Hello all. I just finished another one of my commutes from Southern Cal to Southern Oregon. I've been monitoring this oddity for several trips now. This may seem like a dumb question, but I noticed it several times. I have made sure I was not in a DPF Regen cycle the last three trips. I have noticed my MPG drops while the engine brake is turned on going down long grades. Does the 6.7L engine burn diesel when the engine brake is engaged? If anything, I would have guessed the MPG would have actually increased thinking that for all intents and purposes I was sorta coasting, foot off the pedal so to speak. Not that it's a big issue. In my much younger days, my cars got the best MPG's coasting in a "poor kids" overdrive at the time.

 

Engine brake needs RPM to work well so it stays locked to the drivetrain instead of coasting. More RPM = more fuel usage. Makes sense to me.

 

but coasting still yields better fuel economy (engine brake is just like using brakes as far as fuel economy is concerned).

 

I've noticed it tends to run 1-2 gears lower when I leave the engine brake on. Even cruising flat.

 

Engine braking is achieved by either allowing the compression stroke to go right out the exhaust instead of that force being returned to the engine and keeping the crankshaft rotating, or a simpler design of just closing off a valve in the exhaust causing extreme back pressure lugging down the engine. Both methods are designed to waste the energy from the PowerStroke (pun intended) of the engine.

If the computer actually takes into account engine efficiency, then I could see it registering lower fuel economy because the engine is essentially lugging under extreme load.

 

It should be using less fuel... Foot off the feed the diesel should go full lean and use almost no fuel. The exhaust brake on your truck does no more than block the airflow out of the engine. The engine is a big air pump, close off the flow and you get drag from the engines compression strokes.

 

When the exhaust brake is commanded by the engine computer, the vanes of the turbocharger hot side are closed. This forces each piston to push air against the closed vanes during the exhaust stroke, creating backpressure and a braking force.

No fuel is used during the exhaust braking sequence, but I agree that the transmission shift map seems to be different and could account for the change in fuel mileage..

 

While in theory you're right, however if the PCM tells it to run in lower gear, higher RPMs cause higher fuel consumption. Like I said, I noticed mine runs a gear or 2 lower when I run with the brake on. Running flat, my truck will cruise at 53mph in 10th. With the brake on, it'll run in 9th r 8th.

 

Well not really. The way diesels work, when they drop to idle for instance they can run super lean and use virtually no fuel. This is why they wont warm up at idle, they need some throttle or they use almost no fuel to run. The few hundred rpm increase should not cause anywhere near a 1-2 mpg change as reported, even a 1000 rpm jump with the throttle closed should not change fuel consumption by any measurable amount. Maybe it is but I do not think anybody has actually hit on "why?" yet. Now when the exhaust brake engages during warm up it forces the engine to burn more fuel to stay running against the back pressure, maybe the 6.7 is programmed to burn fuel with the throttle closed and the brake on, even when not in warm up mode?

 

You are right it does seem to hold gear’s longer.

But the op’s question was when coasting down a long grade with foot off of the throttle his fuel mileage appears to decline.

I do not have that long of down grades to have noticed that.

 

I always run the exhaust brake, EXHAUST BRAKE, it is not actually an engine brake, all braking affect is on the exhaust side of the turbo, outside of the engine.

I have a long downhill stretch along one of my routes, run it often.
As to the fuel economy dropping off while rolling down the hill on the exhaust brake, getting to the top of the long downhill stretch means that the truck had a long uphill pull, and the result is the fuel economy dropping off after cresting the hill and going down.
Sometimes it will gain back a tenth just after bottoming out and starting to pull going up the next hill(smaller hill), then loses that as the pull evens out.

Now, on my previous truck, a RAM Cummins, it would always pick up a tenth or even two going down that same hill on the exhaust brake, and might lose a tenth going up that next hill before evening out. The exhaust brake on the Cummins is much stronger though than the PowerStroke.

It is just a difference in setup and tuning.

But, the exhaust brake blocks off the exhaust flow, so it is not fuel efficiency friendly, but the fuel is cut off when rolling with the exhaust brake engaged, so it isn't using extra fuel, so at best it would be fuel economy neutral.

Now, I will say that I never run in tow/haul mode, ever, not when towing or when bobtailing, if you do then that may make the truck downshift sooner on the exhaust brake engagement.

 

Lots of great responses. The most curious is the "exhaust Brake" vs. "engine Brake". This is my first diesel engine and to be embarrassingly honest to admit, the damn thing scares the heck out of me when I pop open the hood. If the CEL light ever came on, bout all I could do would be to open up the hood to verify, "yep, I checked- the engine is still there", then close the hood. My weak understanding and a greatly ignorant over simplification of the braking types, engine vs. exhaust, is that with an engine brake the compression stroke is used to slow down, that's why I down shifted, grant you that was a 460cid backed by a 4 speed manual shift trans. But if the turbo needs RPM to work, I am not really understanding how, when the fuel component is removed from the process how does it cause excessive back pressure to slow. I kinda understand the exhaust brake valve concept. Isn't that what the old "Jake" brakes were, a butterfly kinda valve in the exhaust stream that when closed create back pressure.

 

The Jacobs brake system is an engine braking system. The exhaust valve is opened at the top of the compression stroke allowing all the power created to escape out the exhaust.

An exhaust braking system is a plate in the exhaust path (much like a throttle body plate on a gasoline engine) that can be closed, forcing the engine to choke down.

Smaller diesels in lightweight applications use exhaust brakes. Semi trucks and heavy duty applications use an engine brake/engine retarder system.

 

Mine cruises and tows in 10th gear even with the exhaust break on.

 

I reluctantly read some of the owners manual online last night, since I don't have my vehicle yet (T minus 10 days?) and it sounds like there are a couple of modes for the exhaust brake, depending on if you are in tow/haul mode. It's worth a peek.