derived data The drive means is preferably disposed outside of the assembly and arranged to rotate the shaft and closure member The drive means may be any suitable component arranged to rotate the closure member between the first and second positions The drive means may be arranged so as to position the closure member at any angle between the first and second positions described above Thus, the air/fuel mixture entering the engine can be controlled in a conventional way Preferably, the closure member and drive means are arranged such that the closure member can be maintained at discrete angular positions between the first and second positions This conveniently allows a stepper motor arrangement to he employed Preferably, the closure member is controlled and positioned by an electrically powered motor connected to a shaft which is in turn connected to the closure member Most preferably the motor is a stepper motor arranged to rotate the closure member between the first and second position and to locate the closure member at discrete positions between said two positions in response to control signals Preferably, the stepper motor is coupled directly to a closure means actuator component such as a drive shaft coupled to the closure member as described above Most preferably the stepper motor is directly coupled to the closure member and not by means of a reduction gear This thereby allows the stepper motor to move the valve into a fail safe position using a suitable biasing means which biases the closure member to a closed position Thus, if there is a power failure the throttle valve moves into a fail safe position where fuel supply is cut off from the engine Furthermore, coupling a stepper motor directly to a shaft improves the response time of the valve Preferably, the stepper motor is provided with a plurality of steps between the first and second positions of the closure member Most preferably the stepper motor is arranged to provide a resolution of between 50 and 100 steps between the first and second positions of the closure members It will be appreciated that although the closure member has been generally described with reference to a rotatably mounted closure member disposed between the inlet port and outlet port other forms of closure member may be used in which the closure member cooperates with the inlet port For example a linear motor connected to a suitable closure member and port arrangement may be employed Preferably, the outlet port has a generally uniform cross-section so as to facilitate connection with an engine inlet manifold or the like The outlet port may for example have a circular cross-section.

The inlet port is preferably provided with a first portion arranged to communicate with an exterior surface of the valve housing and to provide a connection to an air/fuel mixing box (or the like) The inlet port is also preferably provided with a second portion extending into the valve housing and arranged to cooperate with the closure member In one arrangement the profile of the passage may be determined by the profile of the second portion of the inlet port The profile of the second portion may then be arranged such that there is a relationship between the position of the closure member relative to the second portion of the inlet port and the cross-sectional area of the inlet port which is open i e which permits an air/fuel mixture to flow Thus, there is provided an approximately linear relationship between the angular position of the closure member relative to the inlet port and the open cross-section through which and air/fuel mixture can flow The profile of the second portion of the inlet port may generally be of a circular cross-section and may comprise an inverted U' portion extending from the periphery of the circle towards the centre thereby defining a profile having a circular cross-section with a bell shaped portion extending towards the centre Preferably, a further relationship is defined between the flow of air/fuel mixture into an engine, resulting from a particular open cross- section of inlet port, and at least one characteristic of the engine to which the assembly is to be connected. For example, a relationship between the flow of air/fuel mixture into an engine may be determined with respect to engine temperature Preferably, in an arrangement where the engine is part of a CI-IP unit, a relationship (as discussed above) is determined between the flow of air/fuel into the engine (i e from the throttle valve) and the power output of the engine Thus, in combination with the relationship described above, a relationship can be determined between power output of a particular engine and the angular position of the closure member relative to the inlet port in the throttle valve.

