WO2001083966A1

WO2001083966A1 - Air valve control method

Info

Publication number: WO2001083966A1
Application number: PCT/AU2001/000517
Authority: WO
Inventors: David Michael Cooper; Daniel James Kammermann; Andrew Michael Tilmouth
Original assignee: Orbital Engine Company (Australia) Pty Limited
Priority date: 2000-05-04
Filing date: 2001-05-04
Publication date: 2001-11-08
Also published as: AUPQ729500A0; TW495579B

Abstract

A method of controlling airflow to an engine having an intake air throttle (9), an air bypass line bypassing the throttle, and an air valve (8) supported on the bypass line for controlling the passage of air therethrough, the method including opening the air valve (8) as a function of a crankangle of the engine at least at some point of the engine operating load range.

Description

AIR VALVE CONTROL METHOD

The present invention is directed to the control of the airflow to an internal combustion engine. Although the invention will be described in relation to four- stroke port injected engines, it is to be appreciated that the engine is applicable for other engine types such as direct injected and two-stroke engines.

In four-stroke port injected engines, the load control for the engine is typically achieved by controlling the airflow to the engine. In a typical arrangement, the main airflow control is by an engine throttle valve, with a bypass valve providing more accurate and/or trimming control of the airflow, in particular during idle and start up engine conditions. The bypass valve is typically located on a bypass line bridging the throttle valve.

In some engines, typically larger engines, an airflow meter, manifold absolute pressure (MAP) sensor or other sensor is generally provided to enable the airflow to the engine to be determined. The engine therefore has some feedback of the actual airflow to the engine, and hence the bypass valve can be accordingly controlled on the basis of that determined airflow. This is commonly known as "closed loop control" of the airflow to the engine. In smaller engines however, for example, a single cylinder engine used on a scooter, the cost of having such a feedback arrangement may not be warranted. Therefore, the bypass valve needs to be controlled with no airflow feedback in what is known as "open loop control" of the airflow to the engine.

In such open loop control, it is therefore highly desirable that the bypass valve be controlled to provide a known repeatable airflow. One way of achieving this is by using a stepper motor actuated bypass valve wherein a direct relationship for a nominal pressure drop, exists between the number of steps of the motor and the airflow through the bypass line. This can provide accurate control of the airflow to the engine. However, the problem with having a stepper motor for controlling the bypass valve is that they have a relatively low response speed relative to an engine cycle and are relatively expensive, particularly in relation to use on a low cost small engine application such as a single cylinder scooter or motorcycle.

An alternative arrangement is to use a suitable solenoid actuated valve driven with a Pulse Width Modulation (PWM) signal. In such an arrangement, the airflow through the bypass line is a direct function of the PWM signal provided to the valve. That is, the PWM signal can typically be varied to regulate the position of the valve between maximum and minimum opening positions so as to control the airflow to the engine. The problem with such an arrangement however is that battery voltage fluctuations affect the relationship between the PWM signal and the valve position. Furthermore, the operational temperature of the solenoid valve can also affect the performance of the solenoid coil.

It would therefore be advantageous to provide an air by pass system and a control method for controlling an air valve using open loop control which is relatively unaffected by variations in the battery voltage, coil temperature, or pressure drop and yet can be relatively inexpensive and have a fast response time.

It is therefore an object of the present invention to provide for improved control of an air valve of an engine. With this in mind, there is provided a method of controlling airflow to an engine having an intake air throttle, an air bypass line bypassing the throttle, and an air valve supported on the bypass line for controlling the passage of air therethrough, the method including opening the air valve as a function of a crankangle of the engine at least at some point of the engine operating load range.

Preferably the air valve is driven between known positions, such as fully open and fully closed.

Preferably, the air valve is opened synchronously according to a function of a top dead centre (TDC) position of a crankshaft of the engine. Preferably, the opening of the valve in synchronous relation to a crankangle of the engine occurs sequentially over a number of engine cylinder cycles.

