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"in piston" combustion

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Posted by: nephron

I'd like to discuss any serious personal experience involving building a motor with in piston combustion. Outcomes, etc. Beyond the tremendous turbulance and squish-piston area, what other benefits are we talking about?

I plan on building another big block chevy, and ran across a head called the "Merlin X" 16 degree head that uses standard bbc valvetrain and has an essentially non-existant combustion chamber (looks big enough only to unshroud the valves). I can get 10-11:1 (pumpgas friendly) by dishing the pistons 25cc. What are the thoughts on how much better this would be than a large, open chamber using dome pistons.

Oppenheim's book will be a starter for me.


Posted by: MikeS

Not sure on the specs but there is one of those killer merlins in a 69 camaro. 560ci with those heads. Holley dominator, 6 speed trans ... all I can say is hold on.


Posted by: crazy4nitro

Oh This thread is gonna be good...*tunes in*....I love this kinda stuff....


Posted by: nephron

Somebody had mentioned this before, so I'm asking. I'm not trying to start a BBC thread. This is about combustion physics and...of course, philosophy.


Posted by: jmics

oldsmobile preferred flat or concaved pistons and they tended to offer better low rpm/torque figures than chevy.


Posted by: whenfoxforks-ruled

So, lowering the elevation of the combustion dome gives the engine more torque? Is the shape of the piston the important factor? Does this change the temperature of the head, block or piston? I'm not for sure on the swirl patterns, direction and what not , but a dished piston sounds like it would help this process more than flat or domed.


Posted by: 76GMC1500

It's bad to think of individual parts giving more torque or horsepower. It isn't as straight forward as that. An example, a 350 with a 400 crank (383) often makes less torque than a long rod 400 with a 350 crank (377) even though the 383 has a longer stroke. Torque is actually a function of displacement and cylinder pressure, not stroke. An engine with a shorter stroke but equal displacement will have a larger bore. The torque generated by a given force on the connecting rod will be less because of the shorter stroke, but the larger bore engine will generate a larger force for a given cylinder pressure.

This is my understanding of how things work:
On to the subject of combustion. New engines run higher compression ratios on lower octane than ever before thanks to advances in combustion chamber science. Compression ratio = power and efficiency, but it is limited by application and fuel. The current trend combustion chamber design is fast burn. GM has their Vortec heads (previously swirl port), Dodge the Magnum, Ford the Triton. All of these heads are designed to increase combustion chamber turbulence. The old swirl port heads had a step in the intake runner to generate turbulence, but turulence in the intake restricts flow. These heads made good power at low engine speeds, but could not flow enough to make the high horsepower numbers at high speeds. The Vortecs were an improvement because they left the runners smooth and straight and used the combustion chamber to generate the turbulence. The new heads have a kidney shape while the old ones were all D-shaped.


Posted by: 76GMC1500

Combustion chamber swirl allows higher compression ratios to be run on lower octane because fast combustion prevents detonation. My interpretation of detonation is the explosive ignition of fuel. If fuel combusts at normal compression pressures, it burns steadily and controlably. At high pressures, it burns explosively. When the spark plug fires, fuel burns radially outwards from the plug like a wave. As the fuel burns it expands and cylinder pressures increase. By the time the flame front reaches the far ends of the cylinder, the pressures and temperatures are already very high. The remaining fuel detonates. The swirl increase the speed at which the flame front travels, it also promotes a more homogenous fuel/air mixture. In the combustion chamber there are also pockets of fuel/air that are richer and leaner than ideal. These pockets don't always ignite right away, which means the ignite explosively later. A swirl reduces the size and numbers of these pocket.


Posted by: 76GMC1500

New combustion chambers have large squish areas. When the piston travels towards the cylinder head at high speed, the fuel in these squish areas gets kicked out into the main combustion chamber area, near the spark plug. This kick creates a large amount of turbulence, but having a tight clearance between the piston and cylinder head in the squish area also reduces the amount of fuel in the outskirts of the cylinder which reduces the time required for the fuel/air charge to combust.

