d89's Free Fuel Mileage Tip - 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 #1 of 26 (permalink) Old 02-25-2017, 07:39 PM Thread Starter
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d89's Free Fuel Mileage Tip

Doing some testing the other day for my injection pump conversion, I learned something about how the P7100 works that can save some fuel.

When descending a hill at cruising speeds, some prefer to pull the truck out of gear and letting it idle under the assumption that less rpm equals less fuel injected if the rack is at the "idle" position. With the P7100, you will actually see reduced fuel consumption by leaving the truck in gear. When coasting with no throttle input at cruise RPM, the governor weights will move the rack past "idle" to it's parked position, stopping injection altogether.

Seeing this for yourself is easy. Next time you're coasting down a hill with your foot completely off the throttle and no if other motorists will be affected, key your engine off. You will notice that your rate of deceleration doesn't change. If the pump had been injecting fuel, your rate of deceleration would have increased. Clutch in to restart the engine as the pull coil in the FSS requires bumping the starter, and continue with your journey. Auto trucks will have a harder time doing this test due to the lockup/OD solenoids losing power, but it may still work. Couldn't say.

I don't know exactly what RPM the governor will begin to park the rack at no throttle, it probably varies depending on your GSK, adjustment, idle speed, and other factors, but mine will do it as low as 1250 rpm.




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post #2 of 26 (permalink) Old 02-25-2017, 07:42 PM
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i also noticed this, because going down a hill with revs at 800 its a lot quieter than at 800rpm idle, the turbo sounds really nice too with no other noise

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post #3 of 26 (permalink) Old 02-25-2017, 09:45 PM
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This works for almost any vehicle. It's how governor's work.
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post #4 of 26 (permalink) Old 02-25-2017, 09:58 PM
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Why would anybody put their transmission in neutral while going down hill to begin with? All you do is pick up speed and use the hell out of the brakes.
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post #5 of 26 (permalink) Old 02-25-2017, 10:11 PM
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Quote:
Originally Posted by dauntless89 View Post

seeing this for yourself is easy. Next time you're coasting down a hill with your foot completely off the throttle and no other motorists will be affected, key your engine off.
What!!!!!!!!!!!!!!

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post #6 of 26 (permalink) Old 02-26-2017, 01:08 AM Thread Starter
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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.

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Next time you're coasting down a hill with your foot completely off the throttle and if no other motorists will be affected, key your engine off.
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.

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post #7 of 26 (permalink) Old 02-26-2017, 01:08 PM
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I wondered about the injection events when going down hill or even slowing down, when I installed a Tiny Tach on my Fummins to check the dash tach's accuracy I noticed that whenever I let up on the fuel at speed or going down hill the Tiny Tach quits reading, when I touch the throttle it goes back to reading rpm's.

FYI, Tiny Tach's get their signal from the injection pulse, a sensor clamped to one of the injector lines picks up the signal.

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post #8 of 26 (permalink) Old 02-26-2017, 07:21 PM
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My engine developed an air leak and shut down. I was still able to steer the truck and through a few turns with no power steering, then tune 90* into my driveway and stop the truck.

I'm 5'6 and 170lbs, not exactly the biggest or strongest guy around.

I wonder if a manual truck would still have power steering because the engines are connected to the road.





Also in a new car you see the instant mpg goes to 99 when you let go of the gas. If you pop the clutch in or out in neutral it goes to zero.
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post #9 of 26 (permalink) Old 02-26-2017, 08:25 PM
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I wonder if a manual truck would still have power steering because the engines are connected to the road.
It would as long as you didn't push in the clutch and kept the motor spinning.

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post #10 of 26 (permalink) Old 04-06-2017, 03:34 PM
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Good info.
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post #11 of 26 (permalink) Old 05-01-2017, 05:25 PM
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OK, I know this thread is old, but I just seen it today... So yeah. I also over analyze everything, so come prepared.

Tony I am glad brought this up! The whole governor being constantly active thing was something @OK12V and I were trying to convince the world years ago to little avail.

However, I'm not sure I completely agree with the fuel saving aspect of it.

2 case scenarios:

1st. You're going a down hill and you don't hit the clutch.

Your engine will be running at the same RPM you were cruising at, say 1800RPM which obviously takes power to spin whether that power is being created by fuel or or drained from your momentum. The power to spin the engine will also be going through the differential, T-case and Transmission, so there will be friction losses there as well.

So at the bottom of this hill, you won't have used any fuel, but you will have lost speed due to the engine drag.


2nd scenario. You're going down a hill and you do hit the clutch.

Your engine will be spinning at ~800RPM, and there will be very minimal friction loss going through the transmission. You are however burning fuel to maintain your idle RPM.

So at the bottom of this hill, you will have used some fuel, but you won't have lost speed due to engine drag.


So it becomes a balancing act between fuel used, and momentum lost. It would be interesting to do testing on this, measuring fuel usage on a hot engine, and then seeing how much speed you lose with and without clutching. This would be an entertaining experiment, However simply using the Law of Conservation of Energy, we can come to a basic conclusion.

In scenario #1, the engine is spinning faster thus creating more friction, and so is the drivetrain. So by not hitting the clutch, more energy gets wasted to friction and thus heat. Now there's the argument of the efficiency that the fuel is being burned at, and whether that makes up for the extra friction. However we must remember that this is the same fuel that must be burned to make up for speed lost while coasting, making it somewhat of a null-argument.

Now, if this hill is rather steep, so riding the brakes is required, or if you'll be braking shortly after the hill for a turn or stop sign, then the speed lost while coasting down the hill has no effect on the overall fuel usage of your trip and it becomes a no-brainer to coast the engine without hitting the clutch, might as well slap the exhaust brake on as well.

We are also truck people, so it's worth mentioning that if you're hauling a heavy load up this hill, than crest the top and are going back down. If you hit the clutch letting the engine return to idle, this CAN cause the engine to overheat due to your cooling system performance being directly proportional to engine RPM.

I guess the proof is in the pudding, so do I clutch down every hill??? Heck No lol. I'm not a MPG junky, nor do I believe that the extra hassle or clutch wear is worth the cost of fuel.

Now, if you've made it this far, you're probably wondering where all this crap came from, how can this poor soul devote so much thought into such a pointless topic. The answer is simple. You try driving around in circles all day on a tractor going 8 mph. The mind wanders. And this tractor has a mechanical fuel shut-off, which makes playing around/testing really easy.

Back to the governor involvement, Tony's test makes it pretty clear that it decreases fueling when the RPM is over the RPM set by the throttle pedal, even to the point of cutting off fueling completely. Here's a vidier showing that the opposite is true as well, that the engine will increase fueling if the RPM drops below the RPM set by the throttle pedal.



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post #12 of 26 (permalink) Old 05-02-2017, 05:42 AM Thread Starter
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Losing speed due to engine drag is kinda the point. If the rate of speed loss is too great, I add just enough throttle to maintain my desired speed. I am not disagreeing with you, but I think fuel consumption at idle versus fuel consumption at ever-so-slight throttle and 1800 rpm is probably splitting hairs.

My FADEC system will have the ability to measure fuel volume consumption (gal/hr) with .5% accuracy and fuel mass consumption (lb/hr) with about 1.25% accuracy. I'll be sure to add this test to the list.

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