GB2163492A - Fuel injection pump for internal combustion engines - Google Patents

Abstract

In a fuel injection pump wherein the front control edge (25) of a plunger (14) closes an inlet port (18) to cause fuel to be delivered from a working chamber (16), at the commencement of the delivery stroke a peripheral groove (28) on the plunger (14) positioned a small distance from the edge (25) allow fuel to leak from the working chamber into the inlet port (18). Fuel flows from the pump working chamber (16) into a longitudinal groove (29) connected to the peripheral groove (28). The fuel volume returning from the pump working chamber (16) depends on the cross-section of the groove and the angular position of the plunger, the latter determining the effective length of the peripheral groove. The distance of the peripheral groove (28) from the end face (15) is preferably 0.2 to 0.4 times the diameter of the inlet port, and the width and depth of the peripheral groove (28) is preferably 3 to 6 per cent of the piston diameter d. <IMAGE>

Description

SPECIFICATION Fuel injection pump for internal combustion engines The invention relates to a fuel injection pump and, in particular, but not exclusively, to a fuel injection pump for an internal combustion engine.

Already known from Swiss Patent Specification No. 269 597 is a fuel injection pump, where at the commencement of the delivery stroke a leakage duct between the pump chamber and the inlet opening in the pump cylinder wall becomes active. During the first part of the delivery stroke, fuel returns through this leakage duct to the inlet opening, and the flow restriction formed by a leakage hole between groove and pump working chamber reduces this return flow with increasing speed, so that the delivery is increased with speed (negative adaptation). At the same time a slow delivery pressure increase is obtained and consequently a later commencement of delivery at low speeds than at high speeds.In combination these measures result mainly, within the modern direct injection engine, in a reduced combustion noise, especially at low loads and low speeds, and have altogether a favourable effect upon fuel consumption and upon smoke emission.

In the known fuel injection pump the leakage hole serving as flow restriction penetrates one flank of the groove between the end face of the plunger and the annular groove machined in the piston skirt. This provides the same throttling action independent of the full load position of the pump piston and thus the fuel delivery quantity and, owing to the layout of the leakage hole, the groove must be relatively deep, which considerably weakens the region of the piston defined by the flank of the groove.This is not permitted for high pressure injection pumps and leads to deformation and rapid wear at the end of the pump piston face controlling the commencement of injection An object of the invention is to provide a fuel injection pump having a leakage duct effective between the pump working chamber and the inlet port only at the commencement of the delivery stroke which, in spite of the required narrow tolerances, can be manufactured safely and cheaply and which without additional measures provides a load sensitive control of the flow restriction effect.

In accordance with the invention, a fuel injection pump for an internal combustion engine comprises a pump piston guided in a pump cylinder for delivering fuel from a pump working chamber, which chamber is defined by the end face of the pump piston and by a delivery valve, a fuel inlet port in the wall of the pump cylinder, a control surface on a skirt of the pump piston, which control surface is defined by a control edge at the front end and at the drive end by a leakage connection comprising a peripheral groove in the control surface substantially parallel to, and spaced apart from, the front control edge, between the pump working chamber and the inlet port, which leakage connection by its relative position to the inlet port determines the commencement of delivery and the end of delivery, and which leakage connection is only active at the commencement of delivery stroke, and a connection duct between the peripheral groove and the pump working chamber, controlled by a flow restrictor, wherein the connection duct comprises a longitudinal slot which extends from the end face of the pump piston in the direction of its longitudinal axis at least up to the peripheral groove, the flow cross section and the active length of the peripheral groove, covered by the wall of the pump cylinder depending upon the set fuel delivery, form the flow restrictor of the leakage connection, and the distance between the confining edge of the peripheral slot, remote from the end face of the pump plunger and the end face is smaller than the diameter of the inlet port.

Such a fuel injection pump has the advantage that the connection passage, formed by a longitudinal groove, between the pump working chamber and the peripheral groove can be manufactured in a simple and well known manner without the close dimension tolerances which make manufacture difficult, and that furthermore the flow cross section directly, i.e. without additional means of flow restriction, and the active length L of the peripheral groove, covered by the piston cylinder wall, provides a throttling action depending upon load or delivery quantity set. Therefore, no additional measures are necessary to cut off or change the throttling action dependent upon the quantity of fuel delivered.Finally, the distance a between the peripheral groove and the end face has been made smaller than the diameter of the inlet port, and this ensures that the measures aiming at a gradual pressure increase during commencement of delivery, at a speed related timing of the commencement of delivery and at a change of the delivered quantity become inoperative after commencement of delivery. Thus they are no longer operative at the end of injection and have no negative effect upon the necessary high injection pressure at the end of injection.

