VW Reality

On Mon, 04 Feb 2008 13:06:29 -0600, Charles Vincent
wrote:

RST Engineering wrote:
I could be very, very wrong, but my understanding of efficiency is that 25%
efficiency means that a quarter of the energy goes to torque and 3/4 to
heat. That would mean you throw away THREE horsepower's-worth instead of
four, no?

Jim



since such engines are no more than 25% efficient when it comes to
converting the heat of combustion into torque at the crankshaft. That
means that for every horsepower measured at the crank you must
generate at least four horsepower's-worth of heat in combustion.


You are correct, but that is also exactly what he said ---. i.e.
generate four hp in heat, only one available at the shaft.

Charles
What he's forgetting is the 1 available at the shaft is STILL heat
energy.
Fuel makes 4
Pistons catch 1
cooling and exhaust loose the other 3.


--
Posted via a free Usenet account from http://www.teranews.com

Charles Vincent wrote:
RST Engineering wrote:
I could be very, very wrong, but my understanding of efficiency is
that 25% efficiency means that a quarter of the energy goes to torque
and 3/4 to heat. That would mean you throw away THREE
horsepower's-worth instead of four, no?

Jim



since such engines are no more than 25% efficient when it comes to
converting the heat of combustion into torque at the crankshaft. That
means that for every horsepower measured at the crank you must
generate at least four horsepower's-worth of heat in combustion.


You are correct, but that is also exactly what he said ---. i.e.
generate four hp in heat, only one available at the shaft.

Charles


Well that's better than a 100 watt electric light bulb, which is a 90
watt heater and 10 watt light source...

John

the modern 'VW engine': http://www.ulpower.com/

"John" schrieb im Newsbeitrag
...
Charles Vincent wrote:
RST Engineering wrote:
I could be very, very wrong, but my understanding of efficiency is that
25% efficiency means that a quarter of the energy goes to torque and 3/4
to heat. That would mean you throw away THREE horsepower's-worth
instead of four, no?

Jim



since such engines are no more than 25% efficient when it comes to
converting the heat of combustion into torque at the crankshaft. That
means that for every horsepower measured at the crank you must
generate at least four horsepower's-worth of heat in combustion.


You are correct, but that is also exactly what he said ---. i.e. generate
four hp in heat, only one available at the shaft.

Charles


Well that's better than a 100 watt electric light bulb, which is a 90 watt
heater and 10 watt light source...

John

On Feb 6, 1:28 am, "oilsardine" wrote:
the modern 'VW engine':http://www.ulpower.com/
-------------------------------------------------------------------------------

Nice.

It's a real shame that the typical VW 'expert' knows so little about
engines :-) The ulpower people have gone out of their way to provide
easily understood explanations of how to measure torque; of torque vs
power, of Specific Fuel Consumption and so forth.

Indeed, these things are so simple -- and so fundamental to engines --
that it is difficult to understand why the EAA does NOT endorse a
program of publicly testing engines at its annual convention. The
'Experimental - Amateur-built' license is supposed to foster EDUCATION
and there are few things more educational than allowing the public to
see such demonstrations with their own eyes.

-R.S.Hoover
-EAA 58400 (Life Member)

oilsardine wrote:

the modern 'VW engine': http://www.ulpower.com/


Now that is a sweet little motor!

165 pounds
2600 cc
claims 81 HP at 2800 RPM
500 hour TBO
FADEC
No Carb Ice problems

Only problem - I didn't catch the price?

The maximum SUSTAINABLE power available from ANY air-cooled engine is
determined by the engine's ability to cool itself -- to couple its
waste-heat to the atmosphere. And with a carburetted, spark-ignited,
gasoline-fueled engine there is a LOT of waste heat to be managed
since such engines are no more than 25% efficient when it comes to
converting the heat of combustion into torque at the crankshaft. That
means that for every horsepower measured at the crank you must
generate at least four horsepower's-worth of heat in combustion.
These basic rules of thermodynamics are made even worse by two
additional factors, the first being 'Economy of Scale' in that smaller
engines are LESS thermally efficient than larger engines, and the
basic definition of Standard Day conditions -- 59.9 degrees on the
Fahrenheit scale and an atmospheric pressure of 29.92 inches of
mercury -- a fairly cool day.

