Hone pattern, rockwell hardness, ring tension
- Thread starter Fahlin Racing
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kjpcummins
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After the cylinder walls are coated there wouldn't be a hone pattern left. So what else does a hone pattern do?
biggy238
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Matty, I understand your analogy. I believe you are interchanging surface "area" with surface "contact area".
You have decreased surface area at one micron level (metal to metal) and wildly increased surface area (metal to liquid)
At another, through honing.
If you take your two pieces of glass, with perfectly parallel retaining surfaces, and then scratch them, you have taken them from "imaginary" flat, two dimensional surfaces, to what would now be 3 dimensional surfaces. Adding another dimension instantaneously adds additional area.
Pleats in a oil or air filter are an example.
I just woke up. I will come back and proof read when I get to work.
From my Not-So-Smart phone
You have decreased surface area at one micron level (metal to metal) and wildly increased surface area (metal to liquid)
At another, through honing.
If you take your two pieces of glass, with perfectly parallel retaining surfaces, and then scratch them, you have taken them from "imaginary" flat, two dimensional surfaces, to what would now be 3 dimensional surfaces. Adding another dimension instantaneously adds additional area.
Pleats in a oil or air filter are an example.
I just woke up. I will come back and proof read when I get to work.
From my Not-So-Smart phone
While compression ring design typically gets lots of attention, most people fail to appreciate just how critical oil ring function is. In fact, improvements in oil control ring design are a big factor in longer production engine life and reduced HC emissions.
These ring assemblies are properly described as "oil control rings". Their function is to wipe excess oil from the cylinder wall on the downstroke and leave behind a very carefully controlled micro-thin layer of oil. This layer of oil left on the bore surface each downstroke is just thick enough to provide adequate lubrication for the compression ring contact sliding, but also thin enough so that it can transfer heat to the liner surface rapidly enough to keep from flashing off and being burned.
Getting any compression ring to work properly requires both a well engineered oil control ring assembly and careful attention to heat transfer across the liner wall. Without proper lubrication any compression ring material will scuff the bore surface.
These ring assemblies are properly described as "oil control rings". Their function is to wipe excess oil from the cylinder wall on the downstroke and leave behind a very carefully controlled micro-thin layer of oil. This layer of oil left on the bore surface each downstroke is just thick enough to provide adequate lubrication for the compression ring contact sliding, but also thin enough so that it can transfer heat to the liner surface rapidly enough to keep from flashing off and being burned.
Getting any compression ring to work properly requires both a well engineered oil control ring assembly and careful attention to heat transfer across the liner wall. Without proper lubrication any compression ring material will scuff the bore surface.
Pistons rings are items that we are perhaps not accustomed to having to worry too much about, especially if our race engine is not of bespoke design. Once ‘bedded-in’, they normally form a reliable seal between the piston and bore.
However, there is a particular case where piston rings can cause a problem owing to a vibration condition which is due to a combination of factors centred around the fundamental geometry of the engine, the mass of the piston rings, engine speed and the pressure differential across the piston ring.
As the piston approaches bottom dead centre on the exhaust stroke, the combination of inertia forces acting on the piston ring and forces acting on the ring owing to the pressure differential across it act to push the piston ring up against the top side of the ring groove. This effectively causes a seal between the ring and the top of the groove. For the piston ring to work effectively it is energised against the cylinder wall by a combination of its own inherent radial forces when compressed into the bore (often referred to as piston ring ‘tension’) and a force owing to the radial force exerted as a result of the gas pressure acting on the inside of the piston ring.
Let us take the example of a piston ring with an 80 mm inside diameter, and which is 1 mm thick. The cross-sectional area is (80 x π x 1) = 251mm2, and if the cylinder pressure is 5 bar then the force on the inside of the ring is 5 x 0.101325 x 251 = 127 N. Robbed of this force, the radial force of the piston ring can become insufficient to allow the piston to seal effectively. This leads to a vibration condition known as ring flutter.
As the speed of the engine increases, so the piston acceleration increases as the square of engine speed, and even a small increase in maximum engine speed cause the onset of ring flutter. If we say that the fundamental geometry of the cranktrain and the new maximum engine speed is fixed (this defines the piston acceleration), and that the pressure differential across the ring is also fixed (this can be a dangerous assumption), then if we want to eliminate ring flutter we can do so using a few different methods, but the aim is to increase the speed at which flutter will occur so that it is above the operating speed of the engine.
