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Title:
METHOD OF DELIVERING START-UP FUEL TO AN INTERNAL COMBUSTION ENGINE
Document Type and Number:
WIPO Patent Application WO/2012/002859
Kind Code:
A1
Abstract:
A method for controlling the fuel supply to an internal combustion engine at startup, the engine having a fuel supply system which can be set in at least two start modes, a lean mode, and a rich mode, the rich mode providing extra fuel during start-up of the engine, the method including: a) during a start attempt, determining if the next start attempt should be executed in lean or rich mode based on an evaluation of at least one engine parameter/s from the previous start attempt and/or at least one engine parameter/s from the last successful run, and/or at least one engine parameters/s of the present start attempt, and b) setting the fuel supply system in rich or lean mode depending of the evaluation in such a way that the next start attempt is executed in said rich or lean mode.

Inventors:
LARSSON, Mikael (Rörliden 8, Jönköping, S-556 28, SE)
OTTOSSON, Magnus (Östra Storgatan 89B, Jönköping, S-554 52, SE)
CARLSSON, Bo (Hemsjö 2470, Alingsås, S-441 96, SE)
Application Number:
SE2010/050758
Publication Date:
January 05, 2012
Filing Date:
July 01, 2010
Export Citation:
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Assignee:
HUSQVARNA AB (Drottninggatan 2, Huskvarna, S-561 82, SE)
LARSSON, Mikael (Rörliden 8, Jönköping, S-556 28, SE)
OTTOSSON, Magnus (Östra Storgatan 89B, Jönköping, S-554 52, SE)
CARLSSON, Bo (Hemsjö 2470, Alingsås, S-441 96, SE)
International Classes:
F02D41/06; F02D35/00; F02M1/08; F02M17/04
Foreign References:
US5852998A1998-12-29
US20090013951A12009-01-15
US4723523A1988-02-09
US6848405B12005-02-01
Attorney, Agent or Firm:
ANDERSSON, Jonas (Husqvarna AB, Intellectual Property- EM-LPHDrottninggatan 2, Huskvarna, SE-561 82, SE)
Download PDF:
Claims:
CLAIMS:

1. A method for controlling the fuel supply to an internal combustion engine at startup, the engine having a fuel supply system which can be set in at least two start modes, a lean mode, and a rich mode, the rich mode providing extra fuel during start-up of the engine, the method including:

a) during a start attempt, determining if the next start attempt should be

executed in lean or rich mode based on an evaluation of at least one engine parameter/s from the previous start attempt and/or at least one engine parameter/s from the last successful run, and/or at least one engine parameters/s of the present start attempt, and

b) setting the fuel supply system in rich or lean mode depending of the

evaluation in such a way that the next start attempt is executed in said rich or lean mode.

2. The method according to claim 1 wherein the fuel supply system is set in lean mode when the engine is stopped after a successful run so that a first start attempt is always executed in lean mode.

3. The method according to claim 1 or 2 wherein a start attempt is determined in that the engine is started when set in a start position, and that the method includes the step of detecting that the engine is started in the start position, where preferably the start position is having a throttle valve in a start gas position, e.g. having a throttle ratio in the interval 5-20%, and a choke valve in closed position.

4. A method according to any one of claims 1-3 or wherein in step a) the evaluation includes determining if an ignition indication has occurred in the present start attempt based on at least one monitored engine parameter/s of the present start attempt, and wherein if an ignition indication is determined to have occurred, in step b) the fuel supply system is set or maintained in lean mode.

5. A method according to claim 4 wherein the ignition indication is determined by monitoring the engine speed and evaluating the engine speed behavior during said start attempt, for instance a sudden increase in engine speed could indicate an ignition.

6. A method according to claim 4 or 5 wherein the ignition indication is determined if an ignition quotient is larger than a predetermined ignition threshold value, the ignition quotient based on the quotient between the time from the lower dead point to upper dead point and the time from the upper dead point to the lower dead point.

