Date: Sat, 14 Sep 2013 14:42:16 -0500
Reply-To: mcneely4@COX.NET
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Dave Mcneely <mcneely4@COX.NET>
Subject: Re: Why Bigger Brakes Was Cause of brake rotor warping
In-Reply-To: <R7Yh1m00G08X5Fr017YidN>
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http://en.wikipedia.org/wiki/Compression_release_engine_brake
If Wikipedia is an acceptable source, then this sounds authoritative to me. So, seems that a diesel engine has little or no engine braking capacity in its raw form, because it gives it up as the pistons move to compress the air previously entrained, creating a spring effect. The engine does brake the vehicle, but then it accelerates it, obviating the braking effect.
What jake brakes (more generically termed engine brakes or compression brakes) do is release the air before compression, preventing the accelerator effect, thus conserving the engine braking.
So, diesel trucks do indeed have engine braking.
mcneely
---- Scott Daniel <scottdaniel@TURBOVANS.COM> wrote:
> Diesel engines do have a fair amount of engine braking affect, just not
> as much as gasoline engines do, not 'horribly less' ..
> just somewhat less.
> Jake brakes are very noisy and often illegal to use in some areas.
>
> Some may find this interesting ..there's 6 or 7 mile long Interstate
> freeway decent near me ( Syskiyou Pass on I-5, top of California,
> southern end of Oregon )
> there's a sign for trucks saying what their speed limit is for that
> descent, by weight, this is for northbound trucks heading into Oregon. I
> don't thinkthere is a similar sign for the California side descent.
>
> The highest wieght truck mentioned is 80,000 lbs . ...
> their speed limit is 18 mph.
>
> And there's a gravel-trap escape area too, for trucks that can't stopor
> control their speed.
> Many of ushave seen a big truck with badly smoking brakes trying to get
> slowed down I imagine.
>
> On 9/14/2013 11:16 AM, Karl Wolz wrote:
> > Not sure 'bout that.
> >
> > My DoKa slows when I lift my foot; if I gear down, it slows more. I'm sure this is an oversimplification, but it's good enough for me.
> >
> > Karl Wolz
> > Sent from my electronic umbilicus
> >
> > On 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
--
David McNeely
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