Well, it's my understanding the jake brakes on trucks are engine-speed
dependent.   So, the truck is shifted to a gear that spins the engine at
maximum effective braking speeds (using the jake brake)...I could be wrong,
since I have never driven a big rig nor ridden in one descending a grade.

 The truckers in Mexico are not required to have noise suppressors on their
truck brakes, I don't think.. When one comes to a complete stop with the
Jake Brake still on, it sounds like a jackhammer...The logging trucks
around my house, they like to bypass their jake brake mufflers and wake us
all up as they descend out of the woods at daybreak ( beginning around
3:30am in high summer) using their gears and compression (or "Jake" in
slang) brakes to control their speed.

  Wiki is sort of right, I guess...but my Ford turbo diesel powerstroke
diesel has plenty of engine braking....No, it does not 'create' intake
vacuum...it has a vacuum pump to power- assist the brakes.   But it still
engine brakes....I've towed an 18,000lb load, in the Ford turbo diesel,
over Sonora Pass in the Sierras and descended in a very low gear without
using much brake pedal at all......no jake brake either...

On Sat, Sep 14, 2013 at 10:55 AM, Ben <syncro@gmail.com> wrote:

> On Sep 14, 2013, at 8:37 AM, Don Hanson <dhanson928@GMAIL.COM> wrote:
>
> > That is why we encounter very slow trucks on the downhills
> > as well as the climbs....They've shifted to a low enough to hold their
> > trucks from gaining speed...and thereby they can control the heat they
> > create with their brakes.....
>
>
> If this statement is about very large trucks such as diesel rigs, this
> would be less than an accurate statement. Excerpt taken for Wikipedia which
> is accurate enough for this discussion.
>
> "Diesel engines do not have engine braking in the above sense. Unlike
> petrol (gasoline) engines, diesel engines vary fuel flow to control power
> rather than throttling air intake and maintaining a constant fuel ratio as
> petrol engines do. As they do not maintain a throttle vacuum, they are not
> subjected to the same engine braking effects."
>
> So, NO. Those trucks are not downshifting to slow down. They are using
> alternative mechanisms such as jake brakes. Ever notice that the diesel in
> your Vanagon & other VW's have an auxiliary pump to create vacuum for your
> brake booster? Same deal.
>
>
>
> BenT
>