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-   2014+ Jeep Grand Cherokee Ecodiesel 3.0 (http://www.jeepgarage.org/f222/)
-   -   Engine braking (http://www.jeepgarage.org/f222/engine-braking-70136.html)

netboy 01-08-2014 12:43 AM

Engine braking
 
I didn't have a chance yet to drive down a lengthy slope and try to slow down the SFU with engine braking. Considering the general lack of engine braking qualities in diesels, I was wondering if anyone that already drove the SFU down the hilly roads has had good/bad experience and cares to share.

SNUKE 01-08-2014 05:28 AM

Re: Engine braking
 
We have had mixed results, so it both worked and did not.

I set cruise at 62kmh and started down a steep hill, it pretty quickly dropped from 6th to 3rd and maintained 62.
Later in a section set at 82kmh, it got up to 100kmh before I engaged the brakes.

Same 60 section a week ago, it took longer to engage, a lot longer.
Same issue again at 80, it just kept speeding up.

fun2drive 01-08-2014 09:34 AM

Re: Engine braking
 
Most diesels until recently had no throttle body which means there would be no engine braking. I think all new diesels have a throttle body and it does provide engine braking. You can always put an exhaust brake in place called a Jake Brake but I just prefer to use my brake pedal.
Looking forward to others regarding the engine braking but with the 8 spd ZF transmission and the algorithms it uses for fuel economy I would be it would do everything possible to not do engine braking...

crabman 01-08-2014 10:51 AM

Re: Engine braking
 
They are two different things, jakes use the valves while exhaust brakes impede the flow of the exhaust post turbo. I had little engine braking on mine when crossing the Cascades unloaded and needed to brake.

mtnmerlin 01-08-2014 11:17 AM

Re: Engine braking
 
I have noticed that taking my foot off the accelerator doesn't have the same slowing affect as does my V8 vehicles. Going down steep hills I usually have the cruise control off and use the paddle shifters to lower the gear to achieve the needed engine braking. It takes a little practice to get to the right gear.

netboy 01-08-2014 11:34 AM

Re: Engine braking
 
Quote:

Originally Posted by fun2drive (Post 956593)
Most diesels until recently had no throttle body which means there would be no engine braking. I think all new diesels have a throttle body and it does provide engine braking. You can always put an exhaust brake in place called a Jake Brake but I just prefer to use my brake pedal.
Looking forward to others regarding the engine braking but with the 8 spd ZF transmission and the algorithms it uses for fuel economy I would be it would do everything possible to not do engine braking...

I'm not sure our diesel has a throttle. The reason we get a bit of engine breaking I think might be attributed to the turbo, creating intake vacuum on low revs and the DPF creating exhaust backpressure. Combining the two should provide some reverse torque to the drivetrain. BTW, Jake Brake is not an exhaust brake but rather a compression brake. Exhaust brakes are restrictor valves in the exhaust chain and with our overly complicated exhaust system it might be a challenge to install. Jake Brake, on the other hand, is almost impossible to retrofit as it requires a completely redesigned camshafts and valve timing system. There are situations such as extremely long down slopes where the only way to descent is with engine breaking and I'm not sure our SFU is up to those (I'm not planning to do the Waipio Valley Access Road any time soon, but want to be prepared...).

f1anatic 01-08-2014 02:42 PM

Re: Engine braking
 
Quote:

Originally Posted by fun2drive (Post 956593)
Looking forward to others regarding the engine braking but with the 8 spd ZF transmission and the algorithms it uses for fuel economy I would be it would do everything possible to not do engine braking...

I am not sure whether it is different for diesels, but for gas engines, engine braking is the way to save gas. Or to be specific, coasting in gear and slowing down (without hitting the brakes). That's when your AFR goes to lean burning mode or injectors shut off altogether. Whereas when braking or placing the granny in neutral causes the engine to run at stoichiometric conditions (AFR 14.7).

Also, using the cruise control downhill doesn't seem to be the right way to use the engine brake. The proper technique was explained by mtnmerlin above.

crabman 01-08-2014 02:52 PM

Re: Engine braking
 
Never place your granny in neutral. :) lol Phone right?

netboy 01-08-2014 02:57 PM

Re: Engine braking
 
Quote:

Originally Posted by f1anatic (Post 956706)
I am not sure whether it is different for diesels, but for gas engines, engine braking is the way to save gas. Or to be specific, coasting in gear (without hitting the brakes).

It is very different for Diesels. Diesel engines do not have a throttle like gassers do. Ignition-Compression engines (gas engines) control their power by restricting the amount of air coming into the engine. If you pull your foot of the accelerator, you block air coming into the engine and create a vacuum. This vacuum causes the engine to resist the drivetrain move - hence engine braking. Diesels control their power by the rate of fuel injection. They aspirate air freely without any restriction. Pulling your foot off the accelerator in a Diesel engine will simply stop fuel injection. The engine will act as an air pump, pulling air from the inlet and pushing it to the exhaust with no substantial resistance.

MuSh 01-08-2014 09:38 PM

Re: Engine braking
 
I thought the engine braking worked great. I went down several steep grades from elevations just shy of 5k feet. Being my first diesel, it scared me for a second the 1st time it kicked in.

talltale 01-08-2014 10:51 PM

Re: Engine braking
 
I've used engine braking a lot, both with and without the trailer attached. I use the paddle shifters to to downshift. I have noticed at times I get a considerable amount of white smoke after a long down hill grade using engine braking. It dissipates quickly after I resume normal engine operation.

I've also noticed at times on steep downhill grades the transmission automatically downshifts and at other times on the same grade at the same speed it doesn't. It must be tied to the rate of acceleration, brake pressure being applied, engine RPM and torque, and how steep the grade is. At other times it acts as if it is in Hill descent mode even though it is clearly outside Hill descent mode parameters. It downshifts and applies brake pressure to hold a constant speed.

raptor5618 01-10-2014 12:37 PM

Re: Engine braking
 
I am picking one up in less than 24 hours so I will find out how well or poorly it works. I have a Jetta Diesel and it really does a good amount of breaking. I rarely have to hit the brake when I know that I am coming to a stop because the car just will slow down on its own. On long down hills just let it go and it will usually hold it back good enough unless it is long and steep. But because it works so well I am at 85K and still on the original pads.

I hope this works good enough because I drive a lot of hilly highways where I hate where your vehicle speeds up on every down hill. That is why the cops sit at the bottom. With my car I just let up, my gas usage goes to zero and I usually maintain the same speed.


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