GB2198785A

GB2198785A - I.C. engine fuel injector

Info

Publication number: GB2198785A
Application number: GB08628630A
Authority: GB
Inventors: Malcolm Kevin Hurford; Cedric Paul Davies
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1986-12-01
Filing date: 1986-12-01
Publication date: 1988-06-22
Also published as: GB8628630D0

Abstract

The sac volume 24 is reduced by adding an extension 26 to the end of the valve needle 18, so that the extension occupies a substantial part of the sac volume when the valve is closed. <IMAGE>

Description

P1 XASga & This invention relates to a fuel injector for an internal combustion engine, more particularly a diesel engine and still more particularly a direct injection diesel engine where fuel is injected directly into the engine combustion chambers.

Conventionally, fuel injectors have a hollow cylindrical housing with fuel inlet openings at one end and fuel outlet openings at the end which, in use, will extend into the chamber. A valve or needle reciprocates inside the housing so as to open and close a fuel passage from the inlet openings to the outlet openings.

Figures 1 and 2 show a conventional direct injection fuel injector. Figure 1 is a general, schematic view while Figure 2 shows a section through the injector tip.

The hollow housing 10 has an inlet 12 for fuel under pressure. The housing ends in a tip 14 from which passages 16 extend from the inside of the housing outward to allow fuel to flow from the injector into the combustion chamber. Fuel flow is controlled by a needle 18 which fits inside the housing and has a valve 20 which sits on a valve seat 22. The needle 18 can be lifted off the seat 22 to allow fuel to be pumped through the passages 16.

In this injector construction, there is a small volume inside the housing but downstream of the valve 20. When the valve closes, a small amount of fuel is trapped in this so-called "sac volume" which is denoted by the refence numeral 24. This sac volume is associated with undesirable hydrocarbon emissions in the engine exhaust.

Much effort is currently directed to reducing such emissions.

It has been proposed to construct an injector with a zero sac volume by positioning the passages 16 so that they communicate with the housing interior at the valve seat 22. In this way, when the valve is closed, there will be no volume inside the injector communicating with these passages. This proposal (known as VCO calve closes orifice}) has been shown to be effective in reducing hydrocarbon emissions, but has not been widely accepted for production purposes because the concept of allowing high localised pressure to be produced under the valve seat appears likely to lead to reliability problems.

According to the invention, there is provided a fuel injector wherein the tip of the valve needle has an extension beyond the valve seat and, the shape of the extension is such that it occupies substantially the whole of the sac volume of the injector.

By "substantially the whole of the sac volume", we mean that the sac volume is filled by the injector to the greatest extent practicable. There are two main restraints which affect this. The first is the practicability of actually forming the extension with available machining techniques. The second is the requirement for fuel to flow past the extension to reach the outlet openings when the valve is open.

It would be possible for the shape of the sac also to be modified so that the closest possible fit, bearing in mind the second requirement above, is obtained.

In one suitable embodiment, the extension is frustoconical, this being a shape which can be satisfactorily machined and which provides a good approximation to the shape of the internal wall of the injector.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figures 1 and 2 show features of prior art injectors and have already been described above; and Figure 3 is a sectional view, corresponding to that of Figure 2, of an injector in accordance with the invention.

The injector of Figure 3 differs from that of Figure 2 in that the valve head 20 has a nose 26 which extends into the sac volume 24 and substantially reduces this volume.

The nose 26 is machined from the same piece of material as the rest of the needle 18. Although it may not be practically possible to machine an ideal shape on this nose, a close approximation to ideal can be made by using a generally frusto-conical shape. Alternatively or additionally, the interior shape of the sac wall can be modified to conform to the shapes which can be obtained on the nose 26. It has to be considered that the dimensions of the injector in this region are very small. The sac volume for the conventional injector shown in Figure 2 is typically about 0.24 cu.mm. The volume is reduced for the injector shown in Figure 3 to 0.104 cu.mm.

Tests have shown that hydrocarbon emissions resulting from use of the injector as shown in Figure 3 are 13% better than with the standard injector of Figure

Claims

Claims 1. A fuel injector wherein the tip of the valve needle has an extension beyond the valve seat and, the shape of the extension is such that it occupies substantially the whole of the sac volume of the injector.
2. A fuel injector as claimed in Claim 1, wherein the shape of the sac is modified so that the closest possible fit of the extension to the internal sac wall is obtained.
3. A fuel injector as claimed in Claim 1 or Claim 2, wherein the extension is frusto-conical.
4. A fuel injector substantially as herein described with reference to Figure 3 of the accompanying drawings.