EP2078843B1

EP2078843B1 - A carburettor for supplying an internal combustion engine

Info

Publication number: EP2078843B1
Application number: EP20090150155
Authority: EP
Inventors: Marco Ferrari
Original assignee: Emak SpA
Current assignee: Emak SpA
Priority date: 2008-01-10
Filing date: 2009-01-07
Publication date: 2014-09-17
Anticipated expiration: 2029-01-07
Also published as: EP2078843A1; ITRE20080002A1

Description

The invention relates to a carburettor for supplying an air/fuel mixture to an internal combustion engine, either two- or four-stroke, in particular a two-stroke internal combustion engine, of the type destined to be installed on portable tools, for example motor-saws, brush-cutters, motor-blowers and the like.
It is known that portable work tools are usually provided with a two-stroke internal combustion engine, which not only exhibits a lighter weight with respect to four-stroke engines, but is also constructionally simpler and more economical.
A two-stroke internal combustion engine schematically comprises an outer casing in which a lower casing is destined to contain a crank shaft, and at least an upper cylinder housing a piston connected to the crank shaft. The piston defines, with the cylinder head, a variable-volume combustion chamber, sealingly separating it from the casing chamber.
In two-stroke engines the casing chamber is provided with a fresh-mixture inlet port, while the combustion chamber is provided with an exhaust opening for the combusted gases. The casing and combustion chambers are reciprocally connected by at least a transfer conduit, afforded in the engine body, which exhibits an inlet port which opens in the casing chamber, and an outlet port which opens into the combustion chamber.
In proximity of the upper endstroke, the piston closes the discharge outlet port of the combustion chamber and the outlet port of the transfer conduit, leaving the inlet port open, which port opens into the casing chamber, while in the lower endstroke position, it leaves the combustion chamber and the outlet port of the transfer conduit open while closing the inlet port which opens into the casing chamber.
In this way, the operating cycle of the two-stroke internal combustion engine is performed in only two operating strokes of the piston in the cylinder, which correspond to a single complete rotation of the crank shaft.
The first operating stroke begins with the ignition of the mixture in the combustion chamber when the piston is in the upper endstroke position, and continues with the expansion of the gases which push the piston towards the lower endstroke position, compressing the fresh air/fuel mixture contained in the casing chamber. During this descent, the piston first opens the discharge outlet port, so that the combusted gases begin to exit from the combustion chamber, and almost at the same time it opens the outlet port of the transfer conduit, such that the fresh air/fuel mixture compressed in the casing chamber begins to flow into the combustion chamber until it fills the chamber completely, while the discharge gases exit. The ability to retain the combustible mixture entering from the transfer conduit while the discharge outlet is still open is commonly known as "trapping" and is measured by the percentage quantity of mixture retained with respect to mixture transferred.
During the following rising stroke, the piston compresses the fresh mixture in the combustion chamber and before reaching the upper endstroke position opens the inlet hole through which the fresh air/fuel mixture enters the casing chamber by effect of the depression created therein.
When the piston reaches the upper endstroke position, the sparking plug creates the spark which sets off the ignition of the mixture in the combustion chamber and the cycle is repeated.
The fresh air/fuel mixture is supplied into the casing chamber via a device that comprises an air intake line, connected to the inlet port and opening on the outside, along which the following are installed in series: an air filter and a carburettor in which the filtered air coming from the filter is mixed with the fuel before entering the motor.
The carburettor schematically comprises a diffuser conduit in the form of a venturi tube which is crossed by air running through the intake line, a separate tank in which the fuel is kept at a constant level, and a conduit which places the fuel tank in communication with a sprayer, which is located at the narrowest section of the venturi tube.
The above is all in reference to known diaphragm carburettors.
Thus, when an air draught is aspirated internally of the engine, in the narrowest section of the venturi tube a depression is generated which in turn aspirates a flow of fuel from the tank via the sprayer, mixing the fuel with the comburent air to form the mixture.
Downstream of the engine and towards the engine, the carburettor generally comprises a valve, typically a butterfly valve, which is activated by a manual organ (accelerator) and which serves to regulate the quantity of mixture to be introduced into the engine during the aspirating stage.
In the portable work tools of the above-described type, the internal combustion engine generally works at a constant power level under conditions close to full speed, i.e. with the butterfly valve completely open. It has been found, however, that the engine performance and its functioning regularity are better when the engine functions at less than full speed; performance and regularity indeed suffer unacceptably at full power, and there is also an increase in consumption and an undesirable rise in working temperature.
In other words, with a carburettor of a correct size for ensuring supply to the cylinder or cylinders of an air/fuel flow in a correct stoichiometric ratio, i.e. having a given inlet diameter, a given diameter of the narrow section of the venturi tube and a given length thereof, engine performance diminishes when the engine functions under close-to-maximum conditions, i.e. with the butterfly valve completely open.
Naturally one would consider reducing this phenomenon by running the engine at an advantageously lower rate than full power, but this is not possible in practice, as experts in the sector will testify, due to the difficulty in finding a stable position for the butterfly that is not the position of maximum opening in which the butterfly is parallel to the axis of the conduit of the carburettor.
If it were possible, it would also be possible to stabilise the functioning of the engine with a load corresponding to the maximum power requested, simply by over-sizing the carburettor and operating it with the butterfly not fully open. The same problems obtain for a four-stroke engine supplied by a carburettor.