Conveniently, the air valve is actuated such that it is either in an opened or closed position. That is, unlike a stepper motor which is typically arranged to vary the positional characteristics of the air valve, the air valve in accordance with the present method is either fully opened or fully closed. Preferably, the air valve is solenoid controlled and may form part of an electromechanical device wherein energisation of a solenoid coil results in the valve being opened. Conveniently, the air bypass line is arranged so as to bridge the intake air throttle of the engine. Preferably, the air valve is provided in an idle speed control device for the engine.

Preferably, the method of controlling airflow to the engine is effected in such a manner so as to provide for open loop control of the airflow. That is, no MAP sensor, airflow meter or the like is used and so no feedback control of the air bypass valve is performed.

Synchronous operation of the air valve with respect to the crankangle position of the engine crankshaft enables the air valve to be opened when a known pressure differential exists across the air bypass valve. This results from the pressure within the air intake system being cyclic and repeatable as a consequence of the cyclic operation of the engine.

Accordingly, the opening of the air valve may be timed to correspond to a gas exchange period of a cylinder of the engine. More particularly, the opening of the air valve may be synchronised with an air induction event of the engine. To this end, the air valve may be timed to open at a preset engine crankangle and may be timed to close at another preset engine crankangle. In this manner, as the engine speed drops, the effective duration of opening of the air valve increases, thereby providing an inbuilt proportional control. Alternatively, the air valve may be timed to open at a preset engine crankangle and close after a preset duration.

Conveniently, the opening angle, closing angle or time and/or duration of opening for the air valve may be stored in a look-up map or table provided within an electronic control unit (ECU) which manages the operation of the engine. These air valve events or valves may conveniently be mapped against engine speed or any other suitable engine operating parameter. The use and application of such look-up tables or maps and ECUs is well known in the field of engine management and will not be elaborated upon herein in any further detail.

Conveniently, the crankangle at which the air valve is opened and closed and/or the duration of opening of the air valve may be controlled as a function of the engine temperature. The engine temperature may conveniently be determined as or from the coolant temperature in the case where the engine is liquid cooled. The method is applicable for use when the engine is at idle, the air valve providing the principal control for the idle speed of the engine.

Alternatively, the method may be used at start-up of the engine to provide additional air to the engine during engine starting. In the case of a four stroke port injected engine, the air valve may be opened and closed twice per firing cycle or once per 720° of crank rotation. In the latter situation, the TDC timing must be established to ensure that the air valve opening coincides with the gas exchange period of the cylinder.

The repeatable accuracy of the airflow is achieved by opening the air valve at a known differential pressure within the engine air intake. As this differential pressure is repeatable, the airflow to the engine can therefore be reliably controlled as a function of the engine crankangle. Conveniently, the known differential pressure at which the air valve is opened is preferably sonic. When the airflow in the air intake system is sonic, the flow rate is ultimately determined by the area of the valve or orifice through which the air flows. That is, once the flow rate is sonic, any increases in the differential pressure will not effect the flow rate. Hence, operation of the air valve during such sonic conditions renders the airflow across the valve essentially duration based. Sonic conditions are understood to occur where the pressure differential across an orifice, such as a valve, is in the ratio of 1.895 or greater.

Unlike the case with an air valve driven by a PWM signal, the air valve is driven in an "on/off" manner and the opening and closing thereof is synchronised with the engine position and speed. As the air valve is operated in an "on/off" manner, the flow through the air bypass line is determined by the duration for which the valve is held open. That is, the operation of the air valve is analogous to that of a pressure-time metering device such as a fuel or air delivery injector. Accordingly, the control of the operation of the air valve, for example via ECU maps, may compensate for any changes in battery voltage and/or coil temperature which may effect the turn-on and turn-off times and hence the opening and closing times of the air valve. In this way, the airflow to the engine is able to be accurately controlled in an open loop manner regardless of certain variables not being constant. Still further, the control method is able to take account of any distinction that may exist between the electrical turn-on time of the solenoid coil and the mechanical opening point of the air valve.

