GB2128684A - Carburettor float chamber - Google Patents

Abstract

The float travel is restricted by a projection 17 or an adjustable abutment (21), Fig. 2 (not shown), or the flow cross-section, Fig. 3 (not shown), or movement of the float-operated valve, Fig. 4 (not shown), is limited, so that a maximum fuel flow into the float chamber is not exceeded. <IMAGE>

Description

SPECIFICATION Carburettor float chamber The invention relates to a fuel supply system comprising a carburettor for an internal combustion gasoline engine and, more particularly, to a carburettor provided with a float chamber.

A carburettor incorporated in a gasoline engine of an automobile is normally cooled during operation of the engine by the draught of the radiator, the flow of intake air and vapourization of liquid fuel. When a car is however operated under conditions such that the temperatures attained by the fuel system are too high relative to the volatility of the fuel several problems can occur which are generally indicated with the expression "hot-fuel handling problems." The classical hot fuel handling problem is that of vapour lock in the fuel pump of the fuel supply system, in which excessive vapour generation, due to overheating of the pump or to high fuel volatility inhibits the replenishment of liquid fuel taken from the carburettor float chamber and causes malfunctions due to fuel starvation.Poor restarting, idle quality and poor acceleration for example are typical manifestations of vapour lock problems following a hot soak and are usually associated with the aspiration of an overlean mixture into the engine. Vapour lock problems have been extensively studied over many years and have been the subject of many publications.

Volatility expressions generally used to control hot fuel handling performance generally feature a vapour lock index or they involve the use of a vapour:liquid ratio expression. A common feature of all these expressions is that they are primarily intended for fuel pump vapour lock control.

Apart from hot fuel handling problems due to vapour lock in the fuel pump there is another important group of hot fuel handling problems, viz.

those originating in the carburettor. The latter group is generally indicated with the expression "percolation".

Percolation problems are well-known but have been assumed to be covered by the same control expressions as those used for fuel pump vapour lock. This is consistent with traditional beliefs in the relative importance of the fuel pump as opposed to the carburettor as the source of hot fuel handling problems.

Carburettor malfunctions are thought to be due to the ingestion of unmetered fuel into the engine through the internal vent, if any, of the float chamber or through the carburettor jet system as a consequence of local boiling of the liquid fuel in the carburettor float chamber during or following a hot soak. Contrary to vapour lock, this type of problem is associated with the engine operating in an over-rich regime. This phenomenon is wellknown to carburettor designers, who pay special attention to fuel passage configuration and to the location and the shape of the jet. It is known to use an anti-percolation device with the objective of preventing the ingress of fuel into the inlet manifold during a hot soak.Various designs have been described to overcome percolation problems, e.g. featuring a fuel cut-off valve in the float chamber which blocks the main jet when the engine is switched off, or a system which closes the internal vent during an engine-off soak and routes the fuel vapour from the float chamber to a charcoal canister. The above measures are intended to overcome the problems of difficult hot restarting and persistent stalling at idle, being recognized as the main consequences of percolation.

Up to now, acceleration problems were thought to be caused by fuel vapour lock. It has now been found that there is a further very important feature causing acceleration problems.

Tests have been carried out which clearly showed engine malfunctions during acceleration tests although the occurrence of fuel pump vapour lock was inhibited. The engine malfunctions were now associated with the engine operating in an over-rich regime, which clearly could not have been caused by fuel pump vapour lock. Further tests demonstrated that these engine malfunctions coincided with a large pressure build-up in the float chamber of the carburettor, although the float chamber was equipped with a vapour vent line. It has now been found that the large pressure build up in the float chamber of a carburettor during acceleration after a hot soak is caused by the formation of fuel foam in the carburettor float chamber.

It was found that under certain conditions a substantial volume of fuel foam built up in the carburettor, to such an extent that it filled the float chamber, blocked the vent, sank the float and permitted the fuel pump to force a huge excess of liquid fuel into the float chamber and into the engine, either through the main jet or through the internal vent system, if any is applied.

The object of the invention is to overcome the above problems of engine malfunctions, due to an over-rich mixture of liquid and air entering into the engine, by providing an improved fuel supply system.

The fuel supply system according to the invention thereto comprises a carburettor having a float chamber, a fuel inlet opening for supplying fuel into the float chamber, a fuel outlet opening for discharging fuel from the float chamber, a float inside the float chamber, an inlet valve member to open or close the fuel inlet opening in response to a rise and fall of the float, a vent opening at an upper portion of the float chamber to vent gases therefrom, wherein the carburettor comprises means arranged within the float chamber for limiting the maximum supply of fuel into the float chamber. In a suitable embodiment of the present invention these means for limiting the maximum fuel supply are formed by means for restricting sinking of the float during foam formation.