The second portion of the inlet port i e the port which cooperates with the closure member is most preferably provided (i e manufactured) with a profile such that there is a linear relationship between engine power output and rotational position of the closure member relative to the inlet port Thus, a predetermined relationship between the rotational position of the closure member (and closure member drive shaft) and engine power output can be used to control an engine in response to power output demands by rotating the closure member drive shaft to specific predetermined positions For example, engine power can be controlled using a stepper motor arrangement such that each discrete position of the stepper motor (and thereby position of the closure member relative to the inlet port) provides a pre-determined power output from an engine The respective power outputs (or other characteristic(s) of the engine) for each position of the stepper motor may be stored in memory, look-up table and the like or may be determined within a suitable control arrangement using a suitable algorithm Viewed from another aspect there is provided a method of operating an internal combustion engine wherein there is provided an internal combustion engine connected to a throttle valve, the throttle valve comprising an inlet port allowing an air/fuel mixture into the valve, an outlet port arranged to allow an air/fuel mixture to flow to an engine and a closure member arranged between the inlet port and outlet port and arranged to cooperate with the inlet port to define a passage between the inlet and outlet port with an adjustable cross- sectional area, wherein the profile(s) of the inlet port and/or closure member is(are) adapted in accordance with at least one operating characteristic of an engine Viewed from a still further aspect there is provided a method of operating a combined heat and power unit comprising an internal combustion engine and a controllable throttle valve connected thereto, wherein the throttle valve receives a control signal representing a required power output from the engine and adjusts the flow of an air/fuel mixture to the engine in response to said control signal Viewed from another aspect there is provided a combined heat and power unit comprising an internal combustion engine and a controllable throttle valve connected thereto, wherein the throttle valve is arranged to receive a control signal representing a required power output from the engine and to adjust the flow of an air/fuel mixture to the engine in response to said control signal Viewed from yet another aspect there is provided a combined heat and power unit comprising an internal combustion engine connected to a throttle valve assembly, wherein a control unit is arranged to receive an indication of the required power output from said unit and to control a throttle valve assembly to in response to said indication S The combined heat and power unit is preferably provided with a control apparatus arranged to store data indicated pre-determined power outputs for each position of the throttle valve i e the position of a closure member The data may be stored in a suitable control apparatus, computer or in a look-up table Thus, the control apparatus can receive an indication of the required power output from the engine and determine, by looking-up or calculation, the required position of the closure member relative to the inlet port to meet the power demand In the arrangement described above problems associated with conventional butterfly throttle valves can be overcome For example, if a traditional butterfly valve is used and is directly actuated by a stepper motor, the relation between step angle and engine power output will be substantially non-linear i e one step at low power will be 10 times bigger then one step at high power Defining a relationship between the profiles within the valves as described in accordance with the present invention can compensate for this non-linear relationship A further problem associated with combined heat and power (CI-IP) installations relates to their use in confined spaces and specifically in sealed units of the type used in homes and industrial applications In these situations the internal combustion engine, fuel mixing unit and throttle valve (with associated control) are all disposed within a sealed sound-proofed box The only inlets to the unit are the fuel line in (for example petrol, diesel or natural gas) and air The only outlets are the exhaust gases and conduits and cables carrying the heat and electricity to the user Stnct regulations define the use of internal combustion engines and the like in sealed units (or confined spaces) particularly where gas is used so as to avoid conditions developing within confined spaces which could result in explosions or fires For example, in Denmark the law requires that a sealed unit containing an internal combustion engine, for example in the application of a CHP unit, is flushed with air both before and after operating the engine This removes any combustible gases from the box or casing which could, as discussed above, result in an explosion orfire One way to meet legal requirements such as those in Denmark is to include a valve and pump within the unit which can be used to purge the casing before and after use This arrangement, although complying with the regulations, generally adds to the expenses of the CHP unit and/or requires intervention by a user on start-up and shut-down or some form of suitable control arrangement This is particularly undesirable for situations where un-scheduled stops occur, which may for example be caused by engine failure and so forth In these situations combustible gases may remain in the casing when the engine stops This will also be the case where a gas removal arrangement is controlled by the CHP control unit as opposed to manual control by a user There is therefore a need for an arrangement which allows the casing to be conveniently purged before and after an internal combustion engine has operated without the intervention of the user and which does not require additional complex valve(s) and/or control apparatus Preferably, the throttle valve arrangement described above is conveniently provided with a bypass port arranged to communicate air from an exterior of the throttle valve to the outlet port, wherein the closure member has a first position connecting the inlet port to the outlet port and sealing the bypass port and a second position connecting the bypass port to the outlet port and sealing the inlet port Thus, a throttle valve assembly as described can be used to introduce air surrounding a valve e g air which may contain combustible gases within the casing

- U -

of the CI-IP unit into the engine via an engine inlet port In this arrangement when the engine is shut-down the air/fuel mix is closed i e the passage between the inlet port and outlet port is sealed The bypass port is simultaneously connected to the outlet port allowing air from the CHP housing into the engine As the engine slows down (as a result of the rotational inertia of the engine) air is drawn through the bypass valve, through the engine and to the exhaust This thereby uses the momentum of the engine itself to draw air from the casing The concept of a throttle valve comprising a bypass valve is both novel and inventive in its own right and thus, viewed from another aspect, an invention disclosed herein provides a throttle valve assembly for an internal combustion engine, comprising an inlet port arranged to receive an air/fuel mix, an outlet port arranged to communicate and air/fuel mix to an engine, a closure member arranged in a passage between the inlet and outlet port, and a bypass port arranged to communicate air from an exterior of the throttle valve to the outlet port, wherein the closure member has a first position connecting the inlet port to the outlet port and substantially sealing the bypass port and a second position connecting the bypass port to the outlet port and substantially sealing the inlet port Preferably the closure member is rotatably mounted in said passage In an application where the engine is connected to the electricity grid, the engine may be arranged to continue to rotate by means of rotation of an electrical generator coupled to the engine, and to the electricity grid, over a sufficient period of time to purge a substantial portion of the air from the CI-IP casing Alternatively, the engine momentum on slow-down may be sufficient to purge the air from the casing.