7. The method according to any one of claims 1-6 wherein the engine parameter/s includes at least one of:

- a stop time t2 indicating the time has passed since the last successful run,

- a run time tl indicating the duration of the last successful run,

- a stop temperature Tlof the last engine stop,

- a start temperature T2 of the present start attempt.

8. A method according to any one of claims 1-7 wherein in the fuel supply system includes two fuel paths, a first and a second, wherein the first fuel path is closed during lean mode and open during rich mode, whereas the second fuel path is open during both the lean and the rich mode.

9. A method according to claim 8 wherein in the first fuel path is controlled by a bistable two position valve, having a first, open position and a second, closed position.

10. A method according to any one of claims 1-9 wherein the engine is a crank case scavenged engine.

11. A method according to any one of claims 1-10 wherein the engine is a two-stroke engine.

12. A carburetor (10) including:

an intake channel (30) with a venturi section (31),

a throttle valve (33) mounted in the intake channel (30), downstream the venturi section (31), a choke valve (32) mounted in the intake channel (30), upstream the venturi section (31), and

A fuel supply system including a main fuel path (13) connecting a diaphragm controlled regulating chamber (11) to a main outlet (22) in the region of the venturi section (31), the main fuel path (13) including a electronically controlled valve (26), and an idling fuel path (14-18) branching off from the main fuel path (13) downstream the valve (26) and ending in at least one idling outlet (19-21) in the region of the throttle valve (33),

characterized in that the fuel supply system further includes a bypass line (23) starting upstream the valve (26) and ending in at least one bypass outlet (25) to the intake channel (30), and in that the fuel supply system only have one valve, between the regulating chamber (11) and the intake channel (30), that is actively controlled during operation of the engine.

13. A carburetor according to claim 12 wherein carburetor includes an air channel (24) that connects ambient air to the bypass line (23)so that it can draw fuel from the regulating chamber (11) and air from the air channel (24), thereby diluting the fuel concentration supplied from the bypass outlet to the intake channel (30) during operation of the engine.

14. A carburetor according to claim 12 or 13 wherein the electronically controlled

valve is a bistable two position valve, having a first, open position and a second, closed position.

15. A carburetor according to any one of claim 12-14 wherein at least one of the bypass outlet/s (25) is located upstream the venturi section (31), preferably in the region of the choke valve (32) and downstream of it, for supplying fuel to the intake channel (30).

Description:
Method of delivering start-up fuel to an internal combustion engine

TECHNICAL FIELD

The invention concerns a method for controlling the fuel supply to an internal combustion engine at start-up, the engine having a fuel supply system.

The invention also concerns a carburetor having an intake channel with a venturi section, a throttle valve mounted in the intake channel downstream the venturi section, a choke valve mounted in the intake channel upstream the venturi section, and a fuel supply system including a main fuel path connecting a diaphragm controlled regulating chamber to a main outlet in the region of the venturi section, the main fuel path including a electronically controlled fuel valve, and an idling fuel path branching off from the main fuel path downstream the valve and ending in at least one idling outlet in the region of the throttle valve.

BACKGROUND

Internal combustion engines of two-stroke or four-stroke type usually are equipped with a fuel supply system of carburettor type or injection type. In a carburettor, the throttle of the carburettor is affected by the operator's demand, so that wide open throttle produces a minimum throttling in the carburettor barrel. The depression created by the passing air in the carburettor venturi draws fuel into the engine.

Diaphragm-type carburettors are particularly useful for hand held engine applications wherein the engine may be operated in substantially any orientation, including upside down. Such carburettors typically include a fuel pump that draws fuel from a fuel tank and feeds the fuel to a fuel pressure regulator via a needle valve. The fuel pressure regulator usually includes a fuel metering chamber that stores fuel fed from the fuel pump and the fuel metering chamber is generally separated from atmosphere by a diaphragm that adjusts the fuel pressure to a constant pressure. The needle valve opens and closes the fuel passage from the fuel pump to the fuel metering chamber as the diaphragm moves. From the fuel metering chamber fuel is delivered to the main air passage via a main channel and an idle channel. The main channel leads to a main nozzle in the main air passage fluidly prior to the throttle valve, whereas the idle channel leads to an idle nozzle fluidly shortly after the throttle valve.