We could consider an increase in ring tension, so that the ring still seals even when it is forced against the top of the ring groove. This can lead to significantly higher engine friction though, and adds to the forces which push the ring toward the top of the ring groove.
The level of piston acceleration at which ring flutter begins can be raised by decreasing the mass of the ring. For a given cranktrain geometry, piston acceleration increases in proportion to the square of engine speed, so by increasing the piston acceleration at high flutter starts we can take it out of the engine running range. Engineers tend to like reducing the mass of reciprocating engine components, so this is a solution that will find favour among designers and developers. The problem though is that it means having to change the design of the piston ring and piston. The elimination of flutter is one reason why we have seen a constant decrease in top ring widths in recent years. As engine speeds increase, a thin low-mass ring is important for eliminating flutter as well as reducing the inertia forces acting on the cranktrain.
Another option is to modify the piston so that the cylinder pressure is not ‘denied access’ to the inside diameter of the piston ring. There are machined design features that can be added to pistons to allow the cylinder pressure to act on the inside diameter of the top ring, even when the ring is forced against the top of the ring groove.
However, there is a particular case where piston rings can cause a problem owing to a vibration condition which is due to a combination of factors centred around the fundamental geometry of the engine, the mass of the piston rings, engine speed and the pressure differential across the piston ring.
As the piston approaches bottom dead centre on the exhaust stroke, the combination of inertia forces acting on the piston ring and forces acting on the ring owing to the pressure differential across it act to push the piston ring up against the top side of the ring groove. This effectively causes a seal between the ring and the top of the groove. For the piston ring to work effectively it is energised against the cylinder wall by a combination of its own inherent radial forces when compressed into the bore (often referred to as piston ring ‘tension’) and a force owing to the radial force exerted as a result of the gas pressure acting on the inside of the piston ring.
Let us take the example of a piston ring with an 80 mm inside diameter, and which is 1 mm thick. The cross-sectional area is (80 x π x 1) = 251mm2, and if the cylinder pressure is 5 bar then the force on the inside of the ring is 5 x 0.101325 x 251 = 127 N. Robbed of this force, the radial force of the piston ring can become insufficient to allow the piston to seal effectively. This leads to a vibration condition known as ring flutter.
As the speed of the engine increases, so the piston acceleration increases as the square of engine speed, and even a small increase in maximum engine speed cause the onset of ring flutter. If we say that the fundamental geometry of the cranktrain and the new maximum engine speed is fixed (this defines the piston acceleration), and that the pressure differential across the ring is also fixed (this can be a dangerous assumption), then if we want to eliminate ring flutter we can do so using a few different methods, but the aim is to increase the speed at which flutter will occur so that it is above the operating speed of the engine.
We could consider an increase in ring tension, so that the ring still seals even when it is forced against the top of the ring groove. This can lead to significantly higher engine friction though, and adds to the forces which push the ring toward the top of the ring groove.
The level of piston acceleration at which ring flutter begins can be raised by decreasing the mass of the ring. For a given cranktrain geometry, piston acceleration increases in proportion to the square of engine speed, so by increasing the piston acceleration at high flutter starts we can take it out of the engine running range. Engineers tend to like reducing the mass of reciprocating engine components, so this is a solution that will find favour among designers and developers. The problem though is that it means having to change the design of the piston ring and piston. The elimination of flutter is one reason why we have seen a constant decrease in top ring widths in recent years. As engine speeds increase, a thin low-mass ring is important for eliminating flutter as well as reducing the inertia forces acting on the cranktrain.
Another option is to modify the piston so that the cylinder pressure is not ‘denied access’ to the inside diameter of the piston ring. There are machined design features that can be added to pistons to allow the cylinder pressure to act on the inside diameter of the top ring, even when the ring is forced against the top of the ring groove.
Hone pattern
I sort of mis-spoke. I was referring to the final cylinder finish pattern. IF that is what you are referring to, then here is a little info.
