EP0024890B1

EP0024890B1 - Exhaust valve for an internal combustion engine

Info

Publication number: EP0024890B1
Application number: EP80302900A
Authority: EP
Inventors: Kai Emil Fursund
Original assignee: MAN B&W Diesel AS
Current assignee: MAN B&W Diesel AS
Priority date: 1979-08-29
Filing date: 1980-08-21
Publication date: 1984-03-14
Also published as: DK361279A; ES494569A0; BR8005413A; DK144217C; IE50116B1; ES8105441A1; JPS5634915A; FI67252C; NO154807C; FI802721A; NO802533L; YU41940B; FI67252B; PL226485A1; YU213080A; NO154807B; DE3066986D1; JPS6336403B2; PL124723B1; DK144217B

Description

The present invention relates to an exhaust valve for an internal combustion engine, comprising a stationary valve part and an axially movable valve part, said valve parts having cooperating seating surfaces and, upstream of the seating surfaces, opposed surfaces spaced apart so as to define a narrow annular gap when the valve is closed; and an annular chamber provided in at least one of said valve parts and communicating with the combustion chamber of the engine cylinder through said annular gap when the valve is closed.
An object of the invention is to obtain a prolonged lifetime of the valve by reducing the rate at which the seating surfaces of the valve gradually deteriorate due to corrosive and erosive attacks by aggressive constituents present in the exhaust gases, in particular sodium and vanadium compounds occurring when the engine is running on heavy fuel oil.
It is well-known that the aggressivity of said constituents increases rapidly with increasing temperature and based on that knowledge it has been proposed to provide a direct or indirect air cooling of the seating surfaces, in some cases as a supplement to a liquid cooling of the stationary valve part.
Thus, U.S. Patent Specification No. 1,873,119 discloses a valve of the kind referred to above in which an annular chamber in the stationary valve part and a chamber centrally located within the movable valve part both communicate with the combustion chamber through bores opening adjacent the edges of the seating surfaces oriented towards the combustion chamber, into an annular gap between the valve parts. Through non-return valves said two chambers within the valve parts communicate with a source of pressurised air and when the exhaust valve opens, the outflowing exhaust gases entrain, by ejector action, the relatively cool air from the chambers, which is located in the annular gap, which air thus effects a certain cooling of the seating surfaces during the exhaust period. It must be appreciated however, that this relatively cool air has little cooling effect, because the walls of the two valve parts defining the annular gap are generally convex, and thus define a flow passage for the exhaust gases leading directly to the seating surfaces.
U.S. Patent Specification No. 4,106,466 discloses an exhaust valve, the stationary and movable parts of which are shaped with opposed, cylindrical surfaces which define a narrow annular gap upstream of the seating surfaces, i.e. between those surfaces and the combustion chamber of the engine cylinder. The purpose of the gap is to prevent effectively an outflow of combustion gases from the cylinder during the initial part of the valve lift until the lower end of the seating surface on the movable valve part is clear of the cylindrical surface on the stationary valve part. An exhaust valve of similar design is disclosed in German Patent Specification No. 744250.
An exhaust valve according to the present invention differs from the valve known for U.S. Patent Specification No. 1,873,119 by the features that the opposed surfaces are conical and that the annular chamber is a continuous, circumferential recess formed directly in one of said opposed surfaces and located intermediate the seating surfaces and the annular gap so that gases flowing from the combustion chamber through the annular gap when the valve is in the closed position pass into said annular chamber and any gases which flow through a leak between the seating surfaces during a combustion period will first have passed through said annular chamber.
