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-   -   could a tow vehicle pulse and glide? (

stillsearching 02-20-2013 03:00 AM

could a tow vehicle pulse and glide?
If you have 10,000lbs hanging off the back end of a pickup, would it still be feasible to pulse and glide to improve under load mileage? This is more theoretical/wondering if at some point P&G ceases to work, due to factors like turbo spool up time, and the extra load pulling your speed down so quickly there's not alot of time gliding off your pulse...

I'm also wondering if 40mph steady state might be superior to pulsing up to 70mph and gliding down each time too if it's while towing..

Frank Lee 02-20-2013 03:18 AM

I suppose it depends on how much excess, or reserve power there is. At times my lil 302 is working pretty hard just to keep the whole mess going at cruise speed; part of the P&G thing is to work the engine hard to put it in the meat of it's best BSFC zone- probably there already. The glides would be nice and long but the pulses would be a bear and I don't think the transmission would like it.

Piwoslaw 02-20-2013 03:49 PM

Someone here tested P&G going uphill and it turned out that it still works, ie it still saves fuel even if the pulse is long and the glide is short. I guess this would work for towing if the engine would otherwise be below optimal BSFC, like Frank mentioned.

redpoint5 02-20-2013 11:43 PM

I advise against trying this for safety and reliability reasons. It can't be good for a transmission, and it's not safe to be constantly racing up to 70mph pulling 10k lbs.

As Frank said, the answer is. it depends.

Gasoline Fumes 02-22-2013 03:53 PM

I've done a little pulse and glide in a 1980 Chevy pickup (manual trans) pulling as much as 5000 lbs. But with a peak speed of about 45 MPH, not 70. Glides are loooooooooong. :D

pete c 02-22-2013 07:17 PM

I would think that it would actually work better on level ground. Your pulses would be long, but so would your glides. I don't see it being a problem for your tranny, so long as you aren't too agressive on the pulses.

stillsearching 02-22-2013 09:28 PM

Part of what i've wondered is whether pulse and glide is good for reliability even under normal conditions though. >_> It's one thing that's kept me from trying it yet on my high mileage normally babied vehicles, saving $500 a year on gas doesn't pay for a new transmission... i've wondered whether the savings in gas outweigh the potential for increased repair costs over the long run. Even just the endless engaging and disengaging of the clutch I would think would wear it out - mine last almost 200k miles when "average" is said to be 100k. I don't think i'd like putting in one every 40k.

High power output combined with heavy weight is the 2nd hardest stress you can put on parts. (the hardest are shocks, ie suddenly losing then regaining traction, stabbing the throttle then dropping it then stabbing) I would think that if the machinery were overbuilt enough it would work better, perhaps one could build a deliberate P&G vehicle with an overstrong powertrain? (one downside there being that large heavy parts take more parasitic drag to turn normally as well, hmm)

G-fumes can you expand? You pulsed up to 45 then glided down to what speed roughly?

piwoslaw - could you expand with some more details? I'm just curious how it worked for them.

It would seem that its an area worth some more research in any case - I may very well end up doing that kind of research with my Project Tow10k when I get it together. I'm deliberately choosing to make it out of 'cheap' parts that if I lunch a few it's not the end of the world, like a transmission maybe costing $400 to replace instead of $4000, same with rear axles, etc.

slowmover 02-22-2013 10:15 PM

Towing is about steering and handling safety. Predictability. The actions, the potential actions of the trailer on the tow vehicle, mitigate against anything like P&G, IMO.

It's hard enough that in normal driving the trailer is trying to pass the tow vehicle any time the throttle goes closed. Rig instability is very, very high at that point. And the actions of crosswinds, truck bow winds, etc, make these transitory moments scarier.

Fuel mpg while towing is about the proper trip plan. Known stops. Those stops themselves finessed as to parking, highway re-entry, etc. That is where the money is while towing (past vehicle mechanical baseline and equipment).

Past this are concerns about fluid temps, tire temps, etc that change with P&G while towing a heavy trailer. The coolant is taking it in the keister in every acceleration event . . so planned acceleration events (trip plan) are the things one wants.

Deceleration to exit ramp speed can start a full half-mile out via turn signal (and knowing the distance to any full-stop at the end of that) and that is where one wants to watch mirrors closely for overtaking/passing traffic when one is vulnerable (slack in the system).

Mountain downgrades are the scariest throttle-closed minutes. Very little control over the trailer at that time.

Trailer towing safety trumps all else. Loss-of-control accidents are more likely at any time when the combined vehicle is not under the tension of constant throttle.

A trip plan is a highly detailed itinerary of every action from intial engine start to engine kill. No wasted motions . . much harder than it appears. What truck drivers spend years in acquiring.


northroad 02-22-2013 11:30 PM

I team drive with my cousin a few times a month in his truck and I've tried P&G'ing with it but the glide is so short when loaded that any gain become a lot of work. Just something I tried off hand, didn't document anything.

pete c 02-23-2013 07:19 PM

I assume that team driving is in a semi? If i am not mistaken, P&G doesn't help any. Sure, gliding down hills works, but not regular cruising.

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