US 4 834 784 shows a system to control airflow in an air filter, this latter having an air inlet for the carburettor. Such system has an element that can rotate between two positions of which one is configured in such a way that a disc provided with a hole is interposed between the air filter and the air inlet. The diameter of the hole that is suitable for intercept the airflow is inferior to the diameter of the air inlet. Such device is used in the mentioned configuration only in special conditions of engine use, such as engine start or engine stop.
An aim of the invention is to improve the efficiency of internal combustion engines, both two-stroke and four-stroke, supplied by a carburettor, especially in full-power functioning, i.e. with the butterfly parallel to the axis of the conduit.
A further aim of the invention is to attain the above-set objective with a solution which is simple, rational and relatively inexpensive.
These aims are attained by the characteristics of the invention as reported in the independent claim. The dependent claims delineate preferred and/or especially advantageous aspects of the invention.
In particular, a carburettor is provided which comprises a diffuser in the form of a venturi tube, which is provided with an air intake port and a fuel outlet port, in which the intake port of the diffuser is partially closed by a diaphragm having a passage hole exhibiting a smaller diameter than the intake port.
Particularly the ratio between a diameter of the inlet port and the diameter of the passage hole of the diaphragm is comprised between 1.5 and 2.8, and the ratio between a diameter of a narrowed section of the venturi tube and the diameter of the passage hole of the diaphragm is comprised between 0.8 and 1.6.
In this way, the diaphragm effects a constant partialisation of the air in intake to the carburettor, so that the engine always functions at lower than full power, even if the butterfly valve is completely open.
It is thus possible to stabilise the functioning of the engine with a power load corresponding to the maximum power requested simply by over-sizing the carburettor, and making the diaphragm passage hole of such a size as to make the engine function at a lower power than the maximum power but corresponding to the requested power, and causing the engine to function with the butterfly totally open.
Thanks to this situation, the efficiency of the engine is improved, by reducing the consumption and functioning temperature with respect to a solution without a diaphragm. Further, the diaphragm enables a precise definition of the flow rate of the comburent air, which depends on the diameter of the passage hole.
Further characteristics and advantages of the invention will emerge from a reading of the following description, provided by way of non-limiting example, with the aid of the figures illustrated in the drawings.
Figure 1 schematically shows a two-stroke internal combustion engine on which a carburettor according to the invention is installed.
Figures 2 and 3 show the engine of figure 1 in two distinct stages of the functioning cycle.
Figure 4 is an alternative embodiment of the carburettor of the invention.
Figure 5 is a comparative diagram of the power.
Figure 6 is a comparative diagram of the temperature.
Figure 7 is a comparative diagram of the trapping.
Figure 8 is a comparative diagram of the He + NO_x toxic emissions.
Figure 1 schematically illustrates a two-stroke typical internal combustion engine 1, of a type usually installed on portable work tools, such as for example motor saws, debushing devices, motor-blowers and the like.
The engine 1 comprises an external casing, denoted in its entirety by 2, which defines a lower casing 20 destined to contain a crank shaft 3, and at least an upper cylinder 21 in which a piston 4 is slidably housed, which piston 4 is connected to the crank shaft 3 by means of a con rod 40.
The piston 4 defines, with the cylinder head 21, a combustion chamber 22, hermetically sealing it from an underlying compression chamber 23 defined internally of the casing 20. The combustion chamber 22 and the compression chamber 23 are both provided with variable volume, by effect of the sliding of the piston 4 in the cylinder 21.
The combustion chamber 22 and the compression chamber 23 are reciprocally connected by means of a transfer conduit 5, afforded in the body of the motor, which exhibits an intake port 50 which opens into the compression chamber 23, and an outlet port 51 which opens into the combustion chamber 22.
The compression chamber 23 is further provided with an inlet port 24, located higher than the intake port 50 of the transfer conduit 5, through which the air and fuel mixture, typically a mixture of oil and petrol, is introduced into the casing 20.
The combustion chamber 22 is provided with a discharge outlet 25, located at a slightly higher level than the outlet port 51 of the transfer conduit 5, through which the combustion gases are discharged to the outside environment.
A sparking plug projects internally of the combustion chamber 22, which sparking plug is fixed to the cylinder head 21, and is destined to strike a spark necessary for igniting the air-fuel mixture.
As illustrated in figures 2 and 3, the piston 4 is conformed and of such a size that when it is in proximity of the upper endstroke position, in which the volume of the combustion chamber 22 is at a minimum, it closes the discharge outlet port 25 of the combusted gases and the outlet port 51 of the transfer conduit 5, leaving the intake port 24 open, which intake port 24 opens into the compression chamber 23. Differently, when in proximity of the lower endstroke position, in which the combustion chamber 22 is at its maximum volume, the piston 4 leaves the discharge outlet port 25 of the combusted gases and the outlet port 51 of the transfer conduit 5 open, closing the intake port 24. The inlet port 50 of the transfer conduit 5 is always open.
In this way, the engine 1 performs its functioning cycle in two operative strokes of the piston 4 in the cylinder 21, i.e. in a single complete rotation of the crank shaft 3.
The first functioning stroke begins with the ignition of the mixture in the combustion chamber 22 when the piston is about in the upper endstroke position (see figure 3), and proceeds with the expansion of the gases which push the piston 4 towards the lower endstroke position (see figure 2), compressing the fresh air-fuel mixture contained in the compression chamber 23 internally of the casing 20. During this descent, the piston opens first the discharge outlet port 25, such that the combusted gases begin to exit the combustion chamber 22, and immediately after opens the outlet port 51 of the transfer conduit 5, such that the fresh mixture compressed in the compression chamber 23 flows into the combustion chamber 22 until it completely fills the chamber 22.
During the successive upstroke from the lower endstroke position to the upper endstroke position, the piston 4 compresses the fresh mixture in the combustion chamber 22 and, before reaching the upper endstroke, opens the inlet port 24, from which the new mixture is aspirated into the engine 1, by effect of the depression created in the compression chamber 23 due to the increase in volume.
