The piston crowns are actually insulated with a very thin film of burnt exhaust gas during normal operation. That's why you can run gas temperatures higher than the melting point of aluminum. Anything that disturbs the even gas flow through the combustion chamber, such as detonation or bad injectors, is bad. The insulating film gets swept away and exposes the bare aluminum to the combustion flame. That's when the aluminum melts.

John

 

I oftened wondered how that worked exactly. Is this a boundary layer caused by surface tension and how does carbon build up affect it? The oil and a fresh air charge cool the piston but 550 psi compression has got to generate 450 degrees plus on its own. Then add boost and the temps way up there and you haven't shot in the fuel yet. It amazes me that aluminum pistons can hold up to all that pressure. I would guess that fully loaded a stock engine is pumping 700 to 800 psi for firing pressures. Imagine what a 600 plus HP 24v is seeing.

 

The gas film sticks to the surfaces by friction, forming the boundary layer over the surface which then acts as an insulator. The film forms on anything, even the carbon on the piston crown is insulated. Sharp points on the deposited carbon sometimes penetrate this film, and in gas engines, can then create hot spots causing detonation or pre-ignition.

The pressure in the chamber doesn't amount to all that much stress in the piston crown (if it's designed right), unless you get detonation. When that happens, I've seen collapsed piston crowns and crimped top rings. High temperatures are bad because they both reduce the strength of the metal in the piston, leading to distortion, or in the worst case, even melting the aluminum.

John

 

Do you have any idea how much a water/meth system will drop combustion chamber temperatures as I am thinking that for me this is the way to go since the only time the temp gets to 1200* or more is with the trailer at the top of long hills??

 

It is amazing. Pure aluminum melts at 1220F, the aluminum alloys in the pistons melt around 1270F. The trick is as stated before, oil cooling from underneath, the piston cycles in different temps.

But it is still aluminum. Getting the top hotter than engineered for will result in the top weakening until the pressure blows a hole through it. But kept to normal temps they last seemingly forever. Them engineers are thinking a lot. Pretty neat stuff if you ask me.

 

Oil cooling certainly helps, but is not the whole answer. You have two competing mechanisms that determine the temperature of the piston. The combustion flame temperature pumping the heat in, and the conduction of heat away through the piston by heat loss through the piston skirt, and oil cooling from underneath. Anything that changes one of those mechanisms changes the temperature of the piston crown. If you push more heat in, by either changing the temperature of the flame, or by destroying the insulating film, it drives up the temperature. If you cut down on the heat flow through the piston, by interfering with the heat loss through the skirt, or oil cooling on the underside, it again drives up piston temperatures.

That's why ceramic coatings on the top of the piston crown are very successful. It acts as an extra insulator to prevent heat flow. Also a few years ago there was a trend to go to cut away piston skirts to reduce friction between the piston and the cylinder bore. But that was found to raise piston temperatures by cutting down on the conduction of heat through the sides of the piston. It was a tight line for the designers to walk.

John

 

Wow, this has turned into quite the informative thread, I have definitly learned what I wanted to know plus some, thanks for all the good info ya'll!!


Chris

 

Ceramic coatings are usually applied by a flame/metalizing procedure. At least that is how we do it in the repair field. They are a pain to machine/grind. Has anyone been successful using Titanium Nitrite coatings. That is a coating used to protect tungston carbide cutting tools from heat and abrasion. I have even seen it used on automatics in the hand gun industry. IMI's Desert Eagle.

 

Water/meth will drop about 200 degrees off the EGT when you use 20-25% meth.

 

I know that Titanium nitride can be applied using plasma arc, but the high heat of the process anneals the titanium, so the properties are poor. I've heard that good success has recently been experienced with nitriding titanium at lower temperatures using a fluidized bed. That seems to work well, but I don't know if everything gets coated, or whether you can selectively coat surfaces.

What repair field do you work in?

John