Piston Driven Fuel Pumps

Field of the Invention

This disclosure relates to a method of controlling and limiting the problem of self- latching in high pressure fuel pump systems of the type which use a reciprocating plunger piston. It has particular but not exclusive application to such system which use an electrically or electronically controlled fuel feed pump to feed the high pressure pump.

Background to the Invention

In modern fuel injection engines, fuel is supplied from a tank to a high pressure pump, typically v ia a mechanical low pressure fuel feed pump. The high pressure, usually a piston/plunger type pump driv en by a cam mechanism, prov ides pressurised fuel to a fuel accumulator volume such as a common rail, which in turn supplies fuel to indiv idual fuel injectors. Typically a common rail includes a pressure relief v alv e. Currently, most

'normally open' high pressure pumps are designed to ran up to a maximum allowable speed while retaining control of the amount of high pressure fuel that is deliv ered to the rail. There is usually an outlet metering v alve located e.g. integral with the high pressure pump which is biased in the open position by a spring but can be closed by operation of e.g. a solenoid mechanism.

If the pump is driv en beyond this maximum speed then uncontrolled deliv ery of high pressure fuel to the rail can occur due to the outlet metering v alv e closing without receiving any assistance from the solenoid the stator , i.e. when the outlet metering valve is still supposed to be nominally open. This is known as self-latching.

Design compromises therefore have to be made in order to accommodate the high spring force required to resist self-latching. These can include, but are not limited to, needing a large stator to ov ercome the high spring force, and/or expensive features or geometry around the valve to improve flow characteristics and minimise plunger bore pressure build up, as well as a requirement to hav e a rail pressure relief valve to control ov er-pressurisation ev ents.

It is an object of the inv ention to provide a system and method which ov ercomes problems with self-latching Statement of the Invention

In one aspect of the invention is provided a method of controlling an engine fuel supply system, said system comprising a high pressure fuel pump, said high pressure fuel pump including a cam driven piston/plunger adapted to pressurise fuel in a chamber so as to supply consequently pressurized fuel to one or more fuel injectors, said high pressure fuel pump being supplied by fuel from a low pressure feed pump, a method of controlling said system to avoid self-latching of said high pressure pump comprising;

a) measuring or determining the operational speed of said high pressure fuel pump,

b) ensuring that the supply pressure outlet from said low pressure feed pump is below a limit value, said limit value being dependent on high pressure pump speed.

Said high pressure fuel pump may include an outlet metering valve located integral or adjacent to the high pressure pump.

Said outlet metering valve may be biased to an opening position by a spring means

Said low pressure feed pump may be an electrically or electronically controlled/regulated pump.

The method may include storing a map, said map correlating a maximum or limit value of supply pressure from said low pressure pump with the speed of said high pressure pump.

The fuel temperature may be determined and also used along with high pressure pump speed in determining said limit value.

Said high pressure pump speed may be determined from engine speed

Said low pressure supply may be controlled by controlling one or more electrical operational parameters of the fuel feed pump.

Brief Description of the Figures

The invention will now be described by way of example and with reference to the following figures of which:

Figure 1 shows a partial cross sectional view of a high pressure fuel pump;

Figure 2 shows a plot of cam profile (cam lift against cam angle) as well as associated plunger velocity at a set speed, of a cam driven plunger type high pressure fuel pump;

Figure 3 shows a chart of fuel feed pressure characteristics against pump speed for a both mechanical and electrical low pressure pumps. Figure 4 shows an arrangement of a system adapted to supply an engine with high pressure fuel according to one aspect.

As mentioned, self-latching is basically uncontrolled delivery of high pressure fuel to the rail due to the pump being driven at a higher speed than a nominal maximum.

Figure 1 shows a partial cross sectional view of a high pressure fuel pump where fuel is pressurised by a plunger 1 reciprocating in a pumping chamber 2 and flow of the spilled fuel is via conduits 3 to a fuel inlet gallery. The plunger is operated by a cam mechanism, of which there are various types that would be known to the skilled person. An Outlet Metering Valve (OMV) 4 is provided, and includes an OMV pin 5, pin guide 6, and an armature 7. The valve is biased by a spring 8 to open the valve to allow the spilled fuel to flow through the conduits. The valve can be shut by movement of the armature (e.g. via solenoid) to close or latch the valve.

As the speed of the pump increases, the ability to spill the fuel past the OMV pin, through the OMV guide and out towards the fuel rail (fuel gallery) becomes too low to keep up with the flow rate of fuel being generated by the plunger. As this effect becomes more pronounced with increasing pump speed, pressure builds up in the plunger bore, which in turn creates a force on the OMV pin head that can overcome the opposing force from the OMV spring. This is referred to as self-latching.

Figure 2 shows a plot of cam profile (cam lift against cam angle) as well as associated plunger velocity at a set speed, of a cam-driven plunger type high pressure fuel pump.

The position on the cam where self-latching is most likely to occur is the point with the highest plunger velocity, which is 45 degrees from the start of the nominal cam profile in this case.

