Piston structure design
Piston structure design
(OP)
After having combustion on the mind for a short time again lead me into thinking about pistons and heat transfer and everything else involved. After reading and thinking, the heat distribution of a forged unit will be better than that of a cast unit. I am more interested in the Diesel piston design but am still open to gasser piston design as well. Recently I have been reading on the trunk type, as well as the articulated types both heavy truck and light duty diesel engines. Currently the hot light duty unit seems to be the Mahle Monotherm which is up against Federal Mogul's MonoSteel. As far as what goes into a piston and what we can to to influence condition is the chamber as far as temperature and power production and emissions too.
Right now I am thinking, through the selection of the right material parts and materials they are made of, we can control our heat absorption and dissipation in the crown and other surfaces to get the last little bit of power ouf of the chamber without melting things. Pretty much thinking about just moving slightly closer to a adiabatic process. I know that won't be achieved through the ICE fully but I am thinking we can come closer and be more efficient. However I am not sure how the longevity is going to be effected directly even when/if we have a finer tuned level of control on the temperature of our piston crown.
I would like pick anyone's brain on what they thought of my thoughts.
Thanks,
Jim
Right now I am thinking, through the selection of the right material parts and materials they are made of, we can control our heat absorption and dissipation in the crown and other surfaces to get the last little bit of power ouf of the chamber without melting things. Pretty much thinking about just moving slightly closer to a adiabatic process. I know that won't be achieved through the ICE fully but I am thinking we can come closer and be more efficient. However I am not sure how the longevity is going to be effected directly even when/if we have a finer tuned level of control on the temperature of our piston crown.
I would like pick anyone's brain on what they thought of my thoughts.
Thanks,
Jim





RE: Piston structure design
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Piston structure design
Also the lower the surface area the better.
Steel for the crown would seem a good choice, but a really tough ceramic might be better in a multi piece piston.
While a flat top design offers lowest area for a piston, when you consider the piston and chamber combined, a small flatish dome chamber and a small dished piston would be best. You also need to consider squish and then you really start to make compromises and the differences between diesel and SI start to become more apparent as flame travel or retaining stratified layers become important in one or the other.
Regards
Pat
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RE: Piston structure design
I'm not sure I'd think that was necessarily true, even for the improbability of identically dimensioned parts, and after reading the TRW literature of the 1960s that made that very claim for their "powerforged" pistons.
RE: Piston structure design
Tmoose, what you are saying, with a forged piece there could be a cast unit (or casting process) with just as well of a heat flow even though that grain structure is tighter in a forged piece or some forgings just dont flow as well as one would think? Thats definitely something for me to check out some more.
RE: Piston structure design
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.g...
It also says that "By the late 1980's, the production of hypereutectic AI-Si pistons has dominated the manufacture of AIuminum
pistons. Presently, the standard hypereutectic AI-Si alloy for the industry is the A390 alloy. "
It lists some chemistry and tensile strength info, but no thermal conductivity info.
A quick search for thermal conductivity (and expansion) of MS75 ( a dark gray forging alloy used by TRW/SpeedPro/ etc)., Mahle's forging alloys ( they say Power Pak pistons are predominately made from 4032 aluminum alloy) 2618-T61 (Bill Miller's stated favorite) , came up pretty thin.
RE: Piston structure design
Aluminium pistons have been around for about 100 years I think, so there has been plenty of development time and I am sure intense interest from a huge number of people in gaining improvements.
There are quite a few different alloys used and they certainly have quite a range of different thermal characteristics re co-efficient of expansion initial strength and degradation of strength with increase of temperature. These differences are well documented, especially the co-efficient of thermal expansion as they are critical to successful application.
A lack of data re thermal conductivity leads me to PRESUME that it is not a critical property or at least there is not enough variation to make the variation critical.
Regards
Pat
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RE: Piston structure design
The advantage of steel pistons for high BMEP DI diesel service is their much greater elevated temp fatigue strength, especially around the bowl edges. The Monotherm and Monosteel steel pistons are both excellent designs, with good reliability, light weight and low cost. The only real difference being the Monosteel design is friction welded from two pieces and the Monotherm design is a single piece forging. Aluminum pistons are acceptable for small bore DI diesel service with BMEP rates up to about 200 bar. But these aluminum pistons still require a ferrous insert for the top ring groove.
Both the Monotherm and Monosteel pistons have a large "cocktail shaker" type oil gallery for cooling. While one purpose for this oil gallery is cooling of the crown, the most important function is keeping the top ring cool enough so that the lube oil in the groove does not form deposits.
Good luck.
Terry
RE: Piston structure design
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Piston structure design
RE: Piston structure design
RE: Piston structure design
FR- Not exactly. The piston conductive heat transfer path for the ring is through the lower groove land surface where it contacts the ring, across the piston wall thickness behind the groove, and into the agitated oil mass passing over the gallery surface. The piston conductive heat transfer path from the crown to the oil gallery is simply across the crown thickness. However, there is some benefit to increasing the distance from the piston deck to the top ring, because it allows the gallery oil to more effective cool the outer circumference of the piston crown, thus minimizing the heat flow into the top ring area. The steel piston crown can tolerate much higher operating temperatures than the top ring can.
