diesel truck help can't decide what to get

[hudr]

"That is why diesels do not hold you back worth snot when coasting."
I may be stepping in it here, but when I step off the footfeed in my 95 Chevy 6.5TD, I immediately get some engine braking. Some of that I will attribute to the manual tranny since I notice my BIL's auto equipped truck (same motor) doesn't do it as much. But, I thought this had a lot to do w/ engine compression ratios.
Example: We have 2 tractors of roughly the same hp rating, same approx. weight, (both 2 cyl JDs). One is diesel and one is LP Gas. The LP Gas model will always "out coast" the diesel. i.e. the diesel slows at a greater rate. I always assumed this was due to the greater compression in the diesel engine. Thoughts? Comments?

[Pat]

I was thinking of typical diesel powered pickup trucks, not tractors. Anyway the diesel typically has no control over the intake configuration. It is wide open all the time and power output is changed with fueling, spraying more or less fuel.

The diesel has higher compression. OK, so what? It makes it harder to take the piston to TDC on the compression stroke but then the compressed air pushes the piston back down and ignoring friction as a simplification, you get the effort back again. In a gas (oline or propane) rig when you let up on the throtle less air gets into the cylinder because you are pulling a vacuum against the restricted intake. (Carb's butterflly or throttle body is closed.) This is what causes engine braking in a typical gas engine, doing the work of pulling that vacuum.

Find someone with a diesel pickup and coast it down a grade and you'll soon see just how little engine braking there is. This is why there are numerous after market suppliers of exhaust brakes for diesels. Ever hear of an exhaust brake on a gas pickup?

Have no clue regarding your observations with tractors, I'd have to be there. Of course when yo take your foot off the accelerator of your diesel pickup the friction of spining all those parts holds you back a little but precious little compared to a gas rig.

Side note: the Toyota folk have biased the regenerative braking of the Prius to simulate engine braking so the car feels like a standard gas car when coasting. Otherwise it woiuld coast too good and make folks nervous.

[img]/forums/images/icons/smile.gif[/img] Pat [img]/forums/images/icons/smile.gif[/img]

[hudr]

I understand the diesel having no intake control. Hence the vacuum pump (no buterfly valve, no vacuum) on a lot of diesels. Here is my hypothesis, I may be wrong, but hey [img]/forums/images/icons/grin.gif[/img]
Foot comes off throttle, pump goes to idle as far as fuel delivery, piston comes up against compression, no fuel to support increased burn so lots smaller fire, no acceleration. next piston comes up against compression, and so on and so on.
Just seem from my experience w/ diesel p/u (I started w/ a 5.7 chevy) that they slow down faster than their gas powered counterparts. Just an observation from having driven both engine types. Like I said in my earlier post, my P/U (6.5TD manual) slows down faster than my Brother In Law's P/U (6.5TD automatic) and I am sure it has something to do w/ the manual vs. auto BUT my P/U also coasts LESS than my gas powered suburban (auto) and less than my gas powered Bronco (manual). Maybe there are other forces at work here, but like I said that is just my seat of the pants, non-scientific observation/opinion. And you know what they say about opinions [img]/forums/images/icons/grin.gif[/img]

[CJDave]

[img]/forums/images/icons/tongue.gif[/img] I drove the GM early diesels from day one, and those cars and pickups definitely did coast more than their gasoline-powered counterparts. A lot of what seems like engine braking is actually due to various parasitic loads both on the engine and in the drive train. Things like compressors, oil pumps, and vacuum pumps; it all takes HP to run and it does help slow the vehicle. I used to play the "no brakes" game in the mountains and use the air conditioning system as a grade retarder. I only cut in the compressor on downgrades so I got "free" air conditioning. [img]/forums/images/icons/crazy.gif[/img] PAT....did you know that before the advent of the Jacobs system the logging trucks had 11-inch brakes, two fifty-gallon barrels of water to drip onto the brake drums, and a lot of really nervous drivers? When those trucks went down a very long downgrade, the air through the engine often cooled it so much that if would not fire the fuel when the throttle was opened for the next upgrade. [img]/forums/images/icons/tongue.gif[/img]

[Pat]

hudr, I am at a loss to explain your experience.

As regards the effort that is required to push the piston up on the compression stroke. You get that back (minus frictional losses) when the piston comes back down on the power stroke. This is true even if ALL fuel is cut off, not just reduced as in coasting.

All I can say is that in my experience, your experience is unique. I have never known of a normally aspirated gasolline engine of equivalent displacement that would out coast a diesel. Goes back to my comment about exhaust brakes being offered from several manufacturers for diesel application but I have never heard of one for a gas engine. The reason is... diesels don't have significant engine braking without an exhaust brake (or jake brake.)

[img]/forums/images/icons/smile.gif[/img] PaT [img]/forums/images/icons/smile.gif[/img]