Preferably, the closure member is rotatably mounted on a shaft passing through the valve assembly and connected to a drive means disposed outside of the assembly and arranged to rotate the shaft and closure member The drive means - 12 - may be any suitable component arranged to rotate the closure member between the first and second positions The drive means may he arranged so as to position the closure means at any angle between the first and second positions Thus, the air/fuel mix entering the engine is controllable in a conventional way by increasing or decreasing the size of the passage Preferably, the closure member and drive means arc arranged such that the closure member can be maintained at discrete positions between the first and second position.

The drive means may be any suitable means arranged to rotate and to position the closure member Preferably, the closure member is controlled and positioned by an electrically powered motor connected to a shaft which is in turn connected to the closure member Most preferably the motor is a stepper motor 1 5 arranged to rotate the closure member between the first and second position and to locate the closure member at discrete positions between said two positions in response to control signals The closure member is preferably arranged such that it is biased towards the position where the bypass port is connected to the outlet port Thus, when there is no control of the closure member by the drive means the closure means is automatically biased so as to permit gas from the exterior of the assembly to enter the engine In this arrangement any loss of power to the drive means results in a connection of the bypass and outlet ports thereby sealing the inlet port and air/fuel mixture.

The biasing may be by any suitable means Preferably the biasing is achieved using a suitable resilient member disposed around and or proximate to the shaft so as to bias the shaft in an appropriate direction The biasing means may be provided by a spring for example - 13 Preferably, the inlet port and outlet port are coaxial so to minimise the flow resistance generated by the valve assembly The bypass port may be arranged to communicate with the outlet port by any suitable arrangement of conduits within the assembly casing and may be arranged at any suitable angle The bypass port maybe in the form of a single port connecting the outlet port to the exterior of the assembly Alternatively, the bypass port may be in the form of a plurality of ports commonly connected to the outlet port The cross-sectional areas of the bypass port and outlet port may be selected such that the entire volume of gas defined within a casing surrounding the assembly and/or engine is drawn through the engine as the engine slows to a stop following and engine shut-down I 5 Viewed from another aspect there is provided a combined heat and power unit comprising an internal combustion engine within a casing and a throttle valve assembly comprising an inlet port arranged to receive an air/fuel mix, an outlet port arranged to communicate and air/fuel mix to an engine further comprising a bypass port arranged to communicate air from the interior of the casing to the outlet port such that in such rotation of the engine pumps gas within the casing through the engine Thus, potentially combustible gases within a casing surrounding an engine can be drawn from the casing using the rotation of the engine The casing may be sealed but is preferably provided with an air inlet port connected to atmosphere such that as gas is drawn through the engine air is drawn into the casing from the exterior of the casing It will be appreciated that according to other aspects there is provided an internal combustion engine incorporating a throttle valve as described and a combined heat and power unit incorporating such an internal combustion engine - 14- Furthermore, other aspects provide a throttle valve as described in combination with a suitable fuel mixing box and a combined heat and power unit incorporating such a combination Aspects of inventions described herein may be used in isolation or in any

suitable combination

Embodiments of inventions disclosed herein will now be described, byway of example only, with reference to the accompanying drawings in which Figure 1 shows a first cross-section of a throttle valve Figure 2 shows a second cross-section of a throttle valve Figure 3 shows a closure member Figures 4A and 4B show an inlet port profile Figures 5A to 5D show the movement of the closure member Figures 6A to 6C show a cross-section of the bypass port Figure 7 shows an exploded view of an entire throttle valve assembly.

