Dear Science

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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