Gravity works

[Ms. Bee]

(04-05-2013 23:37)davidmbrady Wrote:  Anyone ever heard of droop? Okay, stop looking in the mirror - not that kind of droop!

Ever wonder why truckers drive the way they do? One minute they're on your tail next minute your passing them like they're standing still.

Well, it turns out this is by design. See, a good trucker or more correctly a trucker who can't pass fuel surcharges off to an unsuspecting public knows how to conserve fuel. He also knows how to get a million miles out of his engine.

A good trucker plays a perpetual motion game. This kinda game works well on the right side of this country (east of the Mississippi).

What a good trucker does is let gravity work for him. The hills are rolling so he's not afraid of running out of brake before the next up hill. He lets his speed build on the downhill and he aids it if necessary with added boost. As he progresses thru the depths of the valley his speeds may reach 70 mph and his turbo is freewheeling. He's building his momentum.

As Sir Isaac Newton taught us, momentum is conserved. The momentum built coming down the hill is given back when climbing the hill. Well, not completely given back or we'd have a perpetual motion machine and God would never allow that or we may become Gods ourselves!

So we have this head of steam coming down the hill. As the next hill approaches he lets this gift of nature propel him up. Now he feathers the boost to get the most out of his gift. He lets his speed drop slowly as he dials in more boost, but he's careful to maximize his gift. He doesn't want to peg the boost and loose all his advantage, and why waste fuel fighting air, so he lets his speed diminish to 65, then 60, then 55 where rolling resistance exceeds air resistance, then he feathers on the boost. As the crest approaches he completely backs off letting his momentum propel him over.

He dances a delicate dance. Time and again he trades off a thirsty steady speed for a frugal undulating one. Like a pendulum he swings from peak to peak letting gravity return to him a portion of the fuel he burned to make the next peak.

So, how is this related to droop? Well, turns out our cruise controls are fuel burners. If you want to save a half mile per gallon then you need to get off the cruise and drive the bus. The problem with our cruise control is that it wants to always maintain a constant speed. If you also have the jake on then it's a double whammy. That gift of nature is wasted and is being turned into global warming.

See, our cruise controls are too good. They dial in full boost at the slightest decrease in speed. If we know that we're close to cresting the hill, and we know that momentum will provide us a free ride, then we don't need full boost.

Droop is a DDEC parameter. It's the number of rpm's that the DDEC will allow the engine to fall before full boost is applied. It's programmable and your local Detroit Diesel Allison Service Center can change it for you. This modification will allow you to use cruise control to drive the hills like a good trucker without the attentiveness required. But, if you really want to do it right and gain half a mile per gallon, turn off the cruise

For the layman, David could one liken this to not wanting to apply one's brakes going downhill on a bicycle in order to have more momentum going back up the next hill?