Diesels and Engine Braking: Ken vs. His Transmission

[kzemach]

Originally Posted by Ford_6L_E350

Gas engines seem to inject more fuel when engine braking than they do when idling. Diesels shut off the fuel when engine braking.

Mike

Interesting. That's what I thought, although, as noted, my Scangauge still tells me I'm burning fuel on one screen, but no fuel on the other. If I ever get less lazy, I'll send them a note on that.

[geoffff]

Originally Posted by Ford_6L_E350

Gas engines seem to inject more fuel when engine braking than they do when idling. Diesels shut off the fuel when engine braking.

Mike

Mike, can you point me at references to show this? I did not believe this to be correct.

I can't find a direct reference for my 2004 Ford 6.8L gas v10 in the Service Manual other than something about a computer "Profile Correction" that happens during "decel fuel cutout ... in the 60 to 40 mph range ... likely to correspond to a freeway exit condition."

Googling for ["injectors are turned off" "engine braking"] or "decel fuel cutout" or "fuel cut off strategy" seems to turn up some discussion of fuel cut-off for gas engines on deceleration.

For instance, this Mopar PCM Modes of Operation document talks about fuel cut-off in Jeep engines:

If the vehicle is under hard deceleration with the proper rpm and closed throttle conditions, the PCM will ignore the oxygen sensor input signal. The PCM will enter a fuel cut-off strategy in which it will not supply a ground to the injectors. If a hard deceleration does not exist, the PCM will determine the proper injector pulse width and continue injection.

And this Ford Mustang 1989-1993 Repair Guide mentions using the Throttle Position Sensor for "Fuel cut off during deceleration."

These are gas engines.

Googling also turns up some other examples of people who believe the ScanGauge is erroneously showing fuel consumption (based on calculated data rather than actual fuel flow) on deceleration. This Hydrogen-Boost April 2005 Newsletter implies that the ScanGauge calculates fuel consumption from the air flow rate:

I find it important to note here that the Scangauge does not calculate actual miles per gallon. The Scangauge does a calculation based on the assumption that the air/fuel mixture is always the 14.7 to 1 goal of closed loop operation.

-- Geoff

[Ford_6L_E350]

Geoff,

Can't point you in any direction.

Just monitoring the fuel flow on my vehicles. The diesel fuel flow goes to Zero. My pickup (Chevy) shows lower fuel flow if I shift into neutral and coast than it does when I downshift.

I'm guessing that more current gas engines need to keep add fuel during engine braking to cut down on pollutants. Since the diesel doesn't have a throttle to pull a vacuum, it probably won't pull as much oil into the cylinder. So, maybe it can just shut of the fuel flow completely?

On a diesel, the Scanguage can't do the same calculation. Diesels never change the air flow, just the amount of fuel injected.

Mike

[mmocken]

Diesel engines have to cut the fuel off, they have no throttle plate to slow the airflow.

Gas engine must maintain a proper fuel/air ratio (as controlled by the O2 sensor). A closed throttle(idle circut on) will limit the airflow and the O2 sensor will reduce fuel proportionately.

Wikipedia has a good write up...and actually calls out 'cut off fuel'
http://en.wikipedia.org/wiki/Exhaust_brake

As for transmission wear, with very shift the brake band will engage/disengage. A little wear occurs. Brake band wear is a function of load during the shift and frequency of shifting. Hard shifting = no good. Slipping brake bands = bad. Get them adjusted, change trans oil and your problems should be few.

Originally Posted by Ford_6L_E350

Geoff,

Can't point you in any direction.

Just monitoring the fuel flow on my vehicles. The diesel fuel flow goes to Zero. My pickup (Chevy) shows lower fuel flow if I shift into neutral and coast than it does when I downshift.

I'm guessing that more current gas engines need to keep add fuel during engine braking to cut down on pollutants. Since the diesel doesn't have a throttle to pull a vacuum, it probably won't pull as much oil into the cylinder. So, maybe it can just shut of the fuel flow completely?

On a diesel, the Scanguage can't do the same calculation. Diesels never change the air flow, just the amount of fuel injected.

Mike

[mmocken]

As long as your trans temps are OK, you should be OK. An overheated brake is a safety issue.

If too hot, you can always pull over and let the trans cool. Also, since your engine is starved for fuel, it actually runs cooler. The will help the radiator temps and thus your trans cooler.

You should change the trans fluid following 'severe service' schedule and adjust brake bands.

I have heard of torque converters wearing out...but I have no experience with them. I would assume your torque converted is heavy duty and thus OK.

Originally Posted by kzemach

...Again, am I sacrificing my transmission only to save the brakes?

[kzemach]

OK, thanks, that helps.

[daveb]

Our shop suggests that constant shifting is not a good thing to do. They claim the same as Marty just said, especially problems with heat from shifting very often in short periods of time. They recommend down-shifting on long grades, but not in and out all the time. In other words, use the brakes when you can and when you feel a grade might be too much downshift instead. The 7.3 fleet had trannys that had constant problems. They installed a device that keeps the transmission out of overdrive until the speed reaches 50mph. After 50 mph the tranny shifts into overdrive. This helped with the old transmissions. When driving in areas (such as town) you don't want the transmission to run thru the gears and into overdrive with a stop sign in site. The same goes for driving in the hills. Thats why they put the device in, to keep us from forgetting to turn off the overdrive. This is more of an overdrive issue though. I downshift often when needed but also use my brakes for what they are made for whenever possible. Because I pull a SMB trailer most of the time, downshifting is unavoidable. Still, I try to keep it to a minimum whether the tranny can handle it or not.
Dave Boyer
(Although I would like to have electric brakes on the trailer so far I don't think this little trailer needs them).
Dave Boyer

[kzemach]

Cool, that was also helpful. Sounds like I need to be less of a downshift maniac. But you have to admit, it is fun to use no fuel on the downhills as much as possible.

[Steve C]

Thanks Dave. I just purchased a 2001 7.3 and the previous owner told me to always turn the overdrive off until I reach 50-55 mph and then turn it off when slowing to less than 45-50 mph. Sounds like Ford installed a "device" to do this instead. Guess i'll call Ford for further guidance.

[carringb]

You won't hurt the transmission by downshifting. On the contrary, it won't even let you over-rev. You can throw in into "1" on the freeway, and it will progressively downshift as you slow.

As mentioned, an already hot transmission can take longer to cool when you're also scrubbing speed. If I'm maxed out on trans temps, I'll use the service brakes for the 1st mile or two before using the transmission.

I don't recommend aggressive engine braking on a cold engine. Normal operating temps are needed for lifters and cam tensioners to be able to react to sudden surges in engine speed.

Steve - with the 7.3L/4R100, factory programming had a tendency to shift early, but then unlock the torque converter to prevent lugging. So yes, locking out O/D can be beneficial, at least when towing or just running heavy. (The gassers had a better programmer, because the V10 rarely runs unlocked). But the stock programming won't do this for you. You either need to run a tune, or add an external controller.