GB2134185A - Twin-barrel carburettor - Google Patents

Abstract

The secondary throttle valve 7 is opened when the pressure downstream of the valve exceeds the venturi pressure, a valve 26 responsive to these pressures changing the connection of the throttle valve actuator chamber 20 from the upstream barrel chamber 37 to the barrel downstream of the secondary throttle valve. Closing of the valve 6 causes closing of the valve 7 through cooperating levers 42, 43. <IMAGE>

Description

SPECIFICATION Carburettor The invention relatesto a multiple-induction-channel carburettor for internal combustion engines, with at least one first and one second induction channel, each containing a venturi and, downstream ofthis, a throttle valve which in the case ofthe first channel is driver actuated; and with an eccentrically pivoted chokevalve located in thefirstinduction channel, upstream ofthe venturi; and with a pneumatically actuated positioning device whose control rod is pivoted to a lever which rotates the second throttle valve.

From the German Auslegeschrift 14 26 164 and the German PatentSpecification 12 50 680, carburettors ofthis general kind are known in which the pneumatic positioning device is subjected, on the one hand, to atmospheric pressure and also subjected, on the other hand to the venturi pressure at the narrowest region of at leastone of the venturis. The known carburettor has a firstthrottle valve which is actuated by the driver of the vehicle. This throttle valve is mechanically linked to a second throttle valve in such a way that the second throttle valve is held closed as long as the degree of opening ofthe firstthrottle valve remains below a certain specified value.

The degree of opening ofthesecond throttle valve therefore depends on two criteria. At moderate engine loads (moderate delivered torques) the degree of opening of the second throttle valve depends,via the mechanical linkage, on howfarthefirstthrottle valve has been opened. At high engine loads the degree of opening of the second throttle valve depends on venturi pressure.

Nowadays internal combustion engines are designed with the particular intentions of achieving low fuel consumption and a low level oftoxic emissions in the exhaust gases. Forthis purpose it is intended that the engine should burn a lean mixture at moderate engine loads, but a richer mixture at the higher engine loads In a two-induction-channel (twin-choke) carburettorthe leaner mixture is produced in the first of the two induction channels, the richer mixture in the second induction channel, which engages when engine load increases beyond a certain threshold value.

To minimise fuel consumption and toxic emissions, the engagement ofthe second induction channel should be deferred,with increasing engine load, until engine load has increased to as high a value as possible,thelimittowhatcan be achieved in this direction being set by thermal considerations in that at high loads the mixture must be rich enough to prevent the engine from overheating. Consequently the linkage between the two throttle valves must hold the second throttle valve closed until the first throttle valve has opened quite wide even though withoutthe mechanical linkage venturi pressure would open the second throttle valve much sooner.A disadvantage of this arrangement is that the pneumatic positioning device constantly thrusts the second throttle valve in the opening direction and consequently this is held open to a degree corresponding to the unavoidable free play in the linkage between the two throttle valves. As a result, an imprecise extra flow ofair is aspirated through the second induction channel, not enough to draw a flow offuel from the second venturi but enough to weaken the mixture and upset good operation of the engine, impairing both the road performance of the vehicle and exhaust gas quality.

During cold starting a choke valve in the first induction channel is closed, reducing venturi pressure. This shifts the point of engagement ofthe second induction channel, either making the cold-start arrangements inoperative or supplying enriched mixture to the engine before this is necessary and so making the engine run uneconomically.

The intention in the present invention is to provide a carburettor ofthe general kind described at the beginning, but arranged so that no mechanical linkage is required between the two throttle valves for holding the second throttlevalve closed when the engine is operating at moderate loads, and so that when the choke valve is closed this does not shift the point of engagement ofthe second induction channel.

The problem is solved, according to the invention, in that the pneumatic positioning device, which is subjected, on the one hand, to atmospheric pressure, is also subjected, on the other hand, through a differential pressure valve, to the pressure in the induction channel downstream ofthetwothrottle valves, when the induction-channel pressure is higher than the venturi pressure in the narrowest region of at least one of the venturis, but is subjected entirelyto atmospheric pressure when induction-channel pressureis lower than venturi pressure, the differentialpressure valve having two valve ports arranged so that when the one opens, the other closes.