The method of the present invention is equally applicable to two and four stroke engines and can be arranged to control airflow to the engine regardless of whether the associated fuel system renders the engine manifold injected or direct injected. Further, the method has applicability to both single fluid and dual fluid fuel injection systems. An example of such a dual fluid fuel injection system is detailed in the Applicant's US Patent No. 4934329 and PCT Patent Application No. PCT/AU98/01004, the contents of which are incorporated herein by way of reference.

Further, whilst the method of airflow control is applicable to single cylinder and multi-cylinder engines alike, it has particular applicability to single cylinder engines wherein greater cost benefits may be achieved.

According to another aspect of the present invention, there is provided an air intake system for an engine, the air intake system comprising an intake air throttle, an air bypass line bypassing the throttle, and an air valve supported on the bypass line for controlling the passage of air therethrough wherein the air valve is opened as a function of a crankangle of the engine at least at some point during the engine operating load range. Preferably, the air bypass line bridges the intake air throttle.

Preferably, the air valve is opened synchronously on a crankangle basis relative to the TDC position of a crankshaft of the engine.

According to a further aspect of the present invention, there is provided an electronic control unit (ECU) for controlling an internal combustion engine having an air intake system comprising an intake air throttle, an air bypass line bypassing the throttle, and an air valve supported on the bypass line for controlling the passage of air therethrough; the ECU being adapted to open the air bypass valve as a function of a crankangle of the engine at least at some point during the engine operating load range. Preferably said air bypass valve is a solenoid valve and said ECU is adapted to actuate said solenoid when opening said valve. Preferably said ECU is adapted to open said air bypass valve on a crankangle basis relative to the top dead centre position of a crankshaft of the engine.

Preferably said engine further comprises crankshaft position apparatus; said ECU adapted to receive crankshaft position data from said crankshaft position apparatus to utilise said data in opening said air bypass valve on a crankangle basis.

It will be convenient to further describe the invention by reference to the accompanying drawings which illustrate one preferred embodiment of the present invention. Other embodiments of the invention are possible and consequently the particularity of the accompanying drawings is not to be understood as superceding the generality of the preceding description of the invention.

In the drawings:

Figure 1 is a schematic representation of a scooter in which an air valve may be employed;

Figure 2 is a schematic view of a port injected four-stroke spark ignited engine having an air valve and being suitable for use with the scooter of Figure 1 ; and

Figure 3 is a graph showing the timing of the opening of the air valve in the engine of Figure 2.

Figure 4 is a timing diagram showing a typical relationship between electrical excitation of a solenoid and mechanical actuation of an associated valve member.

The present embodiments are directed to operation of a valve in an inlet manifold of an internal combustion engine in order to enable an air flow to the engine to by pass a throttle. The embodiments utilise manifold pressure waves which have been observed to be relatively stable on a cycle to cycle basis, for fixed operating conditions. This stability in manifold pressure waves has enabled the valve to be operated synchronously with crank angle. Particular embodiments operate the valve at predetermined crank angles where sonic air flow is induced in the valve as this enables pressure time metering of air flow to the engine. Present embodiments have particular application to engines having small inlet manifolds relative to the swept volume of the engine. Such engine typically have a manifold with a volume that is less than one or two engine swept volumes.

Single cylinder engines typically have such small engine inlet manifolds as do other engines with individual inlet runners for each cylidner.

To better understand the environments within which the embodiments are employed, Figures 1 & 2 will now be described. Figure 1 is a schematic illustration of a scooter 300 being a representative small engine application in which an air by pass valve may be utilised. The scooter has a front wheel 305 and a rear wheel 310 which support a chassis and associated panel work 315 off of a road surface 320. The chassis and panel work 315 comprises a rider area 325 which typically consists of a seat that is capable of supporting two riders. The rider area 325 is located above the rear wheel 310. An engine and associated drive mechanism 330 is mounted intermediate the rider area 325 and the rear wheel 310. Handle bars 335 are rotationally mounted to the chassis and panel work 315 and further support shock absorbers 340 that located the front wheel 305 onto the scooter 300.

