I've always wondered why adding extra fuel to a diesel engine increases the temperature of the exhaust gas. Well after a little research I came across something a little fascinating.

A quick Google search yielded that during the compression stroke the air can reach around 600 degrees Celsius, roughly 1,100 Fahrenheit. Awful close to 1200 isn't it?

Well upon further research the actual combustion temperature of a gasoline engine can reach 1,000 degrees Celsius, a diesel on the other hand can reach 2,500 degrees Celsius.

Adding extra fuel to the combustion chamber has the following effects,

It actually cools the combustion temperature because it's more cold fuel soaking up heat

It then takes this heat it stole and carries it out of the combustion chamber and wears on the valve, exhaust port, manifold and gives you a high egt reading at your probe.

So I'm going to go out on a limb here and say adding excess fuel does not burn holes in pistons, but I imagine it can ruin heads, valves, and manifolds once it reaches an appropriate temperature to affect steel.

I suppose the oil jets under the pistons must make one hell of a difference, some early diesel engines never had them..

I imagine every pyrometer company will sue me now that they don't have a fear tactic to keep food on the table.

Holes in pistons you say? I say blame defective injectors that pour a stream of fuel thus disrupting the bubble shield of gases on top of the piston.

DISCLAIMER: Everything here is simply my opinion and uneducated guess, you're welcome to do your own research before trusting me. Here's a simple article for starters

DynamicMenu

I'm going to throw my pyrometer away now..

 

Just thinking outside of the box, I'd say that the majority of the heat comes from compression of the exhaust gasses, which forces it thru the valves (which subsequently affects the manifolds), and the down-stroke makes a negative pressure that aides in the cooling. I'm not convinced the "cool" injected fuel affects the chamber temp as much as the negative pressure in the chamber would, but I'm sure it helps. Remember, the fluid is compressed to hundreds of PSI in the lines and injector, which probably increases its temps a bit relative to the IP reservoir.

 

A pyrometer is just a tuning instrument like a timing light. How else are you going to know how much to turn up the screw on your IP?

 

Injectors can practically **** a stream of fuel and perform well enough, arguably a less atomized mix can offer greater max power output at the cost of efficiency. The case has been made in various builds and threads. I fail to understand how a stream of fuel from an injector is going to "disrupt the bubble shield of gases on top of the piston".

 

My point was I don't see how egt are even relative at this point. If the cylinder has a 2,500 degree celcius explosion in it then why even worry about an egt temp that's much further below it.

 

Because the EGT temp is relative to the average air temperature in the engine. What you mention is well known, this is why the super high hp pullers and what not literally black out the lights. They have no intention of burning all the fuel, the fuel is for keeping the valves and such from turning to liquid.

If you dont think running over 1200 egt is bad thing, give is shot, then let me know how big the bill is, im curious. Or you can find the vid of the duramax running at 1600 on youtube and talking about how bad *** duramaxes are right before something REALLY bad happens. Id doesnt say in the vid, but my guess is he cracked a piston, sucked a valve or something. Or you can call a diesel shop, and ask them why theyre alwasy working on cummins.

 

Duracraps use an aluminum head, they are already well known for warping under normal circumstances which in that case I could see why you would want to keep the temps down. But in an all steel motor minus the pistons I'm sceptical. And besides why pick 1200 as a safe number? If aluminum was anywhere near 1000 degrees it would be soft enough for the wristpin to break it. I just don't see the point of the magic number of 1200

How can pushing 1500 degree air out of a cylinder be any more hard on a piston that see's 4500 every single power stroke.

An internal combustion engine by design is supposed to transfer heat exceptionally well. And they do exactly that.

I'm sure with enough added fuel and or boost cylinder pressure could blow a head gasket or deform a cylinder wall if it was thin enough but melting pistons. I just don't see it.

Auber inc makes a thermocouple adapter that is the right thread for the glow plug hole. I'd be curious to see what kind of temps it would show if the response would be fast enough..

 

What is not really visualized in that example is Energy. When you burn 20cc of Diesel, and get the 4500 degree in cylinder temperature. You are forgetting that relates to a given amount of energy. More energy in the form of heat, and more heat will be retained as the engine can only get rid of so much heat in a given moment. Convection is rather inefficient, which is why they are cooled with water and then air. Still this process is not perfect, and more energy again will mean more heat.

As for why a Piston or Valve might be the first to bite the bullet, it comes down to wear and size. The Piston is already moving up and down which creates friction, even with oil. There is still some friction and heat from compression, do that thing, then do it 2,000 times a minute and you see some energy building up. Add more heat and they will be the parts that achieve the highest temperature the fastest. This is due to heat being built up on them first, and no giant mass to radiate heat through. Pistons and valves are small, picture the energy required to melt a paperclip vs the energy to melt a hammer.

