600 Degree EGT = Best Fuel Mileage?

retmil46 · **Posted:** Wed Feb 08, 2006 1:05 am

Well, here I go again, trying to pick up clues from the Cummins crowd.

In the latest issue of TDR magazine, a gent with a late model Dodge Cummins recalled something he read in a written history of the same company. Way back when, when they were testing Cummins diesels in cars, they found that maintaining an EGT reading of 600 degrees (measured in the exhaust manifold, normally aspirated engine) gave the best fuel economy.

This same gent did a 350 mile trip, towing a Tolman skiff (no idea how big a boat that is). With this setup, 600 degrees EGT worked out to 55 mph on level ground for the Cummins while towing the boat. He managed to achieve 17.5 MPG for the trip.

Hmmm, sounds like another good reason to install an EGT gauge - figure out what temp gives the best MPG, and use it for a fuel economy gauge as well.

greiswig · **Posted:** Wed Feb 08, 2006 1:51 am

Retmil,

I'd be a little surprised if it were that easy.

For one thing, the lower the EGT, the less fuel is getting injected...so an EGT that reflects ambient temperature would be the "best" mileage. :wink:

For another thing, in addition to an EGT sensor, my Unimog has what I call a poor man's cruise control: pump is mechanical, and there is a throttle control arm in the cab that I can set and leave. The pump is a tractor pump, so it injects as much fuel as is needed to maintain a given engine RPM no matter what the load...so the accelerator pedal and this control arm don't act exactly like most accelerators. But the pump reaches its limit fairly quickly of how much it is set to inject at highway speeds. So if I'm going at a decent clip and hit any level of grade, the pump is injecting its max. I start going up a higher grade, and it's still injecting the max, but the engine RPM are coming down because the truck is slowing down. The EGT's peak at some point in there, but it is neither the top speed nor the lowest speed under that maximum injection volume. So it isn't clear to me what correlation there is between EGT and load.

So it may be that for that particular engine 600 degrees corresponds to the torque peak at some steady load, but even if that were true for the Cummins, I wouldn't expect it to apply also to our engines.

Color me skeptical...

Taz · **Posted:** Wed Feb 08, 2006 8:15 am

So I don't want to drill into my exhaust. Where can a guy get an infrared temp indicator?

retmil46 · **Posted:** Wed Feb 08, 2006 12:28 pm

Taz wrote:

So I don't want to drill into my exhaust. Where can a guy get an infrared temp indicator?

So far, searching thru all the magazines and aftermarket websites, I haven't found any advertised as using infared. Probably due to the expense. A hand-held infared thermometer from Northern Tool, with the temp range you'd need for EGT's, runs close to $300. A standard thermocouple setup runs half that or less. Westach makes a hose clamp affair that straps onto the outside of the pipe and has a fitting on it to hold the thermocouple in contact with the pipe, but I couldn't begin to guess how accurate that would be compared to actually having the probe in the exhaust stream. Same for infared, no telling how different environmental factors would affect the reading.

I'd agree that 600 degrees MAY not be anywhere close to the optimum point on the operating curve for engine load versus fuel economy. The gent with the Dodge actually called up Lyle Cummins, who'd written the said history of his father's company. He did verify the part about EGT's, when testing Cummins diesels in an Auburn automobile in the 30's they found that, allowing for conditions, maitaining a 600 degree EGT yielded the best fuel economy. However, even Lyle Cummins cautioned him that this might not apply to all diesels, especially a modern turbo diesel designed 70 years after the fact.

But during his off the cuff test, he found that trying to maintain a 600 degree temp did produce a noticeable increase in fuel economy, and that adjusting his speed and driving habits to maintain it only added 30 minutes to a 350 mile trip he'd made several times before, while towing a sailboat no less.

Further testing might prove that while 600 degrees is still in the ballpark, that it isn't the optimum temp any more even for a Cummins diesel. The engine in said pickup was non-EGR, open CCV to atmosphere, manual trans. How having an EGR system, closed CCV, auto tranny, and even the upcoming ULSD fuel would affect this is anyone's guess.

Basically, what they're saying is, is that EGT can be used as an indicator for finding the optimum conditions where the engine is operating most efficiently and giving the best fuel economy. As you said, with our setup it may not be that simple, but I think it's worth looking into.

LibertyCRD · **Posted:** Wed Feb 08, 2006 12:57 pm

This may be totally unrelated, but last year I attended a local state fair. At this fair, they had pickup truck sled pulling contests. We watched tons of guys bring their diesel trucks out there and make a go with the sled. Finally this one guy who was a veteran sled puller came out and hooked up to the sled. And when they gave him the green flag, he put his truck in neutral and hit the throttle. We watched him sit there for several seconds with black smoke rolling out of the stacks but he wasn't going anywhere. Everyone thought he had broken his truck. But then he threw it in drive and took off down the track like a beast and he ended up with a full pull.

