Reverse Combustion Chambers in a DOHC head

[MNRaptor]

I work exclusively on mitsubishi 4 and 6 cylinder turbo engines and I have a few questions concerning combustion chamber and piston design. The stock combustion chamber on the engine is a pent roof design and the typical piston design used to date in these motors has been a round dish design with standard valve reliefs cut in. I want to try using a dish that is a reverse of the combustion chamber as opposed to just being round. My thoughts were that it would increase the quench area on the piston and if done right remove the edges associated with the valve reliefs. The other thing I would like to do is raise the top ring land closer the the crown to reduce the amount of dead area above it. My question is just to see if anyone has any input on design considerations etc that I should watch for or other ideas etc. The majority of my customers are drag racing as opposed to street or endurance use.

Thanks in advance for any ideas etc.

 

[Warpspeed]

Moving the top ring up may not be such a good idea in a turbocharged engine. If the engine detonates it is usually the ring lands that are the first things that break.

If a top land comes adrift, there may be broken pieces of piston and ring that could get trapped in the quench space causing catastrophic damage.

At least if a lower ring lands fracture, the broken pieces usually cannot escape into the engine.

 

[MNRaptor]

Thanks for the input, I had considered that, the typical pistons for these motors at the moment have the ring at .300" below the crown which is good for an all around engine, but seems a bit much for a drag only setup. Detonation is a concern only to a point, The engines are tuned for very little to no knock and running very high octane fuel only. My bigger concern was that heat would be more of a problem for the ring at the higher location. I can run a bit wider gap to prevent some of the issues there and I was also thinking there should be some benefit from the different dish design possibly lowering the requirement for advanced ignition timing etc. Considering that information, has anyone done any work with piston design for this type of application? Is there enough of an advantage to moving it up to justify it? I am basically trying to get the most efficient piston design for this application with as few revisions as possible considering the turn around time for custom pistons as well as cost. All input is welcome related to the questions of course.

 

[DesignOutsourcing]

Raising the top ring will give you more metal under the top ring (which is where they break with high boost as you probably know.) I can't think of why you would need more metal above the ring than is needed to pull the ring down (and perhaps "shelter" the ring from combustion temps.)


Why are you concerned about the "dead space" above the ring? As a percentage of the expanding gas in the combustion chamber it is very small.

As for the better quench, if there are flat areas around the combustion chamber (that are within the bore), it would be good to have matching flats on the top of the pistons.

hope this helps. cheers, derek

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[kenre]

Have you looked into using the Dykes type ring as they are situated at the very top of the piston. Also called a head land ring.

 

[EngJW]

The trend for the last few years has been to move the top ring upward, .250 inches or less, but the reason has usually been to reduce the crevice volume which contributes to emissions. It is an extra advantage if it is a new engine design and you can shorten the piston and decrease the deck height.

If you have a standard size of ring you might have some options. One is to convert to a nitrided steel ring. They withstand the heat better than ductile iron rings with moly fill. They are also thinner and lighter, although this reduces inertia loading at the expense of less heat transfer area.

 

[Rob45]

Moving the top ring down is a good and commonly-used way of reducing the operating temperature and therefore increasing the life of the top ring.

Putting the combustion chamber in the piston top is likewise old technology: it was called I believe a "Heron" (Herrin?) head or Heron combustion chamber.

 

[richdubbya]

Moving the top ring land up on a turbo engine will give no measureable difference in power output but will increase the possibility of burning a piston edge, especially near the valve cutouts. Dishing a piston on the intake side is counter productive to combustion efficency IMO.

 

[rpmag]

A Heron head ala Ford 'Kent' and Brabham F1 engines ustilsed a flat head face with no combustion chamber and a combusion chamber in the piston. Perhaps best suited to low speed diesels?
I believe MNRaptor is thinking of a CC in the head and then matching the shape in the piston bowl.

 

[bettonracing17]

Volkswagen VR6 (12v not sure about 24v) also utilize the flat combustion chamber and dished piston setup.

Concerning raising the top ring: A probelm we were running into on a N2O Pro Mod motor is the low heat capacity of the upper ring land (i.e. the low mass relative to other areas on the piston), creating a hot spot... Ceramic coating cured the problem temporarily but this is a N2O motor... If u could somehow apply a forging to the piston face (read: more proce$$ing and turn around time) it would also alleviate this problem.

I see another application for temp-driven FEA... Maybe try simulating your current pistons and compare to actual results?

 

[patprimmer]

The Chev "W" head I think had a flat head and a piston top that was at an angle to the deck.

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[strokersix]

Also, remember that the heat transfer out of the piston through the top ring should be close to the water jacket for least resistance. Moving the ring up will put it into the deck surface and farther away from the cooling water.

 

[EHudson]

You mentioned valve reliefs cuts in the standard piston. In raising the top ring groove you must be sure to keep from cutting into the valve relief area. A minimum of 1/16" of material between the relief and the groove at the closest point is recommended. You almost have to make a CAD model to check this.

The top land must be stout enough to transfer heat away so that it doesn't melt. Some non turbo passenger cars used 3 mm top lands but the trend is to use a larger land for durability even in non turbo applications. A turbo engine piston may see higher temps and thus need a larger top land or in extreme cases a ring groove insert. Consider spraying lots of oil at the piston from below.