To start with, let's get our terminology correct. Although the title to this article is 'glaze busting', bore glazing is a phenomenon normally observed only on diesel engines that run a light load for long periods of time. The 'glaze' is in fact a lacquer coming from the oil or products of combustion that condense on the cold surface of the cylinder bore and fill in all the ridges and furrows (however invisible to the naked eye) that represent the surface topology. Hard and almost impossible to remove, the ring will pass directly over it, producing excessive exhaust gas blow-by and high oil consumption.
In the case of race engines, however, what I am referring to is 'bore polishing' - when the peaks and valleys are worn away gradually by the passage of the piston ring. In diesel engines this can be caused by the use of high-detergent oils that produce ash which acts as a grinding paste in the ring bore contact zone. In race gasoline engines, however, when these detergent additives are present in lower concentrations, this is attributed to just general wear and tear.
In the case of our Formula Ford engine, when new rings have been fitted there are a number of possible approaches. One option may be to do absolutely nothing. After only less than 1000 miles the amount of wear against the standard Ford ring pack would, it is assumed, be almost negligible, so the plateau finish produced at the Ford factory under closely controlled procedures would be hardly affected. At the other extreme, re-honing is surely out of the question. This would remove too much material, increasing the piston-to-bore clearance, which would increase even further after bedding in.
The middle option, and one used by some engine builders known to me, is to use what is known as a 'Flex hone' or 'bog brush'. A resilient flexible cutting tool with a soft cutting action, the Flex hone consists of a number of abrasive globules bonded to the ends of nylon filaments, mounted spirally around the axis of the tool. Placed in a simple pillar drill or even handheld electric drill, the tool is run in and out of the bore at about 350-500 rpm and 30-200 strokes per minute for no more than 10-20 s. Depending on the abrasives selected, this produces a surface finish that is less harsh than that of the traditional hone and, done correctly, once the engine has been run-in it can restore leak-down test results to their original values.
I sort of mis-spoke. I was referring to the final cylinder finish pattern. IF that is what you are referring to, then here is a little info.
To start with, let's get our terminology correct. Although the title to this article is 'glaze busting', bore glazing is a phenomenon normally observed only on diesel engines that run a light load for long periods of time. The 'glaze' is in fact a lacquer coming from the oil or products of combustion that condense on the cold surface of the cylinder bore and fill in all the ridges and furrows (however invisible to the naked eye) that represent the surface topology. Hard and almost impossible to remove, the ring will pass directly over it, producing excessive exhaust gas blow-by and high oil consumption.
In the case of race engines, however, what I am referring to is 'bore polishing' - when the peaks and valleys are worn away gradually by the passage of the piston ring. In diesel engines this can be caused by the use of high-detergent oils that produce ash which acts as a grinding paste in the ring bore contact zone. In race gasoline engines, however, when these detergent additives are present in lower concentrations, this is attributed to just general wear and tear.
In the case of our Formula Ford engine, when new rings have been fitted there are a number of possible approaches. One option may be to do absolutely nothing. After only less than 1000 miles the amount of wear against the standard Ford ring pack would, it is assumed, be almost negligible, so the plateau finish produced at the Ford factory under closely controlled procedures would be hardly affected. At the other extreme, re-honing is surely out of the question. This would remove too much material, increasing the piston-to-bore clearance, which would increase even further after bedding in.
The middle option, and one used by some engine builders known to me, is to use what is known as a 'Flex hone' or 'bog brush'. A resilient flexible cutting tool with a soft cutting action, the Flex hone consists of a number of abrasive globules bonded to the ends of nylon filaments, mounted spirally around the axis of the tool. Placed in a simple pillar drill or even handheld electric drill, the tool is run in and out of the bore at about 350-500 rpm and 30-200 strokes per minute for no more than 10-20 s. Depending on the abrasives selected, this produces a surface finish that is less harsh than that of the traditional hone and, done correctly, once the engine has been run-in it can restore leak-down test results to their original values.
Liner distortion.....
Spare a thought for the poor cylinder liner. As well as expecting it to be perfectly round from the outset, we then go on to expect it to remain so throughout the rest of its life, and under the most arduous conditions.
But are we making it as round as it could be? Take for instance the typical case of a replacement dry liner. We'll measure its external diameter in at least three positions around its axis, and then again in another three places up and down the bore. Averaging these and allowing something like 0.001 in per inch diameter of interference, we'll happily bore away, taking care of squareness and roundness of the boring head until we finally have any number of liners firmly seated in the block. This, however, is only the starting point.