Later that night when it was down to a few trucks to decide the winner, this guy did the same thing. He'd run his diesel engine at full throttle for several seconds before taking off. No one else did that. The guy ended up winning the competition.

The announcer later told the crowd over the loudspeaker what was going on. That guy would pull onto the track, hook up to the sled, and then he would proceed to throttle his engine until his EGT reached exactly 600 degrees on the pyrometer. Once he hit exactly 600 degrees he would let off the throttle, immediately put the truck in gear and take off. It was said that through extensive testing the guy figured up that 600 degrees EGT was the optimum temp. for the engine to make peak power and it gave him an advantage off the line that the others didn't have or know about.

Intersting... 600 the magic number?

greiswig · **Posted:** Wed Feb 08, 2006 1:13 pm

retmil46 wrote:

(SNIP)He did verify the part about EGT's, when testing Cummins diesels in an Auburn automobile in the 30's they found that, allowing for conditions, maitaining a 600 degree EGT yielded the best fuel economy. However, even Lyle Cummins cautioned him that this might not apply to all diesels, especially a modern turbo diesel designed 70 years after the fact.

But during his off the cuff test, he found that trying to maintain a 600 degree temp did produce a noticeable increase in fuel economy, and that adjusting his speed and driving habits to maintain it only added 30 minutes to a 350 mile trip he'd made several times before, while towing a sailboat no less.(MORE SNIP)

Don't read me wrong here...this is interesting, and although I'm skeptical I am curious!

The other interesting question to have asked Lyle would have been "in order to maintain 600F, did you primarily have to *back out* of the throttle, or *increase* throttle?" I would bet that he had to back off the throttle more often...in other words, he may have meant 600F or less, rather than going from "the usual 500F and increasing it to 600F." (Arbitrary number, that 500...) The added trip time indicates that, too.

Under those circumstances, I think I could read "maintaining 600F" as being synonymous with "having less of a lead foot." So I'd expect mileage to increase.

If he also tried a trip maintaining 550F and got *worse* mileage, now that would be interesting. Hmmm....

greiswig · **Posted:** Wed Feb 08, 2006 1:20 pm

LibertyCRD wrote:

(SNIP)That guy would pull onto the track, hook up to the sled, and then he would proceed to throttle his engine until his EGT reached exactly 600 degrees on the pyrometer. Once he hit exactly 600 degrees he would let off the throttle, immediately put the truck in gear and take off. It was said that through extensive testing the guy figured up that 600 degrees EGT was the optimum temp. for the engine to make peak power and it gave him an advantage off the line that the others didn't have or know about.

Intersting... 600 the magic number?

Okay, here's the skeptic again...

He'd hit 600F, then let off the throttle...at which point his EGT would drop dramatically, and according to this theory he would no longer be making peak power. Not only that, but if he *did* manage to rev the engine to peak power and somehow shift from neutral to in gear, he'd probably break or fry something. I dunno...sounds like an old Mrs. Cummins tale to me. :wink:

But here's an interesting possibility: if this actually *did* work, manufacturers would have the diesel equivalent of an O2 sensor...able to monitor the exhaust stream to look for optimal mixture. No, wait...that wouldn't work, either. There's no throttle plate on a diesel to restrict air, so all you do when you go from stop to start is vary the mixture from lean to richer. Which, come to think of it, is another reason that I wonder why EGT would be much more than a loosely-correlated factor around mileage...

FarmDiesel · **Joined:** Thu Sep 15, 2005 8:49 am **Posts:** 132

From what I've seen with my Pyro-equipped CRD, even with the pyro installed post-turbo (where the temps are cooler), and even with a freer flowing exhaust, you'll never be able to keep your EGTs at or below 600 in a CRD.

It's a moot point anyhow.

dog_party · **Posted:** Wed Feb 08, 2006 2:01 pm

I'm curious too. We need an accurate EGT gauge. I would also be very interested to find out if the mileage went down at 550 (or any other number less than 600). HmmmmMMMmmmm.

I don't see keeping my temp down that low either, but it's a good start and info I'm certainly interested in.

And, speaking of info we're all interested in, here's some info that no one will probably care about: A Tolman skiff is a homebuilt skiff/boat built using plans from Renn Tolman's book, "Tolman Alaskan Skiffs; Building Plans for Three Plywood/Epoxy Skiffs; How To Build a Better Boat Than You Can Buy." The largest is 24' and weighs in at about 1850 lbs (with motor).

retmil46 · **Posted:** Wed Feb 08, 2006 2:05 pm

Yep, as the man himself said, it may not apply to all diesels. Especially one with an EGR system that's programmed to control emissions and basically ignores the effect on engine performance (thanks EPA). On all three examples, the original 30's test, the Dodge pickup, and the tractor pull, we're talking non-EGR engines.