In a further suitable embodiment of the invention the inlet valve member is at least partly arranged in the fuel inlet opening irrespective of the position of the float.

All the above-mentioned measures are intended to restrict the supply of fuel into the float chamber, when the liquid is foaming and therefore to prevent the formation of an over-rich mixture resulting in an engine malfunction.

The invention will now be described by way of example only with reference to the accompanying drawings, wherein Figure 1 shows a longitudinal section of a carburettor according to the invention; Figure 2 shows a longitudinal section of a first alternative of the float chamber shown in Figure 1; Figure 3 shows a longitudinal section of a second alternative of the float chamber shown in Figure 1; and Figure 4 shows longitudinal section of a third alternative of the float chamber shown in Figure 1.

It will be appreciated that identical elements of the shown float chambers have been indicated by the same reference numeral. Figure 1 illustrates a first embodiment of the present invention, wherein a carburettor 1 is shown comprising a carburettor housing 2 defining an air passage 3 having an air inlet 4 and an air outlet 5 which is connected to an intake manifold (not shown) of an internal combustion engine. For increasing the air flow velocity within the air passage 3 a venturi 6 is provided therein in the normal fashion. A conventional throttle plate 7 and choke plate 8 selectively open and block the air flow through the air passage 3.

The carburettor housing 2 further defines a float chamber or fuel bowl 9, supplied with fuel from a fuel supply line 10 connected to the pump side of a fuel pump 11. A float 12 is disposed within the float chamber 9, and is pivotably connected to the carburettor housing 2 via an arm 13. The float 12 controls the supply of fuel from the fuel supply line 10 via a fuel cut-off needle element 14 mechanically coupled to the arm 13 and cooperating with a needle seat 1 5 in needle valve means 16. Downward movement of the float 12 is restricted by the presence of an protrusion 1 7 connected to the lower side of the float.

The shown carburettor is further provided with a fuel discharge line 18 for discharging fuel from the bottom part of the float chamber 9 into the venturi 6. Fuel gases generated in the float chamber 9 are discharged through an inner air vent line 1 9 forming a fluid communication between the upper part of the float chamber 9 and the air passage 3 at a location disposed upstream of both the throttle plate 7 and venturi 6. Tests carried out with a carburettor of substantially the above shape but without a protrusion 1 7 for restricting downward movement of the float 12 gave the following results.

Under normal accelerating conditions, i.e.

without a hot soak, the fuel entering the float chamber 9 via fuel supply line 10 formed a small amount of foam (i.e. vapour with fuel droplets dispersed therein), which disappeared very quickly and did not give rise to engine problems. After a hot soak it appeared that the volume of foam formed was much greater and, when the flow of fuel entering the carburettor was high enough (i.e.

when the speed of the test car was high enough), the level of foam in the flat chamber progressively increased. The presence of foam in the float chamber did not have an immediate effect on the acceleration performance.

The formed foam, however, lowered the level of the liquid fuel in the float chamber and the float 12 did not shut off the needle valve means 16.

Fresh fuel therefore continued to flow into the float chamber and the foam level continued to rise until it eventually reached the top of the float chamber 9. At this point the foam appeared to block the vent line 1 9 and the pressure increased inside the float chamber 9, forcing excess fuel through the fuel discharge line 18 into the air passage 3 and expelling foaming fuel through the vent line 19. The level of liquid fuel in the float chamber 9 continued to decrease and the float 12, surrounded only by foam, sank thereby allowing more fuel to enter the float chamber to form more foam. At this point severe engine malfunctions were observed due to the over-rich mixture of fuel and air formed in the carburettor.

By providing the float 12 with a protrusion 17 at the lower side thereof, sinking of the float during foam formation in the float chamber is restricted.

The protrusion should be of such a length that in the lowermost position of the float 12, the needle element 14 is so positioned relative to the needle seat 1 5 that the amount of fuel entering into the float chamber is insufficient for causing foam formation to such an extent that the vent line is blocked and the pressure in the float chamber increases.

Reference is now made to Figure 2 showing an alternative of the float chamber shown in Figure 1.

In this second embodiment of the invention the internal air vent passage 1 9 has been replaced by an external vent passage 20 for venting vapour formed in the float chamber 9 into the atmosphere. The float 12 is restricted in its downward movement by the presence of a vertically displaceable element 21 provided with external screw thread and passing through a screw threaded opening in the bottom of the float chamber 9 below the float 12.