The thin metal liner may have been distorted from careless handling (such as leaving it lying on its side for a long time instead of upright) but no matter what its initial shape, in the end it will conform to the shape of the hole into which it was pushed. Too little interference and the thing will just rattle around in use; too much and the surface could end up being convoluted and even more distorted. Ever tried to get one of those plastic wiring grommets through a hole that was too small for it? Well the situation is much the same here, except that the forces are so great that in the process the edges of the liner could even be damaged.
Often it's a case of 'suck it and see' and for each engine the machine shop will find a way that works for them. But whichever way you look at it, further machining/honing will be necessary.
For anyone concerned about the clamp loads from the cylinder head, a boring plate will be needed. This consists of a metal plate of equivalent stiffness to the actual cylinder head, with holes drilled through it above the bores to enable access by the boring bar/honing tool, and should be clamped into place - ideally using the similar bolts to those used in service - and tightened into yield using the same torque and angle values. This should simulate the loads introduced at build.
In addition, and to get even closer, a similar gasket/fire ring arranged should be inserted to replicate the real geometry at the head face. While many good quality machine shops will use a simple 1 in-thick boring plate, it is always difficult to know how representative such a plate might be. In using similar geometries and actual components as far as is practicable, the loads should be closer to those encountered in use.
But what about the thermal input, I hear you ask? Won't that cause distortion as well? Unsurprisingly, the best machine shops have thought about this and it's not uncommon for them to use hot oil circulating through the cylinder block during final machining. The oil will allow temperatures of up to 150º C, perhaps more if you really want, but I would have thought 100-120º C would be enough of a problem - after all, some poor machinist may have get close to it at some time.
But despite all this, are we not forgetting the effect of combustion pressure on all this distortion? Perhaps we need to introduce nitrogen at high pressure to the cylinder bore which, when sealed top and bottom, will use a specialised boring head. I haven't heard of anyone doing this as yet - although that doesn't mean it hasn't been done - and the pressure couldn't be anywhere near that encountered in a fired engine. Hang on a minute though, am I not taking this to the extreme?
Spare a thought for the poor cylinder liner. As well as expecting it to be perfectly round from the outset, we then go on to expect it to remain so throughout the rest of its life, and under the most arduous conditions.
But are we making it as round as it could be? Take for instance the typical case of a replacement dry liner. We'll measure its external diameter in at least three positions around its axis, and then again in another three places up and down the bore. Averaging these and allowing something like 0.001 in per inch diameter of interference, we'll happily bore away, taking care of squareness and roundness of the boring head until we finally have any number of liners firmly seated in the block. This, however, is only the starting point.
The thin metal liner may have been distorted from careless handling (such as leaving it lying on its side for a long time instead of upright) but no matter what its initial shape, in the end it will conform to the shape of the hole into which it was pushed. Too little interference and the thing will just rattle around in use; too much and the surface could end up being convoluted and even more distorted. Ever tried to get one of those plastic wiring grommets through a hole that was too small for it? Well the situation is much the same here, except that the forces are so great that in the process the edges of the liner could even be damaged.
Often it's a case of 'suck it and see' and for each engine the machine shop will find a way that works for them. But whichever way you look at it, further machining/honing will be necessary.
For anyone concerned about the clamp loads from the cylinder head, a boring plate will be needed. This consists of a metal plate of equivalent stiffness to the actual cylinder head, with holes drilled through it above the bores to enable access by the boring bar/honing tool, and should be clamped into place - ideally using the similar bolts to those used in service - and tightened into yield using the same torque and angle values. This should simulate the loads introduced at build.
In addition, and to get even closer, a similar gasket/fire ring arranged should be inserted to replicate the real geometry at the head face. While many good quality machine shops will use a simple 1 in-thick boring plate, it is always difficult to know how representative such a plate might be. In using similar geometries and actual components as far as is practicable, the loads should be closer to those encountered in use.
But what about the thermal input, I hear you ask? Won't that cause distortion as well? Unsurprisingly, the best machine shops have thought about this and it's not uncommon for them to use hot oil circulating through the cylinder block during final machining. The oil will allow temperatures of up to 150º C, perhaps more if you really want, but I would have thought 100-120º C would be enough of a problem - after all, some poor machinist may have get close to it at some time.