Farmdiesel, didn't you mention that there was a couple of times you saw a decrease in EGT's you couldn't explain? Might be you hit a point where the EGR system was throttling down on recirc.

gsbrockman · **Posted:** Wed Feb 08, 2006 4:28 pm

What about brake specific fuel consumption ?

(Brake Specific Fuel Consumption - pounds of diesel fuel consumed to produce 1 HP for 1 hour)

I'll try to find other links (primarily geared toward the evolution of the CTD 12V's, CTD 24V's and lastly the 3rd GEN CTD HPCR 24V's.

http://en.wikipedia.org/wiki/Brake_spec ... onsumption

Greg

greiswig · **Posted:** Wed Feb 08, 2006 7:13 pm

gsbrockman wrote:

What about brake specific fuel consumption ?

(Brake Specific Fuel Consumption - pounds of diesel fuel consumed to produce 1 HP for 1 hour)

I'll try to find other links (primarily geared toward the evolution of the CTD 12V's, CTD 24V's and lastly the 3rd GEN CTD HPCR 24V's.

http://en.wikipedia.org/wiki/Brake_spec ... onsumption

Greg

Good term to be familiar with. I would assume this does have a positive correlation with EGT: more fuel in for the same HP would mean more unburnt fuel in the exhaust, and less energy transfer to the piston, hence higher EGT. But is there some "nominal" EGT that reflects the highest BSFC for a given displacement? This is, if nothing else, an excellent distraction from my job. DOH! Here comes the boss...gotta look busy.

FarmDiesel · **Joined:** Thu Sep 15, 2005 8:49 am **Posts:** 132

Retmil,

I don't know if that's possible. The EGR system is there not only to ensure complete burn, but more importantly to introduce an inert gas to the combustion chamber and reduce temps therein (from what I understand). I'm not sure if the added heat (recirculating hot exhaust) counteracts the reduced heat (inert, noncombustible gas) or not???

In any event, 550 or 600 degrees isn't gonna happen.

retmil46 · **Posted:** Thu Feb 09, 2006 11:47 am

FarmDiesel wrote:

Retmil,

I don't know if that's possible. The EGR system is there not only to ensure complete burn, but more importantly to introduce an inert gas to the combustion chamber and reduce temps therein (from what I understand). I'm not sure if the added heat (recirculating hot exhaust) counteracts the reduced heat (inert, noncombustible gas) or not???

In any event, 550 or 600 degrees isn't gonna happen.

I'll agree about the temp, 600 may be the "magic" number for a non-EGR diesel, but certainly not for our CRD. I think the question now is, is there a similar point on the operating curve for the CRD, where for a certain EGT, we're getting the best BSFC, or does the EGR and other systems added on to our engines make tracking EGT for MPG an exercise in futility?

I went back and checked what you said about EGT's on the "Cherry Bomb Muffler" thread. I noticed that you said that at 70 mph EGT was running at 780, but when you sped up to 75 mph, it dropped to 700, nearly a 100 degree drop, and was at a loss to explain why.

One possible theory - at some point, due to the physical constraints of the system and/or the way it's programmed to operate, or increasing pressure from turbo boost, the EGR system is going to be at max flow. As engine speed increases above this point, the additional air flow is going to have to come from the fresh air intake. The gas being recirced thru the EGR system is going to become a smaller percentage of the total air charge being delivered to the cylinder intake. This could end up lowering intake air temp and therefore EGT.

Then again, I could be totally out to lunch.

From what I've heard over the past few months, both from some of the aftermarket companies and others, is that the EGT's on the CRD do run considerably higher than most other diesels, and they attribute this to DC making fairly aggressive use of the EGR to meet USA emission standards. Then again, they could just be making a SWAG and know as little as we do. It could just be this engine is designed to operate at these temps, EGR or not.

FarmDiesel · **Joined:** Thu Sep 15, 2005 8:49 am **Posts:** 132

Retmil,

I like that theory...if outside air reduces EGT, which I'm not sure it does??

From what I know of the EGR system on our CRDs, when there is insufficient EGR flow detected and the EGR valve is open, the PCM closes the anti-shudder valve somewhat, to increase the restriction in the 'outside' air intake, and thereby increase the draw from the EGR system.

So if there is sufficient EGR flow, the anti-shudder valve stays entirely open...and at a high enough speed, the engine should start drawing more outside air...

The question remains: is that a more efficient speed based on drag/fuel consumption, or just a speed at which the engine has cooler EGTs?

retmil46 · **Posted:** Fri Feb 10, 2006 12:48 pm

Reason that I believe outside air would reduce EGT at high flow rates, consider what temp the EGR gas is probably running at. The EGR gas cooler is water cooled, tied into the main cooling system, which runs at 170 degrees or slightly more. That means the temp of the recirc is well over 200 degrees at least, probably considerably more.

600 Degree EGT = Best Fuel Mileage?

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