Piston expansion
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Piston expansion
A piston engine generally has iron/iron lined bores and aluminum pistons. These two components have two different thermal expansion coefficients. The common thought is that when both bores are cold upon start up, you do not want to load or over rev the engine because the engine has not reached proper operating temperature and also operating clearances. The idea is that the bore is larger than the piston, with a higher thermal expansion, and that once warm, the piston will expand and reach its proper operating clearance.
That's fine.
My thoughts are this: Since aluminum has a significantly higher rate of thermal transfer and the flame front is pushing directly on the piston, there must be a brief period where the aluminum piston absorbed far more heat energy than the cylinder wall. For this short period, the piston is actually tight within the cylinder even though the engine has just been started. The iron cylinder has barely changed diameter while the aluminum piston has absorbed far more heat and has a much higher rate of expansion.
Is there theoretically a brief period, a few seconds to a minute, where the cold engine is actually running like its hot or overheated?
That's fine.
My thoughts are this: Since aluminum has a significantly higher rate of thermal transfer and the flame front is pushing directly on the piston, there must be a brief period where the aluminum piston absorbed far more heat energy than the cylinder wall. For this short period, the piston is actually tight within the cylinder even though the engine has just been started. The iron cylinder has barely changed diameter while the aluminum piston has absorbed far more heat and has a much higher rate of expansion.
Is there theoretically a brief period, a few seconds to a minute, where the cold engine is actually running like its hot or overheated?
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My 
based on what I have read.....
I don't think expansion has to do with engine health though really. As that articles says, a loose engine just makes noise and waste power and causes a bit more blowby, maybe burns slight bit more oil or contaminates the oilf with a bit more fuel. This one reason it is bad to start a cold car and turn it off right away, you don't have time to burn off the moisture and fuel in the oil. You should drive it once at temp for a good 10 mins before you turn it off, or change your oil much more frequently if you have short trips, or rev high once up to temp to get some good heat.
Once clearances are met, you have good seals and are making good power, good mpg, and contaminating your oil much less and getting less blowby. But I can't think of any direct harm done from too much clearance.
But this is agian for more aggresive sports car, which is very few OEM cars but mostly aftermarket rebuilds with forged.
But I think the reason why you don't want to overload or over rev has more to do with oil temp than it does engine temp and clearances. I could go on an entirely different and probably longer rant about oil temps and starting...
And I could aslo go on of how IMO you never want to overload your under reved car when up to temp or not. lol But don't want to get too far off topic.
But you do make a good point on your theory on the alum piston heating up faster than the block does make sense, never thought of that before. It would almost make me think you shouldn't beat on your car after almost warmed up as the clearances at that point are tighter than operating temp clearances. Interesting....
Then again, what are piston rings made of? They expand too, within the piston right? If they are iron maybe they take a while like the cylinder to heat up and though the aluminum is already up to temp, the rings and cylinders arn't and that is equivillant to all of them being up to temp? Though it isn't jsut material, but ammount of it. A tiny ring might heat up much faster than aluminum maybe?
I guess it really doens't matter though as said above, loose clearances won't really cause damge, but yo ugotta wait anyway for your oil temps, so it is really a moot point, though interesting thought.
based on what I have read.....
Casted alloys have gained silicon, a material that gives the pistons natural lubricity and limits heat expansion. All modern pistons have silicon in them. However, cast pistons have historically had the most. Some of them have as much as 25 percent silicon by volume. Silicon does bring a disadvantage however. It makes the piston brittle. Dropping a modern cast piston will usually crack it, so the piston must be handled carefully.
