EGT Gauges

[ntsqd]

:sombrero:

 

[James86004]

Curious minds want to know...

...how closely correlated is the EGT to the mixture reading on an wideband O2 sensor?

 

[R_Lefebvre]

There is absolutely no correlation whatsoever (assuming gasoline, I dunno about diesels). I have both on my track car. While it is true that EGT's tend to go down as you richen the mixture, EGT's are also largely affected by ignition timing. So two variables, giving one result, you could never go back and use that result to isolate one of the variables. I tune AFR with the wideband, and only use the EGT as an indicator or impending meltdown.

 

[James86004]

Thanks - I was assuming gasoline.

So if I want to avoid burning valves the EGT is more useful.

 

[chasespeed]

With Diesels... its pretty much opposite, though, not directly, if that makes sense....

If its rich, the temps will go UP. Adding air, lowers EGTs....

Advancing the timing will also reduce EGTs...

Load, will increase EGTs....

SO, adding a nice big set of injectors, is really only beneficial if you have a turbo that can move enough air.... and at some point, a more efficient aftercooler.... adding a little water to the mix will also reduce EGTs....

Its not really a tuning aid, but, an engine monitor... I can and do run all day towing, at 1150-1200* when towing HEAVY... no problem.... but, if the temps start to go above that, while I have no shortage of power, its time to drop a gear, or risk melting a piston, or the turbo...

I HAVE spiked my temps to about 1600* before letting off... but, its usually brief full throttle runs....

Chase

 

[R_Lefebvre]

Thanks - I was assuming gasoline.

So if I want to avoid burning valves the EGT is more useful.

Yes. Helps prevents burning valves or the turbo mostly. But, knowing where the limit is is hard. I've run over 1600F sometimes, but try to tune so 1600F is the max. That would be the death of some engines, but not for others. There are so many variables. One of them is the valves themselves. Something as simple as a 3 or 5 angle valve cut actually makes things worse. The exhaust valve gets hot from the hot gas running over it when it's open. The main way it can shed heat is by dumping it into the valve seat when it's closed. How much heat it can shed is related to how much surface contact area it has, and how long it's on that seat. A basic single angle valve cut has a lot of contact area. A 3 or 5 angle reduces it to a thin band. Also, a high performance long duration cam keeps it off the seat longer. So both of these common high performance naturally aspirated modifications make the engine less tollerant of EGT's.

When tuning with EGT's in mind, you're kind of between a rock and a hard place. Assuming everything else is the same and running right, you're balancing off the EGT's vs. Detonation. Retarding the timing to eliminate detonation increases the EGT's. That's where the Air/Fuel ratio comes in. Running richer decreases both the EGT's and the chance of detonation. But, it's usually at the expense of power. But sometimes it's all you can do.

Now, there's something else I wanted to point out. There's an interesting thing that happens when you start detonating. The EGT's actually drop. Normally there's a boundary layer of cool air blanketing the combustion chamber walls. When you get even a little detonation, the boundary layer is broken up, and the piston, head and cylinder walls are exposed to much hotter gas than normal. This is what damages them. But, this is also pulling more heat out of the hot gas charge. That is why the EGT's drop suddenly. So if you're tuning, increasing the timing or reducing fuel, and all the sudden the EGT's start dropping, STOP! This can be an indicator of detonation before you can hear it.

I tuned my car on the street, not the dyno. I don't care about the max power, I just want it to run right and be safe. I just set the AFR under boost to 11:1, and run as much timing as I can without detonation, and make sure the EGT is not too high.

Sometimes dyno tuners are too aggressive. Too close to the edge. When they're looking at that power figure... it's addictive. Result is often an engine that can only run full throttle for a 1/4 mile, or one dyno pull. Any more and it melts down.

I've been pushing 12psi of boost and getting about 240hp out of a 130hp economy car engine that has no internal mods, for about 60,000km and a whole lotta track time using this simply approach.

Sorry for the ramble, but I love combustion science and you got me started.

 

[cruiser guy]

I've been running an Isspro EGT "pre-turbo" for the last 6 years and probably 250,000 kms. I run it pre-turbo because I want to protect my engine from meltdown. I HAVE seen engines where the rings have been melted into the piston lands and I don't want the bill from that repair. If a thermocouple breaks and damages the turbo impeller I'd rather pay that bill than the full on engine rebuild. Aluminum pistons melt at 1350 - 1400 F so I keep my EGT's down to 1250 F for short bursts and less if possible.

The Isspro gauge that I have has a colored dial, green = safe, yellow = take care and red (which starts at 1250 F) = danger. A quick glance tells me where I am.

On a diesel, richer = more heat, the opposite of what a gasser does.

 

[R_Lefebvre]

On a diesel, richer = more heat, the opposite of what a gasser does.

Yes... most of the time. Diesels are running below stoichiometric most of the time. Adding more fuel puts more heat into the gases. I think there might be a point somewhere beyond the smoke limit where more fuel would actually decrease EGT? But, I don't know diesels that well and don't know if you could even get there before meltdown?

In any case, the reason I was theorizing that is because gas engines do run that way, but only when you're also leaner that 14.7:1. And obviously you can only do that at light loads.

I experimented with running very lean during cruise. The Zetec will fire down to about 18:1. Beyond that it starts to misfire. At 18:1, the EGT's were actually lower than at 14.7:1. EGT will peak somewhere around 13:1, and then start going down again.

The lean burn worked very well for cruising fuel economy, but I couldn't pass emissions that way, due to NOx formation.

 

[cruiser guy]

I know on my truck if the hose from the turbo to the intake manifold pops off (giving me a non-turbo'd engine), the EGT's hit the roof. It's easy to tell though because power drops right off too.

It's only happened a couple of times. The first time it happened I glanced at the EGT's and it was 1500 and rising fast! Needless to say I let off the skinny pedal immediately.

 

[R_Lefebvre]

Interesting, ok. I wasn't sure.

 

[craig333]

Pre turbo also. I'll also add that towing nothing heavier than my Jeep, I've never had temps go high enough that I had to back off the throttle. Sure is nice for shutdown though. Around town 30 secs is usually plenty, after a good run up the mountains it really does take five six minutes.

 

[chasespeed]

Yes... most of the time. Diesels are running below stoichiometric most of the time. Adding more fuel puts more heat into the gases. I think there might be a point somewhere beyond the smoke limit where more fuel would actually decrease EGT? But, I don't know diesels that well and don't know if you could even get there before meltdown?

No, I doubt EGTs would drop. Eventually, you will add so much fuel, the engine will lag, and possibly stall.... "Putting the fire out"... too much fuel, and not enough air....

If you ever watch diesel drags... you can tell the trucks with that problem.. they smoke like a chimney, then, at the green light, nothing.... let off the throttle, and re-apply, and good bye....

You need to lean a diesel out to drop EGTs.

Chase

 

[R_Lefebvre]

Ok, thanks for clearing that up.

Anybody have any idea why a diesel engine can't live with the same EGT as a gasoline engine? ie: gas engine the danger point is about 1600, diesel it's about 1200. I wonder if it's because of the way combustion occurs in diesel, the piston is getting sprayed with a flaming stream of fuel, effectively, so it doesn't have the boundary layer of cooler gas like a gasoline engine?

 

[chasespeed]

It has to do with pistons, heat soak, and durability. Most manufacturers, say somewhere between 1250 and 1300*...

Chase