With this construction and location of the annular chamber it constitutes a reservoir in the flow path from the combustion chamber to the seating surfaces of the valve parts, and when during the compression stroke the annular chamber has been filled by pure and relatively cool air, either from the engine cylinder or from a connected external source of pressurized air, that amount of air will after ignition of the fuel and until the exhaust valve opens function as a barrier between the hot combustion gases and the seating surfaces. As a consequence, the combustion gases cannot as in the previously known valves, flow directly out through leaks formed by local gaps between the seating surfaces. Instead it will be only the reservoir air or, at most, a mixture of that air with a relatively small percentage of combustion gases which penetrates through the leaks.
The invention is based on the recognition that the above mentioned attacks, by the aggressive constituents present in the combustion gases, on the seating surfaces occur predominantly during the period in which the valve is closed without, however, providing a perfect seal due to said local leaks which occur sooner or later, e.g. because deposits previously formed on the seating surfaces by corrosion peel off or because particles of slag, cinder or coke squeezed between the surfaces when the valve closes, have left small indentations in the surfaces. Measurements of the surface temperature of the stationary valve part have shown that around such a local leak there can occur a temperature rise of approximately 200°C immediately after top dead centre and an increase of the mean temperature amounting to approximately 100°C. On the other hand, the temperature rise at the beginning of the exhaust period immediately after opening of the valve amounted to only approximately 20 to 25°C. Therefore, an additional cooling of the seating surfaces during the exhaust period exerts only a marginal influence on the corrosion phenomena and on the resulting gradual enlargement of the initially very small leaks to larger burned regions. These phenomena are delayed to a far greater extent when, by means of the characteristic features of the present invention, both the temperature of the gas leaking out between the seating surfaces when the valve is closed, and the content of aggressive constituents in that gas are reduced. Since with the materials normally employed for the seating surfaces, the corrosion rate can double at a temperature rise of approximately 70°C, it will be understood that the invention materially extends the time which a small local leak needs to grow into a regular run-through of such size that it becomes necessary to refurbish the seating surfaces by regrinding them. The result is therefore, as mentioned above, that the lifetime of the valve is prolonged.
The critical period of each working cycle, during which the temperature and the pressure in the combustion chamber are at so high values that combustion gases leaking out therefrom can cause perceptible attacks on the seating surfaces, is rather short and ends approximately 20° after top dead centre, both with two stroke and four stroke engines. The desired effect of the invention can, therefore, be obtained with an annular chamber of relatively small volume which does not necessitate any inconvenient increase of the dimensions of the valve parts or of the so-called dead space of the engine cylinder. The chamber volume can be chosen somewhat larger than the volume of gas which during said period can flow through a gap having a cross-section of about 0.2 by 3 mm, e.g. 2 to 3 times that gas volume.
In one embdiment of the invention, the annular chamber is formed in the movable valve part and, in the conical surface of the stationary valve part, which defines the narrow annular gap, there is a circumferential fillet located opposite the annular chamber and oriented towards the inlet from the combustion chamber to the gap.
The invention will now be described in more detail, by way of example, with reference to the accompanying schematical drawings, in which:-

