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

 

[Tmoose]

Gordon Jennings 1931-2000

obit

http://www.blasterforum.com/642508-post1.html

 

[FahlinRacing]

Anyone have a thought on how the crown design can affect induction and exhausting processes?

 

[patprimmer]

Well on a 4 stroke it might limit valve opening during overlap. It might also shroud airflow across the chamber during overlap. Once the piston moves say 1/4 of a valve dia down the bore I doubt it has any influence.

Regards
Pat
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[Tmoose]

In addition to affecting intake/exhaust processes a severe piston dome/crown can lengthen the path the flame must travel and change the best ignition timing. Generally requires more advance, in turn requiring a reduction in compression ratio for the same fuel octane.

http://s3.racingjunk.com/ui/2/27/26551272-313-Manley-Big-Block-Chevy-Forged-Pistons.jpg

http://ecx.images-amazon.com/images/I/51w4lXJeDKL._SL500_AA300_.jpg

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.

 

[patprimmer]

Flame propagation and airflow are two quite separate things but Tmoose is correct.

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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[140Airpower]

What Pat says is classic. Domes especially, but all piston top complications tend to dampen swirl, slow flame speed, lengthen the burn time, requiring more advance and losing power and ejecting more heat into the piston and chamber walls. But, there are always trade offs, especially with direct injection which loves a piston bowl and with squish created by piston dome vs chamber wall and other special circumstances.

 

[FahlinRacing]

I would figure tall domes would impede on flow to a point, just curious if deep bowls and motion within the chamber could influence a miss-direction of exiting flow.

 

[patprimmer]

A dome really can't impede airflow all that much unless it also has deep valve relief pockets. When the piston is near the valves, the valve lift is still quite low and unless there is a deep pocket, the piston will be well clear of the chamber in the region close to the valve seat.

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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[140Airpower]

These piston and chamber shape issues require a lot of modeling and burn visualization. Getting this right is one of the biggest contributors to the great advances in engine power and efficiency today compared to the classic musclecar days.

 

[SMOKEY44211]

Consider also top ring placement and piston crown diameter. There's 1-2% power increase and fuel consumption reduction compared to OEM offerings.------Phil

 

[SMOKEY44211]

Forgive me negleted to include that those gains are achieved by crevice volume reduction.----Phil

 

[SMOKEY44211]

Also failed to mentioned reduction in unburned hydrocarbon count.-------Phil

 

[FahlinRacing]

Would you SMokey44211 say that the crevice volume create a area where a quenching action would be created and contain most of the burn in the central crown area?

 

[SMOKEY44211]

What happens is some of the air/fuel charge gets trapped in the cevice volume. As the force of compression rises more is trapped. After ignition event still more. If we are using conventional rings, especially at light throttle, some is going to escape past the rings contaminatting oil supply. One of the reasons I'm a big fan of gapless rings. After exhaust valve opens some of the trapped air/fuel is now liberated and does burn but it doesn't provide any usefull work. If you look at a temperature map of a post combustion piston crown highest temps. are in the center with a bias towards exhaust valve side. One of the ways to reduce crevice volume is to simply put a 30deg. chamfer on the edge of the crown. As of this date on a normally asperated hypertunatic 3.5" bore piston .060" top of ring to edge of chamfer has held up with no signs of distress. Time and budgett hasn't allowed me to test to the point of destruction but any gains at this point will be very small and I don't think its worth the risk. I also think that with FEA and modern machining techniques that some steel alloy should be revisited as the parent piston material for a spark ignition engine. The weight penalty normally associated with its use could be reduced enough to make it a viable option.-------Phil

 

[Lou Scannon]

I don't know for sure but I suspect that the Cummins ISL-G stoichiometric EGR natural gas engine uses steel piston crowns, due to the high operating temperatures. Of course, it is based on a diesel engine design and operates in the same rpm range as the diesel counterpart, so the technology is not a big stretch. The cost would be higher than a conventional Ni-resist insert cast aluminum piston.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[140Airpower]

Smokie, I think the industry trend is to lighter pistons and rods. A piston is a dynamic structure where strength-to-weight is everything. Steel is always going to be more than twice as heavy as Aluminum. The penalty for this should mean heavier rods and crank and a heavier engine overall. Also, the poorer heat conductivity would make for a hotter crown, which would seem to be anathema for a SI engine.

 

[Lou Scannon]

140, your strength to weight argument for aluminum applies for temperatures where aluminum is in posession of its nominal strength. As temperature increases, aluminum's advantage is eroded, relative to steel, and eventually, is forfeit.
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

 

[140Airpower]

hemi, I certainly agree that there could be applications where steel is better. This is in fact the case for some high performance diesel engines like the Audi LeMans diesel. However, what you say about Aluminum losing strength normally applies only to the center of the crown. The rest of the piston has the full strength-to-weight advantage of Aluminum. The weakness at the center of the crown means the whole crown has to be thicker and heavier than otherwise needed, but this is mitigated in many modern designs and in racing designs, like F1, with (perhaps copious) oil cooling of a thin crown. IMVHO, steel will never be a preferred material for SI pistons.

 

[FahlinRacing]

You mean they won't bring back the Chevy Stovebolt's trunk style steel pistons? lol darnit guys. Happy New year! I have some reading an thinking from what you guys have posted. Great thoughts!!

 

[Lou Scannon]

140, can't argue with anything in your last post. Only thing I will add is, steel would be more robust against failure due to knock (i.e. due to localized high temperature & stress), but that by itself would probably not be enough justification for steel in a commmercial (vs. racing) application.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz