Reverse Combustion Chambers in a DOHC head
Reverse Combustion Chambers in a DOHC head
(OP)
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.
Thanks in advance for any ideas etc.





RE: Reverse Combustion Chambers in a DOHC head
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.
RE: Reverse Combustion Chambers in a DOHC head
RE: Reverse Combustion Chambers in a DOHC head
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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RE: Reverse Combustion Chambers in a DOHC head
RE: Reverse Combustion Chambers in a DOHC head
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.
RE: Reverse Combustion Chambers in a DOHC head
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.
RE: Reverse Combustion Chambers in a DOHC head
RE: Reverse Combustion Chambers in a DOHC head
I believe MNRaptor is thinking of a CC in the head and then matching the shape in the piston bowl.
RE: Reverse Combustion Chambers in a DOHC head
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?
RE: Reverse Combustion Chambers in a DOHC head
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RE: Reverse Combustion Chambers in a DOHC head
RE: Reverse Combustion Chambers in a DOHC head
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.