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Old 12-25-2010, 02:43 AM   #11 (permalink)
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Quote:
Originally Posted by moTthediesel View Post
What I'm interested in is how it can better be used as an aid to fuel efficient driving technique.
Sure - for cruising, just keep your EGTs as low as possible. Your thermal efficiency will go down, but your fuel economy will paradoxically go up.

You might also want to consider performing so-called jackrabbit starts from a dead stop, and see what that does with your fuel usage. For your engine, the slight increase in fuel usage, as compared to a more gentle acceleration, may be more than offset by the increased amount of heat energy per combustion event that is turned into useful work. This is how I am able to get well over 16 MPG out of my Dakota during city driving (the EPA city rating for my Dakota is 13 MPG). Just remember to keep your EGTs below maximum safe temperature, or you'll start melting or cracking things.

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Old 12-26-2010, 10:38 AM   #12 (permalink)
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Keep in mind that diesels aren't throttled. As a result, if you downshift and bring the revs up, the EGTs will go down, even if you're burning the same amount of fuel, as you're now pulling more air through with that fuel.
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Old 12-26-2010, 02:57 PM   #13 (permalink)
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Quote:
Originally Posted by comptiger5000 View Post
Keep in mind that diesels aren't throttled.
Usually true, though some older Mercedes and Toyota diesels actually did have throttle plates


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As a result, if you downshift and bring the revs up, the EGTs will go down, even if you're burning the same amount of fuel, as you're now pulling more air through with that fuel.
Yeah, I'm kinda thinking the same thing --
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Old 12-26-2010, 03:47 PM   #14 (permalink)
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True, but those are exceptions, and IIRC, they only used the throttle plate during warmup or something.
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Old 12-26-2010, 04:27 PM   #15 (permalink)
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Originally Posted by comptiger5000 View Post
True, but those are exceptions, and IIRC, they only used the throttle plate during warmup or something.
I used to have a Toyota 3B diesel in a FJ 60 Landcruiser, and it had a full time throttle plate controlled by a cable from the pedal. Sounds weird, but the truck still got pretty good mileage, like mid-twenties most of the time
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Old 12-26-2010, 10:14 PM   #16 (permalink)
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I did a little research, and while strange, it sounds like that was done in some cases for either EGR, or to improve shutdown smoothness, and possibly for engine braking (like an exhaust brake on the intake). It's not common, but Toyota, Nissan, VW and Mercedes have all done it at some point, mostly on old designs.
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Old 12-27-2010, 12:10 AM   #17 (permalink)
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Just 2 things in my mind to add to your conversation, egt, high exhaust piping temperature is good because it helps to expel gases out(hot, less dense, it's one reason why some cars use steinless steel in the exhaust, to keep the temperature high, also steinless steel it's relatively lighter than other material), but if the gases themself are coming out from the engine very, black and dense, it's bad, it means that some fuel is being wasted(not burned in the engine to build mechanic energy) hot it's bad, it may mean that your mixture is too rich(too much fuel burning in the exhaust), or your engine needs to be inspected(intake, dirt on egr, turbo not working good, air leak in intake pipes) or injection needs to be set properly(timing may not be good, dirty nozzles, etc).
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Old 12-27-2010, 08:49 AM   #18 (permalink)
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Keep EGTs low and combustion temps high. Some guys install custom higher temp (203*) thermostats to at least keep the coolant temps high. I used to drive by EGT when I had the old '96 F250 PSD and a set of gages. I tried to keep it below 600* at all times while unloaded. My Dodge 24v 5.9 is currently gageless (!!!).

Keeping the RPMs on the high side while towing and towing uphill will keep the EGTs down, vice keeping it in the peak torque and most fuel efficient area (around 1750 rpm).
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Old 12-28-2010, 05:52 AM   #19 (permalink)
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Quote:
Originally Posted by 99metro View Post
Keep EGTs low and combustion temps high. Some guys install custom higher temp (203*) thermostats to at least keep the coolant temps high. I used to drive by EGT when I had the old '96 F250 PSD and a set of gages. I tried to keep it below 600* at all times while unloaded. My Dodge 24v 5.9 is currently gageless (!!!).

Keeping the RPMs on the high side while towing and towing uphill will keep the EGTs down, vice keeping it in the peak torque and most fuel efficient area (around 1750 rpm).
Keeping intake manifold pressure low, and exhaust gas temperatures low is what works on an empty, solo, Dodge. Rpms between 1,700-1,900. Roughly, if one can maintain 5-psi and 600F then fuel economy is high. (Or, an average speed [elapsed time derived] of 27 mph or higher).

Loaded and towing is different. Cummins tells us that 2,150 rpm is the balance between economy & power. A diesel in a truck is meant to work, so low rpms are not the way to go when loaded/towing AND off the flatlands. Road, load and speed have to be understood to find the best balance.

I would say EGT is an aid to understanding how to efficiently drive a diesel.

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