The intake port 24 is connected to a device for supplying the air-fuel mixture, denoted in its entirety by 7, which schematically comprises an aspiration line 70 which sets the intake port 24 in communication with the external environment, along which aspiration line 70 an air filter 8, of known type, and a carburettor 9 are installed in series, the air coming from the filter 8 being mixed in the carburettor 9 with the oil/petrol mixture before entering the engine 1.
In this way, during the aspiration stage, an air draught directed from the outside environment towards the engine 1 is createdin the aspiration line 70, which draught first crosses the air filter 8, getting filtered free of large particles and impurities, and then flows into the carburettor 9, where it is mixed with the fuel, such as to form the fluid mixture of air and fuel which enters the compression chamber 23 through the intake port 24.
The carburettor 9 comprises a diffuser conduit 90 shaped as a venturi tube, which defines a tract of the aspiration line 70, and is provided with an inlet port 91 for the air and an outlet port 92 for the mixture. The carburettor 9 further comprises a separate tank 93, in which the fuel coming from a tank (not illustrated) is kept at a constant level, and a conduit 94 for conveying the fuel contained in the tank 93 towards a sprayer 95 located at the narrowest section of the venturi tube 90.
In this way, during the aspirating stage, the air draught crossing the diffuser 90 of the inlet port 91 towards the outlet port 92 creates a depression in the narrowest section of the venturi tube at the sprayer 95, which depression aspirates the fuel from the tank 93, mixing it with the air draught in order to obtain the mixture.
Downstream of the sprayer 95 the carburettor 9 comprises a partialising butterfly valve 96 which is manually activated by means of a suitable accelerator organ (not shown), and regulates the flow rate of air/fuel mixture directed towards the engine 1.
As is schematically illustrated in figure 1, the inlet port 91 of the diffuser 90 is closed by a diaphragm 10, which is preferably realised from a plate or rigid sheet of metal material destined to be fixed, for example screwed, to the diffuser body 90.
The diaphragm 10 exhibits a central passage hole 11 which faces internally of the diffuser 90, such as to set the venturi tube in communication with the aspirating line 70 upstream, thus enabling passage of air during the aspirating stage.
The passage hole 11 has a smaller diameter than the diameter of the inlet port 91 of the diffuser 90, such as to partialise the flow of air which enters the venturi tube, reducing it by a fixed quantity which depends on the diameter itself.
The ratio between the diameter of the inlet port 91 and the diameter of the passage hole 11 of the diaphragm is comprised between 1.5 and 2.8; the hole 11 can also be not coaxial with diffuser axis 90.
The ratio between the diameter of the narrowed section of the venturi tube conduit 90 and the diameter of the passage hole 11 of the diaphragm is comprised between 0.8 and 1.6.
Alternatively, the passage hole 11 can be substituted by a plurality of distinct holes afforded in the diaphragm wall 10 and of such a size as to overall define a passage hole which is smaller than the inlet port 91 of the diffuser.
In use, the engine 1 generally functions such as to constantly deliver a predetermined nominal power depending on the specifications of the work tool it is installed on, i.e. parallel to the axis of the diffuser 90 (as shown in the figures), as it is the only position in which the butterfly 96 is stable .
In this context, the diffuser 90 of the carburettor 9 is preferably oversized, i.e. it exhibits a diameter of the inlet port 91, a diameter of the narrowed section of the venturi tube and a length thereof such as to ensure that in full power conditions delivery i.e. with the butterfly 96 completely open, the engine 1 is supplied with an air/fuel load which theoretically enables it to deliver a power which is greater than a power requested.
The passage hole 11 of the diaphragm 10 is correspondingly sized such as to reduce the flow rate of air passing through the carburettor 9 in order that, though working at full power with the butterfly 96 completely open, the air/fuel mixture exiting the diffuser 90 is in the correct stoichiometric ratio for the engine 1 to develop exactly the nominal power requested.
In this way, thanks to the diaphragm 10, the engine 1 always functions at a lower power than maximum, even if the butterfly valve 96 is completely open. Thanks to this solution, the efficiency of the engine is improved, reducing consumption and functioning temperatures with respect to a solution lacking a diaphragm. Further, the diaphragm enables a precise definition to be made of the comburent air flow rate, which depends on the diameter of the passage hole 11.
Figure 4 illustrates an alternative embodiment of the invention, in which the body of the carburettor 9 is fixed directly to the box 80 of the filter 8, and in which the diaphragm 10 is defined by a portion of the filter body 8, which is provided with a passage hole 11 of a smaller diameter than the inlet port 91 of the diffuser 90, which passage hole 11 is destined to set the diffuser in communication with the internal volume of the box 80 of the filter 8.
Figure 6 shows diagram A of the functioning temperatures (°C) without the diaphragm, compared with diagram B, which shows the temperatures with the diaphragm 10. Note that with the diaphragm, and with power being equal, the temperature is lower by about 20%.
Line C shows the progress of the temperatures in a traditional engine with power equal to the power of the engine with the diaphragm.
Figure 5 shows the progress of the power A without the diaphragm in comparison with the diagram of the power B with the diaphragm, from which it can be deduced that in order to have the power desired with the diaphragm, the carburettor should be sized for a maximum power increase of about 15%.
Line C shows the development of the power in a traditional engine with a performance of about equal to the performance of the engine with the diaphragm.
Figure 7 shows the increase in trapping obtained with the diaphragm (line B) with respect to the solution without diaphragm (line A), which is about 13%-15%.
Line C shows the development of trapping in a traditional engine with the same power as the engine with the diaphragm.
Figure 8 shows the diagram C of the toxic emissions of a traditional engine delivering the desired power, diagram A the emissions of an engine delivering about 15% more power without a diaphragm, and diagram B shows the emissions of the same engine with a diaphragm.
Obviously a technical expert in the sector might make numerous modifications of a technical-applicational nature to the invention without its forsaking the ambit of the invention as claimed herein below.

