The Square Four Motorcycle Engine Was Weird And Awesome

Did you know that a square four engine used to be a thing? In fact, early designs date back nearly 100 years to a man named Edward Turner. He was a motorcycle designer in the UK who penned up a compact engine that essentially fused two small twin cylinders into a single four-pot. He sold the design to a company called Aerial Motors, and thus the Aerial Square Four was born.

One doesn’t simply stick a couple of inline twins together for a four-banger. As this video from the YouTube channel Driving 4 Answers demonstrates, this engine is truly a square shape with the cylinders pointing straight down to the crankshafts. Yes, that’s crankshafts—the design uses two counter-rotating crankshafts connected with geared flywheels. The cranks trail each other by 180 degrees, and the pistons are diagonally matched. This results in a firing order identical to an inline-four.

Without getting too technical, this all leads to an engine that’s inherently well-balanced and smooth-running. Based on the video, it doesn’t sound bad either. And the square design makes it supremely compact, essentially taking up the same space normally occupied by a V-twin or even a big single-cylinder mill.

The square four engine wasn’t just a flash in the pan. Aerial Motors built the Square Four motorcycle for nearly 30 years, and square four engines were also used by a few other motorcycle manufacturers. It made good low-end power, but it was an expensive engine for the time, and it suffered from overheating issues.

There was also precious little room to have a thriving intake system to feed the engine, which was fine as long as the powerband stayed low in the rev range. But as motorcycle technology evolved to small, high-RPM engines, the low-speed square four just couldn’t keep up. Aerial ended production of the Square Four in 1959.

The video notes that technology has evolved to the point where a square four could thrive. But with the world slowly moving towards electric power, this is likely an engine design that will exist only in the history books.