Diesel

SWM wrote:
I understand the Ideal Gas Law and was not trying to argue in favor of the Kinetic Molecular Theory. What I was objecting to were the statements:

Then how do you explain the temperature of a gas dropping when it expands into a vacuum? (Assume adiabatic expansion with work equal to zero.)

~Tonto

n7axw wrote:I am not a techy, so I tend to get in trouble when discussions get too technical...

But...could someone explain why diesel explodes under pressure? I know there is heat involved, but where does it come from? How hot does it have to get in a cylinder with pressurized diesel before it explodes? I've looked it up, but so far none of the discussions have explained this...

Easternmystic wrote:Diesel doesn't explode under pressure. In an Otto cycle or diesel engine, Air is drawn into a cylinder, The air intake valve closes and the piston rises compressing the air. When a gas such as air is compressed it heats up. When the piston reaches the top of its travel, diesel is injected into the compressed air. The air is hotter than the ignition temperature of the diesel at his point so the diesel burns spontaneously which is why no spark plugs are required in diesel engines.

Weird Harold wrote:The heat comes from compression. When you compress something, it gets hotter -- someone else can get all sciency and technical to explain why, but Compressed=Hotter is a general principle that should be remembered.

Skipping over the nerd-gasms and sciency stuff...

Like steam, Diesels can run without heavily refined fuels, or electricity. The main obstacle would be a suitable source for liquid or gaseous fuel and lubrication.

The main worry as far as detection by the OBS or Temple is the exhaust and compounds that don't happen without high-pressure combustion -- notably NOx compounds, which contribute to photochemical smog clouds visible from space.

Are the advantages of Diesel sufficient to invest in the infrastructure in parallel to Steam infrastructure and to invent and invest in systems to deal with the high-pressure combustion byproducts -- aka pollution. The investment has to be in parallel because Diesel can't fully compete without Diesel-Electric -- Steam trains are still faster and cheaper than Diesel trucks without Autobahn/Freeway systems.

That was a helpful discussion. But a couple of questions. Phillies noted that the heating process requies a moving wall. Am I right to assume that if you compress air in a cylinder and hold it in its compressed state, the air would eventually cool?

My second question is what is actually creating the heat? Is it the molecules coliding with each other?

Don

Weird Harold wrote:

n7axw wrote:I am not a techy, so I tend to get in trouble when discussions get too technical...

But...could someone explain why diesel explodes under pressure? I know there is heat involved, but where does it come from? How hot does it have to get in a cylinder with pressurized diesel before it explodes? I've looked it up, but so far none of the discussions have explained this...

Easternmystic wrote:Diesel doesn't explode under pressure. In an Otto cycle or diesel engine, Air is drawn into a cylinder, The air intake valve closes and the piston rises compressing the air. When a gas such as air is compressed it heats up. When the piston reaches the top of its travel, diesel is injected into the compressed air. The air is hotter than the ignition temperature of the diesel at his point so the diesel burns spontaneously which is why no spark plugs are required in diesel engines.

Weird Harold wrote:The heat comes from compression. When you compress something, it gets hotter -- someone else can get all sciency and technical to explain why, but Compressed=Hotter is a general principle that should be remembered.

Skipping over the nerd-gasms and sciency stuff...

Like steam, Diesels can run without heavily refined fuels, or electricity. The main obstacle would be a suitable source for liquid or gaseous fuel and lubrication.

The main worry as far as detection by the OBS or Temple is the exhaust and compounds that don't happen without high-pressure combustion -- notably NOx compounds, which contribute to photochemical smog clouds visible from space.

Are the advantages of Diesel sufficient to invest in the infrastructure in parallel to Steam infrastructure and to invent and invest in systems to deal with the high-pressure combustion byproducts -- aka pollution. The investment has to be in parallel because Diesel can't fully compete without Diesel-Electric -- Steam trains are still faster and cheaper than Diesel trucks without Autobahn/Freeway systems.

The key metric for transport of any kind is the gravimetric power density. Diesel beats everything practical except gas turbines by a very wide margin.

n7axw wrote:My second question is what is actually creating the heat? Is it the molecules coliding with each other?

Check this out: http://en.wikipedia.org/wiki/Fire_piston

Tonto Silerheels wrote:SWM wrote:
I understand the Ideal Gas Law and was not trying to argue in favor of the Kinetic Molecular Theory. What I was objecting to were the statements:

Then how do you explain the temperature of a gas dropping when it expands into a vacuum? (Assume adiabatic expansion with work equal to zero.)

~Tonto

That is indeed due to the fact that the gas is not an ideal gas. An ideal gas would not change temperature when expanding into a vacuum, but a real gas can (depending on the physical properties of the gas). Or, depending on the gas, it might actually heat up, as Phillies mentioned.

However, in a compression piston, the gas is heated because work is being done, by the piston. This is not the same situation as a gas expanding into a vacuum. The work is causing the gas to heat, not any intermolecular forces as you suggested.