The damage to Pistons and valves is not a true "melt" as you might think either. It is more of an erosion (they even kinda look spongy around the hole that is made) as the heat slowly breaks away the weaker portions of the Piston surface (typically under the exhaust valve due to the Intake rushing cold air onto a portion of the piston surface). This leads to further erosion and eventually your "melted hole", which might even look as if you took a torch to it, because eventually you might as well have. Having seen engines with "Piston holes" I can safely say that in every case I have seen, Gas and Diesel, the culprit was too much power, without sufficient venting. NOS does this by allowing you to squeeze more fuel into a cylinder with the extra Oxygen charge.

 

Exactly, its not that they literally turn to liquid. If you look at a piston or a valve thats been to hot it looks like its been soaked in acid, or was casted in a poor cast. They get very porous, and also brittle. It really isnt very hard to burn an exhaust valve in the wrong circumstances.

 

If you know so much then why not do some testing for us and share the results? Run your engine at sustained 1300*, 1400*, or higher until you have a failure then do a postmortem writeup on it.

Personally I'm happy to heed the vast amount of advise and reports already available regarding at what EGT damage becomes likely in various diesel engine designs. 1250* is generally regarded as where damage begins, maybe higher for brief bursts but not for sustained periods. Pistons do melt, valves to overheat and get damaged. Valve seat contact area is a critical measurement when doing a valve job, for exhaust valves in particular, as it is the primary means by which the valve is cooled. Also, I've seen a melted turbine wheel. Not sure what the engine it came off of looked like, that one was likely rode very hard for a sustained period of time and I suspect had internal engine damage as well.

If you're going to make extraordinary claims, then put your money where your mouth is. The article you linked isn't particularly relevant. Some reading material:

https://books.google.com/books?id=js...D8gQ6AEI6AEwGQ

Why EGT is Important | Banks Power

http://www.trucktrend.com/how-to/exp...uth-about-egt/

 

Ever seend and idi piston with a 3 inch crack......... Ever seen 6 in one engine?...... My first idi I ever owned was a loggers truck idle all day or pulling like hell. Overheated cylinders makes pistons brittle. This truck had a mere 232000 kms on it and was on its third engine when I got it.

 

Yep. It is about the volume of energy produced by the fuel, no so much the temp that the fuel burns at. The volume of heat energy cannot be dissipated fast enough. It reaches thermal capacity and bad things happen.

 

EGT goes down with distance from the cylinder.
Pre turbo 1250 is the calculated max EGT that is safe.
Post turbo that temp is 950 degrees.

The pistons are not pure aluminum, but a silica aluminum alloy so they can stand the combustion temps in the cylinder.
Since that number is a calculated number that accounts for the block and head mass, cooling system, oil jets and the air flow it is not an exact number.

But how much heat do you want to put in your oil? 300 degree oil does not provide much lubrication and sure is thin to keep bearing surfaces off the crank for example.

If your cooling system is a little weak, do you want to be sitting on the side of the road with a boiling radiator? I prefer to let off a little and be driving down the road for many miles.
Then the next question is how often do you want to rebuild your engine?

12 or 13 or 14 or 15 or 16 hundred degrees do not mean instant catastrophic engine failure, but you can rest assured that extended high EGT will mean your engine will need oil changes at more frequent intervals and rebuilds will be required at much shorter intervals than normal.

 

Very professional answer there Dave, definitely food for thought

I'll have to get an oil temp gauge and see how much it corresponds to egts.

My main problem is the magical 1200 number which is the melting temp of aluminum and just that, no extra though went into that calculation and people are so religious about it..just wanted to bring up many variables that aren't well known to some people.

 

In my mind, try to keep the EGTs (pre-turbo) under 1,000 for extended periods of time.
For a few seconds(<30)? I doubt 1600 would hurt anything. Remember, it's not the gas temps that matter, it's the "heat soak" effect of the gas temps going into what's around it(valves, pistons etc).
This is why you can run it hot for a short period of time without hurting anything, but if you run it that way for minutes on end... something's going to fail.

Now, in my experience, high EGTs correspond with 'rich' fuel ratios(for a diesel) and smoke. Keep it lean, keep it clean, and the EGTs will stay low.
But, you say, leaner mixtures will result in less power!
This is true, but the solution is to have /more/ of a leaner mixture. So... Shove more air in it!
This is why you can have a N/A engine push 1100F and only make 120 at the wheels; turbocharge it and you can get 200 at the wheels... with 1050F EGTs(so slightly less stressed).
Same with even more power - Just make sure you have more than enough air to burn it, keep the timing correct, and you'll be able to push a lot of power through without creating high EGTs to worry about.
With my '88 and Banks turbo hauling a big trailer up a long grade through Washington, I found that I had to back off a bit... Not because of EGTs, but because the water temps were climbing past 220F.
(Now, maby I didn't need to, but when you never see more than 210F /ever/, and suddenly see 220F...). Again, though, that was basically maxing out 200-250RWHP of fuel through the motor, for several minutes on end. Even backed off a bit, I was still pushing far more than a N/A motor at full tilt!