Americans like a lot of things in threes — the number of songs played on Thursdays, patties of burger meat, celebrity deaths. But, generally, we've always regarded four as the lowest limit of cylinders in a dignified car engine. This is the year that's changing for good, and we should all welcome it.
The rest of the world has had more experience with 3-cylinder engines, but in America, the list of even remotely common three cylinders is pretty short: a few 2-stroke Saabs way back in the day, the occasional Subaru Justy, and then Geo Metros (possibly the car most associated with 3 cylinders in America still) and more recently the Smart ForTwo. That's not a lot of cars, and the almost exclusively low-end nature of the cars is a large part of why many Americans still look at triple cylinders with a mix of pity and shame.
Interestingly, odd-numbered cylinder engines have always had a sort of stigma about them, being usually seen as weird compromises. Five-cylinders were made to fit in places a six wouldn't, threes were for people too blighted to afford that final fourth cylinder, and a seven cylinder engine was a mythic beast who's coming could only signal the beginning of the End Times, or perhaps a combine or something.
But things are changing. Recently, there's been a flood of announcements from car makers announcing interesting new three-cylinder engines: Nissan's got a 400 HP one for Le Mans, Ford's little EcoBoost 3 fits in a suitcase and makes 120 HP, BMW has a nice turbo three, VW is already selling 3 cylinder engines in gas and diesel, I just tested a Mitsubishi with 3 cylinders, GM has a new one, and the list goes on.
There's a few reasons why three cylinders have taken so long to really catch on. The first is that in four-stroke engines, they're inherently unbalanced, since with only three cylinders, there's going to be at least one cycle where none of the cylinders is firing. With a crank angle of 120°, cranks are rotationally balanced. But, because the three cylinders are offset from each other, the power strokes in the end cylinders causes a sort of pitching motion lengthwise down the engine. A straight-six compensates for this with the second bank of three timed to cancel this out, but on a three cylinder things can get a bit rocky.
There's ways around this, from super-supple engine mounts to balance arms, to eccentrically-weighted flywheels. The extra complications caused by this motion was a factor in the more widespread use of four-cylinder engines, which don't suffer from this, having a cylinder to match each stroke of a four-stroke engine.
Other reasons why threes never really caught on was that until recently, it just wasn't that easy to get good power out of such a small engine. Thanks to mature turbocharging technology, direct injection, and a vast amount of other improvements over the years, this hurdle appears to be leapt at last as well.
It's time for three cylinders. The tyranny of the quad-piston engine as the default cylinder count for cars is finally coming to an end, and with ever-increasing demands on efficiency and emissions, getting rid of that extra cylinder makes sense. There's less internal friction, less weight, and cheaper manufacturing costs. The biggest remaining hurdle is in the public's perception of the mystical three.
And that's where we, as gearheads, have to help. Many of us understand that these modern three cylinder engines can be as efficient and powerful as run-of-the-mill fours, and it's up to us to help break down meaningless stigmas in the minds of all our car-ignorant friends and family. Explain to your mom that she doesn't need that extra cylinder to drive 32 MPH on a 45 MPH road to go to the "good" Target. Remind your wanna-be gearhead co-workers that most of these modern 3-cylinders have power pretty damn close to V8s in the mid-1970s.
The more 3 cylinder engines that get made, the more small, powerful engines that will be available in junkyards in the upcoming 20 years for your total 12-cylinder, one-engine-per-wheel backyard projects, and we all know what a fantastic thing that will be.
All hail the threes.