It's a brave new world, kids, and in that new world, the number of cylinders an engine has is almost irrelevant to its horsepower output. For that, you can thank the proliferation of turbochargers. These days, the two biggest variables in power output are engine displacement and boost pressure, period.
Though the number of cylinders doesn't much factor into power, reducing the piston count certainly helps efficiency. Not having to schlep a piston up and down a couple of thousand times a minute -- or actuate its valves -- means less frictional losses. So in the interest in fuel economy, it's only normal that as we see specific power go up, we're seeing cylinder counts come down.
One thing that hasn't changed, and never will, is how the layout and number of pistons affects the personality of an engine. The 2.5-liter flat-four in a Subaru STI makes about as much horsepower and torque as the 5.0-liter V-12 in an old BMW 750iL - but the two engines neither sound nor feel alike. Engine layout geeks (and that would be me) can tell what's under the hood just by the sound and feel, and that's such a big part of the differentiation between cars.
There are multiple three-cylinder engines offered around the world, most of which are found in cars too small to make much sense in the U.S., but that's about to change. Ford has announced its plans to bring a three-cylinder Focus to our shores. Right now, the only three-cylinder on the market is found in the teensy Smart ForTwo. We've driven this car enough to tell you one thing: it's not properly sized for U.S. roads. Despite accurate unassisted steering and a 70-hp 1.0-liter three-banger that sounds at high rpms vaguely like a Porsche and is indeed mounted in the rear of the car, it's no fun to drive. In fact, it might be okay were it not for the single-clutch automated manual transmission that wants nothing more to break your neck.
Okay, enough about that little rollerskate. We've recently sampled three-cylinder engines in three cars that are, unlike the Smart, big enough to be called real cars. Here's what they're like.
Ford Focus 1.0 EcoBoostThe newest of the three cars we drove was a prototype of the European-specification Focus with a 999-cc triple with Ford's EcoBoost turbo and direct-injection package.
The engine is a clean-sheet design, and we use the term "sheet" literally, as its engine block is about the size of a sheet of paper. Measuring 11.26 inches long, it weighs less than 213.8 lb despite the block being made of iron. (Aluminum is lighter, but according to Ford, absorbs fifty percent more heat than iron, increasing warmup times and therefore emissions and consumption. Additionally, a two-circuit cooling system bypasses the block during warmup, and using an exhaust manifold integral to the head allows the coolant to pick up heat that would have otherwise been lost to the atmosphere -- also resulting in quicker warmup.)
Ford plans to produce the unit with at least two output levels, but the version we'll receive in the Focus makes, preliminarily, 123 hp @ 6000 rpm and 125 lb-ft between 1400 and 4500 rpm, with an overboost function for momentary spurts of up to 147 lb-ft.
These are similar peak numbers to a 1.6-liter normally aspirated four-cylinder, which typically weighs about 60 extra pounds -- and remember, turbo engines produce near-peak torque over a good portion of their operating range, not just at one spot. Ford expects the 1.0-liter Ecoboost triple to return fuel economy gains of about twenty percent over a similarly powerful four-cylinder, and quotes a combined 5.0 l/100 rating on European fuel economy cycles - and is aiming for a 50-mpg rating from the EPA.
A 3-cylinder's biggest engineering problem is its imbalance -- the engine layout results in a rocking motion that is often counteracted by a balance shaft. For increased efficiency, the Ford 1.0-liter does without such a shaft, using counterweights on both the flywheel and the crank pulley. This strategy results in "near zero" vertical motion of the engine, transferring the vibration to the lateral plane, which is far easier to control. The flywheel is a dual-mass design for additional vibration absorption, at least on high-output versions.
The DOHC 12-valve engine's long-stroke design (71.9-mm bore by 82-mm stroke) and high (10.0:1) compression ratio also contribute to efficiency, and the new engine is one of the first we've seen in a long time that uses a timing belt rather than a chain. The belt-in-oil configuration was chosen for its greater efficiency. Are you getting the point yet? It's all about efficiency.
And power. The EcoBoost 1.0 uses a Continental turbo that pushes an unspecified amount of boost into the engine, spinning to 248,000 rpm. A key metric of the engine design was response time from half to 90 percent of full load, especially at low revs. Or, in other words, minimizing turbo lag. Of course, the EcoBoost package contains variable valve timing, and with 45 degrees of range on each camshaft, the computer can dial in enough valve overlap for a "scavenging" effect at low rpms: having both sets of valves open simultaneously allows pressurized intake air to flow through the cylinder, pushing out hot exhaust gasses and keeping the increasing airflow through the turbo to keep boost elevated.
It all works very well, and though there was definitely noticeable turbo lag in this early prototype, it was no worse than, say, Ford's 2.0-liter EcoBoost. The heavy flywheel contributes to the engine's reluctance to rev, but it does make the Focus hard to stall when moving off the line. Three-cylinder engines' relative scarcity of power strokes makes power delivery at very low rpms vibration-prone, and Ford's 1.0-liter isn't immune. There's a noticeable boom from the engine compartment at 1500 rpm, but by 2000, it's all over, and this becomes a superbly quiet, smooth engine.
Power delivery is, of course, linear -- coming on strong by 2000 rpm in higher gears and remaining constant through most of the powerband. Boost peters off noticeably over 5500 rpm as the turbo reaches its flow limit, but the engine note remains pleasant and distant. Equipped with a six-speed manual, a Focus hatch will accelerate to 62 mph in 11.3 seconds according to Ford, so it's not going to win any drag-race awards. But as a base motor with promising fuel economy numbers -- on standardized tests, at least -- it's pretty darned impressive.