In operation a rider positions themselves onto the rider area and locate their feet on foot rests 345 located on a floor pan 350 of the chassis and associated panel work 315. These foot rests 345 are located intermediate the base of handle bars 335 and the rider area 325. The handle bars 335 contain a mechanical throttle actuation mechanism which may be actuated by the rider rotating their hand. The handle bars 335 also contain an ignition switch which activates an electrical circuit between a battery, located adjacent the engine and associated drive mechanism 330, and an electronic control unit and other electrical components, such as a fuel pump and headlights 355.

The scooter has a single cylinder fuel injected engine with small capacity which may be in the range of 50CC to 100CC though utilisation of an increased capacity engine is also possible. A fuel tank may be located underneath the riders area 325. A fuel pump supplies fuel from the tank to a fuel supply circuit that is in communication with a fuel injector of the engine.

Referring now to Figure 2 which is a schematic representation of an engine and an associate fuel supply circuit suitable for use with the scooter of Figure 1. The engine and associated fuel system is a port injected single cylinder four stroke engine 50 having a fuel tank 1 which communicates fuel to a fuel injector 11 by means of a fuel pump 3 located within fuel tank 1. The fuel supply circuit also includes fuel pressure regulator 4 and fuel supply line 52. The fuel injector 11 meters fuel to an inlet manifold in accordance with metering signals received from engine control unit (ECU) 16.

Airflow is provided to the combustion chamber 61 via air box 18 and air filter 19. The air box 18 is in fluid communication with an inlet manifold 65 intermediate throttle 9 and inlet valve 80. The inlet manifold 65 is in fluid communication with the combustion chamber when inlet valve 80 is open. A cam (not shown) opens and closes both the inlet valve 80 and an exhaust valve 85. The exhaust valve 85 permits egress of exhaust gasses from the combustion chamber.

The fuel injector 11 sprays fuel into the inlet manifold where air flow through the manifold transports the fuel into the combustion chamber. The fuel forms a homogenous charge within the combustion chamber. The homogenous charge of fuel is ignited by spark plug 12 which is activated by ignition coil 13 which operates under the control of ECU 16.

A combined pressure sensor and temperature sensor 7 often referred to as a TP sensor is located upstream of the throttle 9. The TP sensor provides ECU 16 with atmospheric pressure and also temperature signals that may be used for altitude compensation and other control strategies. The TP sensor 7 is an analogue sensor whose signals may be sampled by ECU 16 through use of analogue to digital conversion techniques and digital sampling filtering techniques.

Air flow to the engine is in part controlled by the position of throttle 9. At idle the throttle 9 is closed and minimum air flow to the engine ensues. When the throttle is fully open, the engine typically receives maximum fuelling levels and is said to operate under "wide open throttle" (WOT) conditions. The throttle is manually actuated by an operator and the position of the throttle 9 is indicated to ECU 16 by throttle position sensor 10. Throttle air bypass valve 8 may be activated so that inlet air can bypass the throttle. Under idle conditions, controlling the amount of air which air bypasses the throttle allows engine speed to be controlled. Air by pass valve 8 has a solenoid valve arrangement that is controlled by ECU 16 so as to be in one of two positions, either a fully open position or a fully closed position. This is distinct from stepper motor based air by pass valves and PWM control techniques for air by pass valves. Both stepper motor based valves and PWM control techniques can control the extent to which a valve is open (i.e. they allow a valve to be partially opened and for the extent of this opening to be incrementally controlled).

Electrical power is supplied to the engine (including the fuel pump), at least at cranking, by battery 22 and ignition switch 21. The ECU 16 receives information as to the position of piston 60 within the combustion chamber 61 by crank shaft position sensor 15 and an encoder wheel 66 mounted on fly wheel 67. Encoder wheel 66 comprises a number of teeth, typically 24 (one of which may be missing so as to provide a reference tooth) which pass by position sensor 15. The teeth interact with position sensor 15 so as to generate a square wave, saw wave or similar periodic signal as input to ECU 16. The square wave is commonly edge detected by the ECU 16 resulting in detection of each leading edge of the encoder wheel 66 as it passes position sensor 15.