The advantages obtained are, in the first place, that the action of the carburettor agrees better with the real requirement of the engine. Better fuel economy is achieved, the exhaust gases are less toxic and the motor vehicle performs better. Manufacture ofthe carburettor is simplified by eliminating the mechanical linkage between the two throttle valves. Carburettor maintenance and adjustment is easier and needs to be done less frequently.

The invention will now be described in greater detail with reference to the accompanying drawings in which: Figure lisa longitudinal section th rough the carburettor, and Figure 2 is a diagram describing some operating parameters.

Thecarburettor 1 has a first induction channel 2 and a second induction channel 3, each containing a venturi constriction 4,5 and, downstream ofthis, a throttle valve 6, 7, the firstthrottle valve 6 being actuated by the driver ofthe vehicle, the second throttlevalve 7 being actuated as will be described a little later. Fuel inlets 8,9 are located at the venturi constrictions 4, 5. The first throttle valve 6 is fixed to a shaft 10 rotated by a lever 11 againstthe influence of a return-spring 12, which tends to close the throttle valve 6.

The second throttle valve 7 is fixed to a shaft 14 rotated by a lever 13 whose lower end is pivoted to the lower end ofthe control rod 15 of a pneumatic positioning device 16. The upper end ofthe control rod 15 isfixed to the plate of a main diaphragm 17 which is thrust downwards (to close the second throttle valve 7) by a main diaphragm-closing spring 18. The main diaphragm 17 separates a lower atmospheric chamber 19 from an upper suction chamber 20 contained by a cover 21. The upper portion ofthe cover 21 contains a middle chamber 22 which is separated by an auxiliary diaphragm 25 from an upper chamber 44 contained by a cap 24. The upper chamber 44 communicates through a passage 45 with the venturi constrictions 4 and 5.The two chambers 22 and 44, togetherwith the auxiliary diaphragm 25, form a differential-pressure valve device 26. The cover 21 also contains an upper valve-stem passage 23 surrounding a valve stem 33 whose upper end is fixed to the plate 27 ofthe auxiliary diaphragm 25.

The plate 27 of the auxiliary diaphragm 25 is thrust downwards by an auxiliary diaphragm closing spring 29to obturate a port 28 which forms a communicating passage between the upper chamber 44 and the upper valve-stem passage 23. The upper valve-stem passage 23 communicates through a transverse bore 30 with the main suction chamber 20. The valve stem 33 moves up and down in a valve-stem guide-bearing 32, leaving a minimal clearance gap 31. A lower portion of the valve stem 33 is surrounded by a lower valve-stem passage 34. It will be observedthatthe lower valve-stem passage 34 is separated from the upper valve-stem passage 23 by the minimal clearance gap 31 of the valve-stem guide-bearing 32.As long as the plate 27 ofthe auxiliary diaphragm 25 isfirmly down on its seat, obturating the port 28, the lowervalvestem passage 34 communicates through a port 40 with the main suction chamber 20. At its lower end the valve stem 33 has a valve head 38 capable of obturating the port 40. The lower valve-stem passage 34 also communicates through a transverse bore 35 and through a channel 36with the external atmosphere, in a chamber 37 downstream of an airfilter (not shown).

The middle chamber 22 communicatesthrough a channel 41 with the induction channel ofthe carburettordownstream ofthe second throttle valve 7.

Considering now the action of the valve 33,38, whose upper end is fixed to the auxiliary diaphragm plate 27, itwill be seen that when the valve 33,38 is in its lowered position the main suction chamber20communicates, through the port 40, the bore 35 and the channel 36, with the external atmosphere, whereas when the valve 33,38 is in its raised position the main suction chamber 20 communicates, th rough the bore 33, the middle chamber 22 and the channel 41, with the induction channel of the carburettor downstream ofthe second throttle valve 7.

Thethrottle-valvelevers 11 and 13 can, if necessary, be equipped with safety arms 42,43to ensure that when the firstthrottle valve 6 is closed, this positively closes the second throttle valve 7. Butthis arrange ment is used only if necessary.

In the graph of Figure 2, which shows curves 46,47, 48,49, the abscissa measures engine speeds, whereas the ordinate measures absolute pressures.

