The Vespa ET2, ET4.

Ever wondered what prompted Piaggio to abandon the famous “putt-putt-putt” of the two-stroke engine and move to a four-stroke solution?

We’ve dug up this great Wired article from 2002 that details the transition made by the Vespa with the introduction of the ET2 and ET4 to the US in the early 2000s to meet tough emissions laws set by California.

Here’s a glimpse of the article:

It’s easy to see why 2-stroke engines were once so appealing. By generating power and expelling exhaust with only one revolution of a piston (the upstroke and the downstroke), they create torque quickly. This made the old Vespas fast off the line, a must for urban commuters. But the design’s efficiency is what makes it so filthy. A 2-stroke blends oil with gasoline to provide lubrication and burns the mixture together — then simply dumps the unused portion into the earth, water, and sky.

Cleaning up the Vespa proved tricky. Four-stroke seemed the obvious way to go. A 4-stroke engine, like the one in your car, has separate combustion and exhaust strokes. It generates power in one turn of the piston and expels exhaust through valves in the next; the dedicated cycle is what reduces hydrocarbon emissions. But a conventional 4-stroke is also large and heavy and so would compromise the scooter’s signature design. The Vespa’s original architect, Corradino D’Ascanio — an aeronautical engineer who had a hand in designing the first helicopter — found motorcycles to be inelegant and dirty. He wanted his scooter to be more female-friendly. So he gave it a low-slung body with a leg shield to protect the rider from splashing mud and water. Placing the gearshift on the handlebar and using smaller wheels allowed him to enclose the engine over the rear wheel, beneath the seat. This protected the rider from the engine’s heat, oil, and muck — and, just as important, allowed D’Ascanio to design a step-through frame. Motorcycle riders mount their bikes like they would a horse, and ride leaning forward. D’Ascanio imagined scooter drivers sitting upright, with room between their feet for packages — or a skirt.

To read the full article, click here.

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