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Old 12-24-2010, 09:12 AM   #1 (permalink)
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diesel - drive by EGT?

I've been wondering for some time -- how much should you rely on the EGT to select gears when driving for max mpg?

Of course I know that for the most part, the highest gear you can drive in smoothly will yield the best efficiency, but when watching the pyrometer while climbing hills - I sometimes wonder.

For instance, say I'm climbing a fairly steep hill at 55 mph - I can stay in top gear (4th in my case) and the engine will not be lugging, but it may be showing 900 F on the gauge. I could shift down to 3rd, maintain the same speed, and the EGT would drop to 600 F.

So is the EGT reading a reliable measure of how much fuel is being burned? Or - does the reading go down in 3rd just because the engine is breathing more and cooling the combustion chamber, even though more fuel is being burned?

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Old 12-24-2010, 11:25 AM   #2 (permalink)
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I would think a Scangauge would tell you whether EGT is a good guideline or not. But you can directly measure fuel delivery with the LOD gauge, so you would be able to directly measure one gear vs. another that way. I've found the efficiency is best between 1750 and 2000 rpm, if you've got the torque to maintain your speed.
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Old 12-24-2010, 11:43 AM   #3 (permalink)
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Quote:
Originally Posted by moTthediesel View Post
So is the EGT reading a reliable measure of how much fuel is being burned? Or - does the reading go down in 3rd just because the engine is breathing more and cooling the combustion chamber, even though more fuel is being burned?
In order to get a more efficient extraction of useful work out of an internal combustion engine, you would want a larger temperature differential between the intake and exhaust temperatures. Indeed, the equation that predicts the theoretical maximum efficiency of any given heat engine (such as your diesel) is:

n(th) = 1 - T(hot)/T(cold)

Where the Ts are both given in Kelvins (or both in Rankines).

There is a practical limit, of course, and that is the limitation of the materials making up the heat engine. You don't want your heat engine to turn its pistons into so much melted slag, nor do you want it to suddenly shatter its pistons, or ruin its valves. Therefore, there's a practical upper limit to what you should have your EGTs at.

So, to answer your question from this perspective, you are not really gauging the amount of fuel you're burning. Rather, by increasing your EGTs by shifting into higher gears, you are making your engine more efficient at higher gearing than at lower gearing. At lower gearing under load along with lowering EGTs, your engine can't get as much work extracted because of the lower temperature differential, and therefore has to burn more fuel to get the same amount of performance at the higher gear.
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Old 12-24-2010, 11:44 AM   #4 (permalink)
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Keep in mind also that for real-world conditions, your engine also has to deal with volumetric efficiency, which is directly related to the state of tune of your engine.
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Old 12-24-2010, 12:17 PM   #5 (permalink)
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Quote:
Originally Posted by t vago View Post
In order to get a more efficient extraction of useful work out of an internal combustion engine, you would want a larger temperature differential between the intake and exhaust temperatures.



So, to answer your question from this perspective, you are not really gauging the amount of fuel you're burning. Rather, by increasing your EGTs by shifting into higher gears, you are making your engine more efficient at higher gearing than at lower gearing. At lower gearing under load along with lowering EGTs, your engine can't get as much work extracted because of the lower temperature differential, and therefore has to burn more fuel to get the same amount of performance at the higher gear.
OK - thanks - I think that answers my question

So -- while in a given gear the EGT can be useful as an indicator of how much fuel is being burned, it is not of much use in deciding which gear to use.
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Old 12-24-2010, 12:20 PM   #6 (permalink)
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EGT is an indicator of when you absolutely MUST downshift to avoid damage (if it gets toward the danger zone, around 1200-1500* for many engines).
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Old 12-24-2010, 03:58 PM   #7 (permalink)
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Quote:
Originally Posted by moTthediesel View Post
OK - thanks - I think that answers my question

So -- while in a given gear the EGT can be useful as an indicator of how much fuel is being burned, it is not of much use in deciding which gear to use.
Ah.. No. You can't really estimate how much fuel is being burned with EGT. Temperature is not directly dependent on the amount of fuel. You want to keep your EGTs high, but not so high that engine parts start to break.
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Old 12-24-2010, 08:04 PM   #8 (permalink)
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Quote:
Originally Posted by t vago View Post
Ah.. No. You can't really estimate how much fuel is being burned with EGT. Temperature is not directly dependent on the amount of fuel. You want to keep your EGTs high, but not so high that engine parts start to break.
Now that - I'm not buying Driving in any given gear, give me an example of a way you could increase your EGT temps without burning more fuel? Maybe slowing the revs to the point where it's shaking like a wet dog would do it, but accelerating or hill climbing is the more typical way of making the needle climb. And either of those situations certainly involve burning more fuel, do they not?
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Old 12-25-2010, 12:13 AM   #9 (permalink)
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Quote:
Originally Posted by moTthediesel View Post
Now that - I'm not buying Driving in any given gear, give me an example of a way you could increase your EGT temps without burning more fuel? Maybe slowing the revs to the point where it's shaking like a wet dog would do it, but accelerating or hill climbing is the more typical way of making the needle climb. And either of those situations certainly involve burning more fuel, do they not?
For a spark-ignition engine, it's possible to have high EGTs as a result of a lean AFR or as a result of severely retarded ignition timing. For a diesel engine, restricting airflow on the exhaust side will definitely cause EGTs to shoot up without burning more fuel.

For a properly maintained diesel, the only thing you'd be able to tell from high EGTs is whether or not you're burning a lot of fuel. You'd never be able to tell how much fuel you're burning with any degree of accuracy, simply by looking at EGTs.
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Old 12-25-2010, 12:47 AM   #10 (permalink)
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Quote:
Originally Posted by t vago View Post
For a properly maintained diesel, the only thing you'd be able to tell from high EGTs is whether or not you're burning a lot of fuel. You'd never be able to tell how much fuel you're burning with any degree of accuracy, simply by looking at EGTs.
Perhaps I wasn't clear about my reasons for asking this question. Of course I know that reading the EGT gauge will not quantify how much fuel is being burned per mile. What I'm interested in is how it can better be used as an aid to fuel efficient driving technique.

"Whether or not you're burning a lot of fuel" is EXACTLY what it is I want to know

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