The first automotive engines were one-cylinder wonders. Early on, the question of whether to add cylinders to the equation of engine building, probably did not include the consideration of having an even or odd total-just an increase in power and torque.
Over the last half-century and more, the trend has been toward larger engines, with more cylinders. Today we find the multi-cylinder format to be the norm. In America six and eight cylinders have been the most popular. In Europe the inline four continues to be very popular. And even-numbered arrangements of cylinders are the unmistakable norm throughout the world.
But with "Peak Oil," fuel shortages and skyrocketing prices looming on the very near horizon, there is a new trend today toward smaller, more economical engines. And in the process, another seemingly strange trend has emerged. The odd-numbered cylinder arrangement is making a comeback. Today the odd cylinder configurations are offered in cars and trucks, as well as commercial applications, like tractors. Kubota has a five-cylinder diesel tractor, and Lister-Wilder has several three-cylinder models.
Odd-numbered arrangements have included one, three, five, and even seven cylinders. Although seven-cylinder arrangements appear to have been reserved for aviation applications, using a radial piston arrangement. Radial engines specifically operate best with an odd number of pistons in each "row."
Seven-cylinder layouts have been used on the Continental A-70 aircraft, and the Cameron, both around 1930. This formula is said to be obsolete in today's world of aviation. But interestingly, a small company devoted to manufacturing alternative engines for home-built aircraft, has offered a seven-cylinder as its newest power plant. HCI introduced its new engine in 2001.
This brings us to a brief discussion of the difference between the "rotary" engine and the "radial" arrangement-two configurations quite different from the more usual reciprocating piston engines in most automobiles today.
The descriptive term, radial, refers to the arrangement of conventional cylinders situated like the petals of a daisy, around the center of the engine, that is, the rotating crankshaft. The pistons are all in the same plane, or "row" (although there may be multiple rows). Pistons are arranged in a circle around the crankshaft, their collective plane being perpendicular to the shaft. Their text explanation of this layout is weak, but a good visual animation can be seen at the Wikipedia Free Encyclopedia Website.
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