In a fuel injection pump which is known from the above mentioned publication, and wherein a control groove machined into the piston skirt forms the control edge, the longitudinai groove forming the connection duct serves also as a stop groove and connects both the peripheral groove and the control groove to the pump working chamber. By further measures to determine the flow cross section of the peripheral groove and its dis tance from the piston end face, the effect and action of the peripheral groove is clearly limited to its proposed use, so that there is no lowering of the injection pressure over the whole injection period nor an unintended interruption of the fuel injection.If an injection pump, also known from the above mentioned publication, is equipped with a pressure control valve serving as a constant pressure relief valve, the delivery reduction achieved otherwise by this valve during increasing speed is changed by the leakage duct acting, according to the invention, as flow restriction at the beginning of the delivery stroke, into a fuel delivery characteristic increasing with speed within the low speed range. This combination of features prevents the full load delivery setting from leading to an excess delivery in the lower speed range and a corresponding formation of soot in the exhaust gases, during reducing fuel delivery characteristics at increasing speed.

The adjustable fuel delfvery characteristics, which can be achieved by the invention, which at increasing speed initially increase and then drop (negative and positive adaptation), could otherwise only be realised by exceedingly complicated measures at the governor. The present invention makes such measures unnecessary. Also a speed sensitive timing device is not required.

By way of example only, a specific embodiment of the present invention will now be described, with reference to the accompanying drawings, in which: Fig. 1 is a longitudinal part section through an embodiment of the fuel injection pump according to the invention; Fig.2 is a perspective view of a variant of the embodiment of a pump piston used in the embodiment of Fig.1; and Fig.3 is an enlarged part section along the line Ill-Ill in Fig.2.

The fuel injection pump 10 shown in longitudinal part section in Fig. 1 is a single cylinder fuel injection pump. It is self-evident that the invention would also apply to multi-cylinder fuel injection pumps, so-called in-line fuel injection pumps, as Fig.1 can also be regarded as a longitudinal section through one pump element of such an injection pump.

A pump cylinder 12 is inserted into the only partly illustrated housing 11 of the fuel injection pump 10, and a Pump piston 14, which is free to move axially and rotatably, is guided within the cylinder bore 13. The pump working chamber 16, forming part of the cylinder bore 13, is limited at one end by the end face 15 of the pump piston 14 and is closed at the other end by a delivery valve 17, which in order to achieve a controllable increased pressure build-up and a fuel delivery characteristic dropping at increasing speed, is arranged as a constant pressure relief valve. The pump working chamber 16 is connected by a fuel inlet port 18 in the wall 12a of the pump cylinder 12, serving also as a leakage return bore with the suction chamber 21 being under a lower fuel inlet pressure.

Into the peripheral surface 22 of the pump plunger 14 a metering groove 23, shown in Fig. 1 as a helical groove, is machined, the boundary of which at the surface area 22 of the pump piston 14 and the pump working chamber 16 provides a control edge 24. This control edge 24 at an angle to the pump piston centre line A confines together with a front control edge 25, formed by the front end 15, a control surface 26 which, with its position relative to the inlet port 18, determines the commencement of delivery and the end of delivery.

In deviation from the known pump pistons, the pump piston 14, and also the embodiment variant 14' illustrated in Figs. 2 and 3, is provided with a leakage duct 27 between the pump working chamber 16 and the inlet port 18, being only operative during the commencement of the delivery stroke, and which is provided with a peripheral groove 28 in the form of an annular slot parallel to the control surface 26 and at a small distance a from the front of the control edge 25, and a connection duct 29 connecting the peripheral groove 28 to the pump working chamber 16, whereby the fuel flow through the peripheral groove 28 is determined by a flow restrictor 31.The connection duct 29 is formed by a longitudinal groove, which extends in a direction parallel to the centre line A of the pump piston 14 (or 14') starting from its end face 15 to the metering groove 23 (or 23') and thus also serves as a stop groove, connecting the metering groove 23 (or 23'), and the peripheral groove 28 to the pump working chamber 16. If, instead of a stop groove, the pump piston 14 was provided with a central bore and a cross hole leading into the metering groove 23 (or 23'), it would suffice if the connection duct 29, made in the form of a longitudinal groove, was arranged for a correspondingly smaller flow volume and only extended from the end face 15 to the peripheral groove 28.

The flow restrictor 31 is formed by the flow cross section Q, illustrated cross hatched in Fig.3, and the active length L (see Fig.2) of the peripheral groove covered by the wall 1 2a of the pump cylinder 12, depending upon the angular position of the pump piston 14 (or 14') and thus the set delivery volume, and in this way determines the effectiveness of the leakage connection 27. Owing to the required restriction action the width B and depth T of the flow cross section Q in the peripheral groove 28 only amount to a few tenths of a millimetre and are always limited to approximately 3 to 6 per cent of the piston diameter d.

The distance a between the peripheral groove 28 and the end face 15 is fixed as the distance between the side wall 28a remote to the front face 15 of the peripheral groove 28 and the front face 15 and should always be smaller than the diameter D of the inlet port 18.

In order to prevent the flow restrictor 31 from being effective until the end of injection, a specially advantageous embodiment of the invention determines the described distance a of the peripheral groove 28 at a dimension which is smaller than half the diameter D of the inlet port 18, preferably approximately 0.2 to 0.4 times D.

