Cruise control going downhill

[Jetronic]

Originally Posted By: Donald
I was on the north end of the Taconic Parkway today which is hilly. I had my Ford set to 60 mph for the cruise control. Works fine until I start to pick up some speed going down hills and then it downshifts from 6 to 5 and maybe to 4 in an attempt to keep the speed at 60. If I was driving with my foot on the gas pedal (it's diesel) I would just let it coast in 6.

I understand the logic, but I would prefer it not do that.


Mine doesn't do that, and even with cruise off it'll not downshift while coasting until I dab the brakes. It's all about the cruise control logic I suppose. But with me having a fairly light car, with relatively short gearing and diesel engine, it'll not go much over 60 in 6th unless we're talking 5-6% or more descents.

 

[Jetronic]

Originally Posted By: 440Magnum

A diesel is another matter- diesels have virtually no engine braking by reducing throttle to idle- that's the whole reason "Jake Brakes" and other retarders exist- to give a diesel better engine braking comparable to (or in the case of jake brakes, better than) a spark-ignition engine. Downshifting at idle without a retarder on a diesle will only spin the dang thing faster, not actually increase drag like a gasoline engine will when the throttle butterfly is shut.


Diesels do engine braking really well! Even without retarders or exhaust brakes.

 

[Shannow]

Mine seem to do pretty well also.

 

[Jarlaxle]

Originally Posted By: Jetronic
Originally Posted By: 440Magnum

A diesel is another matter- diesels have virtually no engine braking by reducing throttle to idle- that's the whole reason "Jake Brakes" and other retarders exist- to give a diesel better engine braking comparable to (or in the case of jake brakes, better than) a spark-ignition engine. Downshifting at idle without a retarder on a diesle will only spin the dang thing faster, not actually increase drag like a gasoline engine will when the throttle butterfly is shut.


Diesels do engine braking really well! Even without retarders or exhaust brakes.


Simply not true. Diesels have very little engine braking without a Jake or exhaust brake, because they are unthrottled.

 

[Shannow]

Originally Posted By: Jarlaxle
Originally Posted By: Jetronic
Originally Posted By: 440Magnum

A diesel is another matter- diesels have virtually no engine braking by reducing throttle to idle- that's the whole reason "Jake Brakes" and other retarders exist- to give a diesel better engine braking comparable to (or in the case of jake brakes, better than) a spark-ignition engine. Downshifting at idle without a retarder on a diesle will only spin the dang thing faster, not actually increase drag like a gasoline engine will when the throttle butterfly is shut.


Diesels do engine braking really well! Even without retarders or exhaust brakes.


Simply not true. Diesels have very little engine braking without a Jake or exhaust brake, because they are unthrottled.


I think that you are mistaking your concepts with actual experiences of people who are driving these things (diesels) every day to work.

I imagine that a road train has very poor apparent engine braking due to mass being braked...

My Nissan displays some impressive engine braking in terms of roll on and off throttle, and Jetronic similarly has diesel passenger car experience.

"Throttling" means that there is some power drag by drawing air across the throttle...but then you lose by trying to compress that vacuum.

 

[Jarlaxle]

Actually (sound of hanging curveball getting launched 450 feet), I drive a diesel truck every day.

 

[Surestick]

Originally Posted By: Shannow
Originally Posted By: Jarlaxle
Originally Posted By: Jetronic
Originally Posted By: 440Magnum

A diesel is another matter- diesels have virtually no engine braking by reducing throttle to idle- that's the whole reason "Jake Brakes" and other retarders exist- to give a diesel better engine braking comparable to (or in the case of jake brakes, better than) a spark-ignition engine. Downshifting at idle without a retarder on a diesle will only spin the dang thing faster, not actually increase drag like a gasoline engine will when the throttle butterfly is shut.


Diesels do engine braking really well! Even without retarders or exhaust brakes.


Simply not true. Diesels have very little engine braking without a Jake or exhaust brake, because they are unthrottled.


I think that you are mistaking your concepts with actual experiences of people who are driving these things (diesels) every day to work.

I imagine that a road train has very poor apparent engine braking due to mass being braked...

My Nissan displays some impressive engine braking in terms of roll on and off throttle, and Jetronic similarly has diesel passenger car experience.

"Throttling" means that there is some power drag by drawing air across the throttle...but then you lose by trying to compress that vacuum.


My experience with my VW TDI is the opposite, it has virtually no engine braking compared to an SI engine. Seeing as there's no throttle in a diesel the air going into the cylinder is unrestricted and acts like a spring (the energy used to compress it is returned as it de-compresses), the only work done is to move the air through the intake and exhaust.

In an SI engine the engine pulling a vacuum on the intake manifold which creates work.

A lot of diesels do have a throttle of some sort in the intake that's used to control manifold vacuum for emissions reasons, or to ensure the engine shuts down when the key is turned off (in case of a failure causing it to ingest and burn it's own oil). It's possible some manufacturers use this to create engine braking to control cruise speed.

 

[hpb]

Must be a Northern Hemisphere thing, every diesel I've ever driven (lots!) has had great engine braking. Try a Mazda rotary or 2-stroke bike if you want to experience poor engine braking

 

[supton]

My VW TDI did just fine with engine braking. I never checked to see if it was closing the intake flap (a device meant to both shut the engine off faster, and for helping "choke" the engine under high EGR flow), but it did just fine. I think my Camry does better, but that TDI did manage to slow down in gear just fine. Otherwise I would not have had 3/4 the front brake pad left after 175k (rears were toast at 168k though).

I have to wonder if the modern DBW's leave the throttle valve open under coast, unless commanded like in this case.

 

[Vikas]

You want engine braking? Try Tesla! That is some engine braking

 

[Silverado12]

It's nice to have that feature going across I-68 in western Maryland/northern WV.