[Vwdiesel] Diesel braking - again

[Roger Brown]

Erik Lane wrote:
> 
> ok, now this may be nitpicking, but you were a little
> uneven in your analysis, seems to me.
> 
> if the energy lost in a gasser due to the vaccuum and
> cooling and that is a major cause of the braking
> effect, why do you just dismiss summarily the energy
> lost from the compression process? "The cooling system
> in the engine is removing this heat and releasing it
> to the atmosphere." are they really that many orders
> of magnitude different that they can be ignored?

Yes, it is significant, I don't think I dismissed it, rather that I took it as a given, but limited effect.  If it were not for this you would have no braking at all in a VW diesel, would be the same as kicking the tranny out of gear and coasting.  But
there is only so much braking you can get this way and it is less than an intake restrition.  If you fully insulated the cylinders there would be less heat loss and therefore less compression loss and less braking.  If the engine were somehow cooled
better you would get more compression loss, but the air inside only gets so hot and the coolant only gets so cool and in a short period of time you can only transfer so much heat which sets a limit to how effective you are at removing the heat generated
by compressing the air so that you don't get as much energy back out from the expansion stroke.

> and carb icing is from vaccuum, but not from the
> butterfly valve. the venturi in the carb itself
> creates a vaccuum when the air goes over it. it would
> usually freeze over in cool to cold moist weather with
> an engine is working hard and therefore lots of air
> moving thru the venturi. (that's from memory and i
> hope pretty close to accurate.)

Its from the change in pressure and velocity, it can happen if you pull a valve stem on a high pressure tire in hot, humid weather and there there is no vacuum at all involved, just a high pressure gradient and the resultant adiabatic cooling of the air
escaping.  When you force a high pressure gas through a small opening and let it expand on the other side you get a pressure and temperature drop and its not reversible which would be the case in an ideal gas and insulated cylinder with a straight
compression/expansion cycle.  So its the venturi affect that creates the vacuum that in turn makes the engine work hard at pulling the air through the restriction.  However, since the intake pressure is lower, the volume/mass of air pulled into the engine
is less, so you would have trouble in a diesel getting the fuel to ignite.

> as far as an intake restriction - i can just imagine
> how smoky that would make a diesel!! :) and the
> thought of a jake brake locking up a rabbit't wheels
> really made me smile.

Yes, you would need to cut the fuel at the same time, would be a complicated linkage since you likely would not want this to happen in normal driving but to happen at speed on downhills.

> though i still disagree about the braking being
> EXACTLY the same if all other factors were the same,
> including an intake restriction, except for the
> compression. i find it REALLY hard to believe that it
> just doesn't matter. my number one real world
> experience is that my 1.6 diesel takes a LOT more work
> to spin than my cousins 1.6 honda gasser. and that's
> WITH an intake restriction. (of course that only
> barely comes into play at the speeds i'm talking
> about.)

I stand corrected, I should have used a different wording for that.  The higher compression in a diesel is going to make the compressed air hotter resulting in a more heat transferred to the coolant and thus a bit less energy returned to the piston. 
However, since there is less volume of air entering the engine through the intake restriction, the internal pressure in the diesel (and the gas) engine is less, I commonly see 25" of Hg on my gas engine vacuum gauge near sea level while engine braking,
that means that only 1/6 of the normal air volume is getting in (30" is nearly a perfect vacuum).  So, instead of having say 500 psi (or whatever the normal number is) in the diesel, you only have 500/6 or 83 psi in the cylinders.  Your gas engine would
be 25 psi (assuming 150 psi normally), so there will be a little difference due to the differences in pressure, but neither one will be anything near what it would be with no intake restriction.  So if you have a one factor (i.e. intake restriction) and
another factor (braking due to pumping/heat losses) and the first factor has a bigger effect (everyone seems to agree a gas engine has more braking than a similar sized diesel) and by making the effect of the intake restriction more by restricting the
intake (closing the throttle) more you decrease the effect of the heat and pumping losses, the differences due to the engine internals become smaller.

My main point was that the effect of the intake restriction would dominate the braking effect and that if you were pulling so many CFM of air past a given restriction you would have the same energy loss (due to the venturi) no matter what was pumping or
pulling the air through (again ignoring any losses in the pressure or vacuum source).  So depending on the relative contributions to the engine braking effect, there will be differences if you look at the system as a whole, I ignored the other factors
when I made the "exactly" statement, sorry for not clearly stating my assumptions.

> feel free to try to convince me, but this might be
> something that i won't be talked out of... :)
> 
> thanks for trying to explain to me, tho!


-- 
    Roger