How Good Old Car Engines Got So Efficient

How Good Old Car Engines Got So Efficient
An automobile engine at a Volvo plant in Torslanda, Sweden
Photograph by Kristian Helgesen/Bloomberg

It’s official: Cars in the U.S. are more efficient than they’ve ever been. The average vehicle bought last year covered 23.6 miles on a gallon, up from 22.4 miles the year earlier, according to a new report (PDF) from the U.S. Environmental Protection Agency. Automakers are on track to hit a fleetwide average of almost 55 miles per gallon by 2025.

Many of the gains have been driven by electric technology—better batteries and magical systems that recharge them when drivers step on the brake—but old-fashioned engines that run on dead dinosaurs are moving the needle, too. Almost 700,000 Americans (PDF) are tooling around in new Ford F-series pickups this year, the base model of which gets slightly more than 20 miles per gallon on the highway.

How are engineering wizards getting to mileage numbers like these? We took a look under the hood and talked to people at the EPA to find out. Here are four technical terms that make a difference—and what they mean in layman’s terms:

1) Direct Injection (31 percent of new cars): Engine efficiency surged when carburetors were replaced by fuel injectors. Several new engines come with systems that shoot gas directly into each cylinder, as opposed to mixing it with air outside and then having a piston “pull” the explosive soup inside the combustion chamber. The process is more precise, letting the engine dole out less gas.

2) Cylinder Deactivation (8 percent of new cars): Why choose between a V8 and a V6 when you can have both? When a big engine idles, or cruises, its cylinders are almost starved for air. They aren’t using much gas, but the gas they are using isn’t being burned very efficiently. A growing number of vehicles—typically larger trucks—now have systems that idle two cylinders when the driver is cruising, routing the same amount of air to fewer combustion chambers.

3) Continuously Variable Transmissions (15 percent of new cars): Drivers tend to burn the most gas just before—and right after—a car shifts to higher gear. A variable transmission functions somewhat like a belt around two pulleys that are constantly shifting in size. Since there aren’t gear levels per se, the vehicle always stays in the efficiency sweet spot. (Here’s a nice animation of how it looks.)

4) Turbo (15 percent of new cars): Efficiency is all about what engineers call “specific power,” the output of an engine relative to its size. What they strive for incessantly is greater output from smaller engines. Turbo units do just that. Instead of relying on pistons to “pull” air into a cylinder, the engines with turbo essentially have a  complex little fan that pushes—or compresses—air so that more can be forced into each cylinder, creating more power on the rare occasions that drivers ask for a little more pep. The share of new cars with turbo units has almost doubled in the past year.

U.S. automakers are still playing catch-up on the efficiency front. Trucks aside, Toyota, Honda, and Mazda were the mileage winners on the EPA list, each with an average mpg over 29. Ford finished at 27.4 miles per gallon, followed by GM at 25.6 and Chrysler at 24.7.

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