This invention relates to a compression ignition engine having a piston reciprocable in a cylinder, an end wall to the cylinder and a fuel injection nozzle mounted in said end wall and operable to deliver fuel into a combustion space defined by the crown of the piston, the wall of the cylinder and said end wall.

Such an engine is known as a direct injection engine. It is known with such an engine to mount the injection nozzle in the cylinder head which forms said end wall and to locate it centrally of the cylinder and to form the crown of the piston with an annular recess. The injection nozzle is provided with a plurality of outlet orifices and the fuel is delivered to the nozzle at a very high pressure. The axes of the orifices incline downwardly a small amount from a diametrical plane including the orifices so that the fuel spray issuing from the orifices is directed into the annular space formed by the recess and the end wall of the cylinder.

Engines can be designed and have been designed, so that during the compression strokes of the cylinder there is substantial air motion within the cylinders. However, modern direct injection engines are usually designed so that there is much less air motion and this means that more of the energy required to achieve mixing of the air and fuel has to be supplied by the fuel. The pressure at which the fuel is supplied is therefore relatively high.

It is found that with an engine of the aforesaid type in which the swept volume per cylinder is generally above 1.5 litres, there is little if any impingement of liquid phase fuel from the fuel spray on the surfaces

forming the combustion chamber and as a result there is very little smoke in the engine exhaust. In a typical example the orifice size is 0.2 mm and the nozzle is part of a so-called unit injector. As the swept volume is reduced the diameters of the cylinder and piston become smaller and it is found that with the same orifice size and fuel pressure, there is an increased tendency for the surfaces of the combustion chamber to be wetted by liquid phase fuel and as a result there is an increase in the level of smoke in the engine exhaust. In order to reduce the risk of wetting of the surfaces the size of the orifice should be reduced and/or the fuel pressure increased to improve atomisation of the fuel. The diameter of the orifice needs to be reduced in approximate proportion to the distance of the orifice from the surface of the combustion chamber and such a small orifice would be very difficult to provide on a commercial basis. Moreover, increasing the fuel pressure by the required amount would also be difficult.

The object of the invention is to provide an engine of the kind specified in a simple and convenient form.

According to the invention in a compression ignition engine of the kind specified the fuel injection nozzle is offset from the axis of the cylinder and the surfaces of the crown of the piston and the end wall of the cylinder define between them a recess which extends away from the nozzle to a position close to the opposite wall of the cylinder, said recess having a generally wedge configuration in plan view and the nozzle having an orifice which in use directs a fuel spray into said recess.

The invention will now be illustrated by reference to the accompanying drawings in which:-

Figure 1 shows a part sectional side elevation through the upper portion of the cylinder of an engine,

Figure 2 is a plan view of the piston forming part of the engine illustrated in Figure 1,

Figure 3 shows a section similar to Figure 1 to an engine modified in accordance with the invention,

Figure 4 is a plan view of the piston of the engine seen in Figure 3, and

Figure 5 is a plan view showing an alternative arrangement.

The engine which is illustrated in Figures 1 and 2 is known in the art and it will be seen to comprise a piston 10 which is slidably mounted in a cylinder 11 one end of which is closed by an end wall 12 conveniently the cylinder head of the engine. The piston is shown almost at top dead centre and mounted in the end wall and coincident with the axis of the cylinder is a fuel injection nozzle 13 conveniently forming part of a fuel/injector.

The crown of the piston is provided with an annular recess 14 the depth of which increases as the distance from the axis of the cylinder increases the outer periphery of the recess 14 being defined by a generally cylindrical boundary wall 15.

The fuel injection nozzle is provided with a plurality of orifices 16 located about the injection nozzle and as will be seen from Figure 2, the flow of fuel through the orifices produces radially directed fuel sprays 17 which are directed towards the boundary wall 15. As will be observed from Figure 1, the axes 18 of

the orifices and therefore the fuel sprays, incline downwardly by an angle A relative to the end wall 12 of the cylinder the axes generally following a middle path between the upper end wall 12 and the base wall or surf ce of the recess. As stated the diameter of the orifices in a typical example is 0.2 mm and the engine is of the type having a swept volume greater than or approximately equal to 1.5 litres per cylinder.

With the arrangement described the liquid phase fuel from fuel spray hardly reaches the boundary wall 15 neither does it impinge upon the surfaces of the recess or the end wall 12 and as a result there is very little smoke in the engine exhaust.

Referring now to Figures 3 and 4 these show views similar to Figures 1 and 2 but for an engine having a substantially lower swept volume per cylinder. It will be immediately apparent that the injection nozzle 13 is offset by a distance r from the axis of the cylinder and the recess 14 in the crown of the piston 20 is replaced by a recess 21 having a different shape. As will be seen from Figure 4 the recess has a generally wedge configuration in plan extending away from the axis of the nozzle. The section of the recess roughly corresponds to the shape of one half of the recess seen in Figure 1 and it increases in depth as the distance from the nozzle increases. The recess has a generally part cylindrical boundary wall 22 and the nozzle is provided in the particular example, with three outlet orifices which produce fuel sprays 23 directed towards the boundary wall 22 but so arranged that the sprays remain out of contact with the side walls of the recess. Moreover, as with the example shown in Figure 1 the axes 24 of the orifices and therefore the fuel sprays incline downwardly by the angle A so that liquid phase fuel in the fuel sprays does not impinge upon the surface 12. The recess is constructed

so that the distance X of the cylindrical boundary wall 22 from the axis of the nozzle is as great as possible and substantially greater than the cylinder radius bearing in mind that it is necessary when the recess is formed in the crown of the piston, to leave a rim portion 27 of annular form, beyond the recess to maintain the strength of the piston.

By the arrangement described it is possible to maximise the distance from the injector tip to the opposite wall along the spray axis and retain the same size of orifice in the fuel injection nozzle and to operate at the same fuel pressure and to avoid impingement of liquid phase fuel to any substantial extent, upon the surfaces of the recess and the end wall defining the combustion chamber even when the size of the piston and cylinder are substantially reduced.

In the example the angle A is typically 10° but different orifices can have different angles A so that in effect the fuel sprays partially overlap when viewed in plan and this can be utilised to reduce the angle B between the sides of the recess.

It will be appreciated that in the arrangement shown in Figure 4 the distance between the boundary wall 22 and the fuel injection nozzle 13 measured along the axes of the outer sprays is less than that of the centre spray. This in itself does not raise any particular problems however, if desired the configuration of recess which is shown in Figure 5 may be adopted in which the end wall 22A is formed with a projection 25 which extends towards the fuel injection nozzle. In the arrangements shown in Figures 3-5 inclusive the boundary wall 22 and 22A is substantially parallel to the wall of the cylinder bore however, and as shown in dotted outline in Figure 3, the boundary wall may be provided with a lip 26 extending

part way or about the whole of the recess. This lip in the use of the engine will result in a squish type of air motion as the piston moves towards the top dead centre position during the compression stroke. The lip need not extend to the full height of the boundary wall.

The examples described have the recess formed in the crown of the piston however, it is equally possible to arrange for the recess to be formed in the end wall 12 that is to say in the cylinder head of the engine. In this case the axes of the orifices would incline upwardl .