Why Are 7-Cylinder Engines Not Used in Cars? | Carscoops

      We’ve encountered car engines with every cylinder arrangement from two to twelve, and a few variations beyond that, but seven has never been one of them - here’s why.

      Seventeen hours ago

      by Chris Chilton

      Seven-cylinder engines encounter mechanical shortcomings and overall poor balance.

      The geometry of the crankshaft leads to intricate engineering and vibration challenges.

      Inline seven-cylinder engines struggle with size limitations and possible crankshaft flex.

      If you've ever examined the extensive history of engines and wondered why manufacturers have experimented with everything from tiny three-cylinder engines to massive sixteen-cylinder ones, yet never a seven-cylinder, you're not alone.

      After all, while four-cylinder engines are standard, six- and eight-cylinder engines have been favored for many years. So, why is there no seven-cylinder option?

      The Mathematical Dilemma

      It appears that the simple number seven doesn't align well with the principles of physics—or with financial planning. A recent article from Auto Motor und Sport highlights one issue: the crank journals would need to be set at intervals of 51.42857 degrees, which requires sophisticated machining.

      A straight-six engine boasts easy 60-degree spacing, while a V8 comfortably fits at 45 degrees. That’s simple geometry; seven, on the other hand, complicates matters.

      Another concern arises with balance, or rather the lack thereof. While having more cylinders would make a seven-cylinder engine smoother than an inline three or five, it would produce some unusual harmonic vibrations.

      Inline six-cylinder and cross-plane crank V8 engines are naturally smooth due to the way piston forces cancel each other out, but a seven-cylinder engine would not enjoy the same advantage, as explored by Drive Tribe in the video below.

      What About Other Odd Engines That Work?

      It’s true that some unconventional engines have succeeded. The five-cylinder engine (like those in the Audi Quattro or Volvo 850R) strikes a good balance between smoothness and size. Many city cars and subcompacts utilize three-cylinder engines that have proven to be lively and efficient—often equipped with balance shafts to prevent excessive vibrations.

      However, seven cylinders present a unique challenge: too large for compact efficiency, and too unsteady for a smooth luxury ride.

      You may find seven-cylinder engines in large ships or agricultural machinery, where they operate slowly without needing high revs. At low RPMs, balance can be achieved through sheer mass and large flywheels. But in a 7,000 RPM road car? That's not feasible.

      The Other Reality: Is It Worth It?

      Ultimately, a seven-cylinder engine wouldn’t provide significant advantages over six or eight-cylinder options but would come with numerous drawbacks. If an automobile manufacturer were to ignore common sense and engineering principles to create a seven-cylinder engine, it would have already occurred by now.

      As automakers increasingly transition towards hybrid and fully electric powertrains, the traditional competition for cylinder counts is rapidly diminishing.

      If you're seeking a smooth, powerful, and perfectly balanced engine, electric drivetrains achieve this effortlessly. Meanwhile, three-, four-, six-, and eight-cylinder engines—most featuring turbochargers and more frequently augmented by hybrid technology—are covering every aspect of combustion needs. Want more power? Simply increase the boost without the need for additional cylinders.

      For enthusiasts, the concept of a seven-cylinder engine and its potentially unique sound remains intriguing. However, don’t expect to see one anytime soon.

      If you're in the market for something unusual, consider a rotary-powered Mazda RX-7, a Lancia V4, or seek out an old VW V5 or the same company's problematic W8.

      For a clearer yet accessible explanation of primary and secondary balance issues that determine engine smoothness—including why an inline six excels compared to a V6—check out the video below.

      Sources: Drivetribe, AMS