"David Billington" wrote in message
...
Ed Huntress wrote:
"jeff_wisnia" wrote in message
eonecommunications...
Ed Huntress wrote:
"Ignoramus10802" wrote in message
...
On 2009-10-29, ATP* wrote:
True. Yet I still hear this type of "reasoning" all the time. Should
be a
simple concept even for the technically challenged, for example,
people who
argued here that you can compress air and allow it to expand (while
doing no
useful work) with no loss of energy.
That would be almost possible if compressing and expanding was done
very quickly, before compressed air cools.
i
But it has to be done *awfully* quickly. That's why there's a minimum
cylinder size for diesel engines -- something like 300 cc. Below 3,000
rpm or so, the compressed air cools too quickly to ignite the fuel. And
heat transfer gets worse as compression goes up.
That statement may not be entirely correct, Ed.
Remembering waaaay back to my childhood playing around with control line
model aircraft. before the glow plug engine was developed we had but two
choices, conventional spark plug engines and diesel engines.
g I had one of those. They weren't really diesels (technically, they're
"homogeneous charge, compression-ignition engines), but that isn't the
issue, because they ignite from compression, the same as a true diesel.
The issue is that they use ether for fuel, or an ether-diesel mix in the
larger ones, and ether has a cetane rating of 86. Diesel fuel runs around
40 - 45 cetane. Those little engines can't produce enough heat from
compression to ignite diesel fuel.
The diesels were never very popular, but they are still being made for
thos who want to add a bit of "authenticity" to replicas of vintage
model aircraft. Those are surely far below 300 cc, but the fuel they
burn may not be quite the same as what cars and trucks use. G
http://www.eifflaender.com/enginepics.htm
I also remember my not terribly successful efforts at flying RC models
back then. My ham ticket let me do that legally on 28 Mhz (the 10 meter
ham band) using a one tube receiver in the plane which triggered a
rubber band energized "escapement" that moved a combination
rudder-stabilizer at the rear of the plane.
A tail-wagger. I built one of those, a Cessna Birddog from a kit, with an
.049 Babe Bee engine. I used a salvaged CB walkie-talkie for a
transmitter and a homebrew superregen receiver. The signal was CW. I made
the escapement from a tin can, with bits of HSS hacksaw blade soldered
onto the tips of the arms. It flew pretty well, but I was not much of a
pilot. Fortunately, if you just tossed the transmitter aside, it would
land by itself.
How the heck I just remembered that the tube used in those receivers was
an RK61, a gas filled triode, I'll never understand.
For the same reason that I remember that my first homebrew regen
receiver, a battery-powered rig that I built in 1957, used a 1T4. Our
memories of old stuff tend to hang in there while we forget what we had
for breakfast. d8-)
http://tubedata.milbert.com/sheets/138/r/RK61.pdf
Thaks for the mammaries though.
Jeff
sigh That's most of what I'm good for these days. Sometimes I feel like
an antique. d8-(
A small diesel here
http://www.model-engineer.co.uk/albu...=6778&p=123122 ,
about 28cc, not sure about the relation of lamp oil to pump diesel but it
sounds less volatile than the small "diesel" aircraft engines I used on
occasions which were a blend of ether, caster oil and amyl nitrate. Last
time I used one was about 1981 and on returning to the US from the UK I
asked my chemistry teacher about getting some ether to blend my own and
she almost had a fit, then gave me a lecture about the dangerous nature of
ether. Didn't know then that engine start spray is mostly ether and have
seen a thread elsewhere where a chap detailed using it to refill a
capillary temperature gauge, Lucas IIRC.
"Lamp oil" covers a lot of different products, but there are references in
the literature to using it as a cetane booster for diesels. Unconfirmed
references say that its cetane rating can be as high as 70.
The point is that small engines lose much more heat to the cylinder walls,
cylinder head, and piston crown, because of the relationship between volume
and surface area in a cylinder. When you get below 300 cc or so per
cylinder, conventional diesel fuel will either not run smoothly or reliably,
or not at all. The heat loss of the compressed air is too great and the
resulting temperature is too low to ignite regular diesel fuel.
There are some small specialty engines that use spark ignition or a glowplug
to start and heat up the engine, which is then switched to compression
ignition. I don't know how well they run but it all depends on how well they
can keep the temperature up inside of the cylinder. I would guess that their
efficiency is low, too, because of quenching against the cylinder walls
while the engine is running.
I was not talking about those, or about the models that run on high-cetane
fuels, which typically are too expensive, or, as you say, too dangerous to
use as a primary fuel.
You can buy ether in the US. I had a can of it for years that I used to
start a balky, clapped-out lawnmower.
--
Ed Huntress