Figure 1 is an axial section through a valve according to the invention, only those parts thereof which are deemed necessary for understanding the invention being shown, and
Figure 2 is a fractional view, on a substantially larger scale, of the area marked by 11 in Figure 1.

As shown in Figures 1 and 2, an exhaust valve for a two-stroke diesel engine comprises a stationary valve part (or bottom piece) 1 which together with a valve housing 2 is releasably secured (in a manner not shown in detail) to the cylinder cover 3 of an engine cylinder. The movable part 4 of the exhaust valve is generally formed as a conventional poppet valve which contacts bottom piece 1 on a conical surface. The seating surfaces proper of the two valve parts, which are designated by 5 and 6, respectively, are formed on two coatings or inserts 7 and 8 of a suitable material, such as stellite or hard metal, see Figure 2.
In that surface of the movable valve part 4, which is oriented towards bottom piece 1, there is provided an annular chamber 9 located between the combustion chamber 10 of the engine cylinder and the seating surface 6 of valve part 4. As shown, the axial cross-section of chamber 9, i.e. a section located in a plane through the valve axis, is trapezoid with two substantially parallel sides 11 and 12 extending by and large perpendicular to the seating surface 6 and the opposed surface 13 of the valve part 1. Via a circumferential fillet 14 located somewhat inwardly of the radially outer surface of chamber 9, the generatrix of which is the side 12, surface 13 merges, towards combustion chamber 10, into a surface 15 which is retracted so far from valve part 4 that with the valve closed, as shown in Figure 2, there is formed at this place a circumferential gap of relatively narrow gap width, of about 0.2 mm.
Through said gap, annular chamber 9 will, during the compression stroke, become filled by pure scavenging or charging air at a relatively low temperature, and if one or more small local leaks are present between seating surfaces 5 and 6, it will be this air rather than the hot and highly corrosive combustion gases which is pressed out through the leaks in the seating surfaces in response to the pressure rise resulting from the combustion of the fuel injected into the engine cylinder. The high temperature rise in the material surrounding existing leaks in the seating surfaces, which otherwise occurs during that period of the engine's working cycle in which the temperature in the combustion chamber is high while simultaneously the combustion gases have a high density, due to the high pressure, and a high concentration of corrosive constituents, is consequently avoided. The configuration, as shown and described, of the step or transition between the two surface portions 13 and 15 as a fillet 14, the concavity of which is oriented towards the inflow gap from combustion chamber 10, contributes to limiting undesired mixing of the pure and relatively cool air within annular chamber 9 and the inflowing hot combustion gases.
The reduction of the rate, at which small local leaks between the seating surfaces increase due to corrosive and erosive attacks, results in the further advantage that within the regions surrounding such leaks there is maintained a more effective heat transfer from the movable valve part 4 directly subjected to the high temperatures within combustion chamber 10, to the somewhat colder bottom piece 1 during the periods in which the valve is closed. Consequently, it is possible to maintain a lower mean or average temperature of the regions in question and hence, also a more uniform mean temperature of the entire seating surface. This contributes to ensuring that the corrosion of the seating surfaces, which unavoidably occurs during the lifetime of the valve, will be uniformly distributed so that its effect on the sealing function of the valve is less deleterious as that of selective local corrosion attacks on the surfaces.
While in the embodiment shown and described the annular chamber is formed solely in the movable valve part, it will be understood that alternatively it could be formed in the stationary valve part or be composed of two opposed recesses, one provided in each valve part. As,a substitute for or a supplement to the described use of compression air from the engine cylinder for filling the annular chamber there might be provided an external source of pressurized air which through a non-return valve supplies cool air at a suitable pressure to the annular chamber. Said pressure should not be lower than the compression pressure within the engine cylinder, and it may be of substantially the same magnitude as the maximum pressure occurring in the cylinder. Since the necessary consumption of air is relatively small, the air can be supplied by a correspondingly relatively small compressor.

Claims

1. Exhaust valve for an internal combustion engine, comprising a stationary valve part (1) and an axially movable valve part (4), said valve parts (1, 4) having cooperating seating surfaces (5, 6) and, upstream of the seating surfaces, opposed surfaces (15, 16) spaced apart so as to define a narrow annular gap when the valve is closed; and an annular chamber (9) provided in at least one of said valve parts and communicating with the combustion chamber (10) of the engine cylinder through said annular gap when the valve is closed, characterised in that the opposed surfaces are conical and that the annular chamber (9) is a continuous, circumferential recess formed directly in one of said opposed surfaces and located intermediate the seating surfaces (5, 6) and the annular gap, whereby the chamber (9) forms an enlargement of said annular gap so that gases flowing from the combustion chamber (10) through the annular gap when the vave is in its closed position pass into said annular chamber (9) and any gases which flow through a leak between the seating surfaces (5, 6) during a combustion period will first have passed through said annular chamber (9).

2. Exhaust valve as claimed in claim 1 characterised in that the annular chamber (9) is formed in the movable valve part (4) and that in the conical surface (15) of the stationary valve part (1), which defines the narrow annular gap, there is a circumferential fillet (14) located opposite the annular chamber (9) and oriented towards the inlet from the combustion chamber (10) to the gap.