Internal-combustion engines – Charge forming device – Fuel injection system
Reissue Patent
2002-02-13
2003-07-22
Moulis, Thomas N. (Department: 3747)
Internal-combustion engines
Charge forming device
Fuel injection system
C123S450000, C123S496000
Reissue Patent
active
RE038192
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to an apparatus for producing a variable volumetric flow in a fuel feed system, especially for use in common-rail injection systems.
In common-rail injection systems, it is necessary to pump the fuel out of a tank, to compress it and to hold the compressed fuel ready for injection by injectors in a pressure reservoir referred to as a rail. The pressure in the rail and the quantity of fuel removed from the rail by injection vary with the operating conditions of the engine.
In order to enable the pressure in the rail and therefore also the volumetric flow delivered to the rail to be influenced in an appropriate manner, use is made, for example, of a configuration described in European Patent Applications 0 643 220 and 0 643 221, corresponding to U.S. Pat. Nos. 5,427,066 and 5,746,180. An upstream feed pump draws the fuel from a tank through a filter and supplies the high-pressure pump. The compressed fuel is stored in the rail and injected into combustion chambers by the injectors. In that method, use is made of constant-displacement pumps, which deliver a fixed volume with each revolution of the shaft. A variable delivery rate of the system is achieved by discharging the unrequired but already compressed volumetric flow with the aid of a valve. However, that principle is not advantageous for high-pressure systems in terms of energy.
In terms of energy, preference should be given to systems in which the pump delivers only a volumetric flow limited to the quantity that is actually required. A method known from low-pressure systems, in which the volumetric flow is influenced through the use of the adjustability of the volume of the displacer elements is disadvantageous in the case of diesel injection systems because of the high mechanical outlay and the large control forces that are required. Pumps with constant-volume displacer elements are accordingly more advantageous. In pumps with constant-volume displacer elements, the variable volumetric flow is achieved by differences in the filling ratio of the displacer volumes. One possibility, for example, is to force the volume that is not required in piston pumps out of the initially completely full cylinder before compression begins. The fuel can be forced back into the inlet line or into an additional bypass with the aid of a controllable valve. The disadvantage therein is that a quick-operating valve is necessary for each displacer element.
Instead of filling the cylinder completely at the outset and releasing the quantity that is not required, it is also possible to vary the filling level from the outset by limiting the supply to the displacer elements.
One possibility for limiting the supply is to throttle the entire volumetric flow fed to the pump or the volumetric flow fed to each individual displacer element. In that case, use is made of adjustable throttle valves, which allow proportional variation of the volumetric flow by variation of the throttling cross section. The maximum cross section of the throttle valve is set for the maximum volumetric flow at full load and rated speed. The interaction between the maximum volumetric flow that can be delivered, which is dependent on the rotational speed of the pump, and the adjustability of the throttle valve, yields a relationship between the adjustable volumetric flow and the manipulated variable as a function of the rotational speed of the pump. At low rotational speeds of the pump associated with a low maximum volumetric flow that can be delivered, the useful adjustment range of the throttle valve is severely limited since only a small area of the throttling cross-sectional area can be used with throttling effect. It is only at maximum rotational speed that the full adjustment range of the valve can be exploited. If, for example, a pump is to be operated at a rated speed of 3000 rpm and a delivery rate of 0.5 ml per revolution, the throttle valve must be constructed for a maximum volumetric flow of 1500 ml/min. At a speed of 300 rpm and the maximum volumetric flow delivered resulting therefrom of 150 ml/min, only 10% of the adjustment range of the throttle valve is used for control between zero load and full load.
Another disadvantage of known high-pressure piston pumps with a plurality of cylinders and central limitation of the volumetric flow being fed in, is the outlay associated with ensuring uniform delivery-flow pulsation. Due to the finite number of displacer elements, there is always pulsation in the delivery flow from the pump, due to the very principle involved. The fluctuation in the delivery flow about a mean value, which is referred to as pulsation for short, results from the superposition of the component delivery flows coming from the individual displacer elements to give an overall delivery flow. The minimum pulsation for a fixed number of displacer elements is obtained if all of the displacer elements have the same component delivery flow. Therefore, in the case of piston pumps, it is a matter of filling each individual cylinder equally. If the supply is limited centrally and distribution between the individual cylinders takes place downstream, agreement between the characteristics of the inlet valves in particular is responsible for an equal filling ratio of the individual cylinders and therefore for uniform delivery flow pulsation. Differences in the characteristics of the inlet valves and the resulting differences in the flow rates are particularly noticeable at partial load in the form of nonuniform delivery-flow pulsation. Producing valves with the same inlet valve characteristic is extremely expensive since it is difficult especially to produce springs of identical length required for the inlet valves.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide an apparatus for producing a variable volumetric flow in a fuel feed system, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known apparatuses of this general type and in which it is possible, by limiting the supply, to adjust the delivery flow to all volumetric flows required between zero and full load at any rotational speed between an idling speed and a rated speed. With the foregoing and other objects in view there is provided, in accordance with the invention, an apparatus for producing a variable volumetric flow in a fuel feed system, comprising a high-pressure pump having at least one cylinder, at least one piston respectively moveable in the at least one cylinder, a shaft driving the at least one piston, and at least one inlet defining an angle between opening and closure of the at least one inlet; and a metering device disposed at the at least one inlet of the high-pressure pump, the metering device synchronized with the shaft and feeding a variably meterable volume of fuel to the at least one cylinder during each revolution of the shaft, and the metering device performing metering by varying the angle between opening and closure of the at least one inlet.
The advantage of the invention is that an adjustment range which is constant for all rotational speeds of the pump is ensured for the purpose of influencing the variable volumetric flow. Moreover, uniform, low delivery-flow pulsation is obtained by ensuring uniform distribution of the overall supply to the individual displacer elements. Finally, it is advantageous that the high-pressure pump can be operated at a very low feed pressure.
In particular, the invention makes provision for a predefined, meterable volume to be fed to the delivery elements of the high-pressure pump during each revolution of the pump. The resulting variable filling ratio of the displacer elements thus results in a volumetric flow that can be varied between zero and full load. At the same time, the adjustment of the filling ratio is decoupled from the rotational speed of the high-pressure pump. Consequently, the adjustment range for influencing the filling ratio is the same size for all rotational speeds. Metering is effected by varying the angl
Krüger Hinrich
Werner Martin
Zander Eckbert
Greenberg Laurence A.
Locher Ralph E.
Moulis Thomas N.
Siemens Aktiengesellschaft
Stemer Werner H.
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