Why don't seven-cylinder engines exist in cars?

Except for the seven-cylinder engine, practically every other configuration exists on cars. Why is that?

Have you ever wondered why we don't have seven-cylinder engines in cars? Well, it's certainly not that complicated; in a world with two, three, four, five, six, eight, ten, twelve, and even sixteen-cylinder engines, why don't seven-cylinder engines exist? It all comes down to pretty basic physics and engineering, and Drivetribe recently delved into it in an informative video.

Titled “Why There’s No Such Thing as a Seven-Cylinder Engine?” host Mike Fernie takes us through how engines fire cylinders and the complications that can arise from adding and subtracting cylinders. Vibration is the enemy of a smooth driving experience, and with a seven-cylinder engine, it’s a big one.

The first thing you need to know is that seven cylinder engines do exist. They are used in marine, agricultural and aviation applications so they can work but their limitations make them unsuitable for automotive applications.

When you’re building an engine, opposing forces that cancel each other out are your friends. That’s why even-numbered configurations are so popular. Whether it’s the Subaru WRX and its four-cylinder Boxer engine or BMW ’s iconic inline-six in your M340i, even-numbered cylinder engines are known for being smooth and tractable, making them a perfect choice for most applications.

On the other end of the spectrum you have odd-numbered engines, and this is where things get complicated. Perhaps one of the most unstable setups is your three-cylinder engine, which has three pistons firing at different times and therefore no chance of cancelling out the opposing forces. Furthermore, there is actually a gap in any combustion action of a cylinder pushing a piston, which makes vibration even worse, and only by using a balance shaft can these engines function properly.

Most of us know Audi ’s inline fives and how well they can perform. Part of their success is that when you add two cylinders, you now have some overlap on each cylinder stroke, which helps to reduce vibration. Not to mention, the extra two cylinders add more power, making them a solid choice for those who want to save space but don’t want something too expensive.

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Seven-cylinder engines follow this trend as well. As the video shows, just like the inline-five, adding more cylinders further dampens vibration, again allowing for more power. Additionally, with an inline-seven, you're left with a slightly smaller package than an eight-cylinder, and you'll get the same amount of power. So what's stopping it from making it into an automotive application?

There are a couple of reasons. First, the 7-cylinder package only works in an inline configuration. This creates a problem with the crankshaft. The crankshaft for an engine like that would be unique and heavy. When an automaker builds an engine like a V8, they can use the crankshaft from a 4-cylinder setup to save money. That doesn’t work with an I7; it means more money and more engineering to make it work.

Unfortunately, even if you can get it to work, it's never going to be as smooth as a six- or eight-cylinder, and thanks to forced induction, both of these options do pretty much everything a seven-cylinder can do and more. Seven-cylinder engines don't need to exist, and it seems like every automaker has come to that conclusion since the invention of the automobile.