The curve 46 follows the pressure in the induction pipe of the engine (i.e. in the induction channel ofthe carburettor downstream ofthe throttle valves 6,7) when the engine is delivering moderatetorques.The curve 47 shows the corresponding pressures in the ventu ris ofthe carburettor. The curve 48 describes the behaviour of induction-pipe pressure when the engine is delivering full torque, and the curve 49 shows the corresponding venturi pressures.

The hatched area between the curves48 and 49 indicates the operating conditionswherethesecond throttle valve 7 can advantageously be engaged, for example to give the venturi pressures described by the broken line 50. The point of engagement ofthe second throttle valve 7 depends on the spring force of spring 29 which can, if desired, be adjustable with the help of an adjustment screw (not shown).

The carburettor is dimensioned so that it operates economically and correctly, and with minimal emission of toxic substances in the exhaust gases from the engine, when onlythefirstinduction channel is engaged.

On the other hand, when both induction passages 2 and 3 are engaged, the carburettor enablesthe engine to develop reliablythe highest available output of torque and power.

From Figure 2 it will be seen that at high engine loads venturi pressure is continuously below induction-pipe pressure, and this is true even at moderate engine loads when engine speed is very high. The present invention utilises this fact in controlling the engagement ofthe second induction channel.

Considering first the operation of the carburettor at moderate engine loads (curves 46 and 47), under these circumstances venturi pressure 47 is higher than induction pipe pressure 46, except at very high engine speeds. The pressure acting downwards on the plate 27 (Figure 1) of the auxiliary diaphragm 25 is therefore greaterthan the induction-pipe pressure underneath the diaphragm 25 (passage 41). Consequently the plate 27 is thrust hard down on its seat, obturating the valve port28. The valve port 40 is open. Atmospheric pressure (passage 36) acts on both sides of diaphragm 17 ofthe positioning device 16 and the spring 18 ensures that the second throttle valve 7 remains closed.

But when the engine is operating atthe higher loads, with venturi pressures (curve 49) lower than induction-pipe pressures (curve 48), the plate 27 lifts against the influence of spring 29, closing the port40 and applying, through the transverse bore 30, induction-pipe pressure to the upper surface of diaphragm 17, which responds by opening the second throttle valve7againstthe influence ofthe spring 18.The cross sections 39,52 ofthe venturis are chosen to ensure that hunting (oscillation) ofthe second throttle valve 7 cannot occur. Butthis detail of the technique can be regarded as adopted from the conventional carburettors.

Subsequently, when the driver of the vehicle closes, or partly closes, the firstthrottle valve 6, this lowers induction-pipe pressure and raises venturi pressure, so that the plate 27 moves down onto its seat, applying atmospheric presure, through passage 36, to the upper surface of diaphragm 17, allowing the spring 18 to close the second throttle valve 7.

For cold starting, an engine-starting system (not shown) closes a choke valve 51 situated in the first induction channel 2 upstream of the venturi. This simultaneously lowers both venturi pressure and induction-pipe pressure, the difference between these two pressures remaining unchanged. The engagement ofthe second throttle valve 7 is therefore not influenced by the cold-starting process.

Claims (2)

1. Amultiple-induction-channel carburettorfor internal combustion engines, with at least one first and one second induction channel, each containing a venturi and, downstream of this, a throttle valve which in the case ofthe first channel is driver actuated; and with an eccentrically pivoted choke valve located in thefirstinduction channel, upstream oftheventuri;; and with a pneumatically actuated positioning device whose control rod is pivoted to a lever which rotates the second throttle valve, characterised in that the pneumatic positioning device (16), which is subjected, on the one hand, to atmospheric pressure, is also subjected, on the other hand, through a differential pressure valve (26),to the pressure in the induction channel downstream of the throttle valves (6,7), when the induction-channel pressure is higherthan the venturi pressure in the narrowest region of at least one oftheventuris (4,5), but is subjected entirely to atmospheric pressure when induction-channel pressure is lower than venturi pressure.

2. Acarburettorasclaimed in claim 1,characte- rised in that the differential pressure valve (26) has two valve ports (28,40) arranged so thatwhen the one opens, the other closes.