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In my high school chemistry class I am studying the affect of surface area over the rate of reaction. In this case, using Collision Theory to describe how a high surface area to volume ratio contributes to a greater chance of particles colliding, which leads to more successful collisions per second; given reactants are at room temperature and other variables constant.

Since flour grains (in fine powdered form) is highly flammable through combustion with oxygen and may even explode when particles that are suspended in the air are ignited, could it offer a more energy efficient method of converting from chemical to kinetic energy through a combustion engine? I was thinking that due to flour's high air to fuel character and that it's

35 times more combustible than coal dust.

Would it be possible for the expanding combustion gas to push the piston, which in turn rotates the crankshaft and drives the engine?

I am aware that the mechanical components behind this 'flour driven engine' may not be similar to the traditional combustion engine. However, would the mechanics behind this design be realistic or probable in the future, if so, how might it look like? For example in the form of an air cylinder?

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    $\begingroup$ Of course, volatilized fuel has a much much higher surface area... $\endgroup$
    – Jon Custer
    Jan 25, 2022 at 14:16
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    $\begingroup$ Welcome to Physics! This question is interesting, but it's more about engineering than about physics, and it might get closed because of this. If it does, I would recommend you try asking it on our sister site Engineering. $\endgroup$ Jan 25, 2022 at 15:41

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There is a difference between combustible and energy efficiency.

Coal dust, and other dust-form substances, are explosive in some situations. This is, as mentioned, due to surface area. They can burn quite well becaue the fuel and oxygen are well mixed. This allows the flame front to move quite rapidly through the mixture.

However, this is different from efficiency. An efficient engine is one that converts a large fraction of the energy in the fuel into mechanical energy. Dust is unlikely to be a good candidate since each dust particle must burn completely to release the energy. This is likely to occur over an inconvenient ammount of time, and at differing temepratures during the process. This may produce incomplete combustion, and a wide range of combustion products.

This gets into other performance issues for engines. Getting the fuel in is one as mentioned by niels nielsen. Having the fuel burn completely and cleanly is another.

Having the combustion procede at a useful rate is another. It must not be too slow or the engine cycle is limited. Having it proceed in a detonation fashion is also not desirable since that will probably damage the engine.

For dust you would need to control the size of the dust particles. You would need to get them into the engine. And you would need to keep them suspended in a well mixed condition. And you would have a quite limited range of engine cycle times that would accomodate the burning speed of the suspension.

Compare this with liquids that can be turned to vapor. In a vapor the size of the particle can be made extremely fine. In the case of some fuels such as natural gas, the mixture can be gaseous. It can be mixed quite thoroughly with the oxygen supply. The mixture (ratio of fuel to air) can be controlled quite accurately. This allows the engine to stay in the range where the fuel burns very rapidly but not explosively. That means you can have the energy released very efficiently but not with a detonation that damages the engine.

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People have tried to exploit the explosive flammability of finely-ground solid fuels such as flour or coal dust for years. In fact a lot of work was done on this over 100 years ago to make coal dust a viable fuel for diesel engines.

However, no one came up with a satisfactory method of mechanically metering precise and repeatable amounts of coal dust into the combustion chamber of a diesel engine at precisely the right moment in the engine's cycle, and so this idea was abandoned in favor of using liquid fuel instead.

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