Dear Umbra,

I was thinking of buying a scooter with the idea that (in addition to being fun) I would be using a more fuel-efficient means of transportation on days when the weather was good. Then I got the edition of the Daily Grist telling me how dirty motorcycles are. Now I’m worried: Are scooters the same? I’m talking about a Vespa-type thing. Please tell me that’s not as polluting as a Harley!

Thanks (even if the answer isn’t what I want to hear!),

Audubon, Penn.

Dearest Patricia,

The answer is not what you want to hear. To continue our exploration of the coarser points of engine mechanics and travel safety, which I’m sure is making the experts out there cringe, we will now discuss the two-stroke vs. four-stroke engine. The best way to understand this distinction is to look at the two-stroke diagram and four-stroke diagram on, so go there right now if you wish to skip my crude explanations, or at least understand them. In a four-stroke engine, the explosion that powers your car has four steps, called strokes because of the piston movements: The gas is injected, the gas is compressed, the gas is ignited, and the explosion leaves the chamber. In general, four-stroke engines are found in larger mobile transit sources, like Nissan Rustbuckets, and, increasingly, in motorcycles.

Cute, but polluting.

Two-stroke engines combust in, surprise, two steps: The fuel and oil are mixed and compressed in a separate chamber, then brought into the piston area, where ignition and exhaust happen in two strokes. Compared with four-stroke engines, two-stroke engines are powerful for their size, lighter, shorter lived, and filthy dirty. They are found in scooters, chain saws, lawn mowers, jet skis, and so forth. There is a moment in the two-strokers’ combustion process when the intake and exhaust valves are open simultaneously, allowing some gas/oil mixture to escape into the outside world in the form of liquid (the oil sheen around motorboats), gases (the poisonous/smog-forming/depressing compounds mentioned in the motorcycle question), and asthma-causing particulate matter. As much as 30 percent of the fuel may escape, depending on the engine.

Cheap, powerful, fun scooters and their exhaust are a huge health concern in crowded Asian cities, where they are often the most significant mobile source of pollution. So far, scooters are not as big a concern in U.S. cities, where we are wed to larger vehicles and our sprawling cities are built with cars in mind. But just think about the snowmobiles in Yellowstone and you’ll know what kinds of problems two-stroke engines pose in rural areas. Here are the sobering comparisons you’re looking for: Scooters are more polluting than Harleys. They have equivalent particulate-matter emissions to large diesel trucks, and three times the carbon monoxide and hydrocarbon emissions of those trucks. On the bright side, their low fuel use means they contribute fewer global-warming gases such as carbon dioxide and nitrogen oxides (NOx).

In short, from an environmental standpoint, four-stroke motorcycles with catalytic converters are a far better two-wheeled choice than scooters. The engine is more efficient, fuel economy is comparable or better (meaning less greenhouse gas emissions), the engine will last longer (reducing the manufacturing burden), and exhaust pollutants are lower.

If you’re totally wedded to owning a scooter, there are electric models that you could look into. Otherwise, I have one word for you: bicycle.