In internal combustion engines with reciprocating pistons working inside cylinders it is essential that there is a good seal between the piston and the cylinder wall. This seal is usually accomplished by so called piston rings resting in grooves on the piston circumference. The gas pressure inside the cylinder forces the piston ring both against the cylinder wall and against one side of the piston groove. The clearances between the piston ring and the cylinder wall and between the piston ring and the piston groove side thus both become small. This works as a seal and reduces the gas leakage.
The gas pressure inside the cylinder can be rather high, especially so in engines operated with increased inlet air pressure accomplished by compressors or turbo chargers. The high gas pressure results in a correspondingly high contact pressure between the piston ring and respectively the piston and the cylinder wall. The latter manifests itself in friction opposing movement of the piston in the cylinder. This friction can be quite substantial, even if it is reduced by the application of lubrication oil, and it lowers the engine's energy efficiency. It also causes local heat generation, which may harm the lubrication and cause wear and severely limit operating time between overhauls and shorten total life expectancy of the engine. As the friction depends on contact force and thus on both cylinder gas pressure and geometrical size this local heating effect is more pronounced in large engines.
According to the present invention the friction between the piston seal and the cylinder wall can be reduced. This will increase the energy efficiency of the engine and also reduce wear and increase the time between overhaul and lengthen the life expectancy of the engine. Central to the invention is the realisation that sealing between the piston seal and the cylinder wall is dependant of the contact pressure there, while the friction is dependant on the contact force. A reduction of the contact area reduces friction even if contact pressure is kept constant.
In
In an idealised and simplified analysis supposing non-warping bodies and where the gas pressure difference across the piston ring is P the following can be seen (measures as depicted in
a) Solid to solid contact pressure between piston ring and piston groove (area 4) is equal to:
P·(w−(w−c)/2)/(w−c)>P/2
which gives good contact and a good seal.
b) Solid to solid contact pressure between piston ring and cylinder wall (area 5) is equal to:
P/2
which also gives a good seal.
c) Friction between piston ring and cylinder wall:
P·h·l·u/2 (where u is the friction coefficient and l is the circumferential length of the cylinder wall)
The pressure of the gas oil mixture in the sealing slot is supposed to change linearly from one end to the other. Forces due to springiness of the piston ring are disregarded.
From c it is apparent that the friction and also the friction per circumferential length of piston ring is dependent on the piston ring height h. The smaller this can be made, the smaller the friction will be. More complex modelling including for instance springiness and warping of the reciprocating piston ring will not negate this conclusion.
For a conventional piston ring the height h is determined by need for the piston ring to resist warping due to pressure and friction forces. The clearance (c in
In big ships' engines the friction heat then puts high demands on the lubricating system and lubrication oil quality. A not uncommon failure mode for such engines is so called “scuffing” where the lubrication between piston ring and cylinder wall fails. This may cause severe damage to the engine.
The present invention is concerned with lowering the friction between the piston seal and the cylinder wall. One objective is then to increase the energy efficiency of the engine. A second objective is to reduce the local heating caused by friction between the piston seal and the cylinder wall and thereby improve lubrication and lifetime.
The new invention employs a sealing sleeve rather than a sealing ring. In
In a similar simple analysis as was previously made for the conventional piston ring in
a) Solid to solid contact pressure between sealing sleeve and piston groove (area 4) is equal to:
P·(w−(w−c)/2)/(w−c)>P/2
which gives good contact and a good seal.
b) Solid to solid contact pressure between sealing sleeve and cylinder wall (area 5) is equal to:
P/2
which also gives a good seal.
c) Friction between piston seal and cylinder wall:
P·heff·l·u/2 (where u is the friction coefficient and l is the circumferential length of the cylinder wall)
Since heff can be made much less than h of the conventional piston ring in FIG. 1., the friction of this sealing sleeve can be much lower. The pressure in the annular cut out space 6 is the same as on the backside of the sealing sleeve and on top of the piston. The pressure in the slot 10 can also be considered the same. The pressure difference P is thus working on only a part of the total height of the sealing sleeve. Alternatively the gas in the annular cut out space 6 can be regarded as functioning as a gas cushion that balances much of the force caused by the pressure on the inner side of the sealing sleeve.
