GB2269882A

GB2269882A - Internal combustion engine pistons

Info

Publication number: GB2269882A
Application number: GB9217565A
Authority: GB
Inventors: Timothy Sale
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1992-08-19
Filing date: 1992-08-19
Publication date: 1994-02-23
Also published as: GB9217565D0

Abstract

An aluminium or aluminium alloy piston for use in a ferrous metal cylinder block in an internal combustion engine, particularly a compression ignition engine, that incorporates axial reinforcing struts 17a or wires or fibres 2 of material of lower coefficient of thermal expansion extending between the top of its crown and the level of its piston pin. <IMAGE>

Description

INTERNAL COMBUSTION ENGINE PISTONS This invention relates to pistons for internal combustion engines, and more particularly to pistons for compression-ignition engines.

Internal combustion engines having pistons of non-ferrous metal or alloy, e.g. aluminium or aluminium alloy such as alloys of aluminium and silicon, and reciprocating in ferrous metal or alloy cylinder blocks suffer various disadvantages attributable to the higher coefficient of thermal expansion of the material of the pistons than that of the associated cylinders or cylinder liners. This tends in particular to increase the play of the piston in the cylinder as the engine warms up, which in turn increases noise (piston slap) and impairs sealing.

To counter these problems, it is known to cast annular strips of steel into the piston in the neighbourhood of the piston pin, and also around the circumference of the crown to act as ring carriers. These annular reinforcements serve to reduce the radial expansion of the piston.

Another consequence of the use of aluminium or aluminium alloy pistons is that the compression ratio changes as the engine warms up, increasing as the small clearance between the crown of the piston, and/or the faces of the valves, and the cylinder head is reduced. To obtain a desired compression ratio when the engine is fully warmed up, it is necessary to accept a lower ratio during cold starting. This problem is particularly acute in the case of compression ignition engines, for which the designed compression ratio, for example from 18 to 23:1, is higher than for spark ignition engines, in which it is generally not more than about 9:1.

Reduction of the clearance not only reduces the total volume of the combustion chamber, but also significantly changes its geometry. Thus in a direct injection system, the relative volumes of the cavity in the piston crown and the clearance space above the part of the crown surrounding the cavity are changed, which affects the swirl of the gases in the combustion chamber.

In an indirect injection system the relative volumes of the cavity in the crown of the piston and the swirl chamber in the cylinder head are affected.

These changes in combustion chamber volume and geometry lead both to lower torque being developed on starting, and thus to poorer starting performance, and to poorer emission performance until the engine has warmed up. Various remedies have been adopted, including the use of heavier-duty starter motors, adjustment of the mixture ratio on starting and the use of supplementary injection means for use during starting, and the use of heating devices, e.g. manifold heaters, to raise the inlet air temperature.

It is an object of the invention to overcome these disadvantages.

To this end, according to the invention pistons of aluminium or aluminium alloy have incorporated therein reinforcements extending generally axially from the top of the piston crown at least to the region of the piston pin and comprising material of lower coefficient of expansion.

The reinforcements, which may be in the form of axial struts or fibres, are advantageously of material having a coefficient of thermal expansion similar- to that of the cylinder block or liners. Thus when these are of ferrous metal such as cast iron, the reinforcements are advantageously of steel. The reinforcements are metallurgically bonded or otherwise keyed to the surrounding metal, and serve to limit the thermal expansion of the upper part of the piston in the axial direction, or even to reduce it to a value comparable to that of an iron or steel cylinder or cylinder liner. As a result, changes in the clearance between the crown of the piston and the cylinder head, and consequently in the compression ratio and in combustion chamber geometry, as the engine temperature changes are reduced or eliminated.

While the axial reinforcements may extend from the top of the piston crown to beyond the piston pin, this is not necessary if, as is usual, the connecting rod is of steel, since the differential expansion only occurs in the part of the piston between the piston pin and the top of the crown. Moreover, the use of such further amounts of higher density material will reduce the weight advantage which is one of the main reasons for employing non-ferrous metal pistons.