When starting a crank case scavenged engine having a conventional carburettor, the choke valve is closed by the operator using a choke button and the throttle valve is set in a start gas position. When pulling the pulling cord to start the engine, an air and fuel mixture is delivered to the crank case of the engine. When a first ignition is heard by the operator, the choke valve is opened so not to flood the engine with too much fuel. However, sometimes the operator misses the first ignition and the engine is flooded and the product cannot be started as desired.

US6932058 discloses a carburetor including a fuel supply system for supplying fuel from a diaphragm controlled regulating chamber to the intake channel of the carburetor. The fuel supply system includes a main fuel path having a control valve and an idling fuel path that branches off from the main fuel path downstream the control valve. The control valve thereby controls all fuel supplied to the intake channel. It has however been found out that this solution provides an inadequate fuel supply in certain situations. In particular it is difficult to control the fuel supply at start up.

US7603983 shows a carburetor including a fuel supply system having two independent fuel paths for supplying fuel from a diaphragm controlled regulating chamber to the intake channel of the carburetor. The first fuel path includes a main fuel path having a control valve and an idling fuel path that branches off from the main fuel path downstream the control valve. A first bypass line bypasses the control valve. The second fuel path connects the regulating chamber to a outlet in the region of the throttle valve and provides a second bypass line. A second valve is mounted in the second bypass line or alternatively in the first bypass line. The opening and closing of the second valve is controlled by the position of the choke valve. The carburetor further includes an accelerator pump for supplying additional fuel to the main fuel path downstream the control valve during acceleration. This solution improves the operational range of the fuel supply. It is however costly and includes several additional components compared to e.g. US6932058.

US 6 880 812 discloses a carburetor having two independent fuel supply systems, each including an electromagnetically driven control valve. A control system controls the opening and closing of the valves by using input from an engine speed sensor and a temperature sensor. Also this solution is costly and complex.

US 2009/0013951 shows a carburetor including a fuel supply system having two fuel paths for supplying fuel from a diaphragm controlled regulating to the intake channel of the carburetor. A main path supplies fuel to the intake channel during normal operations. A startup fuel supply passage has a solenoid valve to control the timing of startup fuel delivery. In this carburetor the fuel supply cannot be electronically controlled during normal operations since the solenoid valve only operates on the startup fuel supply passage. This is inadequate.

OBJECT OF THE INVENTION

One object of the invention is to provide a method to control the fuel supply when attempting to start a crank case scavenged engine.

Another object is to provide a carburetor which reduces risk for flooding the engine at start up while being capable of delivering extra fuel during a start attempt.

SUMMARY OF THE INVENTION

At least one of these objects or problems mentioned above is addressed by a method for controlling the fuel supply to an internal combustion engine at start-up, the engine having a fuel supply system which can be set in at least two start modes, a lean mode, and a rich mode, the rich mode providing extra fuel during start-up of the engine, the method including:

-a) during a start attempt, determining if the next start attempt should be executed in lean or rich mode based on an evaluation of at least one engine parameter/s from the previous start attempt and/or at least one engine parameter/s from the last successful run, and/or at least one engine parameters/s of the present start attempt, and b) setting the fuel supply system in rich or lean mode depending of the evaluation in such way that the next start attempt is executed in said rich or lean mode. Thereby the fuel supply at start up can be optimized.

Preferably the fuel supply system is set in lean mode when the engine is stopped after a successful run so that a first start attempt is always executed in lean mode. Thereby the risk of flooding then engine at start up is reduced.

Preferably a start attempt is determined in that the engine is started when set in a start position, and that the method includes the step of detecting that the engine is started in the start position, and where preferably the start position is having a throttle valve in a start gas position, e.g. having a throttle ratio in the interval 5-20%, and a choke valve in closed position.