This is where you D-cup/dished pistons come in. Pistons with a dish shape that matches the combustion chamber contain the fuel/air charge close to and directly below the spark plug. They also have large squish areas to generate turbulance. Also, every time the flame front hits a metal surface it has to change direction. When the front changes direction, it looses heat and slows down. A dished piston moves the crown of the piston further from the spark plug and the flame front can travel further and faster before it makes its first direction changes. Domed pistons are not ideal for this reason and should only be used to gain compression if there is no other method available.


Posted by: 76GMC1500

Most new heads locate the spark plug closer to the hot exhaust valve. Pinging is when a second flame front initiates from a source other than the spark plug. A hot exhaust valve is a common second ignition source. Putting the plug near the valve means that all of the fuel/air near the valve will be combusted before the valve has a chance to ignite it.

New heads also have deeper spark plug threads. The longer thread length gives greater area for the plug to dissipate heat into the water jacket. The greater thread length is achieved by projecting the plug furter into the combustion chamber, exposing the electrode to more fuel/air to get ignition started faster.

New pistons moved the top compression ring closer to the crown. Pistons cool through the rings into the water jacket and bringing the top ring higher up allows the crown to cool faster and results in lower crown temperatures.

Faster combustion also means the engine is exposed to the high temepratures of combustion for less time. Engines with high compression ratios and fast burning heads run cooler and don't melt down piston crowns as readily.


Posted by: whenfoxforks-ruled

Nice! So has anyone toyed with this applied to dirtbikes?


Posted by: 76GMC1500

Yes. If you compare old to new, 4-stroke motorcycles all used to have hemispherical combustion chambers with high domed pistons. Now, 4-strokes use flat or near flat top pistons with a flat combustion chamber as well. Apparently, newer 2-strokes have flat top pistons while previous generations had a slightly domed piston.


Posted by: whenfoxforks-ruled

I could be wrong but those are domed and flat, what about a dished or d-shaped,I've never seen a piston like that, I'm waiting on the plastic piston with a metal or glass coating!


Posted by: 76GMC1500

Motorcycles rarely run dished pistons because they wont be able to make the compression ratio they need. Short stroke dirt bike engines have difficulty making high compression ratios. When the combustion chambers get too small, the valves get shrouded and the engine can't breath properly. Therefore, a flat top piston must be run in order to yield the desired compression ratio.

I recently built an engine using some of these principles. It makes 9.26:1 compression and runs on 87 octane. The 350 inch Chevy makes enough torque to get a 4100lb truck with 3.07 gears and 29.6 inch tires rolling through the quarter at 15.3 seconds. If I put 3.73's in the rear, the truck should run mid-14's. The truck still idles at 600 rpm and makes plenty of vacuum to operate accessories (I have to sneak the motor through smog).

http://i24.photobucket.com/albums/c...266243806_l.jpg

D-cup pistons are exclusive to the wedge motor. It matches the combustion chamber. OHC, hemi, and canted valve motors use a different combustion chamber shape, therefore a different dish shape is also used. You will never see a D-shaped dish in a piston in a dirt bike.


Posted by: whenfoxforks-ruled

Well them pistons looked relieved for valve clearance, I'm thinking something like the head being hemispherical and the piston also, If a combustion process goes radially then go with it and blow it out the exhaust, ASAP!!


Posted by: 76GMC1500

That was an example of a d-cup piston being used uner a wedge type head.

Because of the shape of a true hemi head, a dished piston will result in too low of a compression ratio.


Posted by: Rich Rohrich

Quote:
Originally Posted by 76GMC1500
Motorcycles rarely run dished pistons because they wont be able to make the compression ratio they need.


Concave (dished) piston crowns are pretty common on late model four-strokes with compression ratios less than 12:1.


Posted by: gwcrim

Domed pistons are not really beneficial from my experience. They tend to either shroud the spark from the far side of the cylinder or at least cause the flame front to travel farther, thus taking longer to completely burn.

Top Fuel Hemis use three spark plugs now to promote full burn. From what I understand, modern engine design theory likes flat top pistons.

What is the lucky recipient of the new engine, Neph?


Posted by: whenfoxforks-ruled

I don't know if this idea could ever be applied to any 4t I've seen, but a 2t , why couldn't the combustion chamber be lowered to accommodating a dished piston? How about configuring the top of the piston to help flow out the exhaust? Wasn't that why they started with exhaust valves?


