h a l f b a k e r yNot so much a thought experiment as a single neuron misfire.
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In older style diesel indirect injection engines, there was a swirl chamber in the head (a small spherical area with a very small port into the cylinder) was utilized so that air was compression into the cavity, and then fuel was injected into it, quickly igniting due to the high pressure and temperature
of the air.
This idea uses a sliding valve between the port and the cylinder along with two poppet valves into the small spherical area. One valve opens an exhaust port in which a small positive displacement supercharger eventually sucks out exhaust gasses and creates a partial vaccuum in the swirl chamber. The other valve leades to the exit tube of a centrifugal atomizer that is atached to a very small aluminum melter heated by biodiesel, which is also used to provide a pilot combustion at the top of the compression stroke (via a high pressure injector at the center of the head, outside of the precombustion chamber) that ignites the vaporized aluminum in the swirl port. The swirl port is sealed off from the cylinder until the top of its compression stroke, so that relatively low pressure atomized aluminum has sufficient time to enter the chamber (from the end of the combustion stroke to only a few degrees from TDC before the next combustion). The sliding valve between the swirl chamber and the cylinder is sanwiched in a small slot between the head and the block and is cam-controlled (as are both lobes for the swirl chamber, in addition to pushrod actuated parallel intake and exhaust valves on the other side of the head relative to the swirl chamber (pushrod actuation allows for more room for all those other cams up top).
Issues with conventional solid fuel rockets that use aluminum don't really apply here, as no solid aluminum is being melted in the combustion chamber, but merely atomized and very weakly pressurized into a prechamber before it is ignited by a lean pilot injection. Thus, no large droplets of aluminum need to be accelerated, nor do they have the tendency to pool on any surface.
Engine components, including perhaps the swirl chamber, might need to be made from something that can handle the added heat from combusting aluminum vs. fossil fuels. Perhaps tungsten carbide or something along those lines would work.
Upside to this would be virtually no carbon emissions (only the very small amount of biodiesel needed for pilot injection). The only real exhaust product would be Al2O3 (aluminum oxide) and perhaps some other aluminum/nitrogen compounds, but no CO2. Obviously because of the solid particulates that would form, a new exhaust system would have to be designed, but I don't see that as much of a problem. Any thoughts?
Illustration (paint modified; forgot to show pilot injector)
http://i210.photobu...hj.jpg?t=1292747385
original image credit to http://www.4crawler.com/Diesel/SAE/figure_12.jpg [acurafan07, Dec 19 2010]
[link]
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Aluminium oxide is very hard, and therefore abrasive. Since it is an insoluble solid, it would become entrained in any lubricant and work as a grinding paste, causing very rapid, indeed unsustainable, wear on any bearing surfaces. |
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Potassium or sodium might be better. And they could be supplied as a stable suspension in oil. |
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Great idea [id], right up to the moment where you hit the switch for the water injection .... |
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Hmm, if the aluminum oxide would create lubrication problems perhaps this would be a better add on to a gas turbine then, perhaps as an afterburner? |
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Is it hot enough in the exhaust for that to ignite the aluminum in an afterburner? All the nifty pressure stuff you mention in your idea would not apply. |
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I do like the idea of using a big hopper full of old lawn chairs and pepsi cans for fuel. Sort of post-apocalyptic: feeding on the remains of civilization. |
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//Sort of post-apocalyptic: feeding on the remains of civilization.// |
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Next: an engine feeding on powdered zombies. |
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...or un-powdered mummies. Mark Twain: “The fuel use for the locomotive is composed of mummies three thousand years old, purchased by the ton or by the graveyard for that purpose, and sometimes one hears the profane engineer call out pettishly, ‘D–n these plebeians, they don’t burn worth a cent--pass out a King!’” |
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// hot enough ... to ignite the aluminum // |
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I believe [8th of 7] is correct. I'm pretty sure that finely powdered Aluminum is quite flammable in air at room temperature. In other words, if you tried this it would combust long before you got it into the cylinder. |
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Maybe you should look at electric motors and aluminum-air fuel cells instead. |
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Also, searching for "powdered aluminum combustion" on Google brings up articles labelled "Thermite" at the top of the list. |
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I am sure I have seen heaps of aluminum powder lying blithely about in the air at room temperature, complaining about the weather, checking stocks and generally doing the things a pile of powder does when it is not rapidly oxidizing. |
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If contact with air in the vicinity of the engine is a bad thing one could mix the powder with some light hydrocarbon. Maybe kerosene. Converting it into a liquid would make it easier to handle as well. I envision the Junkyard Wars afterburner rocket car model system as a fine place to test this idea. |
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Pondering this more, I wonder if there are any internal combustion engines which operate completely within high pressure fluids. The change in volume from the liquid to the gas phase is big, and a gas bubble which quickly cavitated back to the fluid would allow waste particles (like the oxides mentioned by 8th) to be better swept along by moving fluids. |
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Also the cavitating bubble might pull the piston back down. |
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There was some serious engineering thought to try to develop a thermite engine at one point. The problem was known material was able to withstand the intense heat, and mechanical stresses of a thermite reaction. |