Advantageously the reinforcing struts or fibres are incorporated in a cast piston during its manufacture, e.g. by squeeze casting (mould forging), by placing them in the mould and casting the aluminium or aluminium alloy of the piston around them. It is also advantageous for the struts or fibres to extend continuously in the piston for the whole of their desired axial length.

The invention will now be described in more detail, by way of example, with reference to embodiments of the invention shown in the accompanying diagrammatic drawings, in which: Fig. lis a sectional view of a piston and the associated cylinder of a direct injection compression ignition engine; and Fig. 2is a similar sectional view of a piston and cylinder of an indirect injection engine.

Referring first to Fig. 1, a piston 1 of aluminium or aluminium alloy comprising a crown 2 surrounded by piston rings 3 and a skirt 4 having therein a boss 5 for a piston pin 6 carrying a connecting rod (not shown) is arranged to reciprocate in a cylinder 9 of ferrous metal, e.g. cast iron. A cavity 7 in the top of the crown 2 forms, together with the surrounding clearance from the head 10 of the cylinder 9, a combustion chamber.

The cylinder head 10 also carries inlet and outlet valves 11 and 12 and a fuel injector nozzle 13 directed into the cavity 7. In the top dead centre position of the piston the faces of the valves 11 and 12 are received in recesses 14 in the top of the crown.

Fig. 2 shows a piston and cylinder for an indirect injection engine, using so far as possible the same reference numerals. In this case the injector nozzle 13a opens into a swirl chamber 15 connected by a throat 16 to the cavity 7 and forming together with it the combustion chamber.

In each case reinforcements 17a, 17b of material, in the present embodiments steel, having a lower coefficient of thermal expansion than that of the aluminium or aluminium alloy of the piston extend generally axially of the piston from a level at or near the top of the crown to the level of the piston pin 6. In the left-hand part of Figs. 1 and 2 these reinforcements 17a are shown as axial struts, while in the right-hand part the reinforcements 17b are shown as wires or fibres.

Whatever their form, the reinforcements are incorporated in the piston during its manufacture by casting, and are metallurgically bonded to the piston.

The reinforcing struts or fibres anchored to the metal of the piston serve to restrain the expansion of the piston so as to reduce or largely eliminate the changes in size and geometry of the combustion chamber as the engine warms up. This enables the engine to be designed with a smaller cold clearance between the piston crown and the cylinder head The piston may also include, in known manner, further reinforcements in the form of annular strips of steel extending around the skirt or ring belt of the piston to restrain radial expansion of the piston.

The invention includes an internal combustion engine having a ferrous metal cylinder block and an aluminium alloy piston or pistons reinforced as set forth above and arranged to reciprocate therein.

Claims

1. An aluminium or aluminium alloy piston for an internal combustion engine which includes reinforcements of material having a lower coefficient of thermal expansion than that os the piston and extending generally axially of the piston from the top of the crown at least to the level of the piston pin.

2. A piston according to claim 1 for a compression ignition engine, having a combustion chamber cavity formed in its crown.

3. A piston according to claim 1 or claim 2 in which the reinforcements are of steel and are metallurgically bonded to the metal of the piston.

4. A piston according to any preceding claim in which the reinforcements comprise steel struts.

5. A piston according to any one of claims 1 to 3 in which the reinforcements comprise steel wires or fibres.

6. A piston according to any preceding claim in which the reinforcements extend continuously from the top of the crown to the level of the piston pin.

7. An internal combustion engine having a ferrous metal cylinder block and an aluminium or aluminium alloy piston or pistons according to any preceding claim arranged to reciprocate therein.

8. A compression ignition engine according to claim 7.

9. An engine according to claim 7 or claim 8 in which the material of the reinforcements has substantially the same coefficient of thermal expansion as the metal or alloy of the cylinder block.

Abstract.