GB2307519A - Diaphragm carburettor - Google Patents

Abstract

The fuel chamber 6 supplies fuel into the mixing passage via a main discharge port 7 and primary and secondary idle discharge ports 8A, 8B. To overcome lean pick up, an additional fuel discharge port 20 is provided upstream of the throttle shutter 13 between the main and idle discharge ports. The additional fuel discharge port 20 is operative to commence feeding fuel into the mixing passage 2 when the throttle shutter 13 is opened beyond a point at which substantially only the idle discharge ports 8A, 8B are supplying fuel to the mixing passage but before the throttle shutter becomes fully open.

Description

DIAPHRAGM CARBURETOR The present invention relates to a diaphragm carburetor.

A typical diaphragm carburetor comprises a carburetor body defining a mixing passage or venturi having an air intake side and an engine outlet side, and a throttle shutter rotatably mounted in the mixing passage. A metering chamber supplies fuel into the mixing passage via a main or high speed discharge port or jet and at least one idle discharge port.

In such a carburetor fuel is supplied to the engine substantially only through the idle discharge port(s) when the engine is idling with the throttle shutter in the idle position, and through both the main and idle discharge ports when the engine is running at high speed with the throttle shutter fully open.

However, as will be described, during the transition to the full open position it is often the case, depending on the application, that the main discharge port begins to feed fuel too late. This results in a so-called lean pick up where the engine does not obtain enough fuel for optimum fuel/air ratio, leading to poor acceleration.

It is an object of the present invention to provide a construction of diaphragm carburetor in which this problem is eliminated or mitigated.

Accordingly, the present invention provides a diaphragm carburetor comprising a carburetor body defining a mixing passage having an air intake side and an engine outlet side, a throttle shutter rotatably mounted in the mixing passage, and a metering chamber for supplying fuel into the mixing passage via a main discharge port and at least one idle discharge port, the carburetor further including an additional fuel discharge port for supplying fuel into the mixing passage from the metering chamber, the additional fuel discharge port being located upstream of the throttle shutter between the main and idle discharge ports and being operative to commence feeding fuel into the mixing passage when the throttle shutter is opened beyond a point at which substantially only the idle discharge port(s) are supplying fuel to the mixing passage but before the throttle shutter becomes fully open.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view of a prior art diaphragm carburetor with its throttle shutter partially cracked open for engine idling, Figures 2 and 3 are views of the prior art carburetor of Figure 1 with its throttle shutter in an intermediate position and the full open position respectively, Figure 4 is a graph illustrating the fuel flow into the mixing passage of the carburetor of Figure 1 individually from the idle and main discharge ports, Figure 5 is a graph illustrating the total fuel flow into the mixing passage of the carburetor of Figure 1 from the idle and main discharge ports, Figure 6 is a cross-sectional view of a diaphragm carburetor according to an embodiment of the invention with its throttle shutter partially cracked open for engine idling, Figures 7 to 9 are views of the carburetor of Figure 6 with its throttle shutter in various stages of opening to the full open position, and Figures 10 and 11 are graphs similar to figures 4 and 5 for the carburetor of Figure 6.

In the drawings the same reference numerals have been used for the same or equivalent components.

The construction and operation of diaphragm carburetors is well known in the art and will not be described in detail herein. Briefly, however, and referring to the known construction of a typical diaphragm carburetor shown in Figures 1 to 3, the carburetor includes a carburetor body 1 defining a mixing passage (venturi) 2 having an air intake side (LHS of Figure 1) and an engine outlet side (RHS of Figure 1). Fuel enters the carburetor at a fuel inlet 3 and is pumped by a fuel pump 4 to a metering chamber 6 via a fuel screen 5. The metering chamber 6 supplies fuel into the mixing passage 2 via primary and secondary idle discharge ports 8A and 8B respectively and a main or high speed discharge port 7 located upstream of the idle discharge ports 8A and 8B.

A high speed adjustment needle 9 cooperating with a main fuel adjustment orifice 10 permits adjustment of the rate of fuel flow from the metering chamber 6 to the main discharge port 7. Similarly, a low speed (idle) adjustment needle 11 cooperating with an idle fuel adjustment orifice 12 permits adjustment of the rate of fuel flow from the metering chamber 6 to the idle discharge ports 8A and 8B.

Finally, a throttle shutter 13 is rotatably mounted within the mixing passage 2 in the region of the idle discharge ports 8A and 8B, being operated by a throttle shaft (not shown) in conventional manner.

When the engine to which the carburetor is attached is idling the throttle shutter 13 is partially cracked open as shown in Figure 1, and engine vacuum is transmitted through the primary idle discharge port 8A.

Fuel is then drawn up through the idle fuel adjustment orifice 12 and delivered to the engine via the primary idle discharge port 8A as shown.

