GB2110299A - I.C. engine poppet valve - Google Patents

Abstract

The valve comprises a single crystal of nickel based super alloy having a face centred cubic structure, the <001> axis of the crystal extending longitudinally of the valve. This provides a lightweight, temperature resistant valve which may be hollow.

Description

SPECIFICATION A poppet valve for an internal combustion engine This invention relates to a poppet valve for an internal combustion engine.

In the development of internal combustion engines there are two main aims. One is to produce the greatest possible fuel economy, while the other is to produce as high a power output as possible for a given weight. There are two major approaches to these aims, one being to increase the overall expansion ratio by e.g. turbocharging and the other being to reduce as far as possible the weight of the reciprocating parts.

The first approach inevitably raises the temperature experienced by those parts of the engine forming the combustion chamber, and of these components the exhaust valves suffer the most extreme conditions. The ability of these valves to cope with increasingly severe operating parameters has been a limiting feature of the development of this type of engine.

The second approach requires that the reciprocating components should be reduced in weight. The poppet valves have a powerful effect on this weight, since a heavy valve will require a correspondingly heavy drive linkage. Significant benefit could therefore be realised if the valves could be as light as possible.

At present, the valves for the higher performance engines are made of wrought nickel based alloys. Attempts to improve the performance of these valves have involved the use of better wrought alloy compositions, the use of sodium cooling and the use of thermal insulation which reduces heat input to the metal.

These expedients have met with some success, but there remains a need to improve the valves.

The use of cast metal for high performance poppet valves has not seemed to be a practical possibility. Thus castings are noted for brittleness and poor quality control, which can lead to considerable variation in properties due to inclusions, voids, porosity and other defects.

These features are at variance with the requirement of the valves for very high duty and repeated hammering.

We have appreciated that in one casting process very high quality control is involved, and in addition very good metal properties may be achieved using the process with carefully chosen alloys. This casting process comprises the single crystal casting process, and its use produces single crystals whose properties are uncharacteristic of normal castings. Thus with the correct choice of alloy it is possible to achieve very high strength at high temperatures, resistance to corrosion and ductility approaching that of the best forged materials.

According to the present invention therefore a poppet valve for an internal combustion engine comprises a single crystal of high temperature resistant metal having a face-centred cubic crystalline structure, the (001 > axis of the crystal extending longitudinally of the valve.

Preferably the valve is made of a nickel-based superalloy, and the stem of the valve may be hollow to reduce weight and hence parasitic metal loads.

The invention will now be particularly described, merely by way of example, with reference to the accompanying drawings in which, Figs. 1-3 show succeeding stages of the apparatus used in the manufacture of valves in accordance with the invention, Fig. 4 is a cross-section through the valve made in the apparatus of Figs. 1-3, and Fig. 5 illustrates the mould used for an alternative embodiment.

In Figure 1 there is shown in section apparatus for producing a single-crystal casting consisting of three chambers, an upper melting chamber 10, a middle casting chamber 11 and a lower withdrawal chamber 12. The upper melting chamber comprises a crucible 13 within which is held a charge of metal 14, which in the present instance is a nickel-based superalloy. The crucible has a pouring hole 1 5 which is closed off by a fusible plug 16, and is surrounded by the axis 1 7 of an induction heating apparatus.

The pouring hole 1 5 of the crucible 13 registers with the inlet passage 1 8 of the casting chamber 11. The chamber 11 basically consists of a furnace, having insulated walls 1 9 and electrical heating elements 20. The roof 21 of the furnace has the inlet passage 1 8 formed therein, while the floor 22 of the furnace has an aperture 23 formed in it, the aperture being sized to permit the passage of a water-cooled chill 24 therethrough. The chill is mounted on a ram 25 for vertical movement before the casting chamber 11 and the withdrawal chamber 12. In use the chill 24 supports a mould described in more detail below.

The withdrawal chamber 12 comprises a wall 26 equipped with water cooling tubes 27. It will be seen that when the heater elements 20 and the cooling tubes 27 are operating, a heat front is produced in the vicinity of the aperture 23 between the hot casting chamber 11 and the cooled withdrawal chamber 12. Operation of the apparatus depends upon the existence of this heat front.

