Internal-combustion engines – Charge forming device – With fuel pump
Patent
1995-09-18
1997-07-15
Miller, Carl S.
Internal-combustion engines
Charge forming device
With fuel pump
123507, 123508, F02M 3704
Patent
active
056473257
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a fuel injection device for internal combustion engines, comprising a unit injector extending into each cylinder chamber and containing a pump piston as well as means driving said pump piston, comprising a rocker arm cooperating with the pump piston and mounted on a rocker arm shaft and a cam shaft with a cam for each rocker arm, said cam providing, via the rocker arm, pump strokes in the pump piston upon rotation of the cam shaft.
In fuel systems with unit injectors, the sprayer and fuel pump are integrated in a single unit. A pump piston driven by a cam and a rocker arm builds up, under the control of a relief valve, a pressure in a pump chamber in the sprayer, and when this pressure exceeds the start-to-discharge pressure of the jet, the injection begins. The injection pressure increases continuously until the injection is terminated by an electrically operated relief valve being opened and conducting the fuel to a return line, whereupon the pressure drops below the discharge pressure of the jet. The length of the injection period and thus the maximum injection pressure, are determined by the period of time (the number of degrees of rotation of the cam) during which the relief valve is closed.
It is a wellknown fact that insufficient dispersion of the fuel during injection into a diesel engine results in black exhaust smoke. It is also a known fact that the higher the injection pressure is; the more finely the fuel will be dispersed. At the same time, however, the pressure increase will increase the mechanical load on the components involved, thus placing greater demands on material and dimensions. In modern diesel engines with unit injectors, the injection pressure is usually limited to about 150 MPa, which is sufficient to prevent black smoke from combustion. The higher pressure of the last injected fuel helps the fuel injected initially at lower pressure to burn completely.
In recent years increasing attempts have been made to raise the power of existing engine models by modifying the injection system, so that a greater volume of fuel can be injected into the combustion chamber during the injection stroke of the piston. This can be achieved in a simple and easily controllable manner by simply increasing the length of the injection. The problem is, however, that the maximum injection pressure is increased as a direct result thereof. This is not, however, a specifically desired effect, since soot-free combustion is already achieved at 150 MPa and increasing the pressure to 200 MPa for example does not provide any improvements as regards combustion, but rather only results in sharply increased mechanical stresses.
In order to increase the injection volume in a simple manner without increasing the mechanical stresses, it would of course be possible to increase the flow-through area of the jet apertures, but then there would be insufficient pressure for soot-free combustion at low rpm and at partial load. Theoretically, this problem could be solved with a sprayer with variable aperture size, but this would hardly be possible to achieve in practice.
The purpose of the present invention is in general to achieve a fuel injection device of the type described by way of introduction which makes it practically possible to increase the length of the injection period from a given period without increasing the maximum injection pressure and to be able to achieve the desired pressure even under partial load and at low engine speed. In particular it is the purpose of the invention to achieve a construction which does not require supplementing existing fuel injection devices with more or less complicated control systems, but which could be achieved with simple mechanical means instead.
This is achieved according to the invention by virtue of the fact that the drive means are so arranged that they limit, by elastic deformation of at least one element thereof at a predetermined force, the speed of the pump piston stroke, whereby the injection pressure can be kept at least essentially con
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Axbrink Goran
H.ang.kansson Nils-Olof
AB Volvo
Miller Carl S.
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