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If you've gone shopping for a car lately, you might have noticed that fuel economy—the number of miles per gallon—is creeping up. One reason is a more efficient type of fuel injection, called gas direct injection. It’s a feature of half the new cars sold in the U.S. last year.

如果你最近买了一辆车，你可能已经注意到——燃油经济，每加仑行驶的总英里数正在蠕升。原因之一在于燃油喷射的效率更高，称为缸内喷气。这是去年所出售的一半新车所具有的一大特征。

"With that kind of engine design you can also introduce turbocharging—that's really what it comes down to: more engine power with a smaller engine." Naomi Zimmerman, an air-quality scientist at Carnegie Mellon University—who has never owned a car. "Probably because I did a whole PhD on engine emissions¹."

Here's how the two injection methods differ. In the old standard, called "port fuel injection," gas is injected into the air intake², where it mixes with air before hitting the cylinder³. In direct injection though, as the name implies, the fuel is injected directly into the cylinder. It's more fuel efficient, meaning lower CO2 emissions, which will help slow climate change. But paradoxically, it also emits more particulate⁴ matter, including black carbon, or soot⁵—a pollutant⁶ implicated⁷ in global warming.

这种类型的引擎设计，也可以引进涡轮增压——这才是真正的原因——更小马力的引擎，更多的发动机功率。内奥米齐默尔曼，卡内基梅隆大学一名空气质量科学家——从未拥有一辆车。“可能是因为我博士研究生阶段做了关于发动机排放的整体研究。这是这两种喷射形式的不同之处。按照旧标准，称之为“气门式燃料喷射”，将气体注入进气口，在冲击汽缸之前与空气混合。然而，直接式燃料喷射，顾名思义，燃料直接注入汽缸，这样更省油，意味着较低的二氧化碳排放，这将有助于减缓气候变化。但是，与此相矛盾的是，也能喷射出更多的颗粒物。包括黑碳或煤灰——与全球变暖有直接关系的污染物。

Zimmerman and her colleagues analyzed⁸ studies of direct injection engines to see how this pollution tradeoff works out for the climate. "The answer is that it's really complicated and it's not certain." But on average, she says, boosting fuel economy five to nine percent would probably cancel out the warming effects of black soot, and score a net positive for the climate. The findings are in the journal Environmental Science and Technology. [Naomi Zimmerman et al., Assessing the Climate Trade-Offs of Gasoline Direct Injection Engines]

齐默尔曼与她的研究团队对直喷式发动机的研究进行分析以查明这一污染权衡怎样使气候受益。“答案是，这真的很复杂，目前还不能确定。”但平均而言，她表示，将燃油经济性由5%提高到9%，可能会消除黑炭的暖化效应，整体而言对气候有益。该项研究结果发表在《环境科学与技术》杂志上。

The fuel economy goal might be different in places with big temperature swings, and where gasoline has more aromatic⁹ compounds—which seem to boost black carbon emissions. One place that fits the bill: Canada. “There we might need to be closer to the 10 percent, 15 percent mark in terms of improving fuel economy. Which is actually a very aggressive increase in fuel economy."

燃油经济性目标可能在那些温度波动较大、汽油中含有更多芳香化合物的地方会有所不同——这似乎促进了黑碳排放。刚好符合要求的一个地方：加拿大。在这里，就改善燃油经济性而言，我们可能需要将其提高接近10%，15%，这的确是燃油经济性的巨大极高。

She says one solution that might reduce black carbon emissions is gasoline particulate filters. Which would also screen out particulate matter dangerous to human health. Meaning a win for the planet—and the people who live on it.

她表示减少黑碳排放的一个可行的解决方案是汽油颗粒过滤器，它也可以筛选出对人类健康有危害的颗粒物。这意味着赢得了这个星球——并且人们在这个星球上生活。