In the past, forged pistons had to be fitted loose, which made it noisy and wasted power. However, silicon has been introduced to the forged piston. A mixture of alloys has been found that together with silicon do not result in defective forgings. For example, nickel has been found to offset the silicon's tendency toward brittleness. However, not very much nickel can be used, as it is a heavy metal, and it affects the mixture in other ways. Lower silicon content of course would result in this, making the forged piston less brittle. However, there is another reason as well. The forging process compresses the alloy's molecules, making the material more dense than a casting. The result is a piston that withstands the pounding of detonation better. This is why OEMs use the forged piston in two-strokes and turbocharged engines. The result is that the modern forged piston is much more dimensionally stable than was true in the past, but I still think they expand much more than cast. However, once again, the forged piston's mass does more to define its characteristics than does even its materials. The forged piston has historically had a crude interior shape. The forging ram is straight, which results in a rectangular rather than an inticate interior. There is too much mass there. Consequently, the forged piston has poor dimensional stability. Its expansion is not very controllable. Many engine builders overcome these two problems (too much weight, unpredictable expansion) at least partly by removing by hand the extra material inside the forged piston. This allows them to fit them tighter and rev them higher. However, many forged pistons also have overly thick skirts as well as unsophisticated interiors. This is because the forging produces a piston blank, remember, and not a finished piston. The piston wholesaler takes this blank, and from it, carves out several different sizes and shapes of pistons. If the piston being made happens to be the largest the blank supports, it ends up with the thickest skirt. While hand reworking (or CNC milling, as many do now) the forged piston can lighten it and make it behave more like an intricately made cast piston, there is still excess weight due to the thick skirt.
In the past, forged pistons had to be fitted loose, which made it noisy and wasted power. However, silicon has been introduced to the forged piston. A mixture of alloys has been found that together with silicon do not result in defective forgings. For example, nickel has been found to offset the silicon's tendency toward brittleness. However, not very much nickel can be used, as it is a heavy metal, and it affects the mixture in other ways. Lower silicon content of course would result in this, making the forged piston less brittle. However, there is another reason as well. The forging process compresses the alloy's molecules, making the material more dense than a casting. The result is a piston that withstands the pounding of detonation better. This is why OEMs use the forged piston in two-strokes and turbocharged engines. The result is that the modern forged piston is much more dimensionally stable than was true in the past, but I still think they expand much more than cast. However, once again, the forged piston's mass does more to define its characteristics than does even its materials. The forged piston has historically had a crude interior shape. The forging ram is straight, which results in a rectangular rather than an inticate interior. There is too much mass there. Consequently, the forged piston has poor dimensional stability. Its expansion is not very controllable. Many engine builders overcome these two problems (too much weight, unpredictable expansion) at least partly by removing by hand the extra material inside the forged piston. This allows them to fit them tighter and rev them higher. However, many forged pistons also have overly thick skirts as well as unsophisticated interiors. This is because the forging produces a piston blank, remember, and not a finished piston. The piston wholesaler takes this blank, and from it, carves out several different sizes and shapes of pistons. If the piston being made happens to be the largest the blank supports, it ends up with the thickest skirt. While hand reworking (or CNC milling, as many do now) the forged piston can lighten it and make it behave more like an intricately made cast piston, there is still excess weight due to the thick skirt.
I don't think expansion has to do with engine health though really. As that articles says, a loose engine just makes noise and waste power and causes a bit more blowby, maybe burns slight bit more oil or contaminates the oilf with a bit more fuel. This one reason it is bad to start a cold car and turn it off right away, you don't have time to burn off the moisture and fuel in the oil. You should drive it once at temp for a good 10 mins before you turn it off, or change your oil much more frequently if you have short trips, or rev high once up to temp to get some good heat.
Once clearances are met, you have good seals and are making good power, good mpg, and contaminating your oil much less and getting less blowby. But I can't think of any direct harm done from too much clearance.
But this is agian for more aggresive sports car, which is very few OEM cars but mostly aftermarket rebuilds with forged.
But I think the reason why you don't want to overload or over rev has more to do with oil temp than it does engine temp and clearances. I could go on an entirely different and probably longer rant about oil temps and starting...
And I could aslo go on of how IMO you never want to overload your under reved car when up to temp or not. lol But don't want to get too far off topic.
But you do make a good point on your theory on the alum piston heating up faster than the block does make sense, never thought of that before. It would almost make me think you shouldn't beat on your car after almost warmed up as the clearances at that point are tighter than operating temp clearances. Interesting....
Then again, what are piston rings made of? They expand too, within the piston right? If they are iron maybe they take a while like the cylinder to heat up and though the aluminum is already up to temp, the rings and cylinders arn't and that is equivillant to all of them being up to temp? Though it isn't jsut material, but ammount of it. A tiny ring might heat up much faster than aluminum maybe?
I guess it really doens't matter though as said above, loose clearances won't really cause damge, but yo ugotta wait anyway for your oil temps, so it is really a moot point, though interesting thought.