As the throttle shutter 13 opens to the intermediate position shown in Figure 2, the engine speed increases and more fuel is supplied to the engine by valving in the secondary idle discharge port 8B located immediately behind throttle shutter 13. In this intermediate position fuel is delivered through the idle fuel adjustment orifice 12 in the same manner as when the engine is idling.

As the throttle shutter 13 progressively opens from the intermediate position of Figure 2 to the full open position of Figure 3 the air velocity through the mixing passage 2 increases and fuel is metered up through the main fuel adjustment orifice 10 and the main discharge port 7 in accordance with the power requirements of the engine.

However, during the transition from the intermediate position (Figure 2) to the full open position (Figure 3) it is often the case, depending on the application, that the main discharge port 7 begins to feed fuel too late. This results in a so-called lean pick up where the engine does not obtain enough fuel for optimum fuel/air ratio, leading to poor acceleration.

Figures 4 and 5 show this lean transition condition in the form of graphs showing fuel flow vs throttle shutter position. In Figure 4 the line 15 shows the combined fuel flow from the primary and secondary idle discharge ports 8A and 8B over the range of throttle shutter positions from idle to full open, and the line 14 shows the fuel flow from the main discharge port 7 over the same range of throttle shutter positions. Figure 5 (not to the same scale as Figure 4) shows the total fuel flow 16 resulting from the sum of these two individual fuel flows, from which the lean transition 17 is clearly seen.

It is common to set a rich idle condition which helps to provide for additional fuel required to eliminate this lean pickup. This can, however, result in excessive emissions of carbon monoxide and hydrocarbons.

Figures 6 to 9 illustrate an embodiment of the invention in which this problem is mitigated.

In the embodiment of the invention, an additional discharge port or jet 20 is provided in the carburetor body 1 behind (i.e. upstream of) the throttle shutter 13, between the main discharge port 7 and the idle discharge ports 8A and 8B. The additional discharge port 20 is in communication with the metering chamber 6 via a one way check valve 21. The additional discharge port 20 is fixed, meaning that the fuel flow therethrough is not externally adjustable by, for example, an adjustment needle such as that shown at 9 or 11 for the main and idle discharge ports. The check valve 21 prevents back bleeding of air from the mixing passage 2 into the metering chamber 6 when the engine is in the idle condition.

Figure 6 shows the throttle shutter 6 partially cracked open for the engine idle condition, as in Figure 1, and fuel is delivered to the engine via the primary idle discharge port 8A as before. Figure 7 corresponds to Figure 2 and shows the intermediate position of the throttle shutter 13 wherein more fuel is supplied to the engine by valving in the secondary idle discharge port 8B. Figure 8 shows the position of the throttle shutter 13 during the transition from the intermediate throttle shutter position of Figure 7 to the full open position of Figure 9 (corresponding to Figure 3). During this transition fuel is fed into the mixing passage 2 from the additional discharge port 20.

This additional fuel enriches the mixture providing the correct additional amount of fuel to eliminate or mitigate the lean pick up/acceleration problem which existed in the prior art. Finally, Figure 9 shows the full open position of throttle shutter 13, in which the total fuel requirement is now provided by the main discharge port 7, additional discharge port 20 and primary and secondary idle discharge ports 8A and BB.

The additional discharge port 20 is calibrated to give an exact quantity of fuel and its distance behind (i.e. upstream of) the throttle shutter 13 will determine at what point in the transition it will begin to feed fuel to the mixing passage 2. These two variables will be selected in the initial design of the carburetor depending on the application.

Figures 10 and 11 show the improved fuel flow to the mixing passage 2 in the form of graphs similar to those of Figures 4 and 5. The fuel flow from the additional discharge port 20 is shown at 19 in figure 10, and this combined with the fuel flow 14, 15 from the main and idle discharge ports leads to elimination of the lean transition in the total fuel flow 16.

Claims (4)

CLAIMS:

1. A diaphragm carburetor comprising a carburetor body defining a mixing passage having an air intake side and an engine outlet side, a throttle shutter rotatably mounted in the mixing passage, and a metering chamber for supplying fuel into the mixing passage via a main discharge port and at least one idle discharge port, the carburetor further including an additional fuel discharge port for supplying fuel into the mixing passage from the metering chamber, the additional fuel discharge port being located upstream of the throttle shutter between the main and idle discharge ports and being operative to commence feeding fuel into the mixing passage when the throttle shutter is opened beyond a point at which substantially only the idle discharge port(s) are supplying fuel to the mixing passage but before the throttle shutter becomes fully open.

2. A carburetor as claimed in claim 1, wherein the additional fuel discharge port is fixed.

3. A carburetor as claimed in claim 1 or 2, wherein the additional fuel discharge port is in communication with the metering chamber via a one way check valve.

4. A diaphragm carburetor substantially as described with reference to Figures 6 to 9 of the accompanying drawings.