Preferably, in step a) the evaluation includes determining an ignition indication has occurred in the present start attempt based on at least one monitored engine parameter/s of the present start attempt, and wherein if an ignition indication is determined to have occurred, in step b) the fuel supply system is set or maintained in lean mode.

Preferably the ignition indication is determined by monitoring the engine speed and evaluating the engine speed behavior during said start attempt, for instance a sudden increase in engine speed could indicate an ignition.

Preferably the ignition indication is determined if an ignition quotient is larger than a predetermined ignition threshold value, the ignition quotient based on the quotient between the time from the lower dead point to upper dead point and the time from the upper dead point to the lower dead point.

Preferably the engine parameter/s includes at least one of:

- a stop time t2 indicating the time has passed since the last successful run,

- a run time tl indicating the duration of the last successful run,

- a stop temperature Tlof the last engine stop,

- a start temperature T2 of the present start attempt. Preferably the fuel supply system includes two fuel paths, a first and a second, wherein the first fuel path is closed during lean mode and open during rich mode, whereas the second fuel path is open during both the lean and the rich mode. Preferably first fuel path is controlled by a bistable two position valve, having a first, open position and a second, closed position.

Preferably the engine is a crank case scavenged engine. Preferably the engine is a two-stroke engine. However the engine may also be a four stroke engine.

The invention also concerns a carburetor mentioned initially wherein the fuel supply system further includes a bypass line starting upstream the valve and ending in at least one bypass outlet to the intake channel, and in that the fuel supply system only have one valve, between the regulating chamber and the intake channel, that is actively controlled during operation of the engine. Thereby a simple and robust fuel supply system can be achieved, still being able to have an adaptive fuel supply at start up. Preferably the carburetor includes an air channel that connects ambient air to the bypass line so that it can draw fuel from the regulating chamber and air from the air channel, thereby diluting the fuel concentration supplied from the bypass outlet to the intake channel during operation of the engine.

Preferably the electronically controlled 1 valve is a bistable two position valve, having a first, open position and a second, closed position.

Preferably at least one of the bypass outlet/s is located upstream the venturi section, preferably in the region of the choke valve and downstream of it, for supplying fuel to the intake channel.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a schematic drawing of a fuel supply system of a carburetor,

Fig. 2 shows flow chart representing a process for controlling the fuel supply at start up, and

Fig. 3 is shows an example, of a start attempt. DESCRIPTION OF THE INVENTION

The present invention primarily concerns crank case scavenged, spark ignited, two- or four-stroke engines and any general reference to engines in the following description concerns these type of engines, although also non-crank case scavenged engines are possible.

Fig. 1 is a schematic view showing a fuel supply unit in the form of a diaphragm carburettor. The carburettor main body 10 has an intake channel 30 extending from an air inlet side 34 to an air outlet side 35. A choke valve 32 is mounted in the intake channel 30, at the air inlet side 34 thereof, and a throttle valve 33 is mounted in the intake channel 30, at the air outlet side 35 thereof. In between the throttle valve 33 and the choke valve 32 a venturi 31 is formed in the intake channel 30. During operation air is drawn from the air inlet side 34, via an air filter (not shown), and an air/fuel mixture is delivered to the engine (not shown) connected to the air outlet side 35.

A fuel pump 8 draws fuel from a fuel tank 9. The fuel pump 8 may be a known pulsation controlled diaphragm pump, driven by the pressure pulse generated by a crankcase of the engine that the carburettor is supplying air and fuel mixture to. The fuel pump 8 delivers fuel, via a needle valve (not shown), to a fuel metering chamber 12 of a fuel regulator 11. The fuel metering chamber 12 is separated from atmospheric pressure by a diaphragm 15 and can hold a predetermined amount of fuel. A main fuel path 13 connects the fuel metering chamber 12 to a main outlet 22 in the intake channel 30, located in the region of the venturi 31. An electronically controlled fuel valve 26 is mounted in the main fuel path 13. The electronically controlled fuel valve 26 is preferably a bistable valve that can switch between an open and closed position.

