d89's Free Fuel Mileage Tip - Page 2 - Dodge Cummins Diesel Forum
94-98 Powertrain Discussion of components that are directly involved in the power production and all that is needed to get and keep the truck moving . Engine , Transmission Ect...NO ADVERTISING

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post #13 of 26 (permalink) Old 05-02-2017, 10:45 AM
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Of course we're splitting hairs! MPG Junkies are known for splitting hairs, and "what to do while coasting" is towards the bottom their list of tricks, so this topic by definition is splitting hairs regardless of the conversations content. I think we can agree that if you're applying ever-so-light throttle to remove the engine drag, you are effectively idling the engine to 1800 RPM, if the fuel you are giving the engine is not enough to idle the engine to 1800 RPM, then the difference will still be drained from your momentum. The difference in fuel consumption between idling the engine at 800 RPM versus 1800 RPM is at least double. I think that's a safe guess, but it will be your job to confirm with your electro-fueling wizardry.

My point in all this is simple. This rack parking phenomenon is valuable to know regardless of what you do with the information, but If someone is still willing to pop the tranny out of gear when coasting, than that is still the most MPG friendly option.

I know you're The Uppity 12V Guy, but you must remember that I'm The 10th Man, it's my job to disturb crap.



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post #14 of 26 (permalink) Old 05-02-2017, 03:50 PM Thread Starter
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I don't know if it is or not. I would think you'd have less rack at 1800 rpm with partial downhill throttle than sitting in the driveway, as the powertrain is still providing a light braking action. I mean, the slope of the hill is a huge factor so our arguments may be diverging.

I haven't practiced my own takeaway of Israeli military doctrine in a while, but your 10th Man takeaway compels me to post this:



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post #15 of 26 (permalink) Old 05-08-2017, 01:25 AM
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You pose an interesting topic, the difference in transmission drag when rolling versus sitting. After thinking about it further, you would seem to have a point. When sitting, you're powering the transmission from the engine, when rolling, the transmission is being backfed by the wheels and thus powered by your momentum. Now It would be really easy to overthink all this, so instead, I want to give you a little backstory that also ties in with my 10th man comment.

When I'm trying to learn or understand something, I try to pass it through 3 filters.

1st, break it down into smaller parts, understand each part, then reconstruct it.
2nd, over exaggerate the numbers to see if the answer is clearer, then work my way back and see if the results are still applicable.
3rd, once I think I'm right, try to prove myself wrong. This is where the 10th man thing comes in, as when I developed this method, I had no idea what the 10th man even was.

So let's do that here.

1st. With the tranny in neutral, the engine is spinning slower = less friction. The transmission is no longer having to spin the engine = less friction, and all the gears will be spinning slower = less friction. The differential is no longer having to spin the engine = less friction.

2nd. Right now we're talking 800 RPM idle, and 1800 RPM cruise. What if that was 800 RPM and 4000 RPM? Kinda makes the answer a no-brainer, you don't typically associate 4k RPM and MPG. Now it's easy to think "4000 RPM is an unrealistic example", and that's true. But what's the difference? The number is exaggerated and so is the results. If you scale that number back, how does it change in a way that makes 1800 RPM more fuel efficient than 800. Another way to exaggerate the numbers would be to change the transmissions gear, instead of OD, maybe 3rd. Again a no-brainer, however this result could be a little more flaky as it's effectively overdriving the engine by a lot, which will suffer from increased friction. Still serves as mind-candy.

3rd. I'm going to give this an honest effort. Ideas are welcome.

A) Idling down the hill uses fuel.
This is true, but like we said, it's the same fuel the engine will end up using to re-gain the speed lost by coasting down the hill.

B) While Idling, the Injection Pump is having to compress fuel for it to reach injection pressure. Compressing any substance takes work.
Answer is basically the same as the above.

C) Idling fuel efficiency is less than the efficiency while cruising down the highway, making up for the extra friction caused by coasting.
Diesels are known for there idling efficiency, burning 1/2 GPH, and because the engine is hot, and this is only a short idling sesion, efficiency will be even greater due to an increased burn temperature and warmer oil. So I don't have a definite answer, but I just don't see it. But I could be wrong.

D) Lower RPM causes the Alternator to work harder to charge the batteries increasing load.
.... Ummm, No.

E) Lower Drivetrain friction allows for the fluid temperature of the transmission, T-case and Dif to decrease thus increasing friction.
Ummmmm.... A very long hill maybe!

F) Pressing the clutch engages the throwout bearing increasing friction.
Well, first off, the throwout bearing is always "engaged" it's just not pressing that hard. Hitting the clutch for such a short duration combined with very minimal drag makes it into somewhat of a null issue.

G) With the engine coasting at 1800 RPM, the cooling fan is spinning faster and is thus pulling the truck through the air like a propped airplane also reducing aerodynamic drag.
Does it look like I know!

I think we ended up overthinking it after all. lol

My viewpoint on this is still the same. I'm not super-crazy about MPG, so it's not worth it to me to have to clutch on every hill. If someone is, then I believe that throwing the tranny in neutral on downhills is still the most MPG friendly option.