With those laws of physics as preamble the next factor worthy of note
is the physical dimensions of the Volkswagen cylinder head and the
fact that ALL VW heads have the same exterior dimensions. This is
because they must fit under the stock VW engine shrouding. No
manufacturer of VW heads, either stock or after-market, offers a head
having more fin area. Indeed, most after-market heads have LESS, due
either to thickening of the combustion chamber wall or even
eliminating one of the eight fins -- and in a few cases they have done
both.

I think the Scat single unit heads have somewhat more as do the 356
Porsche heads after '59 or so. Both require different sheet metal in
the car.

All -- ALL -- Volkswagen heads in common use today are derived from
the heads developed for the 1300cc engine; their external physical
dimensions remained exactly the same for the later 1500 and 1600
engines. The output of the 1300 engine was approximately 40hp and
could SUSTAIN that level of output indefinitely under Standard Day
conditions. This engine was bored-out to 83mm to produce the 1500
engine, then over-bored to 85.5mm to create the 1600 (actual
displacement 1584cc), the maximum output of which was 57bhp for
carburetted models, achieved in the 1971 model year. But that level
of output could only be sustained for a bit less than FOUR MINUTES,
until the cylinder head temperature exceeded safe levels, again under
Standard Day conditions.

So what's this 'safe level' of CHT? About 450 degrees on the
Fahrenheit scale. This reflects the fact that VW heads are made of
CAST aluminum (as opposed to a forging) and the fact aluminum is a
'white short' metal, meaning it becomes frangible when its temperature
enters the 'plastic' range. A characteristic of white-short metals is
that when heated they fracture like a cube of sugar when subjected to
stress. The floor of the frangible range is a bit higher for a
forging -- about 550F according to Pratt-Whitney -- but can be as low
as 400F in a casting, depending upon the alloy.

You could always go to an iron or bronze (if TEL is eschewed)
head.....


A common thread used to impress technologically naive buyers is tales
of driving a Volkswagen bug or bus for hours on end with the throttle
wide open. The fact the engine was was probably producing LESS THAN
TWENTY HORSEPOWER goes unsaid. This involves the Horsepower Myth and
generally leaves a large black question mark hanging over the heads of
those without an engineering background but it needs to be touched
upon since ignorance can be as deadly as a machine gun when it comes
to aviation.

The Horsepower Myth was create by James Watt in order to sell his
modified Newcommen steam engine to mine owners. To do so he added the
element of TIME to the power equation and from that day to this the
general public has been comfortable with the idea that 'horsepower'
represents a given quanta of energy... which it does... but only
within a defined unit of TIME. And from that day to this, that
arithmetical loophole has been used by those eager to prey upon
technologically naive consumers.

Indeed, in the early days of aviation those predations cost so many
lives that government agencies had to step in, requiring the
manufacturers of aircraft engines to justify their claims of power and
durability.

------------------------------------------------------

All of which tends to leave the average homebuilder with more
questions than answers. Fortunately, the ENGINES themselves are
incapable of lying, especially when it comes to FUEL CONSUMPTION.

The Specific Fuel Consumption (SFC) of all -- ALL -- air-cooled,
gasoline-fueled, normally aspirated Otto-cycle engines is clumped near
the 0.5 mark, meaning it takes about 0.5 POUNDS of 'gasoline' (*) per
HOUR to produce ONE HORSEPOWER'S worth of torque at the crankshaft.
For aviation gasoline that works out to about 12bhp per gallon per
hour. For a 103hp engine that works out to 8.58 gph.

Liquid cooling is simply so superior that the air cooled gasoline-and
even (see Deutz) diesel-is a museum piece per se. That said the Harley
conversion seems to be working out better than anyone would have
thought.