Random Thoughts: Syrup Energy

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Instead of Tony the Tiger in the tank, how about Aunt Jemima? Would it be possible to use a simple sugar syrup (about 50% water and 50% sugar) as a vehicle fuel?

Why Syrup?

One of the biggest challenges of large-scale use of biofuels is that refining the fuel is often extremely energy-intensive. Most products of biological processes are water-soluable, since biological process all take place in a water medium. Unfortunately, however, most current internal combustion engines can't run on a fuel+water mixture, so it is necessary to remove the water from the fuel as part of the process of refining the biofuel. This can take almost as much energy as is present in the fuel to begin with.

(Note that oil-based biofuels, like biodiesel, don't have this problem since the oil will naturally separate from the water. However, oil-producing plants tend to have a much lower yield of oil than sugar-producing plants have of sugar.)

So if you can build an engine capable of running efficiently on a fuel+water mixture, you can get a lot more biofuel for the amount of energy you put into growing and refining the fuel. In addition to making the biofuel much more sustainable, this also makes the economics of producing biofuels much more compelling since it's no longer necessary to buy massive amounts of fuel to separate the fuel from the water.

Once you've decided to use a fuel+water mixture, sugar becomes a much more compelling fuel choice than ethanol. Ethanol production always begins by fermenting sugar anyway (even cellulose-derived ethanol, since that uses enzymes to break the cellulose down into simple sugars), and sugar has a significantly higher energy density than ethanol. Sugar is a lot cheaper, too.

The only reasons to prefer ethanol over sugar are (a) ethanol can be used in existing engines with little or no modification, and (b) ethanol is a liquid, and sugar is a solid, and solid fuels are really hard to deal with in an internal combustion engine. But if we're designing a new engine specifically to run on a fuel+water mixture, we've already decided that compatibility with existing engines doesn't matter; and a sugar syrup is a liquid.

Sugar syrup has some other advantages: it's readily available from a wide variety of sources, it has a low freezing point and high boiling point, and the desired 50% mixture can be achieved fairly readily by removing water from certain plant saps (no need to dry it all the way to granulated sugar). You can even make the stuff at home, cheaply and easily.

Can It Be Done?

I don't know if a syrup-powered engine is possible, but I think it would be.  The challenge is that before the fuel can burn, the water has to boil completely inside the cylinder, since the water boils (even at high pressure) at a lower temperature than the ignition point of the sugar. Boiling the water takes energy and cools the gas inside the cylinder, making it harder for the fuel to ignite.

This isn't an insurmountable problem: you just have to get the cylinder that much hotter to overcome to cooling effect of the water in the mixture. The trick is to design the engine so that the energy used to boil the water can be recovered to help turn the engine. Since the role of the water in the syrup is essentially to vaporize and cool the combustion gasses, the engine has to be designed for a slightly higher volume of slightly cooler gas.

Thinking in terms of modifying an existing engine design, I would think that a diesel engine would be ideal, since it's intended to operate with very high compression and hot cylinders, and fuel which burns as a mist rather than a vapor. Somewhat higher compression (to yield a hot enough gas to ignite the syrup) may be the only change necessary.

One final note: sugar actually is used as a rocket fuel for some model rockets, typically mixed with potassium nitrate (saltpeter), but this is normally done with solid dry sugar, not syrup, since if the mixture has any water in it it becomes difficult to ignite. I did find, however, some YouTube videos of experiments with including sugar syrup in a rocket propellant.