Figure 1 shows a cross-section of a throttle valve 1 according to an embodiment of the present invention The throttle valve 1 comprises an inlet port 2 arranged to receive a mixture of air and fuel from a suitable mixing unit or box (not shown) For example the fuel may be a natural gas which has been mixed with a suitable quantity of air and introduced into the throttle valve I via inlet port 2 The valve 1 is provided with an outlet port 3 arranged to communicated the air/fuel mixture to an internal combustion engine via an inlet manifold to the combustion chambers (not shown) A passage 4 is provided between the inlet and outlet port which permits flow of the air/gas between the inlet and outlet ports A cylindrical chamber 5 is defined between the inlet and outlet in which a valve closure member 6 is located The member 6 is pivotally mounted on shaft 7 extending through the valve casing The closure member 6 is arranged to rotate between position A where the inlet port is sealed by the member 6 and position B where the inlet port is substantially unrestricted Rotating the member 6 between the two positions acts to open the inlet port and thereby allow a greater amount of air/fuel mixture into the engine via the outlet port 3 Figure 2 shows a second cross-section plan view of the throttle valve I illustrating the cylindrical chamber 5 and shaft 7 Figure 2 shows the inlet and outlet ports 2, 3 and the cylindrical chamber 5 into which the closure member 6 is located Figure 3 shows a perspective view of the closure member 6 which is located within the cylindrical chamber 5 shown in figures 1 and 2 Referring again to figure 2, there is shown a stepper motor 8 which is mechanically coupled to the shaft 7 and thereby to the closure member 6 The stepper motor 8 is arranged to rotate and hold the shaft 7 at discrete positions in response to control signals received on control lines 9 and 10 Thus, the closure member can be held so as to define the open cross-sectional area of the inlet port 3.

Figures 4A and 4B show the profile of the inlet port 3 As shown in figure 4A the profile is not a uniform circular cross-section but is provided with a portion 11 extending into the circular area of the inlet port Figure 4A shows a cross-section at A-A through Figure 4B and illustrates the profile of portion II With reference to figure 1,it will he recognised that because of the curvature of the closure member (as also shown infigure 3) the portion ii extends along the inlet port towards the outlet port as shown by reference number 12 The profile of the portion 12 of the - 16 - inlet port is arranged to generally correspond to the arc of movement of the closure member 6 The inlet port 2 is manufactured so has to have a specific profile determined according to the particular stepper motor and internal combustion engine combination The number of steps provided by the stepper motor between points A and B (shown in Figure 1) is used in combination with power output data for the engine The profile of the inlet port is determined such that an incremental step of rotation of the stepper motor corresponds to an increase in open cross-sectional area of the inlet port which in turn corresponds to an incremental increase in power output from the engine The throttle valve and engine are calibrated using the following steps: 1) Determining power output data relating to the mass flow rate of air/fuel mixture required for particular engine power outputs, 2) Determining the cross-sectional area of the inlet port required to supply the necessary mass flow rate of air and fuel corresponding to respective power output data; 3) Determining the number of steps for a particular stepper motor between points A and B as shown in figure 1, and 4) Selecting a profile of inlet port such that an incremental step of the stepper motor corresponds to an incremental step in power output from the engine Thus, the throttle valve can be calibrated for a particular engine such that rotational control of the stepper motor accurately controls the power output of the engine In operation, data is stored in a throttle valve controller linking the rotational position of the stepper motor with engine power output The throttle controller (or 17 - CHP controller where the two are combined) received an indication from a user or installation of a power output required The controller interrogates the data and controls the stepper motor accordingly thereby match power supply with demand Figures 5A to SD show the relative positions of the closure member 6 and the corresponding open crosssection of the inlet port 2 The hatched areas indicate the closed portion of the inlet port which acts to restrict air/gas mixture between the inlet and outlet ports Figures 6A to 6C show an embodiment ofa throttle valve incorporating a bypass valve The valve shown in figure 6A to 6C comprises common features described with reference to figures 1 to 5, but additionally comprises a bypass port 14 arranged to receive air from the exterior of the valve 1 In normal operation the valve operates in a way corresponding to that described above with reference to figures 1 to 5 On engine shut down a biasing spring 15 (shown in figures 2 and 7) acts to move the closure member from the position in figure 6A to the position shown in 6C with figure 6B illustrating the intermediate position * Thus, the valve is arranged to automatically move the closure member 6 so as to connect the bypass port 14 with the output port 3 such that continued rotation of the engine acts to draw air through the throttle valve as illustrated by the arrows in figure 6C it will be recognised that the features of figures 1 to 5 and figure 6 can be conveniently used in isolation or in any suitable combination Figure 7 illustrates an exploded via of a throttle valve combining the features of figure 1 to 5 and 6 The reference numerals used in figure 7 correspond to those - 18- used in the previous figures Figure 7 also illustrates locking pin 16 which couples the closure member 6 to the shaft 7 and the connecting screws 17 which are used to couple the components together