But despite all this, are we not forgetting the effect of combustion pressure on all this distortion? Perhaps we need to introduce nitrogen at high pressure to the cylinder bore which, when sealed top and bottom, will use a specialised boring head. I haven't heard of anyone doing this as yet - although that doesn't mean it hasn't been done - and the pressure couldn't be anywhere near that encountered in a fired engine. Hang on a minute though, am I not taking this to the extreme?
Cylinder hone pattern
It is a fact not appreciated by many, except the specialists, but the technology of the cylinder bore surface finish has changed significantly over the years. While oil consumption was perhaps the greatest driver in the past, the push to achieve even less and less exhaust emission at higher and higher mileages, has caused OE engine manufacturers to focus even greater resources into getting the surface finish of the cylinder bore exactly right. And while, to the OEM this means emissions approval, to the racing industry this technology brings less friction and wear and with it, improved performance. Which is where, I guess, we start to take an interest.
The move towards improving cylinder bore technology could only begin once we had the means to accurately quantify the quality of the surface. Until the invention of the electronic surface texture testing machines, surface finish measurement was limited to only a few parameters. The mean Roughness (Ra), the total Roughness (Rt) and the bearing area throughout the various levels in the surface were all that could be reasonably expected. Construction of the Abbot-Firestone bearing area curve (a curve describing the theoretical bearing area at each level throughout the surface) was a complete headache and required much manual graphic endeavour. The subsequent development of electronic surface finish measurement machines automated much of this process and enabled a far greater number of parameters to be established. Ra alone doesn’t reveal that much at all about the surface finish but values like Rpk (peak height), Rvk (depth of the valleys), Rk (the average core roughness based on Rpk and Rvk measurements) and Rz (highest peak to valley height) can be begin to describe the precise surface roughness from which we can get a more complete picture. Thus instead of just specifying a simple Ra value, bore specialists now have to grapple with Ra, Rpk, Rvk, and Rk values as well.
Specialists tend to come up with their own numbers but as a guide, typical values for a cast iron bore suitable for use with chrome-plated rings may be:
Ra 12 to 24
Rpk 2 to 24
Rvk 20 to 80
Rk 28 to 48
Keeping in mind that these values are stated in micro inches (0.000001”), you will appreciate that modern bores are very fine indeed. Established over many years the ideal surface finish should also be perfectly cylindrical, smooth at a micro level and contain a series of grooves in it to retain the lubricant. In practical terms the nearest we can realistically get to this is a finely bored surface with all of the peaks of the asperities removed leaving only the oil retaining grooves behind. Such a surface is described as a ‘plateau’ and will take many honing operations to achieve.
At one time following the boring procedure, the surface finish would consist of a single honing operation. Modern multi-stage honing methods however, use different sizes of grit stone and different grinding materials (silicon carbide, diamond or boron nitride) and may well finish with a nylon brush to remove the last traces of folded metal. This will create a surface much closer to our ideal but with all the tears and folded material from machining removed, together with a correctly angled cross-hatching, piston rings can now be expected to bed-in in a matter of minutes.
When you consider the amount of effort that goes into the design and manufacture of the piston and rings, surely only the most carefully prepared bore surface will suffice?
It is a fact not appreciated by many, except the specialists, but the technology of the cylinder bore surface finish has changed significantly over the years. While oil consumption was perhaps the greatest driver in the past, the push to achieve even less and less exhaust emission at higher and higher mileages, has caused OE engine manufacturers to focus even greater resources into getting the surface finish of the cylinder bore exactly right. And while, to the OEM this means emissions approval, to the racing industry this technology brings less friction and wear and with it, improved performance. Which is where, I guess, we start to take an interest.
The move towards improving cylinder bore technology could only begin once we had the means to accurately quantify the quality of the surface. Until the invention of the electronic surface texture testing machines, surface finish measurement was limited to only a few parameters. The mean Roughness (Ra), the total Roughness (Rt) and the bearing area throughout the various levels in the surface were all that could be reasonably expected. Construction of the Abbot-Firestone bearing area curve (a curve describing the theoretical bearing area at each level throughout the surface) was a complete headache and required much manual graphic endeavour. The subsequent development of electronic surface finish measurement machines automated much of this process and enabled a far greater number of parameters to be established. Ra alone doesn’t reveal that much at all about the surface finish but values like Rpk (peak height), Rvk (depth of the valleys), Rk (the average core roughness based on Rpk and Rvk measurements) and Rz (highest peak to valley height) can be begin to describe the precise surface roughness from which we can get a more complete picture. Thus instead of just specifying a simple Ra value, bore specialists now have to grapple with Ra, Rpk, Rvk, and Rk values as well.