The final part of the apparatus comprises the ceramic mould 28. The mould is made in the normal manner for a shell mould, in that a wax pattern of the required shape is formed, and coated with a shell of ceramic material. The wax is then melted out and the mould fired to develop its strength.

The mould shape is made up of four sections, a starter section 29, a selector 30, a mould cavity proper 31 and a pouring cup 32. The starter section 29 is open-bottomed and registers with the upper surface of the chill 24 which closes off the starter section. Within the starter section, as described in more detail below, a columnar grain structure is formed.

The part of the mould above the starter comprises a narrow helical passage which forms a selector 30. The function of the selector is to allow only one of the plurality of grains growing in the starter 29 to continue to grow. Above the selector 30 is the main mould cavity 31. As might be expected, this cavity has the shape of the valve which is to be made, i.e. it defines a stem portion which smoothly flares to form an enlarged head portion. In the present instance the stem is at the bottom of the cavity while the head is at the top.

Finally a pouring cup 32 is formed above the mould cavity and provides a funnel shaped passage for the ingress of molten metal.

Operation of the apparatus is that initially the mould 28 is located in the hot casting chamber 11 as shown in Fig. 1, with a charge of metal 14 in the crucible 13. The coils 17 are energised to melt the charge of metal, which in turn melts the fusible plug 16 and drops through the pouring hole 1 5, through the inlet passage 18 and into the pouring cup 32 of the mould 28. The molten metal duly fills the mould.

The ram 25 is next actuated to withdraw the filled mould slowly from the casting chamber 11 and into the cooled withdrawal chamber 12, while simultaneously the chill 24 is cooled. This causes the heat to be withdrawn from the base of the mould, and because of the movement of the mould through the heat front between the chambers 11 and 12, the molten contents of the mould are caused to solidify progressively from the base upwards. This provides the conditions necessary for the production of columnar grains in the starter chamber 29.

These columnar grains grow up the starter 29 as the solidification front progresses until the front reaches the helical selector passage 30.

(This is the Fig. 2 condition). Here the conformation of the passage enables the growth of one of the grains to block off that of the remainder, resulting in the selection of only one grain which grows through the passage and into the main cavity 31 where it continues to grow in the desired shape and in the form of a single crystal. The crystal is constricted by this process to grow with its (001) > axis along the longitudinal extent of the mould and thus of the valve produced.

Withdrawal of the mould into the chamber 12 is continued until it is completely withdrawn and the molten metal is completely solidified. The mould and its contents may then be lifted from the chill, and the mould mechanically or otherwise removed from the cast metal. It is then necessary to separate the cast valve from the ancillary portions of cast metal which filled the selector and the pouring cup, and any additional machining can then be carried out. Figure 4 shows the valve when completed.

It may be desirable to make the valve with a hollow stem. This may be done, as shown in Fig. 5 by locating a ceramic core 33 within the mould.

The core 33 defines the cavity to be formed in the finally produced valve, and must be leached out of the valve after casting.

It will be appreciated that alternative methods of production, for instance using seed crystals rather than the selector of Figs. 1-3, may be used. Again the specific shape of valve may be varied from that illustrated, and the metal used need not necessarily be the nickel-based superalloy referred to.

It will be appreciated that the valves of the present invention coupled with other features will allow the engine cylinder to be insulated to reduce the wastage of heat to the cooling medium. This will of course increase the -temperature of the valves, hence necessitating the present invention. However, this reduction of wasted heat will allow the power to weight and power to unit volume ratios to be increased. There will also be a very large improvement in fuel efficiency.

Claims (4)

Claims

1. A poppet valve for an internal combustion engine comprising a single crystal of high temperature resistant metal have a face-centred cubic crystalline structure, the (001) > axis of the crystal extending longitudinally of the valve.

2. A poppet valve as claimed in claim 1 and in which said metal comprises a nickel-based superalloy.

3. A poppet valve as claimed in claim 1 or claim 2 and in which said valve has a hollow stem.

4. A poppet valve substantially as hereinbefore particularly described with reference to the accompanying drawings.