To achieve enough resistance to warping from the forces of friction and gas pressure and still not get too much friction against the cylinder wall a conventional piston ring is made so that the width w of the ring is larger or essentially of the same size as the height h of the ring. A sealing sleeve according to the present invention can be made with a height h which is much larger than its width w. Because of the gas cushion in the cut out space 6 this can be done without causing excessive friction against the cylinder wall. A high total height makes the sleeve resistant to warping even if the width w is small.
A third objective of the present invention is to make a piston seal that can resist excessive warping while still being flexible in the radial direction and able to follow irregularities in the cylinder wall. Such an ability to adjust to a non-perfect cylinder wall reduces leakage. It also reduces the risk for abnormally high contact pressure and dangerous friction conditions at protruding cylinder wall areas. Scuffing is often initiated in such areas. The flexibility of the sealing sleeve of the present invention makes the lubrication situation more forgiving. In such engines where lubrication is accomplished by a continuous supply of fresh lubrication oil, the oil feed might then be reduced, saving operating cost.
When applying force to bend a straight beam the resulting bending (1/r, where r=bending radius) is inversely proportional to the moment of inertia of the beam cross section. This in turn is proportional to the cube of the section's height in the bending direction. For an object that is bent already from the beginning a corresponding relation applies to its deformation from original shape. A sealing sleeve according to the present invention can advantageously be made with a width w which is less than one third of the height h. Decreasing the width w to one third while keeping other circumstances constant will change the moment of inertia of the cross section to 1/27 of its original value. The force needed to adjust the sealing sleeve to irregularities in the cylinder wall will thus be reduced by a factor of 27. This without a need to change from conventional and proven materials of construction.
A piston seal made according to the present invention may preferably be made with a smaller width than a conventional piston ring. For retrofit in old engines there would sometimes be an advantage if the old piston ring grooves could be used even with the new sealing sleeves of smaller width. This can be accomplished by using a non-working ring placed inside the sealing sleeve to fill the empty space and keep the working sleeve in correct position. In other cases the height of outer part of the piston grooves could be increased both upwards and downwards and a new higher seal made according to the present invention fitted into the resulting higher outer groove. The sealing sleeve will then be held in place by the new groove surfaces, while remnants of the old, deeper groove, will cause no harm behind the middle of the sleeve.
An important feature of the new piston seal is that the annular cut out (6) in the outer surface of the sleeve is in gas conveying contact with the high-pressure side of the piston. This makes the annular cut out work as a gas cushion between the sleeve and the cylinder wall. This gas cushion balances part of the force from the gas pressure working on the backside of the sleeve. In the design shown in
A conventional piston ring will tend to wear more at the top and the bottom and thus have a somewhat convex sealing surface. In fact such a convex or barrel-shaped outer surface is often an intentional design feature. Anyhow such a convex surface will give little support against warping of the ring from the friction and the gas pressure and the clearance between piston and cylinder. To resist warping the ring has to rely on internal stiffness and a broad width (w). In a sealing sleeve according to the present invention the situation is different. There is no convex contact surface to roll on but two different and separated contact surfaces at some distance from each other. This applies also to a worn sleeve.
The contact area 8 in
The tilting tendency can be enhanced and made independent of the clearance between piston and cylinder by removing material from the outer part of the bottom (low pressure part) of the sleeve and thereby moving the pivot point for tilting in from the edge of the groove. This is accomplished by bevelling or rounding the outer edge of the low pressure part of the sealing sleeve so that by sealing against the piston groove side its contact surface with the piston groove side ends within the piston groove. See
For simplicity in the descriptions above the situation has been described where only one sealing device has been used and the high pressure in the cylinder has worked from above. The invention is not restricted to this special case, but is equally applicable to other orientations and where several sealing devices are used in series as is usually the case with conventional piston rings. For pressure from above or pressure above the piston or the sealing sleeve then instead read pressure from or on the high-pressure side of the device.
Number | Date | Country | Kind |
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0900073-8 | Jan 2009 | SE | national |
This application is a continuation of U.S. application Ser. No. 13/138,040 filed on Jun. 27, 2011, which is a National Stage Application of App. No. PCT/SE2010/050056 filed on Jan. 21, 2010, which claims priority to Swedish App. No. 0900073-8 filed on Jan. 26, 2009, the contents of each of which is incorporated by reference in their entirety.
Number | Date | Country | |
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Parent | 13138040 | Jun 2011 | US |
Child | 14038261 | US |