Downstream the electronically controlled fuel valve 26, an idle fuel path 14 branches off from the main fuel path 13. The idle fuel path 14 itself branches off into three idling outlets 19, 20, 21 to the intake channel 30, which are successively disposed in the region of the throttle valve 33. A first idling outlet 21 disposed downstream the throttle valve 33, a second idling outlet 20 disposed approximately in the region of a closed throttle valve 33, and a third idling outlet 19 disposed upstream the throttle valve 33 when the latter is closed.

The fuel valve 26 is controlled by an electronic control unit 50 that receives sensor inputs such as throttle position/s from a throttle positions sensor/s, engine speed/s from an engine speed sensor/s, and temperature/s from a temperature sensor/s. The electronic control unit 50 cam e.g. use these sensor inputs to decide when to open or close the fuel valve 26. A bypass line 23 emanates from the fuel metering chamber 12 and has a bypass outlet 25 in the region of the choke valve 32, downstream of it. An optional air channel 24, drawn in phantom lines, connects ambient air to the bypass line 23. The air channel 24 is for diluting the fuel concentration supplied by the bypass line 23 to the intake channel 30 during operation of the engine, i.e. by mixing air to fuel drawn by the bypass outlet 25 due to the pressure variations in the intake channel 30. The bypass line 23 is preferably made by drilling a narrow bore in the carburetor body from the fuel metering chamber 12 to the intake channel. An alternative to the air channel 24 is to reduce the diameter of the bore providing the bypass line 23, or to add other flow restriction means in the bypass line 23. The bypass line 23 could alternatively branch off from the main fuel path 13, upstream the electronically controlled valve 26.

The main fuel path 13, the idle fuel path 14, and the bypass line 23 each have a check valve 16-18 for preventing fuel flowing back to the fuel metering chamber 12.

The carburetor 10 can be set in a start position, as e.g. described in US7611131. In the start position the choke valve 32 is closed and the throttle valve 33 is slightly open (around 5-20 % of a fully opened position). When pulling a pull cord to start the engine while the carburetor 10 is in the start position, pressure variations in the intake channel 30 will draw fuel from the bypass outlet 25. For those revolutions the electronically controlled valve 26 is open fuel will also be drawn from the main fuel outlet 22 as well as from the idle fuel outlet 19, 20, 21, thereby delivering an additional amount of fuel. However, for those revolutions the fuel valve 26 is closed, fuel will only be drawn from the bypass outlet 25. In the preferred embodiment of the invention, the fuel valve 26 is either closed or open for all revolutions during a start attempt (for other operating conditions the fuel valve 26 will open and close frequently to adjust the fuel ratio). In the mode when the fuel valve 26 is closed at the start attempt the fuel supply system is referred to as being in lean mode, and when the fuel valve is open the fuel supply system is referred to as being in rich mode.

Moving from the start position the choke valve 32 is released to a fully opened while the throttle valve 33 can take any position between closed (idle throttle) and fully open (maximum throttle). When the throttle valve 33 is closed fuel will mainly be taken from the first idling outlet 19, and the electronically controlled valve 26 can control the fuel supply during idle by closing and opening the valve 26 according to an idle control scheme as e.g. described in WO2009/038503, hereby incorporated by reference. In the same manner the fuel supply can be controlled by closing and opening the valve 26 to adjust the air fuel ratio of the as described in e.g. WO2007133125 and WO2007133148 hereby incorporated by reference.

A method for controlling the fuel supply to an internal combustion engine at start-up will now be described in more detailed with reference to Fig. 2.

The phantom lined "Set carburetor in start position" 100 indicates that the operator sets the carburetor in a start position, e.g. closed choke valve 32 and slightly opened throttle valve 33. Thereafter the operator actuates the start mechanism at box 101, e.g. pulls the pulling cord, which box 101 is also drawn with phantom lines indicating that these steps do not form part of the method.