The information as to the position of the piston 60 within combustion chamber 61 is commonly referred to as the engine's crank angle. A two stroke engine cycle is said to have 360^Q of crank angle whereas a four stroke engine cycle is said to have 720^Q of crank angle. Thus in operation an engine's crank angle corresponds to the instantaneous position of the engine within its current engine cycle. This position is measured relative to the engines top dead center (TDC) position, which for a two stroke engine is the point of maximum compression on any engine revolution and for a four stroke engine is the point of maximum compression on an intake (i.e. compression) stroke and which is often referred to as TDC firing. A second TDC occurs for a four stroke engine at the end of an exhaust stroke. This TDC may be referred to as TDC exhaust. A 24 tooth encoder provides 15^Q of crank angle resolution for both a two stroke and a four stroke engine.

Various embodiments utilse a solenoid based air by pass valve 8 so that it is actuated into either a full open state or a fully closed state. This allows solenoid based valves with relatively fast actuation times to be used. I.e. a valve 8 whose response time is small compared to an engine revolution can be used to actuate the valve into its open or closed position under idle speed conditions and some higher engine speeds. As the valve 8 can actuate in less than an engine revolution, it can be controlled on a cycle to cycle basis. Typical stepper motor based air by pass valves and PWM controlled valves have response times that are close to or significant compared to one engine revolution. This reduces the degree to which these valves can be controlled on a cycle to cycle basis. This slow response time also results in these valves being held open on a cycle to cycle basis and so are open for the entirety of an intake stroke. In contrast, present embodiments allow the air by pass valve 8 to be closed for a portion of an intake stroke. This allows present embodiments to employ a control strategy with a shorter time constant than the time constant that can be used for stepper motor based valves and PWM controlled valves. Present embodiments allow the air valve 8 to be operated in an open loop manner with no feedback of the actual airflow through the air intake 3. According to the present invention, the air valve 8 may nevertheless be controlled to provide accurate control of the airflow to the engine by sequentially opening the air valve 7 as a function of the crankangle of the engine 1. That is, the air valve 8 is opened and closed at different crankangle position relative to TDC of an engine. Meaning that the air valve 8 is operated synchronously with crank angle.. Such synchronous control relies on the manifold pressure wave being stable (i.e. having a repeatable pattern) on a cycle to cycle basis for fixed operating conditions. This allows the valve 8 to be operated by an open loop control strategy where the quantity of air flow through the valve has either been previously determined or can be calculated as the air flow through the valve is sonic at the selected operating angles.

Figure 2 shows the fluctuation of a manifold pressure signal 205 associated with the air intake or intake manifold pressure of the engine 50 as a function of crankangle. Engine crank angle is indicated by crank angle signal 210 developed by engine position sensor 15. The crank angle signal 210 contains a revolution marker 220 which corresponds with a missing tooth on the encoder wheel 66. The manifold pressure 205 relates to a four stroke engine and accordingly two engine revolutions are indicated by the crank angle signal 210. The first revolution indicates TDC exhaust 225 which is followed by an intake stroke where inlet valve 80 is open. The second cycle indicates TDC Firing 230 which is followed by an expansion stroke and then an exhaust stroke. Exhaust valve 85 is open during such an exhaust stroke. The revolution marker 220 is off set from TDC.

The position of the air by pass valve 8 is indicated by air by pass valve signal 200. The air by pass valve signal contains a step response 235 adjacent TDC exhaust 225 which indicates the crank angle at which the air by pass valve opens and the crank angle at which it closes and the duration in terms of crank angle for which it is open.

