Device for Avoiding Cavitation in Injection Pumps

The present invention relates to a device designed to eliminate cavitation in the excess fuel return orifice(s) in the compression chamber of a fuel injection pump of an internal combustion engine after the end of the injection stage.

This stage in the operation of an injection pump, referred to as "emptying", causes excess fuel to be expelled at very high pressure and at very high speed through return orifices where the fuel that is already present is at low pressure. At the interface between the jet of expelled fuel and the fuel at low pressure, this gives rise to the appearance of bubbles due to degassing which, combined with the travel speed, give rise to erosion of the walls of the return orifices by cavitation, which erosion can lead to destruction of the injection pump. One of the means for eliminating this cavitation is to increase the pressure which obtains in the return orifices of an injection pump when emptying takes place. Devices are known such as that described in document JP08296528 which teaches placing a check valve upstream from the feed to the injection pump and two rated valves downstream from the injection pump, one of the rates valves having a high rating and enabling a large flow rate and the other rated valve having a low rating for passing a low flow rate. In addition, at least one of the rated valves includes an orifice to guarantee continuous circulation of fuel. The drawback of that device is that the permanent link does not enable high and sufficient pressure to be maintained in the orifices before emptying takes place. This pressure arises only when the emptying flow appears, and that is not sufficient for avoiding orifice erosion effectively.

The invention proposes remedying those drawbacks by providing a device for eliminating cavitation in the excess fuel return orifice(s) in the compression chamber of a fuel injection pump of an internal combustion engine after the end of the injection stage, said injection pump being connected firstly to a feed duct including a first check valve having low headloss enabling fuel to reach the compression chamber, and secondly to an excess fuel return duct,

wherein the return duct comprises in parallel and close to the return orifice of the injection pump, a second check valve that is rated to cause the pressure in said return orifice of the injection pump to rise, and a two-port valve that is normally open and that is caused to close by the appearance of pressure in the return orifice greater than the pressure which obtains in the feed duct upstream from said first check valve.

According to another characteristic of the invention, the two-port valve is provided with a spring causing said valve to open when the pressure obtaining upstream from the first check valve is substantially equal to the pressure which obtains in the return orifice.

According to yet another characteristic of the invention, the return duct includes a parallel-connected accumulator upstream from the rated, second check valve and the two-port valve.

The invention also provides the use of said device for implementing fuel injection in an internal combustion engine.

The advantages of the device lie in reduced wear of the components of the injection pump, thus making it possible to perform maintenance at reduced frequency and minimizing the dispersion of metal particles in the fuel.

By way of non-limiting example,

FIG. 1 is a diagram of a device of the invention.

FIGS. 2, 3, and 4 show the piston of the injection pump at various stages in compression.

FIG. 5 shows how pressure varies in the return orifices during the injection stages, curve A showing said variation for a pump that does not have the device of the invention, and curve B showing the same variation, but for a pump that is fitted with the device of the invention.

In FIG. 1, a duct 2 provided with a check valve 3 connects a fuel circulation pump 1 fed from a tank 9 to a fuel injection pump 4 shown in part only, being represented by its feed orifice 4a. The delivery pressure of the pump 1 is limited by a rated check valve 1a. The main return duct 5 and the secondary ducts 5a and 5b connect the return orifice 4b of the injection pump 4 in parallel to a rated check valve 6 and to a two-port valve 7. The two-part valve 7 is pilot controlled via a line 7a by the pressure which obtains in the duct 5b, and via a line 7b by the pressure which obtains in the duct 2 upstream from the rated check valve 3. A spring 7c reinforces the pilot control action due to the pressure in the line 7b, and holds the valve 7 in the open position in the absence of a large pressure difference between the two pilot lines. In the position 7e, the valve 7 puts into operation a restriction that gives rise to headloss for maintaining a certain level of fuel pressure upstream from the valve 7. The orifices 4a and 4b are put selectively into communication with the compression chamber 4k of the injection port 4 by means of a peripheral groove 4c of the envelope 4j and orifices 4d and 4e of the piston jacket 4f as a function of the movements of the piston 4g which has edges 4h and 4i for interrupting delivery. A small volume pressure accumulator 8 is installed on the duct 5 immediately downstream from the return orifice 4b. The rated check valve 6 and the two-port valve 7 are connected to the tank 9 via ducts 5c and 5d.

In FIG. 2, the piston 4g is at bottom dead center and disengages the orifices 4d and 4e to put them into communication with the compression chamber 4k.

In FIG. 3, the piston 4g is substantially halfway along its stroke and it closes the orifices 4d and 4e, thereby interrupting communication with the compression chamber 4k.