After actuating the start mechanism the engine control unit is energized and determines in box "Start position?" 103 whether the carburetor is set in its start position, here, by using a throttle position from a throttle position sensor 113. If the carburetor is not in its start position the fuel supply system is controlled by other controls methods as indicated by the box "Run mode" 104.

On the other hand, if the start position is detected the next box "Idetect=True?" 107 checks if a first ignition was detected in a previous start attempt, by receiving input from box "idetect" 114, i.e. a value symbolizing "True" or "False". If the value is "True" the fuel supply system will be set or maintained in lean mode in box "set/maintain lean mode" 109. On the other hand, if it is "False" the box "cold or warm?" 108 follow, where it is determined whether the engine is considered to be started warm or cold.

In box 108 the decision of warm or cold is determined by using the engine parameters from box 115 which here represents parameters from the present start attempt and/or from the previous start attempt and/or last successful run. For instance engine parameters such as a stop temperature Tl stored when the engine was stopped at the last successful run, a start temperature T2 of the present start attempt, and a duration tl of the last successful run, and a time t2 since the last successful run. The condition in box 108 could e.g.

1) t2 > stop time threshold (e.g. 5 minutes) => cold start, else warm start,

2) tl < duration threshold (e.g. 5 seconds) AND T2 < cold temperature threshold (e.g.

-5 °C) => cold start, else warm start,

3) t2 > f (Tl) => cold start, else warm start, where f(Tl i) > f(Tl 2 ) if Tl j> Tl 2 .

The first example being the simplest; if the engine hasn't been running recently the engine is considered to be cold and else wise warm. In the second example the engine is considered to be cold if the last engines run was short and if the temperature sensor indicates that it is very cold, e.g. when the engine is cooled during a cold winter day. In the third example the time t2 since the last successful run is compared to a value that is dependent of the engine temperature Tl when the engine was stopped, i.e. if the engine is very hot when stopped it will take longer timer for it to cool. The specific conditions are shown as examples, of course more complex conditions could be used, for instance by combining one or more of the examples

If the engine during the start attempt is determined to have been started warm, the fuel supply system is set or maintained in lean mode in box "set/maintain lean mode" 109. If the engine is determined to have been started cold, the box "first ignition?" 110 follows. At the box "first ignition?" 110 a function evaluates engine speed data 116 to detect whether any ignitions has occurred during the start attempt. If an ignition is determined to have occurred the variable idetect is set to be True in box "Idetect=True" 111. Thereafter the fuel supply system is set in lean mode at box "set/maintain lean mode" 109, so that the next start attempt will be performed in lean mode. This is done since if a first ignition has been determined to have occurred, the engine should be close to starting and having a fuel ratio in the crank case close to the optimal. Therefore by setting the fuel supply system in lean mode minimizes the risk of flooding the engine during the next start attempt. On the other hand, if no ignition was detected in box 110, the fuel supply system is set or maintained in rich mode at box "Set/maintain rich mode" 112. Thereby the next start attempt is performed with the fuel supply system in rich mode.

Of course when the engine starts to run as indicated by the phantom lined box "Engine starts to run" 117, there will be no next start attempt, and other control schemes are activated to govern the fuel supply to the engine.

After a successful run of the engine and the engine is stopped as indicated by the phantom lined box 118, the fuel supply system is set in lean mode at box 119. Furthermore during shut down, as indicated by box 120, engine parameters such as engine stop temperature Tl and the duration tl of the successful run are stored, and a timer t2 is started. Also the variable idetect is set to False during shut down, as indicated by box 121. Thus, after a successful run, the engine will be started with a fuel supply system in lean mode and with the ignition detection set to False.