Detailed Description of the Invention

In order to limit the amount f pressure within the plunger chamber, in aspects f the inv ention, the fuel feed pump which supplies the high pressure pump is operated to provide e.g. lower feed pressures in order to deliberately create a cav ity within the plunger chamber and increase the amount f time taken to then collapse that cav ity.

Thus in situations (such as speeds) where self- latching may occur, the feed pressure (outlet from the low pressure fuel feed pump ) is reduced. Operational parameters of the low pressure feed pump may be v aried to implement a reduced feed pressure regimes, when self-latching is likely to occur. In a general aspect the self-latching problem is controlled (mitigated) preferably by using electric/electronically regulated operated low pressure fuel feed pump (rather than a mechanical low pressure fuel feed pump ) to supply fuel to the high pressure fuel pump; controlling low pressure fuel feed pump in an appropriate manner to overcome self- latching is simpler in the case of an electrically operated/controlled pump compared to conventional mechanical (vane) (low pressure fuel feed ) pumps .

So in one aspect a control strategy is used to mitigate the problems and maintain controllability of the pump beyond the typical self-latching speed by using an electric low pressure fuel feed pump, and reducing the feed pressure when self-latching is likely to occur.

In one example, the speed of the high pressure pump is determined, and dependent on this, the fuel feed pump is controlled to supply an appropriate (e.g. reduced ) pressure dependent on this. I n order to facilitate this in one example, a high pressure pump speed versus low pressure pump supply pressure map may be prov ided, and the low pressure supply pump controlled to provide the appropriate pressure. In order to prevent self-latching (e.g. by creating the aforementioned cav itation effect ), the fuel feed pump may be operated such that the fuel feed pressure (outlet from the low pressure feed pump ) lies below a limit value, said limit value being dependent on high pressure pump speed.

It has been observed that when the Volumetric Efficiency is below 50% the cav ity within the chamber of the high pressure plunger pump pumping chamber is collapsing after the peak rate portion of the cam.

Figure 3 shows a chart of fuel feed pressure characteristics against pump speed for a both mechanical and electrical low pressure pumps (this is the pressure between the low and high pressure pumps e.g. at the location e.g. measured by sensor of figure 4 - sec later) and pumping speeds of these pumps. The parameter of Maximum Volumetric Efficiency for the high pressure pump is also shown. In this example it is assumed that under ordinary condition a pump will self-latch at 3500 rpm. This is represented by the line A and shows the high pressure pump speed at which self-latching occurs. Self- latching speed would normally limit high pressure pump operation to <3500rpm.

According to the control strategy, the high pressure pump VE is kept below 50% to ensure self-latching does not occur at peak plunger velocity whilst retaining pump functionality at above 3500 rpm. In practical terms the volumetric efficiency is reduced by varying the feed pressure outlet from the (low pressure ) feed pump supplied to the high pressure pump. One or more charts can be provided which maps the maximum feed pressure outlet from the supply (low pressure) pump against high pressure pump speed. Such charts or maps can be formulated using experimental or empirical methods.

Figure 4 shows an arrangement of a system 10 adapted to supply an engine 1 1 with high pressure fuel according to one aspect. Fuel is drawn from a fuel tank 12 by an electrical or electronically operated or controlled (low pressure ) fuel pump 13 (supply pump ) to a high pressure e.g. piston (plunger-type) pump 14. The high pressure fuel pump is described above and is of a plunger/piston design, and prov ides high pressure fuel to a fuel rail which provide fuel to the engine e.g. fuel injectors. Reference numeral 15 indicates the Engine Control Unit adapted to control v arious engine components. In the example this ECU also controls the electric fuel feed pump. The control of the electrical fuel feed pump may be performed in a v ariety of ways such as controlling the power, current or v oltage across or through a motor, control of speed or pressure and such like.

The outlet pressure of the low pressure supply pump, or the pressure at any point between the low pressure electrical fuel pump and the high pressure fuel portion of the high pressure fuel pump is measured or estimated e.g. by a sensor 16 and fed to a control unit such as the ECU of an engine. In addition the control unit also has an input which is the speed of the high pressure pump (typically operated by a rotating camshaft ). Typically as this camshaft is powered from the engine driv e, as an alternativ e, the engine speed 1 7 is input to the control unit as this parameter that correlates to the high pressure pump speed.

In refined examples other parameters such as the fuel temperature (preferably at a point again between the low pressure electrical fuel feed pump and fuel inlet to the high pressure fuel pump ) is measured e.g. by an appropriate sensor 1 8 and fed to the engine ECU.

The ECU is adapted to process the data such and v ary i.e. control the operational parameters of the fuel feed pump so as to av oid self-latching. This may be performed such that the fuel feed pressure ( outlet from the low pressure fuel feed pump ) is at or below a limit lev el, the limit lev el being dependent on high pressure pump

(engine) speed. This in a simple embodiment a fuel feed pressure ( limit ) against high pressure pump speed map may be prov ided such that it is used to ensure the feed pressure is below or at a maximum value for the pump speed. More complex ( e.g. 2 and 3-d maps ) may be prov ided where parameters such as fuel temperature arc also used as an input along with high pressure pump speed to determine the max allowable suggested maximum feed pressure to av oid self-latching.