The present invention relates to an arrangement of the kind set forth in the preamble of the following claim 1.

An arrangement of this kind is proposed in Swedish Patent Specification 85 05 302-3, this known arrangement being characterized in that when at least a part of the walls of combustion chamber are treated in the manner prescribed, the walls will absorb 90-95 % of all thermal radiation in the energy-rich wave length range of interest for influencing engine combustion. Furthermore, since the radiation which is reflected into the combustion chamber is diffuse, radiation from the walls of the combustion chamber contributes towards the occurrence of knocking in the combustion process to only a very slight extent.

As the energy-rich radiation is absorbed into the combustion chamber walls it converts, however, to so-called joule's heat within the thin layer of material, which is therewith heated rapidly to such high temperatures as to eventually form a so-called "hot spot" during the combus¬ tion process, which initiates knocking. Some of this radia- tion is also reflected back into the combustion chamber as a result of the high surface temperatures that prevail, resulting in heat loss.

The object of the present invention is to provide an improved arrangement of the aforesaid kind in which the aforementioned drawbacks are fully or partially elimina¬ ted. This object is achieved in accordance with the inven¬ tion with an engine having the characteristic features set forth in claim 1.

Thus, the invention is mainly characterized in that a heat buffer which has a special function is provided beneath said surface layer, and in that the nature of this surface layer is such that during combustion the layer is able to "capture" the energy-rich radiation and later, when the temperature of the combustion chamber has fallen to a level beneath the level of the layer temperature, to emit radiation effectively to the combustion chamber. The aforesaid special function of the heat buffer is to accumulate heat rapidly from the surface layer when the layer receives heat from the combustion chamber, while at the same time cooling said surface layer, i.e. the heat buffer must be capable of rapidly leading away heat and have a given thermal capacity. Subsequent hereto, it shall be possible to utilize as much of the stored heat as is possible, to heat the surface layer upon the termination of the combustion process. Cooling of the surface layer during the process of combustion counteracts knocking, while subsequent heating of said surface layer and the radiation of heat to the combustion chamber maintains the temperature level during expansion of the cumbustion gases, so as to obtain an improvement in efficiency. The simplest way of achieving such a transient heat buffer is to incorporate beneath the surface layer a layer of copper or silver having a thickness of about 1 mm.

These metals conduct heat very rapidly, which is the most important property expected of a good transient heat-buffer, i.e. there should be chosen a material which has a high value of the thermal diffusivity according to the formula: λ

D = where λ = thermal conductivity, p X c p = density, and c = specific heat capacity.

In order to utilize the material in the heat buffer to the best extent and to reduce the loss of heat therefrom to the engine cooling channels, the underlying layer is pre¬ ferably arranged on a heat insulating layer, e.g. a thin layer of nickel.

The invention will now be described in more detail with reference to the accompanying schematic drawing, in which Figure 1 is a side view of the top of a piston according to one example of the invention, and Figure 2 is a diagram which illustrates temperature curves for two points in an internal combustion engine during a combustion cycle.

Figure 1 illustrates the upper part of a piston 1 be¬ longing to an internal combustion engine. The piston may be one which has been cast from a suitable aluminium alloy in accordance with conventional techniques. In accordance with the invention, the illustrated upper piston part in¬ corporates a surface layer 2 of black chromium (chromium oxides and chromium) having a thickness of about 4 m, and a thicker copper layer 3. The copper layer has a thickness of about 1 mm and is situated on a thin layer of nickel 4, which forms a heat insulating layer.

The layers are conveniently applied electrolytically, subsequent to sand blasting the underlying surfaces. The copper layer 3 may be made slightly thinner, particularly when it is seated on a nickel layer 4.

The effect of the invention arrangement on the tempera¬ ture conditions t in a combustion chamber during the diffe¬ rent working strokes (PS), compression I, expansion/combus¬ tion II, exhaust III, and suction IV is illustrated dia- grammatically in Figure 2. The full line 10 in Figure 2 indicates very schematically how the temperature varies in the combustion chamber during a combustion cycle. Thus, it is shown that the temperature will first rise slowly during the compression stroke 10a and then rapidly to a peak value during the combustion 10b. The temperature will then fall rapidly during the final part of the combustion process 10b and then at a slower rate during the exhaust stroke 10c and suction stroke 10d.

The broken line curve 11 indicates the temperature of the surface layer 2 and varies in time with the curve 10, although it has other amplitudes. The surprising and signi¬ ficant f ct about the curve 11 is that the rise in tempera- ture during the combustion process is interrupted and that the temperature remains constant during a large part of the combustion process and the exhaust stroke. In the case of a conventional piston, the temperature would follow the course of the line shown in chain. This is because the copper layer 3 located beneath the surface layer 2 stores heat from the surface layer during the combustion process, therewith cooling said surface layer, and delivers stored heat to the surface layer 2 upon completion of the combus¬ tion process, therewith heating the surface layer 2 so as to maintain or sustain the combustion chamber temperature during the expansion of the combustion gases, during which the temperature decreases, thereby maintaining the pressure level and consequently also the engine torque in a more effective manner than was previously the case. The emission properties of a conventional piston are inferior under such conditions. The fact that the surface layer 2 is heated by the underlying layer in this way upon completion of the combustion process probably explains the reason for the marked reduction in carbon deposits observed when comparing an engine which incorporated the inventive arrangement wiht an engine which did not.

This comparison also showed that the emission of hydro¬ carbons was about 30 % lower in the engine which incorpora¬ ted the inventive arrangement, which was surprising. As will be understood materials other than chromium oxide can be used in the surface layer 2, for instance so- called cermet material, as described in Swedish Patent Application No.. 85 05 302-3, or quite generally such metal oxides as those used as selective absorber layers in solar energy collectors.

Furthermore, the cylinder head and valves may also be prepared in accordance with the invention, either in addition to the piston head or alternatively thereto.

The inventive arrangement is intended primarily for Otto-cycle engines, but may also be incorporated in diesel engines, since it dampens knocking in such engines quite considerably.