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Last edited by Cowboy303; 05-08-2017 at 01:36 AM.
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post #16 of 26 (permalink) Old 05-10-2017, 06:37 PM
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I agree more or less, using neutral downhill can get you to your destination on less fuel, but it also depends on the lay of the land. how long/steep the hill is, is there an uphill on the other side to climb, is braking going to be required, (if it is, no point in throwing it in neutral). I tried the coasting downhill in gear off the throttle yesterday, while pulling the shutoff cable, no change in rpms whatsoever. I didn't realize the p pump defueled that way till reading this thread. Interesting.
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post #17 of 26 (permalink) Old 08-02-2017, 01:40 PM
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What is the maximum airspeed velocity of a laden swallow?

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post #18 of 26 (permalink) Old 08-02-2017, 03:51 PM Thread Starter
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I forgot to update this a few days ago, I did more testing and found that the rack will only park with absolutely zero throttle input while coasting. At least with my truck, if you toe the pedal at all, but not even enough to hear a change in engine tone while coasting, injection will begin. This includes the slightly applied throttle that cruise maintains when coasting.

So, it would seem Cowboy303 was on the right track with his idea.

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post #19 of 26 (permalink) Old 08-02-2017, 10:32 PM
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Quote:
Originally Posted by Loud Pedal View Post
What is the maximum airspeed velocity of a laden swallow?
What do you mean?

African or European Swallow?


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post #20 of 26 (permalink) Old 08-10-2017, 10:47 PM
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The fully laden canadian swallow can hit around 21 mph carrying half it's own weight, but struggles badly in headwinds higher than 18 mph. However in a vertical dive, said laden swallow can hit peak speeds of up to 45 mph, with near zero input (for the speed it's travelling) and wings in a neutral outstretched position. This extra speed gained on the downward dive can carry momentum needed to nearly re-gain altitude at will. So the laden swallows speed and wing position is gravity dependent so to speak.
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post #21 of 26 (permalink) Old 08-24-2017, 06:55 PM
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Quote:
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Some people do not assume that the engine controls reduce fuel delivery below idle regardless of the system and whether it's electronic or mechanical. Particularly in the diesel crowd, coasting in neutral seems to popular because they have comparatively limited engine braking compared to a gasser without a compression brake, retarder, or exhaust brake.



I would like to assume you skimmed over this part, but you clearly included it after trimming the rest of the post out.

Life is not a Michael Bay movie. The truck will not suddenly careen out of control, flip over, and burst into flames. The engine will continue spinning and you will still have power assist for the steering and brakes. Any competent motorist should be able to recover from a loss of engine power, particularly one that was initiated on purpose.

This is not a safe test on a public highway by any stretch of the imagination. Imagine someone trying this and oops... since I shut off the ignition I am having a really hard time making this turn when my steering locks and I kill someone else or crash into a tree and since I am now in a panic I don't think to turn the key back on.

The last thing I want is to have someone who thinks chocolate milk comes from brown cows to be trying this little maneuver careering toward me.
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post #22 of 26 (permalink) Old 08-24-2017, 10:01 PM
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@Haoleb, while I see your point, there is also another way of looking at it.

When driving a motor vehicle on public roadway, there is always a chance of engine failure, and that failure is not likely to happen to in an opportune moment. It would be good to have some experience with staying in control of the vehicle in these conditions while maneuvering it safely. Being able to gain that experience on your own terms is not a bad idea. Nobody else around, straight flat stretch of road, hazards on, and because it's self initiated, there's no surprise or reason to panic.

Think of it this way, Have you ever gone hunting? Was that your first time operating a rifle? Likely not. Knowing what you're doing and having experience doing it before putting yourself in a possibly tense situation is always a good idea.

I have done the test Tony mentioned dozens of times. With a manual, you hardly notice the key's not even on, you just won't have any power if you give it throttle. Same with an auto as long as you don't let off the throttle, as that will unlock the TC. I have also had legitimate engine failures, luckily the first couple were in almost perfect conditions. The last one happened about two month ago, the truck would try to die, then be choppy, then get stuck at partial throttle, and then try to die again, the side of the road was a bluff so that wasn't really an option either. I can definitely see where someone could panic and do something stupid in this situation. Me? I was giggling like a little girl, thinking how textbook this problem was and how lucky I was to have a bucket of water in the bed.

Also, FWIW, on these 12V's, just turning the key back on ain't gonna do a darn thing, gotta bump the starter to lift the FSS.

Quote:
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The last thing I want is to have someone who thinks chocolate milk comes from brown cows to be trying this little maneuver careering toward me.
Naa Man, everyone knows to get chocolate milk, you just don't wash their udder before milking it. Now, I've tried to get a milkshake multiple times, using various different techniques, and still haven't mastered it.
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post #23 of 26 (permalink) Old 08-24-2017, 10:39 PM
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You only have to shake the udders, not the hole cow.
Feed her a bucket of strawberries a couple hours in advance if you want a strawberry milk shake.
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post #24 of 26 (permalink) Old 08-24-2017, 11:01 PM
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No kidding? I'll have to give that a shot. I was going for a more full body flavor.

Brilliance! I don't want to waste strawberries, so I wanna master the milkshake technique first. Once I do though, I'm totally going to try it.

Seriously though, DO NOT feed your milk cows onion tops. Never again.


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