Claims

A carburettor for supplying an internal combustion engine, comprising a diffuser in a form of a venturi tube (90), which is provided with an inlet port (91) for air and an outlet port (92) of an air/fuel mixture, wherein the inlet port (91) of the diffuser (90) is partially closed by a diaphragm (10) having at least a passage hole (11) exhibiting a smaller diameter than a diameter of the inlet port (91), characterized in that a ratio between a diameter of the inlet port (91) and the diameter of the passage hole (11) of the diaphragm is comprised between 1.5 and 2.8, and in that a ratio between a diameter of a narrowed section of the venturi tube (90) and the diameter of the passage hole (11) of the diaphragm is comprised between 0.8 and 1.6.
The carburettor of claim 1, characterised in that the hole (11) is not coaxial with the axis of the diffuser (90).
The carburettor of claim 1, characterised in that the diaphragm (10) is a plate or rigid sheet of metal material fixed to the diffuser body (90).
A device for supplying an internal combustion engine (1) comprising the carburettor (9) of claim 1 and an air filter (8) connected in series by at least an aspiration line (70), through which air, filtered by a filter (8), is conveyed internally of the carburettor (9), wherein the diaphragm (10) is a portion of the box (80) of the filter (8).
A method for supplying an air/fuel mixture to an internal combustion engine, using a diffuser in a form of a venturi tube (90), which is provided with an inlet port (91) for air and an outlet port (92) for said air/fuel mixture, wherein the inlet port (91) of the diffuser (90) is partially closed by a diaphragm (10) having at least a passage hole (11) exhibiting a smaller diameter than a diameter of the inlet port (91), characterized in that a ratio between a diameter of the inlet port (91) and the diameter of the passage hole (11) of the diaphragm is comprised between 1.5 and 2.8, and in that a ratio between a diameter of a narrowed section of the venturi tube (90) and the diameter of the passage hole (11) of the diaphragm is comprised between 0.8 and 1.6.