The greatest manifold pressure in the inlet manifold is indicated as 79kPa and the minimum inlet pressure is indicated as 33kPa. Atmospheric pressure 215 is approximately 101 kPa and accordingly it can be seen that the inlet manifold is in partial vacuum over the engine cycle. It is noted that the greatest differential pressure between the manifold pressure and atmospheric pressure occurs at and around bottom dead centre. This also coincides with the gas exchange period of the engine wherein air is inducted into the engine cylinder 61. The air valve 8 is timed to open during this period to thereby allow for accurate supply of air to the cylinder 61 , the mass air flow rate to the engine during this period being a function of the duration of opening of the air valve 8. That is, the air valve 8 is held open for a defined period of crankangle which represents a known quantity of air flow into the engine.

As the differential pressure at which the air valve 8 is open is repeatable at a pre-determined crank angle on a cycle to cycle basis for a particular set of engine operating conditions, accurate control of the airflow to the engine can be provided. In the abovenoted illustration, the air valve opens every 360° of crankangle, or once per engine revolution. Accordingly the air valve opens at crank angles when the inlet valve 80 is closed. Alternate embodiments open the air by pass valve 8 once per engine cycle or once every 720^s. This requires the engine phase to be established relative to control of air valve 8 to thereby coincide the opening of the air valve 8 with the gas exchange period of the cylinder 2. The air valve 8 is preferably opened during a period when the airflow in the intake manifold is sonic. Sonic conditions exist where the ratio of the pressures across the valve is greater than 1.895 and means that any increase in this pressure ratio will not result in increased air flow through the valve. During such a period, the airflow through the valve 8 is only influenced by the duration of the opening time of the air valve 8 and the cross-sectional area of the air valve 8. Thus the air flow during this period can be accurately determined through use of pressure time or pressure duration metering principles. For example, a pressure time principle opens the valve 8 at a set crank angle and holds it open for a fixed time period. A pressure duration principle opens the air valve at a fixed crank angle and closes it at a fixed angle from the opening angle. Such metering allows open loop control to be effected.

Open loop air flow control strategies can be employed in operation of the engine under conditions where the turn on and turn off time of the by pass valve 8 is short compared to engine speed. Typically such conditions exist at lower engine speeds, however it is believed that the principle is effective at higher engine speeds where a valve with suitable flow area and response time is identified.

The crankangle at which the air valve 8 is open and the duration of opening of the air valve 8 or the crankangle at which the air valve 8 is closed may be a function of the engine temperature. When the engine temperature is cold, for example at engine start-up, an increased idle speed is required to maintain good engine stability and a robust idle quality. The opening of the air valve may therefore be controlled to provide extra air at engine start-up. By increasing the air flow to the engine, a greater quantity of fuel is metered which results in higher engine speed.

Alternate embodiments are not restricted to opening the by pass valve 8 only under sonic conditions. These embodiments may provide maps in ECU 16 with predetermined crank angles for actuating the air by pass valve. During calibration of the engine, air flow for each point in the map will be either measured or calculated and accordingly a know quantity of air can be admitted to the cylinder 61 under operational conditions of the engine. This enables fuelling levels to the engine to be determined in preferably a stoichiometric ratio to the quantity of fuel admitted.

Typically these embodiments are employed under idle operating conditions, enabling closed loop speed control to be implemented. Closed loop speed control at idle requires the idle speed of the engine to be monitored and the engine load to be varied so that the engine speed is maintained at a predetermined level. Under closed loop idle conditions throttle 9 is typically closed and typically the air by pass valve 8 is opened and closed at the same crank angle each engine cycle thereby admitting a known quantity of air to the combustion chamber. However if the ECU 16 detects that engine speed has increased, then the duration for which the air by pass valve is open will be reduced, typically on the next engine revolution. Typically the duration is reduced by varying the crank angle at which the air by pass valve 8 closes, however the starting angle or both the starting angle and the closing angle may be varied. By reducing the duration for which the air by pass valve is open, the air flow to the engine is reduced by a known amount. As the quantity of air is reduced, the associated quantity of fuel metered to engine by fuel injector 11 is reduced so that the air fuel ratio within the cylinder remains near stoichiometric. The reduced level of fuel reduces the power delivered to the engine which in turn reduces the engine speed. Similarly if the engine speed falls below a predetermined level then the duration for which the air by pass valve is open may be increased, which results in an increased air flow and so an increased fuelling level to the engine. This increases the engine's speed. Such closed loop speed control can be implemented because the turn on and time off time of the air by pass valve 8, under idle conditions at least, is small compared to the time for one engine revolution. This allows cycle to cycle variations in air flow to the engine to be effected through varying the duration for which the air by pass valve 8 is open in any one cycle. Typical stepper motor based valves and PWM control techniques do not to have response times that are small compared to an engine cycle at idle (i.e. they have a relatively long time constant compared to an engine cycle and they are typically held open on a cycle to cycle basis). Hence air flow to the engine can not be controlled on a cycle to cycle basis as accurately with such stepper motor based valves and PWM controlled valves as can be effected by the present embodiments.