Claims (1)

1. Claims 1 A throttle valve for use with an internal combustion engine

   comprising an inlet port arranged to receive an air/fuel mixture, an outlet port arranged to communicate an air/fuel mixture to an engine and a closure member arranged between the inlet port and outlet port and arranged in use to cooperate with the inlet port to define a passage between the inlet and outlet port with an adjustable cross- sectional area, wherein the profile(s) of the inlet or outlet port and/or closure member is (are) adapted in accordance with at least one operating characteristic of an engine 2. A throttle valve as claimed in claim 1, wherein the profile(s) of the inlet port and/or closure member is(are) adapted in accordance with the power output characteristics of an engine 3. A throttle valve as claimed in claim 2, wherein the profile(s) is(are) adapted such that there is a substantially linear relationship between the position of the inlet port and closure member relative to one another and engine power output 4 A throttle valve as claimed in any preceding claim, wherein the passage is defined by movement of the closure member relative to the inlet port A throttle valve as claimed in claim 4, wherein the closure member is moved relative to the inlet port is by means of a stepper motor 6 A method of operating an internal combustion engine wherein there is provided an internal combustion engine connected to a throttle valve, the throttle valve comprising an inlet port allowing an air/fuel mixture into the valve, an outlet port arranged to allow an air/fuel mixture to flow to an engine and a closure member arranged between the inlet port and outlet port and arranged to cooperate with the inlet port to define a passage between the inlet and outlet port with an adjustable - 20 - cross-sectional area, wherein the profile(s) of the inlet port and/or closure member is(are) adapted in accordance with at least one operating characteristic of an engine 7 A method as claimed in claim 6, wherein the closure member and/or inlet port profile(s) cooperate in use to control the power output characteristics of an engine 8 A method as claimed in claim 7, wherein there is a substantially linear relationship between the position of the inlet port and closure member relative to one another and engine power output.

   9 A method of operating a combined heat and power unit comprising an internal combustion engine and a controllable throttle valve connected thereto, wherein the throttle valve receives a control signal representing a required power output from the engine and adjusts the flow of an air/fuel mixture to the engine in response to said control signal A method as claimed in claim 9, wherein the flow of an air/fuel mixture is controlled by means of a stepper motor.

   11 A combined heat and power unit comprising an internal combustion engine and a controllable throttle valve connected thereto, wherein the throttle valve is arranged to receive a control signal representing a required power output from the engine and to adjust the flow of an air/fuel mixture to the engine in response to said control signal 12 A combined heat and power unit comprising an internal combustion engine connected to a throttle valve assembly, wherein a control unit is arranged to receive an indication of the required power output from said unit and to control a throttle valve assembly to in response to said indication -21 - 13 A throttle valve assembly for an internal combustion engine, comprising an inlet port arranged to receive an air/fuel mix, an outlet port arranged to communicate and air/fuel mix to an engine, a closure member arranged in a passage between the inlet and outlet port, and a bypass port an-anged to communicate air from an exterior of the throttle valve to the outlet port, wherein the closure member has a first position connecting the inlet port to the outlet port and substantially sealing the bypass port and a second position connecting the bypass port to the outlet port and substantially sealing the inlet port 14 A throttle valve as claimed in claim 13, wherein the closure member is arranged to rotate in said passage between said first position and said second position A throttle valve as claimed in claim 14, wherein the closure member is rotated by means of a stepper motor coupled to the closure member 16 A throttle valve as claimed in and of claims 13 to 15, wherein the closure member is biased towards the said second position wherein the bypass port is connected to the outlet port 17 A throttle valve as claimed in claim 16, wherein the closure member is biased by means of a spring cooperating with the closure member 18 A combined heat and power unit comprising an internal combustion engine within a casing and a throttle valve assembly comprising an inlet port arranged to receive an air/fuel mix, an outlet port arranged to communicate and air/fuel mix to an engine further comprising a bypass port arranged to communicate air from the interior of the casing to the outlet port such that in such rotation of the engine pumps gas within the casing through the engine - 22 - 19 A throttle valve for use with an internal combustion engine and claimed in any of claims Ito 5 in combination with a throttle valve as claimed in any of claims 13 to 17