In FIG. 4, the piston 4g has continued its stroke, and the edges 4i and 4h disengage the orifices 4d and 4e, putting them into communication with the compression chamber 4k via a groove 4m formed on a generator line in the side wall of the piston 4g.

In FIG. 5, a graph having an abscissa T representing time and an ordinate P representing pressure, there can be seen a curve A showing how the pressure of the fuel in the return orifices 4d and 4e varies during an injection cycle for a pump that is not provided with the device of the invention, and a curve B showing the same variation for a pump that is provided with the device of the invention.

The operation of the device is described below.

The piston 4g is at the beginning of its compression stroke, as shown in FIG. 2. The check valve 6 is rated to a pressure lying in the range 50 bars to 100 bars, the damper 8 having an inflation pressure that is slightly smaller than the rated pressure of the check valve 6, and in the absence of a large pressure difference between the ducts 7a and 7b, the two-port valve 7 is held in its open position 7e by the spring 7c. The restriction of the valve 7 in its position 7e provides circulation pressure of about 3 bars. The fuel supplied by the pump 1 flows along the duct 2 through the check valve 3, the orifice 4a, the compression chamber 4k, the orifice 4b, the two-port valve 7, and returns to the tank 9 via the duct 5d. This situation corresponds in FIG. 5 to time T.sub.0 of curve B.

The piston 4g follows its compression stroke and the high pressure in the duct (not shown) connecting the compression chamber 4k to the injector (not shown) causes the check valve 3 to close and fuel to be delivered via the orifice 4b. The sudden increase in flow rate in the duct 5b, and the headloss in the two-port valve 7, give rise to a significant increase of pressure in the ducts 5a and 7a, causing the valve 7 to be controlled so as to switch to position 7d. Pressure continues to rise in duct 5a still it reaches the rated value of check valve 6 which begins to open. Simultaneously, the damper 8 fills and its pressure rises, thereby attenuating the hammer on the check valve 6. This situation corresponds in FIG. 5 to the variation of curve B in the vicinity of point B.sub.1.

When the piston 4g reaches the position shown in FIG. 3, the orifices 4a and 4b are closed and the fuel is contained between the check valve 3 and the rated check valve 6 at a pressure close to the rated pressure of the rated check valve 6. This pressure therefore obtains likewise in the circular groove 4c and in the orifices 4d and 4e. Because the compression chamber 4k is isolated from the orifices 4d and 4e, the pressure in said compression chamber can rise until it reaches the value at which injection is to take place, which can be of the order of 1000 bars. This situation corresponds in FIG. 5 to variation in curve B between points B.sub.1 and B.sub.2.

When the piston 4g reaches the positions shown in FIG. 4, the edges 4h and 4i have uncovered the orifices 4d and 4e, putting them again into communication with the compression chamber 4k. The beginning of this "emptying" opening corresponds to time T.sub.1 and to pressure P.sub.2 in FIG. 5. This emptying causes fuel to be transferred suddenly through the orifices 4d and 4e in the form of very high speed jets, giving rise to a rapid rise of pressure in the orifices 4d and 4e, corresponding to pressure peak B.sub.3 in curve B in FIG. 5. The interface of the high speed jet with the fuel already present is the seat of turbulence that generates bubbles of gas if the pressure that obtains in the fuel present in the orifices 4d and 4e is insufficient, with this being minimized by the high level of the pressure P.sub.2 which lies in the range 50 bars to 100 bars.

After reaching top dead center, the piston follows its return stroke to bottom dead center, pressure in the compression chamber 4k drops as its volume increases, and when the orifices 4d and 4e are again in communication with the compression chamber 4k, pressure also drops in the entire circuit extending between the check valve 3, the rated check valve 6, and the two-port valve 7. When the pressure in the duct 7a is close to the pressure in the duct 7b, the spring 7c causes the two-port valve 7 to take up position 7d, the damper 8 empties, and the cycle can restart.

Curve A in FIG. 5 shows the same operating stages for a pump that is not fitted with a device of the invention. The pressure at point A.sub.1 remains close to the pressure P.sub.0, i.e. close to a few bars. The pressure P.sub.1 at point A.sub.2, less than 50 bars, corresponds to the beginning of emptying via the orifices 4d and 4e, and is insufficient to prevent bubbles of gas forming at the peripheries of the jets. These bubbles strike the walls of the orifices 4d and 4e and give rise to erosion which destroys the jacket 4f. In the device of the invention, the residual pressure maintained in the orifices 4d and 4e by the rated check valve 6 considerably reduces the formation of gas bubbles and minimizes erosion.