Posted by: 76GMC1500

The combustion chamber size can be reduced. When I built my 350, I used a deeper dish volume and still yielded a higher compression ratio because I went from a 76cc combustion chamber to a 64cc.


Posted by: nephron

I'm not talkin' dished pistons. I'm talking about what some call a 'bathtub', with the majority of combustion chamber in the piston. Think Diesel. A lot of them have it. I remember resleeving 4840 John Deere's at our Implement Dealer when I was in high school nd marveling at the top of the piston. It looks like when you take an avacado, cut it in half lengthways, and remove the pit. What's left is an inverse hemisphere. That's what I'm talking about. FWIW, from what I've read on various European sites (that love this design), it's this design that makes the motor loud--the 'knock' of combustion loud. Now we all know why Diesels sound the way they do as they're frequently designed this way. Why? I don't know.

From scanning Oppenheim's book and some more advanced literature, diesels are being designed that not only take advantage of the small, bathtub area to directly inject into, but also maintain a multitude of measurement sensors in which to literally affect and control the process of combustion.

Anybody got any real-world experience here?

Quote:
What is the lucky recipient of the new engine, Neph?
I'm not sure yet. I don't have a whole hell of a lot of time to dabble, and the only car I'm continually modifying right now is a 68 big block vette I've had for about 8 years. This one will be anything from a 540 to a 555 and make 700+hp on pump gas.


Posted by: KiwiBird

Diesels use the in piston combustion chamber because (at least in Cummins) they direct inject in to the combustion chamber. The injector spray pattern - determined by the number, size and angle of the injection holes, and the number of injection events combine with the shape of the piston chamber to get the best burn going. Multiple injection events is why diesels are relatively quiet now. My 5.9 liter in line 6 makes around 450 hp to the wheels with 43 lbs of boost on straight diesel.


Posted by: Rich Rohrich

Jack - You'll want to check on the Heron combustion chamber design research. It should answer a lot of the questions floating around in your head.


Posted by: nephron

Thanks, Rich. So far, I'm picking up that they have thermodynamic properties favorable to the motor, that the annular quench/squish area is superior in light of its symmetry, and that control of the combustion process is enhanced. Negatives seem to include the fact that you've got to use heavy pistons due to the thickness above the bath required for heat transfer to the rings, and the 'mass' of piston below the bath, for structural integrity purposes.

Here, all these years, I thought the heaviness of diesel parts had to do solely with the 20:1 compression ratio.


Posted by: 76GMC1500

Diesels do it to prevent fuel impingement on the piston crown which can cause carbon build-up, crown erosion, and cracking.


Posted by: 76GMC1500

Here is something on crown erosion and the dangerous consequences. Check out the picture on page 17.

http://www.gov.im/lib/docs/dti/marineadmin/valour.pdf


Posted by: nephron

Quote:
Diesels do it to prevent fuel impingement on the piston crown which can cause carbon build-up, crown erosion, and cracking.


So what's unique about diesel injection vs gas FI that requires the tub? Just the fuel? Crown area relationship to injector?


Posted by: 76GMC1500

Diesels are direct injection. Diesel is sprayed into the cylinder at extremely high pressure (5000 psi in the case above) and if the fuel particles haven't combusted by the time the reach the piston, they will wear the crown away. Fuel in a fuel injected gas engine lazily enters the cylinder with the fuel air charge and just floats around until it's ready to burn. Direct injected gas engines are a little different. They inject gas at high pressure like a diesel, but the fuel is directed towards the spark plug and not the piston crown.


Posted by: nephron

Ah, yes. Thanks.


Posted by: hot125mod

76GMC1500 that is a darn good explaination we need more of it because i could read this kind of stuff all day keep it up you guys are just full of info so its really cool to hear what your 2 cents. believe me its worth a lot more than two cents. thx


Posted by: john221ex

Rich, it's scary that halfway through the thread, I started thinking of the Heron head---and you bring it up!
Along those lines, wasn't the Gurney-(as in Dan)Weslake head similar?

John



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