In a practical embodiment having a piston diameter D of 9 mm and an inlet port 18 diameter D of 3.5 mm, distance and dimensions of the peripheral groove 28 are established as follows: B equals 0.3 mm, T equals 0.3 mm and a equals 0.7 mm. A groove with this form and dimensions can be produced with very narrow tolerances in mass production; and contrary to the well known cylindrical ground faces, an accurately defined restriction action results at the head of the pump piston extending to its end face, which is practically independent of the stroke position of the pump piston 14 (or 14') during that part of the stroke which is determined by the distance a.

The effects of the leakage duct 27 according to the invention are set out in the following with reference to the drawings.

At the bottom dead centre of the pump piston 14 (see Fig.1) the end face 15 opens a sufficient cross section of the inlet port 18, so that adequate fuel can enter from the suction chamber 21 into the pump working chamber 16. When during the upward motion of the piston 14 (or 14') the inlet port 18 is closed by the control surface 26 (see also Fig.2), the delivery stroke would commence if the leakage duct 27 were not present.During the slow motion of the pump piston 14 (or 14'), or at low speeds of the internal combustion engine, the very narrow flow section 0 (see Fig.3) of the peripheral groove 28 causes a portion of the fuel, determined by the flow cross section 0 and the active length L of the flow restrictor 31, to return through the inlet port into the suction chamber 21, until this is closed by the additional stroke of the pump piston 14 (or 14') through the distance a.

This partial return flow slows down the pressure increase in the pump working chamber 16, retards the commencement of delivery and reduces the fuel delivery volume determined in any angular position of the pump plunger 14 (or 14') by the control surface 26.

With increasing speed also the throttling action of the flow restrictor 31 increases until it is completely eliminated. Owing to the increased length L of the flow restrictor 31 at higher loads this throttle action also increases proportionally to the load, and at simultaneous high speeds it becomes ineffective. Thus the length of the peripheral groove 28 need not be limited to the required active length, but in order to simplify manufacture the groove can be finish ground during manufacture of the pump piston 14.

Thus the invention provides the following advantages: 1. In the lower speed range a fuel delivery characteristic increasing with speed is realised (negative adaptation) in order to reduce the exhaust gas emission; 2. Compared with conventional injection pumps the pressure increase at the commencement of delivery stroke is perceptibly reduced, mainly at low speeds; this also reduces the combustion pressure in the engine cylinders and thus the noise emission; 3. The commencement of delivery retarded owing to the fuel compression in the injection line at increasing speed is readvanced in the required manner by the increasing throttling action of the flow restrictor 31 at increasing speed, so that special timing devices, such as centrifugal advance mechanisms, are not necessary and can be saved.

In pressure discharged diesel engines the fuel delivery increasing with speed can obviate an otherwise necessary manifold pressure compensator. If the fuel injection pump, illustrated in Fig.1, is fitted with a delivery valve 17 acting as a constant pressure relief valve, the combination of this valve with the leakage connection 27, as proposed by the invention, can realise fuel delivery characteristics, which are initially increasing and then dropping back with speed, whereby very expensive and sophisticated compensation devices can be saved.

Claims (6)

1. A fuel injection pump for an internal combustion engine, said pump comprising a pump piston guided in a pump cylinder for delivering fuel from a pump working chamber, which chamber is defined by the end face of the pump piston and by a delivery valve, a fuel inlet port in the wall of the pump cylinder, a control surface on a skirt of the pump piston, which control surface is defined by a control edge at the front end and at the drive end by a leakage connection comprising a peripheral groove in the control surface substantially parallel to, and spaced apart from, the front control edge, between the pump working chamber and the inlet port, which leakage connection by its relative position to the inlet port determines the commencement of delivery and the end of delivery, and which leakage connection is only active at the commencement of delivery stroke, and a connection duct between the peripheral groove and the pump working chamber, controlled by a flow restrictor, wherein the connection duct comprises a longitudinal slot which extends from the end face of the pump piston in the direction of its longitudinal axis at least up to the peripheral groove, the flow cross section and the active length of the peripheral groove, covered by the wall of the pump cylinder depending upon the set fuel delivery, form the flow restrictor of the leakage connection, and the distance between the confining edge of the peripheral slot, remote from the end face of the pump plunger and the end face is smaller than the diameter of the inlet port.

2. A fuel injection pump as claimed in claim 1, wherein the peripheral groove is an annular recess.

3. A fuel injection pump as claimed in claim 1 or claim 2, comprising a control groove machined into the skirt of the pump plunger and forming the drive end control edge, wherein the longitudinal groove, forming the connection duct, serves also as a stop groove and connects both the peripheral groove and the control groove with the pump working chamber.

4. A fuel injection pump as claimed in any of claims 1 to 3, comprising a constant pressure relief delivery valve, wherein the width and the depth of the flow cross section in the peripheral groove are 3 to 6 per cent of the piston diameter.

5. A fuel injection pump as claimed in any of claims 1 to 4, wherein the said distance of the peripheral groove from the front control edge is between 0.2 and 0.4 of the diameter of the inlet port.

6. A fuel injection pump for an internal combustion engine, substantially as herein described, with reference to and as illustrated in the accompanying drawings.