Further tests have been carried out with an engine provided with such an externally vented float chamber wherein no float sinking restrictions were applied. In these tests the possibiiity of encountering fuel pump vapour lock was reduced by submerging the fuel pump in the fuel tank (not shown in Figure 2). Since an external vent system was used it could be assumed that any over-rich mixture provided to the engine could not be due to unmetered fuel coming from an internal carburettor vent. Acceleration after a hot-soaktests resulted in substantially the same engine problems due to an over-rich fuel/air mixture discussed hereinabove with reference to Figure 1.

Also foaming problems during high-speed cruise conditions occurred. Cruise foaming appeared to be a cyclic phenomenon, with pressure peaks occurring every few minutes, making the air/fuel mixture very rich and prompting cruise surge. It was concluded that for a given fuel composition there is a critical temperature regime at which severe foaming can occur but that the evaporative cooling associated with the formation of the foam enables the system to move in and out of this regime in a cyclic manner.

By providing the upwardly extending element 21 in the bottom of the float chamber 9 the downward movement of the float due to lowering of the fuel liquid level at foam formation is restricted to such an extent that the needle element 14 remains in such a position with respect to the needle seat 1 5 that the amount of fuel entering the float chamber is insufficient for generating so much foam that vent blockage and therefore pressure build-up occurs.

The height of the element 21 above the bottorr of the float chamber can be adjusted by the screw thread arrangement. in this manner the lowermos position of the float 12 can be adjusted for the volatility characteristics of the fuel and the temperature conditions in which the engine is assumed to be operated.

Reference is now made to Figure 3, showing a further alternative of the float chamber arrangement shown in Figure 1.

In this further embodiment of the invention the fuel supply system of the float chamber 9 is provided with needle valve means 30 having a substantially elongated needle seat 31 cooperating with a substantially elongated needle element 32. The length of the needle seat 31 and the length of the needle element 32 are so chosen that in any position of the float 12 the needle element 32 is at least partly positioned in the bore of the needle seat 31. In this manner the amount of fuel entering the float chamber is restricted irrespective of the position of the float 12.

In the float chamber shown in the iast Figure a float 35 is arranged directly below a needle element 36 of needle valve means 37. As shown in Figure 4 the needle element 36, being fully separate from the float 35 is slidably mounted in a substantially elongated needle seat 38. The element 36 is thereto provided with laterally extending pins 39 having their free ends arranged in grooves 40 in the needle seat 38. When the liquid level in the float chamber 9 and therefore float 35 rises the lower end of the needle element 36 will mate the upper end of the float 35. Upon a further rise of the liquid level the needle element will be pushed by the float 35 and subsequently close the fuel supply. When the liquid level in the float chamber 9 lowers, the needle element 36 will be pushed upwards by the float 35 and subsequently close the fuel supply. When the liquid level in the float chamber 9 lowers, the needle element 36 will be allowed to go downward thereby permitting fuel to flow into the float chamber. Downward movement of the needle element is however restricted due to the shown pin/groove arrangement of the needle valve means 37. In this manner the amount of fuel entering the float chamber is restricted irrespective of the position of the float 35.

Claims (10)

1. A fuel supply system comprising a carburettor having a float chamber, a fuel inlet opening for supplying fuel into the float chamber, a fuel outlet opening for discharging fuel from the float chamber, a float inside the float chamber, an inlet valve member to open or close the fuel inlet opening in response to a rise and fall of the float, a vent opening at an upper portion of the float chamber to vent gases therefrom, wherein the carburettor comprises means arranged within the float chamber for limiting the maximum supply of fuel into the float chamber.

2. Fuel supply system as claimed in claim 1, wherein the carburettor comprises means for restricting sinking of the float thereby limiting the maximum supply of fuel into the float chamber.

3. Fuel supply system as claimed in claim 2, wherein the means for limiting the maximum fuel supply comprises a downward protrusion at the bottom part of the float.

4. Fuel supply system as claimed in claim 2 or 3, wherein the means for limiting the maximum fuel supply comprises an upward protrustion at the bottom part of the float chamber.

5. Fuel supply system as claimed in claim 4, wherein the upward protrusion is vertically displaceably mounted in the bottom part of the float chamber.

6. Fuel supply system as claimed in claim 1, wherein the inlet valve member is of such a length, that said member is at least partly arranged in the fuel inlet opening irrespective of the level of the float, thereby limiting the maximum fuel supply.

7. Fuel supply system as claimed in claim 1, wherein the inlet valve member is movably secured to a housing defining the fuel inlet opening.

8. Fuel supply system as claimed in any one of the claims 1-7, further comprising a vent line for venting gases from the vent opening to the atmosphere.

9. Fuel supply system as claimed in any one of the claims 1-7, further comprising a vent line for venting gases from the vent opening to an air intake line of the carburettor.

10. Fuel supply system comprising a carburettor having a float chamber substantially as described with particular reference to the accompanying drawings.