[Vwdiesel] RPhysics and Beer

Val Christian val at swamps.roc.ny.us
Tue Jan 6 23:43:33 EST 2004


Here's a note I sent Hagar on the topic.  I can see we're going 
have fun with this one.  I just wish that I had better email
presentation skills.  Then we could do modeling of the
different engine cycles.

Oh, carb icing is due to cooling of the air as it passes through
an orifice ( the throttle valve ), and the evaporative cooling
caused by the vaporization of the gasoline.  I forget the exact 
ratio of each contribution.

And the braking of a diesel or a gasser will likely be the square 
of the rotational velocity.  So comparison of starting currents or
power required to turn over a nearly static engine is not realistic.
The dynamic case is where the heat is given off.  

[See, a jet engine turns very freely, therefore there should be 
very little difference in power between 99% Ng and 100% Ng.]



I'm inclined to do a more formal analysis of this.  First,
let me try to talk through this.

Given a similar size engine, a gasser consumes more BTUs 
of fuel than a diesel in idle mode.  Considerably more.
Not single digit percentage points.  Triple, last I checked.

A jake brake eats energy by giving it off, and not through the
radiator.  Compressed air released from the cylinders near TDC
is HOT; it would otherwise cause ignition of fuel.  As it's 
released, there's some noise, but the real energy is in the
release of the hot air.  As it decompresses, it cools off,
and makes some noise.  But mostly, it just gives off heat.
On top of that, there isn't compressed air to help turn the
In effect the engine, in jake brake mode, becomes a heat pump.

Consider the diesel engine without a jake brake.  It's heating 
and cooling air.  Compress the air, release the air.  The cylinder
is an air chamber element spring.  Much of the energy that went 
into compressing the air now is extracted pushing the piston back
(just like a spring).  It's almost like harmonic motion, except 
there is resistance (rings on cylinders, bearings, etc.).
The heat buildup on the cylinder walls may indeed go out the 
radiator.  But there's not much of it.  
There's also a small heat loss from the conduction of heat from
the compressed air at TDC.  But remember, that air is only hot 
for a moment, so the heat lost is small.

Hagar, I'm sure that you notice when going down that couple of 
mile calibrated hill, that the engine coolant temp drops.  That's 
because the cooling from the airstream takes more energy than
the friction of the engine operation offers.  (We're not counting
the whirling diesel fuel in the injection pump, because that is
very loosely coupled to the engine.  )  

So we've differentiated between a jake brake system and a standard 
NA diesel system.  It is clear that a jake brake has considerable
capability to brake an engine, because it takes work out of a system
with the release of compressed air.  In effect that work is released
to the atmosphere.

The standard diesel system is in effect an airpump, recovering almost 
all the energy invested into it.  More like a resonator.  

Now let's talk through the gasser vs. diesel.  Certainly we can do more
emphirical measurements.  There is one notable difference.  Instead of
each compression cycle being symetric (or nearly so), the compression
cycles are asymetric.  The work done is drawing air against the throttle.
[This doesn't really compare to the jake brake, which is much more 
efficient at converting energy into lost heat.]  In effect, there's
a miniheat pump there.  It works better at higher RPMs, which is to say
that it wastes more heat.  Higher RPMs cause a higher "vacuum" which 
results in more assymetry.

Oh, to be fair, part of the higher BTU consumption of a gasser while 
idling, is the fact that the fuel is less efficiently burned.  More excess
heat, which is ultimately, why a gasser gets the heater going faster
than a diesel, while idling on a cold day.  

If I'm easy on the fuel, I can get to my daughter's school and back 
(18 miles), before the engine coolant is at thermostat temp (195F).
That's with the TDI.  The next couple of cold days will make it even 
easier to keep it cool.  Makes me want a block heater, so that I can
run a warm car in the morning...

Let's have the next round of discussion over a beer...


> I love Physics --- it is good exercise for an old brain.
> A lot of VW drivers say that the diesel does not slow down the
> car going down a hill as well as a gasser.
> Physics tells us that is MOSTLY   myth.    Do a test as follows and
> you will be convinced.Use a "calibrated hill"  (Val). Start test at top =
> of hill
> with the fuel off. ---Check max speed going down.
> Then do it again with fuel on .    If braking is a lot less  with fuel =
> on, reset idle and
> fueling screws a tad towards less idle rpm and less fuel.
> The Bosch pumps are real tricky in that Idle range . Totally different =
> from a carb.
> What slows us down ????   ----heat loss --it takes a lot of heat going =
> in to
> the rad to slow things down.  ----That's where the diesel shines. Higher =
> temp
> in combustion chamber faster transfer of heat.
> A fair amount of force is recovered with piston on downstroke. Air =
> working
> like a spring. More braking would result if valving was changed to =
> Aircompressor
> configuration.( I think that's the Jake --am I right?)
> Let me say it again  ----if a fixed amount of fuel was injected , =
> regardless of  rpm
> there would be braking. The bosch pump is tricky , and may inject enough =
> fuel to
> ruin the slowdown.
> The sum of all friction and powerloss according to Physics tells us that =
> the  VW will
> work as a brake going down that hill. How do I know ??? I live in =
> hillbilly country.
> Jackass Mountain  is a good proving ground. (that's close to Hells =
> Gate).
> The bosch pump is an amazing retarder ---- 4 flyweights thrashing around =
> in fuel,  lots of
> power loss. Turning at slightly faster than engine rpm.--so going down =
> that hill with 5k
> on the clock   the weights fly around at 5000 plus. That's what heats up =
> the fuel in wintertime.
> (and summertime).
> Hagar.
> PS: Here are a couple of physics teasers. ---- Water is a good Insulator =
> both for heat and electricity.
> Water is not heaviest when it is coldest.
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