Why are there no cars with seven-cylinder engines?

37w ago


Since the dawn of the motorcar (and motorised personal transport in general), there have been many successful and unsuccessful attempts to develop and popularise different forms of engines.

The Rolls Royce 100ex concept was powered by a massive 9-litre V16 engine, although it had an already large bay to fill up... Credit - Supercars.net

We've seen three-cylinders, four-cylinders, 16-cylinders and in terms of rotaries - no cylinders. Arranged in 'straight', 'V', 'W' and even 'flat' configurations, the car world has left no stone unturned in its quest for propulsion - whether for the benefit of performance, efficiency or reliability.

So here we are in 2018, with electric being pioneered as the optimum fuel for cars and the belief that all avenues of the combustion engine have been explored. But what about seven-cylinders?

I'm not about to suggest a seven-cylinder engine would be the saving grace for the combustion engine, but it is intriguing why we haven't developed seven or even nine-cylinder-engined cars.

There are radial seven-cylinder engines used in planes and straight seven-cylinders that have been used in boats and submarines, so why not cars? Well, one reason is noise, vibration and harshness (NVH). Because it is inherently off-balance due to being an odd amount of cylinders, seven cylinder engines produce more vibrations than even-numbered V8 engines of a similar displacement.

Another reason is size. The straight seven engines used in ships are gargantuan and would never fit in a road-going car. Of course, you could shrink these down in theory but a straight-seven engine would in still be longer than a V12 and would need a larger engine bay. Although there have been straight eight engines used in race cars such as the Alfa Romeo 158/159 - which one the first two World Championship of Drivers (now known as Formula One) and the Bugatti Type 35.

The Alfa Romeo 159 straight-eight engine. Not the 2000's saloon, obviously. Credit - Ultimatecarpage.com

At this point you might be thinking about Volkswagen's VR5 engine. Why can't we make a V7 where the cylinders are arranged next to each other, making it more compact? The main reason is to do with balancing. Opposing pistons help to cancel out dynamic imbalance (when a piston pushes down on the crank, another piston is there helping balance the force out). With odd numbers, the crankshaft will need a heavy counterweight or dynamic balancer - both expensive options.

A V5 engine showing the 'W' or 'M' cylinder configuration. Credit - Car Guy's Paradise.

Weight, size and cost are just some of the factors why the seven-cylinder engine never made it to production cars. That being said, I can't help but wonder what a straight or 'V' seven-cylinder car would've sounded like...