Specialists tend to come up with their own numbers but as a guide, typical values for a cast iron bore suitable for use with chrome-plated rings may be:
Ra 12 to 24
Rpk 2 to 24
Rvk 20 to 80
Rk 28 to 48
Keeping in mind that these values are stated in micro inches (0.000001”), you will appreciate that modern bores are very fine indeed. Established over many years the ideal surface finish should also be perfectly cylindrical, smooth at a micro level and contain a series of grooves in it to retain the lubricant. In practical terms the nearest we can realistically get to this is a finely bored surface with all of the peaks of the asperities removed leaving only the oil retaining grooves behind. Such a surface is described as a ‘plateau’ and will take many honing operations to achieve.
At one time following the boring procedure, the surface finish would consist of a single honing operation. Modern multi-stage honing methods however, use different sizes of grit stone and different grinding materials (silicon carbide, diamond or boron nitride) and may well finish with a nylon brush to remove the last traces of folded metal. This will create a surface much closer to our ideal but with all the tears and folded material from machining removed, together with a correctly angled cross-hatching, piston rings can now be expected to bed-in in a matter of minutes.
When you consider the amount of effort that goes into the design and manufacture of the piston and rings, surely only the most carefully prepared bore surface will suffice?
Laser honing.....
The surface condition of the cylinder bore has occupied the minds of motorists, engineers and enthusiasts for many years. Often expressed in terms of oil consumption – miles per litre or miles per quart depending upon which side of the Atlantic you reside – provided consumption isn’t excessive, all is generally thought to be fine. In a racing engine however, where cost of ownership is, let’s face it, more or less irrelevant, the real issue here is one of friction.
In recent years and with vehicle emission standards becoming more and more exacting, the surface of the cylinder bore is considered to be an emission critical component. While the technologies developed appear to have more direct benefits to the OE vehicle industry, the spin offs, in this case the bore finish, has helped to reduce engine friction. Modern engines are therefore very much more likely to have what is known as a ‘plateau’ surface finish. Consisting of a finely bored cylinder, bored more or less to size, the surface is then textured to produce the combined characteristics of a smooth, precisely cylindrical surface but with oil retention capability. To generate this texture will require dedicated honing machines when a series of coarse and then fine grit stones will be plunged through the bores in turn and while rotating at the same time produce a series of random grooves at angles varying between 30 and 45 degrees. The rougher stones will gouge out deep grooves in the material while the finer ones in subsequent operations, will effectively ‘top slice’ the surface away producing altogether a surface smoother to the eye but with deep groves spiralling down the bore. It is these deep grooves that are intended to retain the oil, which will subsequently lubricate the piston ring as it passes. When oil is present on the surface and the thickness of the oil film is greater than the height of the surface peaks, then hydrodynamic lubrication will be present and friction will be minimized.
The main concern with all this is that of the initial honing operation. The coarse grit whether it is vitreous stone or diamond rips out the material and can leave small amounts of residue deep in the grooves. While later operations will remove and effectively polish the upper portion, loose or partially removed material can still be retained in the grooves even after a thorough washing. By changing the honing procedure and introducing a laser to remove small but controlled amounts of the surface material later in the cycle, discreet pockets can be eroded which can be deeper than these initial rough grooves, more precisely controlled and can retain much more oil.
Thus the complete cylinder surface is smoother and more homogenous than with conventional practice. Furthermore, these discreet pockets can be machined anywhere on the bore to introduce additional oil to those places most in need. For most engines, that means nearer to the top ring reversal point where lubrication is traditionally difficult as the ring slows down.
With a million engines worldwide claiming to use this sort of technology, maybe it is only a matter of time before we all are using it.
