Is it possible to make a gasoline engine so efficient that it would emit less carbon dioxide per mile than is created by generating electricity to run an electric car over that same mile?

Small Japanese carmaker Mazda says yes.

In an interview published last week with the British magazine Autocar, Mazda claimed that its next generation of SkyActiv engines will be so fuel-efficient that they'll be cleaner to run than electric cars.

MORE: As Gas Mileage Rises, Energy To Make Cars Gets More Important

That's possible. But as always, the devil is in the details.

Power plant

Power plant

Specifically, total emissions of carbon dioxide (CO2) in each case depend on both the test cycles used to determine the cars' emissions and the cleanliness of the electric generating plants used to make the electricity.

In the U.S., the "wells-to-wheels" emissions from running a plug-in electric car 1 mile on even the dirtiest grids in the nation (North Dakota and West Virginia, which burn coal to produce more than 90 percent of their power) equate to those from the best non-hybrid gasoline cars: 35 miles per gallon or more.

The U.S. average for MPG equivalency is far higher, however, and it's roughly three times as high--near 100 mpg--for California, the state expected to buy as many plug-in cars as the next five states combined.

In Europe, however, 35 mpg is a perfectly realistic real-world fuel efficiency for small diesel cars (generally compacts and below). And their official ratings are often higher still.

MORE: Electric-car wells-to-wheels carbon-emission equivalencies in MPG

European test cycles for measuring vehicle emissions (which translate directly to fuel efficiency) are gentler than the adjusted numbers used in the U.S. by the EPA to provide gas-mileage ratings.

Mazda SKYACTIV-G 1.3 direct-injection gasoline engine

Mazda SKYACTIV-G 1.3 direct-injection gasoline engine

On the generation side, some European countries use coal to produce a large proportion of their national electricity. (Some also buy their natural gas from Russia, a supplier that may appear more problematic today than in years past.)

So if Mazda can increase the fuel economy of its next-generation SkyActiv engines by 30 percent in real-world use, as it claims, it's possible that its engines might reach levels approaching 50 mpg or more--without adding pricey hybrid systems.

And those levels would likely be better than the wells-to-wheels carbon profile of an electric car running in a coal-heavy country--Poland, for example.

Mazda will raise its current compression ratio of 14:1 to as much as 18:1 and add elements of homogeneous charge-compression ignition (HCCI) to its new engines.

The HCCI concept uses compression itself to ignite the gas-air mixture--as in a diesel--rather than a spark plug, improving thermal efficiency by as much as 30 percent, though so far only under light loads.

Mazda's next round of SkyActiv engines won't emerge until near the end of the decade, "before 2020." Even its current-generation diesel models still haven't been launched in the U.S.

SkyActiv badge

SkyActiv badge

With rising proportions of renewable sources like wind and solar, and perhaps more natural gas, some European grids will then be cleaner than they are today--making the comparison tougher for Mazda.

But the company's assertion is at least plausible. We'll wait for actual vehicles fitted with the new and even more efficient engines to emerge, and see how they compare to the lastest grid numbers then.

Electric-car advocates may be tempted to pooh-pooh any vehicle with any tailpipe emissions. Or they may point out that electric-car owners in the U.S. appear to have solar panels on their homes at a much higher rate than the country at large--meaning much of their recharging is done with virtually zero carbon emitted.

But every effort to reduce the carbon emissions per mile of the trillions of miles we drive globally every year is a step in the right direction.

Will Mazda lead the march along that path? We look forward to learning more about its next SkyActiv engines.


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