A further benefit of actuating air by pass valve 8 so that it is either fully open or fully closed is that battery voltage compensation techniques can be readily employed. Figure 4 indicates a typical delay that occurs between electrical actuation of a solenoid, indicated by solenoid actuation signal 405, and resultant mechanical actuation of an associated valve element, indicated by valve actuation signal 400. Valve actuation signal 400 is delayed relative to solenoid actuation signal 405 by turn on time 440. Turn on time 440 is the time between electrical excitation of the solenoid, which occurs at solenoid start angle 410, and commencement of movement of the valve element associated with the solenoid, which occurs at mechanical start angle 420. Similarly a delay, referred to as turn off time 455, occurs between de-activation of the solenoid, which occurs at solenoid end angle 415, and commencement of closing of the associated valve member, which commencement occurs at mechanical end angle 430. The valve member takes a finite time to fully open and to fully close as indicated by open ramp 460 and close ramp 465.

To activate and deactivate the air by pass valve 8 at predetermined crank angles, the solenoid is excited by solenoid actuation signal 405 at an earlier crank angle before TDC than the desired crank angle for which the valve member is to be opened. The earlier crank angle corresponds with the turn on time 440 or turn off time 455 of the valve. Thus the solenoid start angle 410 is calculated having regard to turn on time 440 and current engine speed.

The turn on time 440 and the turn off time of the valve varies with battery voltage. Battery voltage can be monitored by ECU 16 and accordingly during calibration of the engine a map for ECU 16 of off set values can be constructed that details how the turn on time 440 and turn off time 455 varies with battery voltage.

The use of an air valve which is controlled in a manner analogous to a fuel injector enables certain advantages to be realised including a fast response time as compared to prior art arrangements and high level of accuracy. The response time for the synchronous air valve of the present invention is on a per cylinder cycle basis unlike the typical slow rate of a stepper motor. This enables better load rejection and enhanced cycle to cycle control. The method of the present invention also enables compensation for different or varying battery voltages and ensures repeatable and reliable airflow control without the need for any costly air flow measuring and feedback arrangements. Further, as the air valve 8 can be opened when the differential pressure is known to be sonic, simple adaptation techniques can be applied to take out any valve to valve differences when closed loop idle control is being effected by the air valve which may be part of an idle speed control device.

Although the present invention has been described with respect to a four stroke single cylinder engine, it is to be appreciated that the invention could also be used on two stroke engines and/or multi cylinder engines.

Modifications and variations as would be deemed to be obvious to the person skilled in the art are included within the ambit of the present invention as defined in the appended claims.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A method of controlling airflow to an engine having an intake air throttle, an air bypass line bypassing the throttle, and an air valve supported on the bypass line for controlling the passage of air therethrough, the method including opening the air valve as a function of a crankangle of the engine at least at some point of the engine operating load range.

2. A method according to claim 1 , wherein the air valve is driven between known positions.

3. A method according to claim 2, wherein the air valve is driven between fully open and fully closed positions.

4. A method according to any one of the preceding claims, wherein the air valve is opened synchronously according to a function of a top dead centre (TDC) position of a crankshaft of the engine.

5. A method according to claim 4, wherein the opening of the valve in synchronous relation to a crankangle of the engine occurs sequentially over a number of engine cylinder cycles.

