Naked Science Forum
Non Life Sciences => Technology => Topic started by: briofuel88 on 18/06/2012 16:58:33
-
How does the Potential energy in the fuel convert to kinetic energy of expanded gas via combustion?
-
The fuel has an excess of energy tied up in its molecular bonds. By rearranging the molecules, meaning breaking them down and forming new ones with lower-energy bonds, energy is released, and that energy shows up as heat which manifests itself primarily as in increase in the speed of the molecules and therefore a temperature/pressure increase. Some of the energy may be released as infrared radiation, depending on the fuel.
-
Two common methods are with:
- A reciprocating piston: Bursts of exploding fuel push a piston up. A crank converts this into rotating motion.
- A rotating turbine: The gas from continuously burning fuel makes the turbine blades rotate in a single direction
Other methods being researched include fuel cells, where the fuel is converted into electricity, without burning it in a flame.
-
- A reciprocating piston: Bursts of exploding fuel push a piston up. A crank converts this into rotating motion.
The fuel air mixture is burning rather than exploding. An explosion would only damage the engine without producing any power.
-
The fuel air mixture is burning rather than exploding. An explosion would only damage the engine without producing any power.
That is debatable.
I would argue that Diesel engines (ie. compression ignition engines) operate in an explosive manner, since the pressure wave is supersonic.
Our friend Wikipedia says (http://simple.wikipedia.org/wiki/Explosion):
"Explosions are categorized as deflagrations if the pressure waves are subsonic and detonation if they are supersonic. When a detonation happens, the supersonic pressure waves are named shock waves."
-
The fuel air mixture is burning rather than exploding. An explosion would only damage the engine without producing any power.
That is debatable.
I would argue that Diesel engines (ie. compression ignition engines) operate in an explosive manner, since the pressure wave is supersonic.
Our friend Wikipedia says (http://simple.wikipedia.org/wiki/Explosion):
"Explosions are categorized as deflagrations if the pressure waves are subsonic and detonation if they are supersonic. When a detonation happens, the supersonic pressure waves are named shock waves."
Yeah, but your car would not work better if you used C4 for fuel. I think the idea is to get the release of thermal energy to match the rate of expansion.
-
Yes, which is why hydrogen is a rubish fuel in ic engines (the flame front is too quick so work is wasted)
-
Yes, which is why hydrogen is a rubish fuel in ic engines (the flame front is too quick so work is wasted)
shhhh.... don't let the Brown's gas people hear you! [;)]
They're very protective of their particular flavour of nonsense!
-
The fuel air mixture is burning rather than exploding. An explosion would only damage the engine without producing any power.
That is debatable.
I would argue that Diesel engines (ie. compression ignition engines) operate in an explosive manner, since the pressure wave is supersonic.
Our friend Wikipedia says (http://simple.wikipedia.org/wiki/Explosion):
"Explosions are categorized as deflagrations if the pressure waves are subsonic and detonation if they are supersonic. When a detonation happens, the supersonic pressure waves are named shock waves."
Hang about! Diesels don't create an explosive mixture. A stream of diesel oil burns as it is injected into hot air.
-
Actually I struggle to see what fundamentally separates the definition of an explosion from any other type of combustion. I used to think that explosions were, by definition, combustion events happening at a supersonic rate, but this appears to be an unfounded assumption.
Is an explosion no more than unpredictable combustion? - I was going to say uncontrolled, but then what's a 'controlled explosion'!
Also I have for a long time pictured the injection event in (single-shot injection) Diesels to begin fractionally earlier on the compression-stoke, prior to the time when the temperature reaches that needed for self-ignition. The thinking being that having multiple well-distributed microscopic sites for self-ignition to occur would ensure the region immediately around the injector was less likely to be staved of oxygen and the pressure reaction through the cylinder would have a more even progressive expansion; leading to more extraction of mechanical work.
-
I don't know much about injectors. With the mechanical kind, does the injection duration vary with engine speed?
-
Burning, exploding, it is just the rate of flame front propagation. Gunpowder burns fast, and if it is confined the pressure evolved by the hot gas is vety high. C4 burns a few magnitudes faster, the flame front speed can be measured in kilometers per second, as opposed to the meters per second of gunpowder.
-
Burning, exploding, it is just the rate of flame front propagation. Gunpowder burns fast, and if it is confined the pressure evolved by the hot gas is vety high. C4 burns a few magnitudes faster, the flame front speed can be measured in kilometers per second, as opposed to the meters per second of gunpowder.
Yes - it seems to be a bit arbitrary. Also, we need to take into account the density of the stuff that the wave travels through. Supersonic in Earth's atmosphere at sea level is not the same as supersonic in air compressed to fifteen atmospheres :)