Dear Science,

Is hydrogen suitable for an internal combustion engine?

Gassy Driver

The answer to your question depends on a few strokes. Most engines rely on pressurized gases to get work done. The energy in the pressurized gas is used to press on a piston, consuming the pressure to expand a chamber, and causing movement to occur. In external combustion engines, the pressurized gas is created elsewhere and piped into the chamber. For example, steam locomotives rely on a system where water is boiled with coal-fired furnaces and the steam is loaded into pipes that eventually fill the pistons. You need a boiler, strong piping, and pistons to create the engine. It's cumbersome. Internal combustion engines turn a liquid (or simple gas) into a whole bunch of gas right there in the piston—typically by burning the fuel with oxygen—removing the need for a separate boiler and high-pressure piping. Clever!

When not invading France—and forcing the Parisians to eat their zoo animals—19th-century Germans were busy inventing many of the pieces that autos and trucks rely on. Nicolaus Otto invented the first four-stroke internal combustion engine. As the piston draws down, vaporized fuel—in little droplets in air or a fuel gas mixed with air—is drawn in (stroke one). Next the piston is pushed upward, compressing the fuel-air mixture (stroke two). Let's think about the fuel-air vapor mix. In gas form, each molecule contributes to pressure roughly equally (regardless of how big the molecules are, idealizing things here a bit). The more molecules, the more pressure is created. Likewise, the hotter the molecules, the more pressure they create. So if we have a way of creating a whole bunch more molecules at this point—say, by breaking up larger molecules into many smaller ones—and heating them up in the chamber, we can create a huge amount of pressure really quickly.

At the end of the compression, the fuel-air mix is pretty miserable, with fuel and air molecules bashing into one another. In Otto's engine, a spark is fired right at this point, and all hell breaks loose. In the case of gasoline, the long chains of hydrogen and carbon are turned into copious amounts of carbon dioxide and water. From a single octane (eight-carbon chain) and oxygen from the air, eight carbon dioxides and nine water molecules are created—plus a whole bunch of heat, all of which vastly increases the pressure in the chamber, pushing the piston rapidly down (stroke three). Finally, the waste gases of the combustion are exhausted out of the chamber, preparing it for the next cycle (stroke four).

Hydrogen gas (H2) is as delighted as the carbon chains in gasoline to combine with oxygen (O2) when compressed and a spark is thrown. Thus, internal combustion engines can be converted to use hydrogen.

Cyclingly Yours,

Science

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