About a week back, I stumbled across a heated online conversation about the quirky contraption that is the inline five-cylinder engine. While one guy continued to refer to the motor as “sheer folly,” his debater argued that oddly numbered engines still serve a purpose, even if they may not be nearly as smooth or capable as modern conventional engines.
But right as I was about to keep scrolling, something intriguing caught my eye. The inline-five guy mentions that on an international scale, there are still plenty of vehicles being made with oddly numbered amounts of pistons pumping inside. Being that I drive a Daihatsu over-cab kei truck with an anemic little 3-banger underneath, I found myself nodding in approval all the way up until his next statement. Which, needless to say, raised at least one eyebrow.
The guy claimed that technically an inline 7-cylinder engine could work, and work well at that, if only someone were to take the time to engineer one. Being that I have never encountered a straight-seven motor, I decided to do a little research.
Apparently, seven-cylinder engines do technically exist, they are just reserved for aircraft and large-scale industrial vehicles. However, no street-legal production automobile in recorded history has sported an inline seven-cylinder engine. Why is that? And more importantly, how badass would that sound?
Overcoming The Odds
While poking around, I came across a recent video upload from DRIVETRIBE. While the amusing antics of James May, Richard Hammond, and the foul-mouthed Orangutan were nowhere to be seen, the insights of contributing host, Mike Fernie, were there in abundance. And low and behold, the topic of conversation was all about seven-cylinder engines.
As Fernie explains, the straight-seven design suffers from many of the same maladies as the previously mentioned inline-five-cylinder motor. Ergo, balancing is more than likely one of the main reasons why the 7-banger never became mainstream, or even attempted by mainstream automakers. When compared to the self-balancing act that is the inline-six cylinder motor, a straight-seven doesn’t have the right number of evenly placed pistons to keep the firing order and corresponding lower-end undulation in… well… order.
According to Fernie and every other paper on the matter we came across, a straight-seven’s firing order would be all odd pots first, followed by the even cylinders. So a 1, 3, 5, 7, 2, 4, 6 sequence. It may sound strange, but this frenzied firing order would be somewhat subdued by the sheer volume of cylinders firing. The 720-degree four-stroke cycle within this engine configuration does benefit from having more pistons firing, which as we know, uneven firing orders can be slightly smoothed by adding additional pistons to the mix. Hence an inline-five is far smoother than my Daihatsu’s crappy little sewing machine of a three-banger.
But even that will only get you so far. Reaching an equilibrium within this sort of setup would require an asinine amount of erroneous engineering. And for what? Just a few extra twists of torque and some additional top-end speed that could be far more easily achieved with a smaller, forced induction motor. That, or just opt for a big-ass V8 and call it a day.
So why not just throw a balance shaft at the equation and call it a day? Unlike my kei truck’s stubby three-cylinder, the level of primary and secondary forces going on within a seven-cylinder are astronomical. To get this to work, a straight seven-cylinder engine would require a rather large (and heavy) balance shaft assembly, and that’s just for starters.
Balancing the crankshaft would only get you so far, as that would likely require its own counterweight, and even then all of those firing gaps between cylinders would still be present. Sure, those extra pistons would cause internal combustion cycles to spike, thus smoothing out the engine’s rotational mass along the bottom end. But at what cost?
Having such a long crankshaft would cause all kinds of additional issues. Not just with clearance, but with torsional imbalance and the risk of distortion or failure under heavy load. Don’t even get me started on what an engine teardown on something like this would be like too, especially if it were turbocharged. Furthermore, forged crankshafts still cost a lot to manufacture, making the performance-per-penny portion of the puzzle pretty perturbing.
Oh Look, We Found Seven-Cylinder Sounds!
Now that’s not to say that there aren’t seven-cylinder engines in existence. It’s just that they are intended for industrial applications, and would not work (or fit) in the average automobile.
Check out the video above, which demonstrates the firing order of the AGCO Power HD 98. A 9.8-liter turbocharged diesel inline-seven engine, that utilizes the aforementioned firing order. Although this engine apparently can only rev to about 2,100 rpm, the video creator decided to push the envelope, just to see what an I7 might sound like blasting down the interstate.
Even when used as intended these engines are pretty bonkers. Relying upon 598 cubic inches of real estate, this I7 engine makes about 470 horsepower at 2,100 rpm and 1,327 lb-ft of torque at 1,500 rpm thanks in part to its compound turbocharger system.
Additionally, each 1.4-liter cylinder in this motor comes with its own personalized cylinder head. It may sound strange, but this modular design allows the engine to be assembled in an array of different configurations for superior fitment and reliability.
The Other Magnificent Seven
Certain submarines and freightliner ships also come powered by seven-cylinder engines. But this is purely because they often operate on a set RPM range, which does not fluctuate rapidly or all that often for that matter. An automobile engine must be able to jump to task at the blip of the throttle, which can be a real challenge for this type of engine design.
As for prop planes, which often rely upon oddly numbered engines, these motors run on a completely different, axial internal combustion process. So good luck stuffing all that beneath the bonnet of your Miata, amigo.
So essentially, seven-piston engines suffer from the same malady as their smaller, inline-five sibling: They’re completely erroneous. But if an even number of cylinders is the key to creating reliable, smooth, linear power, then why not go all-in and whip up a “V14” engine? Let that sink in for a second…