Best regards,
Terry
RE: Piston structure design
I have a old Cummins dump truck piston which is a trunk type. Has the ni-resist top ring land.
RE: Piston structure design
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Piston structure design
Fahlin Racing,
The picture you posted illustrates part of the problem concerning thermal management in a high BMEP diesel piston. While an aluminum piston structure would have better thermal conductivity across the crown wall thickness than a steel piston structure, the aluminum piston would also need to run at less than half the crown surface temperatures as the steel piston, and the crown wall thickness of the aluminum piston would also need to be much greater due to its lower fatigue stress limits. The primary heat transfer path in aluminum piston example you show is from the crown to the skirt. That is why the sections are so heavy.
When comparing aluminum and steel pistons, we must consider the entire process for heat transfer in the piston structure. There are the variables of thermal conductivity in the material, allowable operating temperatures at the crown surface, thickness of the crown wall, and temperature delta across the crown wall.
Lastly, you should also look closely at the design of the Monotherm or Monosteel piston oil galleries. These oil galleries are designed to retain a small volume of oil within the gallery space and agitate it back and forth for an extended period before it is discharged. This way as much heat as possible is transferred into the oil, and the mass flow of oil required for piston cooling is minimized.
RE: Piston structure design
......here the heat removal.... by a liquid...is far more efficient than through metal to metal transfer?
I noticed Charles Fayette Taylor states with aluminum trunk pistons first quote is apparently for small bore unsuprcharged engines... "For this type of cooling aluminum is desirable, with generous sections for heat conduction from the center of the piston head..... Heat conduction to the cylinder walls is promoted by small skirt clearances. Rings run cooler as the top "land" (distance from the top edge of the piston to top ring) is increased."
Now a few sentences later in the next paragraph about supercharged and diesel engine pistons about the oil speed in the gallery of the crown...
"For oil cooled pistons the rate of oil circulation must ne high enough to avoid breakdown of the oil into cabronaceous deposits"
I think the first quote can be used with more than just small bores, should be able to use it with most aluminum trunk style pistons but thats only my thought. You guys have posted some things I will keep reading them over and over and hoepfully some more pops in my brain. Hope this weekend goes well for you guys!
RE: Piston structure design
http://vintagesleds.com/library/manuals/misc/jenni...
RE: Piston structure design
RE: Piston structure design
RE: Piston structure design
RE: Piston structure design
obit http://www.blasterforum.com/642508-post1.html
RE: Piston structure design
RE: Piston structure design
Regards
Pat
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RE: Piston structure design
http://s3.racingjunk.com/ui/2/27/26551272-313-Manl...
http://ecx.images-amazon.com/images/I/51w4lXJeDKL....
Early days racing small block Chevies and Hemis ran over 40 degrees BTDC.
For years recommended 38 max for their big block wedge engines, and 35 for the 426 Hemi.
Nowadays with flatter more compact chambers and organic piston domes mid 30s is just right.
RE: Piston structure design
I once saw a very high dome SBC piston looking for over 15:1 static compression using methanol for fuel.
He needed 60 deg advance and did not make the power of the previous engine with 13:1, same heads cam etc.
We put small slots in the piston dome adjacent to the spark plug tip to get the flame to cross the chamber while approaching TDC. We lost a few decimal points in compression but reduced optimum timing to 40 deg and picked up considerable power.
Regards
Pat
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RE: Piston structure design
RE: Piston structure design
RE: Piston structure design
Of course if there are deep valve reliefs in the piston, the sides of that relief can certainly interfere with flow.
Regards
Pat
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RE: Piston structure design
RE: Piston structure design
RE: Piston structure design
RE: Piston structure design
RE: Piston structure design
RE: Piston structure design
RE: Piston structure design
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Piston structure design
RE: Piston structure design
The thermal conductivity argument is valid, but is trumped by the need for a piston that can at least survive the operating conditions. A hotter bulk temperature of the crown can theoretically be offset by a thinner section combined with judicious oil cooling, resulting in an acceptable combustion surface temperature. Not saying aluminum is always inferior to steel, only that for some applications steel may be a better choice.
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Piston structure design
RE: Piston structure design
RE: Piston structure design
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Piston structure design
http://chevy.oldcarmanualproject.com/shop/1942_47/...
"Chevrolet pistons are cast iron surface treated, with a slipper skirt. "
However it also says "The truck piston may be identified by a small forged boss on the lower center of each piston pin boss."
Forged iron? I don't think so.
The 1949-1953 Chevy Shop Manual says the first use of aluminum pistons (instead of cast iron) was the 1953 Powerglide models.
http://chevy.oldcarmanualproject.com/shop/1949_53/...