6. A method according to claim 5, wherein the air valve is timed to open at a preset engine crankangle and is timed to close at another preset engine crankangle.

7. A method according to claim 5, wherein the air valve is timed to open at a preset engine crankangle and close after a preset duration.

8. A method according to claim 5, wherein the crankangle at which the air valve is opened and closed and/or the duration of opening of the air valve is controlled as a function of the engine temperature.

9. A method according to claim 8, wherein the engine temperature is conveniently determined as or from the coolant temperature in the case where the engine is liquid cooled.

10. A method according to any one of the preceding claims, wherein the air valve is solenoid controlled and forms part of an electromechanical device wherein energisation of a solenoid coil results in the valve being opened.

11. A method according to any one of the preceding claims, wherein the air bypass line is arranged so as to bridge the intake air throttle of the engine.

12. A method according to any one of the preceding claims, wherein the air valve is provided in an idle speed control device for the engine.

13. A method according to any one of the preceding claims, wherein the method of controlling airflow to the engine is effected in such a manner so as to provide for open loop control of the airflow.

14. A method according to any one of the preceding claims, wherein the method is used when the engine is at idle, the air valve providing the principal control for the idle speed of the engine.

15. A method according to any one of claims 1 to 14, wherein the method is used at start-up of the engine to provide additional air to the engine during engine starting.

16. A method according to claim 5, wherein, In the case of a four stroke port injected engine, the air valve is opened and closed twice per firing cycle or once per 720° of crank rotation.

17. A method according to claim 16, wherein the TDC timing is established such that the air valve opening coincides with the gas exchange period of the cylinder.

18. An air intake system for an engine, the air intake system including an intake air throttle, an air bypass line bypassing the throttle, and an air valve supported on the bypass line for controlling the passage of air therethrough wherein the air valve is opened as a function of a crankangle of the engine at least at some point during the engine operating load range.

19. An air intake system according to claim 18, wherein the air bypass line bridges the intake air throttle.

20. An air intake system according to claim 18 or 19, wherein the air valve is opened synchronously on a crankangle basis relative to the TDC position of a crankshaft of the engine.

21. An electronic control unit (ECU) for controlling an internal combustion engine having an air intake system including an intake air throttle, an air bypass line bypassing the throttle, and an air valve supported on the bypass line for controlling the passage of air therethrough; the ECU being adapted to open the air bypass valve as a function of a crankangle of the engine at least at some point during the engine operating load range.

22. An ECU according to claim 21 , wherein said air bypass valve is a solenoid valve and said ECU is adapted to actuate said solenoid when opening said valve.

23. An ECU according to claim 21 or 22, wherein said ECU is adapted to open said air bypass valve on a crankangle basis relative to the top dead centre position of a crankshaft of the engine.

24. An ECU according to any one of claims 21 to 23, wherein said engine further includes crankshaft position apparatus; said ECU being adapted to receive crankshaft position data from said crankshaft position apparatus to utilise said data in opening said air bypass valve on a crankangle basis.

25. A scooter having an engine with a throttle and an air by pass valve wherein said air by pass valve is operated synchronously with crank angle of the engine.

26. A scooter as claimed in claim 25 wherein said synchronous operation of said air by pass valve utilises said air by pass valve in one of two postions, open or closed.

27. A scooter as claimed in claim 25 or 26 wherein air flow to said engine is controlled, over at least a portion of the engine operation range, in open loop.

28. A scooter as claimed in any one of claims 25 to 27 wherein idle speed of said engine is controlled in closed loop using open loop control of air flow.

29. A scooter as claimed in any one of claims 25 to 28 wherein said air by pass valve, at least over a portion of said engine operating range is adapted move between said open and said closed positions in a time period less than one engine revolution.

30. A scooter as claimed in claim 29 wherein said time for moving said valve allows cycle to cycle variations in air flow to said engine.

31. A scooter as claimed in any one of claims 25 to 30 wherein said air by pass valve is compensated for variations to battery voltage through application of off setting values to solenoid electrical excitation start angles and end angles.