Dutch46 wrote:The key metric for transport of any kind is the gravimetric power density. Diesel beats everything practical except gas turbines by a very wide margin.

It isn't quite that simple -- or at least the dozen or so wikipedia pages I got lost in trying to define gravimetric power density weren't anything close to simple.


In more understandable terms, the economics of transportation has to do as much with the infrastructure as it does absolute power density. You might note that I specified "without autobahn/freeway" and Diesel "Trucks."

Diesels couldn't compete with steam until the development of Diesel-Electric locomotives. Direct drive ICE locomotives never matched the power or speed of Steam Locomotives. That leaves Diesel Trucks as competition for trains.

During WWII, before the demise of steam trains, the practice of loading Semi trailers on flatbeds was developed, because one train could move more freight farther and faster for the same amount of fuel. That was largely due to the infrastructure being more efficient for trains than for trucks. The fact trucks could tow at most three trailers safely vs a train's 50-100 rail-cars (roughly the equivalent of two semi-trailers in capacity.)

Trucks of WWII vintage were limited to 35-45 mph by two-lane asphalt roads between cities and 15-25 mph in cities -- which roads did NOT bypass as they do today. Trains averaged over 50 MPH with peak speeds of 70-80.

Safehold has very good roads for dragon-drawn transport, but they're still basically equivalent to RW two-lane asphalt roads of WWII vintage.

Safehold can't replace steam trains with Diesel because without Diesel-Electric technology, Diesel can't put the power on the rails. Safehold can't replace trains with Diesel trucks for long distance transport because the roads won't support the traffic to match rails or canals.

Charis does have the technology to build Diesels, but I can't really see any place they'd be useful in the transport field as anything bigger than a 2.5 ton 6x6 or five-ton 10x10 trucks of WWII vintage -- and for much the same uses those WWII vintage army trucks were used. (see red-ball express) The problem will providing fuel and lubricants and hiding the high-pressure combustion byproducts from OBS/Temple sensors.

The need for flexible transport, a la the Red Ball Express for the military supply system might drive Diesel development, but the same end can be accomplished with narrow gauge steam railroads -- see WWI narrow gauge railroads -- without introducing a second doubtful technology.

n7axw wrote:That was a helpful discussion. But a couple of questions. Phillies noted that the heating process requies a moving wall. Am I right to assume that if you compress air in a cylinder and hold it in its compressed state, the air would eventually cool?

With perfect insulation, the air would stay the same temperature.

However, it is impossible to create a perfect insulator - and so some energy (i.e. heat) would be lost through the walls of the chamber.

My second question is what is actually creating the heat? Is it the molecules coliding with each other?

Don

Kinda - in this case, pressure is energy. i.e. pressure = energy = velocity = heat.

MWadwell wrote:

n7axw wrote:That was a helpful discussion. But a couple of questions. Phillies noted that the heating process requies a moving wall. Am I right to assume that if you compress air in a cylinder and hold it in its compressed state, the air would eventually cool?

With perfect insulation, the air would stay the same temperature.

However, it is impossible to create a perfect insulator - and so some energy (i.e. heat) would be lost through the walls of the chamber.

My second question is what is actually creating the heat? Is it the molecules coliding with each other?

Don

Kinda - in this case, pressure is energy. i.e. pressure = energy = velocity = heat.

That is close, but not quite correct. Pressure is not energy. But Action (e.g. a moving piston) moving against Pressure is Work, which is Energy, which is translated into Kinetic Energy, which is Molecular Velocity, which is Heat.

In the Kinetic Molecular Theory, it could be seen as Phillies described: the moving wall imparts additional kinetic energy to the molecules that bounce off it. Or you can just look at the overall disposition of energy and recognize that pushing a piston against pressure is Work, and that energy has to go somewhere (into heating the gas, in this case).

SWM wrote:

MWadwell wrote:n7axw asked:
My second question is what is actually creating the heat? Is it the molecules coliding with each other?

Don

Kinda - in this case, pressure is energy. i.e. pressure = energy = velocity = heat.

That is close, but not quite correct. Pressure is not energy. But Action (e.g. a moving piston) moving against Pressure is Work, which is Energy, which is translated into Kinetic Energy, which is Molecular Velocity, which is Heat.

In the Kinetic Molecular Theory, it could be seen as Phillies described: the moving wall imparts additional kinetic energy to the molecules that bounce off it. Or you can just look at the overall disposition of energy and recognize that pushing a piston against pressure is Work, and that energy has to go somewhere (into heating the gas, in this case).

Just to address the point I have highlighted above, Pressure is potential energy.

Bleed compressed air out of a cylinder into a turbine, and you have kinetic energy (the turning turbine).

Remember - energy cannot be destroyed (only converted), and so pressure must be potential energy, as otherwise the kinetic energy from the turbine has come out of no-where.....

EDIT - check out the wikipedia article on Enthalpy - http://en.wikipedia.org/wiki/Enthalpy for more information about this form of thermodynamic potential energy.