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I'm not up enough on my chemistry to know if and how aluminum reacts strictly with O2 as opposed to FeO3+Al as it does in a thermite reaction. I would make sure you check the specific heat, coefficient of thermal expansion (as a gas) and other properties of aluminum. While highly reactive it might not be that big of a useable bang as compared to hydrocarbons. I mean flour is explosive as a powder, non dairy creamer is explosive, most powders are. |
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On the metallurgical side, AL at elevated temperatures would act as a interstitial, substitional or low melt point inclusion. Basically it would foul most alloys and destory their properties as they would uptake the aluminum at any temperature high enough to ignite aluminum. |
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Until we invent the diamond block engine, I don't think this is at all feasible. |
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There was some serious engineering thought to try to develop a thermite engine at one point. The problem was known material was able to withstand the intense heat, and mechanical stresses of a thermite reaction. |
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I'm not up enough on my chemistry to know if and how aluminum reacts strictly with O2 as opposed to FeO3+Al as it does in a thermite reaction. I would make sure you check the specific heat, coefficient of thermal expansion (as a gas) and other properties of aluminum. While highly reactive it might not be that big of a useable bang as compared to hydrocarbons. I mean flour is explosive as a powder, non dairy creamer is explosive, most powders are. |
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On the metallurgical side, AL at elevated temperatures would act as a interstitial, substitional or low melt point inclusion. Basically it would foul most alloys and destory their properties as they would uptake the aluminum at any temperature high enough to ignite aluminum. |
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Until we invent the diamond block engine, I don't think this is at all feasible. |
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damn, if it negatively affects alloys would it do the same to a pure metal? like titanium or tungsten? |
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yes titanium is particularly prone to aluminum contamination. Regardless if it's an alloy or a pure metal (pure metals are very rarely used for mechanical or strength reasons, interstitials improve strength so much). |
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Tungsten, I'm not as sure about, I know it's fouled by rhodium, tantalum, zirconium and yttrium. |
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the problem is aluminum is aluminum is not very noble and has a very high coefficient of thermal expansion and low melt point. At temperature an alloy of titanium or tungsten above about 1000f would start to absorb it on the top layer, then break down due to the differences in thermal expansion. |
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You might be able to get there with ceramic coated pistons using the same refractories you use in aluminum melting furnaces, but I'm not sure of their mechanical properties. |
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And again I would check the energy release, piston engines are trying to create the biggest pressure differential , not the highest temperature. Aluminum burns hot, but gasoline and water expand much more I believe. |
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The aluminum afterburner could be combined with water; lots of heat = lots of steam = lots of thrust. |
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I was just thinking the same thing [bungston]. I know that in jet engines the goal is to heat as much air as possible, which seems more fitting to this (probably with introduction of steam as well) than a reciprocating engine. |
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And in thinking about it, the elevated temperature atomized aluminum would never really be in contact with the inside of whatever combustion engine this is added on to. As mentioned, it is stored in prechambers, with valves that open as combustion of the pilot is starting so that the atomized aluminum instantly ignites. Thus, the only parts of the engine that must be resistant to absorption would be the atomizer itself (which I know is possible because they are used to form consistantly sized particles of powdered aluminum) and the prechamber. The only thing that the combustion area of the engine would have to handle would be the combustion itself plus the added heat, but if water injection is used to harness that heat then that probably would not be much of an issue either. |
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"And in thinking about it, the elevated temperature atomized aluminum would never really be in contact with the inside of whatever combustion engine this is added on to." it would be in contact with the combustion chamber as a flue gas, and everything down stream of the chamber such as the exhuast and valves. Metals will uptake aluminum at elevated temperature which will degrade their performance. Ceramics might be more resistant, but more expensive and fragile. |
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acetylene burns at a much hotter temperature than gasoline, but that's really a negative not a positive. i don't see the advantage of running even hotter. |
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Hotter temperature = more usable energy. And if it ignites immediately upon introduction into the combustion chamber, I don't even see it as having the potential to be a flue gas, though maybe I'm missing something. |
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//Until we invent the diamond block engine, I don't think
this is at all feasible.// |
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Well, there you go! You just did! Only the implementation
remains. |
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I would not think diamond would be as heat resistant as ceramics. I thought diamonds burned. |
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There has been stuff about hydrogen peroxide engines around here before. I am thinking about the assertion that anything that can oxidize will oxidize if dipped into a cool mug of H2o2. True for anything carbon, I am sure - but what about aluminum? Is there some energy of activation that must be overcome or would aluminum powder be stable in peroxide? |
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Yes, it would, for one, maybe two picoseconds .... |
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This is one of those ideas it would be simpler not to have. |
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Aluminum has a good energy density, if used as a fuel. Aluminum-air primary batteries could be used to drive an electric car, but then the aluminum oxide would have to be re-refined and the battery rebuilt. |
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