by Main Problems of Seven-Cylinder Engines
Historical Context and Physical Limitations
If you’ve ever studied the long history of engines and wondered why car manufacturers have tried everything from tiny three-cylinders to massive sixteen-cylinder monsters, but never created seven-cylinder ones, you are not alone. After all, four-cylinder engines are the norm, and six- and eight-cylinders have remained popular for decades, so why not seven?
Mathematical and Engineering Challenges
It turns out that the humble number seven simply doesn’t play by the laws of physics – or with the accountants. As Auto Motor und Sport notes in a recent article, the crankpins would have to be arranged every 51.42857 degrees, which would require extremely precise machining. Inline sixes have a neat 60-degree arrangement, while V8s sit comfortably at 45 degrees. It’s simple geometry; a seven throws the protractor out the window.
Another problem is balance, or rather the lack thereof. While a higher cylinder count would make a seven-cylinder engine smoother than an inline three or five, it would generate rather odd harmonic vibrations. Inline sixes and V8s with cross-plane crankshafts are naturally smooth because the piston forces naturally cancel each other out, but a seven cannot do the same.
Why Do Other Odd-Numbered Engines Work?
It’s true that some engines with an odd number of cylinders exist successfully. Five-cylinder engines (think Audi Quattro or Volvo 850R) found a sweet spot between smoothness and size. Numerous city cars and subcompacts use three-cylinder engines, which have proven to be peppy and efficient – often with balance shafts to prevent excessive vibrations. But a seven is a different beast entirely: too large for compact simplicity, too unbalanced for luxury smoothness.
Seven-cylinder engines can be found in massive ships or agricultural machinery, where they rotate slowly and don’t require high RPMs. At a few hundred revolutions per minute, you can balance things out with mass and flywheels the size of coffee tables. But in a road car at 7000 RPM? Forget it.
Economic Feasibility and the Future
When it comes down to it, a seven-cylinder engine offers no significant advantages over six or eight cylinders, but adds plenty of disadvantages. If a car manufacturer were going to throw common sense and engineering logic far enough out the window to build a seven, it would have happened by now. As brands transition to hybrid and fully electric powertrains, the old race for cylinder count is quickly fading.
If you want a smooth, torquey, perfectly balanced engine, an electric powertrain does it effortlessly, while three-, four-, six-, and eight-cylinder engines, almost all turbocharged and increasingly assisted by hybrid help, cover all the combustion bases. Need more power? No need to add cylinders; just turn up the boost.
Alternatives for Enthusiasts
For enthusiasts, the idea of a seven-cylinder engine and its likely strange sound is still captivating. But don’t hold your breath for it. If you really want something rare and weird, try a Mazda RX-7 with a rotary engine, a Lancia V4, or seek out an old VW V5 or the troubled W8 from the same company.
It should be noted that technological progress in the automotive industry in recent years has significantly changed priorities. Instead of exotic engine configurations, engineers are focusing on optimizing existing designs, combining them with electrified components. This allows for better efficiency and performance figures without the need to invent radically new schemes. Furthermore, modern active vibration cancellation systems and computer modeling could theoretically solve some of the problems of seven-cylinder engines, but economic infeasibility remains the deciding factor.