The surface condition of the cylinder bore has occupied the minds of motorists, engineers and enthusiasts for many years. Often expressed in terms of oil consumption – miles per litre or miles per quart depending upon which side of the Atlantic you reside – provided consumption isn’t excessive, all is generally thought to be fine. In a racing engine however, where cost of ownership is, let’s face it, more or less irrelevant, the real issue here is one of friction.
In recent years and with vehicle emission standards becoming more and more exacting, the surface of the cylinder bore is considered to be an emission critical component. While the technologies developed appear to have more direct benefits to the OE vehicle industry, the spin offs, in this case the bore finish, has helped to reduce engine friction. Modern engines are therefore very much more likely to have what is known as a ‘plateau’ surface finish. Consisting of a finely bored cylinder, bored more or less to size, the surface is then textured to produce the combined characteristics of a smooth, precisely cylindrical surface but with oil retention capability. To generate this texture will require dedicated honing machines when a series of coarse and then fine grit stones will be plunged through the bores in turn and while rotating at the same time produce a series of random grooves at angles varying between 30 and 45 degrees. The rougher stones will gouge out deep grooves in the material while the finer ones in subsequent operations, will effectively ‘top slice’ the surface away producing altogether a surface smoother to the eye but with deep groves spiralling down the bore. It is these deep grooves that are intended to retain the oil, which will subsequently lubricate the piston ring as it passes. When oil is present on the surface and the thickness of the oil film is greater than the height of the surface peaks, then hydrodynamic lubrication will be present and friction will be minimized.
The main concern with all this is that of the initial honing operation. The coarse grit whether it is vitreous stone or diamond rips out the material and can leave small amounts of residue deep in the grooves. While later operations will remove and effectively polish the upper portion, loose or partially removed material can still be retained in the grooves even after a thorough washing. By changing the honing procedure and introducing a laser to remove small but controlled amounts of the surface material later in the cycle, discreet pockets can be eroded which can be deeper than these initial rough grooves, more precisely controlled and can retain much more oil.
Thus the complete cylinder surface is smoother and more homogenous than with conventional practice. Furthermore, these discreet pockets can be machined anywhere on the bore to introduce additional oil to those places most in need. For most engines, that means nearer to the top ring reversal point where lubrication is traditionally difficult as the ring slows down.
With a million engines worldwide claiming to use this sort of technology, maybe it is only a matter of time before we all are using it.
biggy238
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And would you say that on the whole, I was more correct, or more incorrect in the discussion I raised after having read that?
Mind you, I wasn't going to type that level of explanation on the ****ter at a quarter after six in the morning.
From my Not-So-Smart phone
Mind you, I wasn't going to type that level of explanation on the ****ter at a quarter after six in the morning.
From my Not-So-Smart phone
biggy238
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This is the very essence of what we are discussing isn't it? How much surface area is exposed to the ring surface, and therefore how much oil it can remove or retain.
From my Not-So-Smart phone
kjpcummins
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Looks like someone can use a search engine.
Maybe im blind but if you lap the rings in to get full contact and reduce tension on the oil ring set. Next coat the rings and cylinder bores there is no finish hone. Its just a really smooth surface left.
EricStaab
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Matty has made many good points.
Here is the deal. You can not have a completely smooth surface because of two things.
1.) You need oil between the ring face, and the sleeve. This creates a seal, pulls heat from the piston, and keeps the ring and sleeve from becoming damaged.
2.) The cross hatch turns the rings to wear them evenly.
Piston ring manufactures have done a lot of testing. They recommend what type of hone will work best for their ring. Unless other data is found, I would use the ring manufacturers recommendations. Now sometimes we deviate just a little to change how fast a set of rings will seat, but it is also a compromise to wear.
Here is a link to our website discussing sleeve honing.
sleeve honing
Here is the deal. You can not have a completely smooth surface because of two things.
1.) You need oil between the ring face, and the sleeve. This creates a seal, pulls heat from the piston, and keeps the ring and sleeve from becoming damaged.
2.) The cross hatch turns the rings to wear them evenly.
Piston ring manufactures have done a lot of testing. They recommend what type of hone will work best for their ring. Unless other data is found, I would use the ring manufacturers recommendations. Now sometimes we deviate just a little to change how fast a set of rings will seat, but it is also a compromise to wear.
Here is a link to our website discussing sleeve honing.
sleeve honing
biggy238
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