Piston structure design 1

[FahlinRacing]

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

 

[Lou Scannon]

Well for a high power density (knock limited) Otto engine at least, you want the piston crown to run as cool as feasible, at least at high load, to help with knock margin.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[patprimmer]

Well obviously the higher the temperature the crown will resist and the lower the thermal conductivity the better.

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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[Tmoose]

"heat distribution of a forged unit will be better than that of a cast unit"

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.

 

[FahlinRacing]

Pat, I totally forgot about surface area in general lol dangit.

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.

 

[Tmoose]

If NASA can be trusted to know or find out about piston manufacturing, the 3 predominant cast piston alloys are on page 4 of this 15 year old document.

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19990019483_1999011669.pdf

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.

 

[patprimmer]

I know this is a bit of a cop out answer and not at all scientific enough to satisfy my normal standards BUT.

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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[tbuelna]

Jim,

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

 

[Lou Scannon]

Aluminum pistons are acceptable for small bore DI diesel service with BMEP rates up to about 20 bar.

FIFY!

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[tbuelna]

hemi- Thanks! I should not attempt to think and type at the same time. What I should have said was aluminum pistons are generally acceptable for small bore DI diesel service with max combustion pressures around 2000 psi.

 

[FahlinRacing]

Yeah, I am just searching to see whats what Pat. Thanks for posting the Nasa doc Tmoose that will help out my curiousity here. If the oil cooling gallery is crucial for crown cooling, wouldn't the distance from the crown surface to top ring be around the same level of importance as the distance between the top ring and the oil gallery too Terry?

 

[tbuelna]

.....If the oil cooling gallery is crucial for crown cooling, wouldn't the distance from the crown surface to top ring be around the same level of importance as the distance between the top ring and the oil gallery too Terry?

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

 

[FahlinRacing]

Interesting, from how I am interpreting here the heat removal is far more efficient than through metal to metal transfer?
I have a old Cummins dump truck piston which is a trunk type. Has the ni-resist top ring land.

 

[Lou Scannon]

The importance of oil gallery cooling is strongly linked to the power density of the application. While undercrown oil squirters are generally used on modern diesel engines, the cooling gallery in the piston is an optional feature that the sectioned piston above evidently does not have. From that I would surmise that the application of this piston is not very high power density, at least by modern standards.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[tbuelna]

Interesting, from how I am interpreting here the heat removal is far more efficient than through metal to metal transfer? I have a old Cummins dump truck piston which is a trunk type. Has the ni-resist top ring land.

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.

 

[FahlinRacing]

Sorry I left out a word or two in my last post...
......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!

 

[Tmoose]

Far from the diesel request by original poster, but a 1973 SAE paper with some detail about 2-stroke mc piston temps with variations in clearance and ring type.

http://vintagesleds.com/library/manuals/misc/jennings/l-ring_effect.pdf

 

[FahlinRacing]

Glad you posted the 2-cycle Tmoose, thank you. I haven't investigated the whole 2-cycle world yet, Pretty kool test piston on page 4. I think I will look into some of that for my bookshelf.

 

[Tmoose]

There are some other 60s and 70s SAE papers, articles by the late great Gordon Jennings, and a copy of his 2 stroke tuner's handbook on that vintagesleds.com link. plenty of engine theory there, much of which has stood up fairly well.

 

[pontiacjack]

Hey Moose- I think you just spoiled my day- I didn't know that Jennings had passed away. Fortunately, though, Kevin Cameron is still cranking out great technical writing- I just read an excellent article by him about the Guzzi 500cc V8 (~1957/58).