ok so i am kinda stumped on egts

my spelling sucks lets not post about it

1. i am using alittle bit of gas job reference but i think its all relivent
2. i have a custom snowmobile and it is possable to hit 1400-1500F if your running alot of fuel to much in fact but when lean it will melt the piston around 12-1300 F
3. aluminium melts at about 1220 F and it flash melts as in now it's here now its gone

4. i may be wrong but diesel pistons are aluminium correct? so why can we see 1400 to 1600 F with out failure? in a gas motor to much fuel can keep it cool but to much diesel will run hot. so the egt readings we get must be a "flame temp" in the manifold not the true egt or the pistons would melt.

Im just looking for info here whatever you guys have let me here it im just trying to understand it all better

thanks

 

I do know that the pistons have jets of oil sprayed ? on underside to cool them.

 

Maybe partly difference between 2-stroke & 4-stroke - we at least get a real intake stroke to cool things down a bit. Also, these temps are after the cylinder.... in our case 6 cylinders feeding into the pipe... plus the fact that if egt is climbing is is because the flame is still burning in the pipe - hot gases being made hotter because the flame is still burning...

 

I don't think your pistons will very last long with those temps.

 

You sled shows high EGT's when rich because the un-burned fuel is igniting when it hits the hot pipe(s)/Y-pipe. It's much cooler in the cylinder(s).

Nick

 

I'm certainly no expert but i dissagree with this statement, egt is exhaust gas temp. so yes it is true, as we are not talking about piston temperature.

Thats mine 2 bits

 

The fuel ignites in the combustion chamber and if the fire is still burning when the exhaust stroke starts then the fire continues while the gasses are in the manifold. This always happens at high HP levels. But the fuel/air is already burning and just continues to burn outside the cylinder. It's not a matter of rich and lean or igniting on the hot pipe. It's the reason the pipes are hot. All of the available air for combustion in the manifold comes from the cylinders. The burn has already started and is just continuing in that area.

EGT is the exhaust gas temp, not the piston temp. But by the time the EGT is running at 1200* I am worried about the pistons. Don't really know what their temps are though. At least, as Thundercraft pointed out, there is a pause between power strokes for them to cool a bit with incomming air, oil being sprayed on their underside and a lot of surface area against the relatively cool cylinder wall. Still, they are under a lot of stress and 1200* EGT is my limit.


John

 

The flame front never contacts the pistons - they are too busy getting out of the way of the increasing pressure as combustion of the 20% O2\fuel mix heats up the 80% non-combustible gasses - anodization and oil-jet cooling protects them as they come back up forcing the hot gases out at which time the intake valve is opening and Boosted fresh air is blowing the residuals out the exhaust valve - if an injector develops bad spray pattern with jets of fuel, then the flame front contacts the crown and you get a serious cavity, increasing on each combustion cycle - doesn't take long to hole a piston at 3000rpm

 

You guys are confusing the difference between heat and temperature.

You can easily have more heat in a swimming pool of lukewarm water than in the tip of an oxy-cet cutting torch.

jh

 

Diesel EGT's of 1300+F should be avoided as they WILL cause damage over time.

 

Hohn,

That comparison is really not valuable when trying to discover the temperature of the pistons with an EGT of 1200*. It offers no relevant conclusion or solution.

GMCTD,

The idea that the flame front never contacts the pistons is another story that I cannot find useful in the explanation. I guess it depends on what you mean by "contact". But the heat of the expanding gas definately gets transferred to the pistons through radiation and conduction. I don't think it's totally proper to say convection in this case because the gasses are not transferring energy from another object. But, so what? They get hot. OK, how hot? Well, if they melted at an EGT of 1600* and we knew the melting point of the alloy used in the pistons we could know how far the piston temp trails the EGT.

 

I read a long article about this subject... can't remember where.

It has to do with physics and laws of latent heat, if I remember correctly. Just because a gas is a certain temperature, in this case our burnt exhaust, does not mean surrounding surfaces are that temperature, especially when we have an intake air charge coming in at a much cooler temp immediately.

1200-1300 degrees will do no harm in a newer Cummins... If that were the case, I'm sure more than half the guys trucks in this entire forum would be screwed to the wall....

 

Fair enough. I had typed a long thermo 101 post and lost it and didn't feel like re-typing it.

The short version is that exhaust temp and piston temps are only loosely correlated. If EGT at your probe is 1200º, what is peak combustion temp? Maybe 2000? But the piston is exposed to that temperature only briefly, the rest of the time you have three much cooler cycles.

Pressure affects the rate of heat transfer. The more pressure, faster heat can travel from the compressed charge into the piston. That's why you can meltdown an engine with radically advanced timing and EGTs of only 1100, but with retarded timing, EGTs of 1300 might be perfectly safe.

Exhaust temps aren't piston temps. They are BOTH products of peak combustion temps, calibration (i.e. timing), and engine loading.

Finally, heat travels much faster within a piston than it does from air to aluminum. That means that a piston that has started to melt will travel quickly from a tiny molten spot to completely meltdown.


There's more to read up on in the tech info section at www.kbb-silvolite.com

JH

 

Here is something to bake your noodle.The flame font on a Nitro burning top fueler runs at around 7000 degrees F. How does that not melt down the piston???

RPM is a big factor how gas motors can run higher egts, timing is another. More timing less egt more heat in the piston, less timeing more egts, this is do to the combustion still burning when the exhaust valves open. Fuel also burns in the exhaust causeing egts. Piston speed helps keep the heat off the piston, low rpms are useally when you will melt the piston. Well unless on NoS or something hehe

If you tune a car based on A/F u can fry it even if you are rich.

 

As mentioned EGT and piston temp are only loosely related. Peak flame kernal temps can be 3000dF or more, but that is for a split second. Aluminum also disperses heat very quickly and evenly, so the entire piston will absorb heat and will also transfer it to the oil it is covered in and to the cylinder walls. This all transfers much better than from the combustion flame to the piston. This is why you can stand over 1500dF for a period with no damage. You just have to keep it to short burst. I also agree that in diesel and gassers timing has a huge effect; both retarded and advanced can have ill effects depending on fuel levels and RPM.