Fig. 3 shows an example for a start procedure. The upper graph showing operator actions, the middle graph showing fuel valve actions, and the lower graph showing fuel supply, each graph following the same time scale. When applicable reference numbers that corresponds to boxes in the control scheme of Fig. 2 have been used, these reference numbers are in the one hundreds. As indicated by reference number 200 in the fuel valve 26 (see Fig. 1) is closed (the fuel supply system is in lean mode) before attempting to start the engine. Also before starting, the engine is set in start position by the operator, corresponding to box "set carburetor in start position" 100 of Fig. 2. After having set the engine in its start position, the operator makes his first start attempt "Pull 1" by pulling the cord, corresponding to box "actuate start mechanism" 101 of Fig. 2. Since the fuel valve 26 (see Fig 1) is closed, only a small amount of bypass fuel 205 from the bypass outlet 25 (see fig. 1) is delivered. I.e. this first start attempt is performed in lean mode. During this start attempt the control scheme of Fig. 2 evaluates if the next start attempt should be executed in lean or rich mode. Here the decision came to that the next start attempt should be executed in rich mode and therefore the fuel valve 26 is opened, corresponding to the box "Set maintain rich mode" 112 of Fig. 2. In the second start attempt "Pull 2" the fuel valve 26 is now open. Hence, in addition to the fuel drawn from the bypass outlet 25, fuel is also drawn from the main and idle outlets 19-22, thereby providing extra start fuel 206 to the engine. Also during the second start attempt the control scheme of Fig. 2 is ran to evaluate if the next start attempt should be executed in lean or rich mode. Here the decision came to that the next start attempt should continue to be executed in rich mode, and hence the fuel valve 26 is remained open. In the third start attempt "pull 3" the fuel valve 26 is open, and fuel is therefore drawn from the bypass outlet 25, and the main and idle outlets 19-22, thereby providing extra start fuel 206 to the engine. As was done during the first and the second start attempt the control scheme of Fig. 2 is ran to evaluate if the next start attempt should be executed in lean or rich mode. Here, a first ignition was detected, and therefore the fuel supply system is set in lean mode by closing the fuel valve 26, corresponding to the box "set/maintain lean mode" 109 of Fig. 2. Thus the forth start attempt "pull 4" is executed in lean mode, here by having then fuel valve 26 closed, and hence only bypass fuel 205 from the bypass outlet 25 is delivered. During this start attempt the engine ignited and started to run, corresponding to the box "Engine starts to run" 117 of Fig. 2. The control scheme is now changed to a start gas control scheme 201 (which is not described in details since it does not form a part of the present invention). The start gas control 201 is active until the throttle trigger is actuated, and the start gas control 201 is replaced by other control schemes, here named as normal control 202 that handles different operating situations such as idle (as e.g. described in WO2009038503) and full throttle (as e.g. described in WO2007133125). During start gas control 201 (see Fig. 1) the main amount of fuel 207 is drawn for the main and idle outlets 19-22 by opening and closing the fuel valve 26. However, since the choke valve 32 (See Fig. 1) is closed during start gas control small amounts of fuel will also be drawn from the bypass outlet 25. During normal the main amount of fuel 208 is drawn from the main or idle outlets 19-22 depending on if it is operating at full throttle or at idle throttle. Since the choke valve 32 is opened during these operating conditions, almost no fuel if any will be drawn from the bypass outlet 25. When the engine is stopped as corresponding to the box "engine stops" 118 the fuel supply system is set in lean mode by closing the fuel valve 26, corresponding to the box "Set lean mode" 119. Whereas the invention has been shown and described in connection with the preferred embodiment thereof it will be understood that many modifications, substitutions, and additions may be made which are within the intended broad scope of the following claims. From the foregoing, it can be seen that the present invention accomplishes at least one of the stated objectives.

Alternatively when shutting down the engine the engine is set in lean or rich mode depending on one ore more engine parameters. One example of conditions could be that if Tl is less than -5 °C, then the engine at the first start attempt is started in rich mode and else in lean mode, i.e. expecting that the next start will be a cold start if Tl gives a low reading. Alternatively, even though it is not preferred, the engine could always be started in rich mode at the first start attempt.

The temperatures Tl and T2 can e.g. be measured by a temperature sensor mounted on a circuit board attached to the carburetor.