| MadSci Network: Physics |
The question: "How does the expansion of gunpowder compare to that
of gasoline? Both gunpowder and gasoline are burnt to create gas to create
pressure to do work. The occasional experiment with powder-powered engines
indicates some folks think they're similar enough in performance to be
somewhat interchangeable (obviously the liquid/solid and oxidizer issues
must be overcome). Information on the performance of the two substances
seems hard to come by, though. What are, and/or where might I find, numbers on:
- gas volume produced per original volume (or whatever is common
useful measure; pressure maybe?), and
- burn rate
for gasoline (and maybe diesel) and modern smokeless powder (and
maybe black powder)?"
Both gasoline/diesel and gunpowders are considered low explosives. (See Explosive Materials at Wikipedia.)
Information on energy density of fuels and gunpowders is hard to find, as you yourself discovered, but I have been able to locate the following information at Energy Density at Wikipedia:
The energy density of gasoline and diesel is about 46 MJ/kgand at Imagiverse:
The energy density of gunpowder is about 3 MJ/kg.But I found some information elsewhere (see this at Patent Storm) which seems to indicate that the energy density of gunpowders is more in the ballpark of 1.2 MJ/kg. At any rate, it is clear that, in terms of energy per mass, gunpowder has less energy than gasoline or diesel. But what about per volume rather than mass? Gasoline has a density of about 737 kg/m3 (see this) while modern gunpowders have densities in the ballpark of 1000 kg/m3 (see this), which means that, per volume, gasoline has an energy density of about 34000 MJ/m3 while gunpowder is about 3000 MJ/m3. However, from Imagiverse:
Although gunpowder yields a (lower) energy density than other high energy substances, it is important to note that gunpowder contains its own oxidant and therefore can release energy extremely quickly and within a highly confined space...thus the elements for danger exist: High temperatures and high pressures... Not to mention that most of this stuff is highly toxic.
Especially because gunpowder contains its own oxidant it might actually be useful in doing work because a proper air/fuel mixture has already been provided in the material itself! Do you remember, though, the episode of Mythbusters in which they tried to get an internal combustion engine to run on gunspowder? From Mythbusters episode 63:
The myth: An engine can run on gunpowder alone.
The result: busted.
Their own quote: "Even though gunpowder has a greater energy density than gasoline, none of the three historical designs worked for more than one cycle; the team could not find a practical or reliable way to feed the gunpowder into the engines (Most likely because black powder is not a liquid and therefore is not transported as easily). They were also unable to convert a modern lawnmower engine to run on gunpowder." (I find it interesting that they say "gunpowder has a greater energy density than gasoline". The data above have it the other way around! You can't believe everything you hear on Mythbusters!)
I don't think, however, that enough engineering work was done on Mythbusters to be able to say positively that an engine could not be made to work on gunpowder. They did, though, discover that one of the biggest engineering problems to overcome is the feeding of the gunpowder into the ignition chamber. That's something that probably could be overcome.
John Link, MadSci Physicist
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