A Climate Tradeoff for Fuel-Efficient Engines?

4:31 minutes

Fuel-efficient direct injection engines work by mixing gasoline in the combustion chamber, allowing a vehicle to burn less for the same amount of power. And they’re quickly taking over the market for new vehicles in the United States as carmakers work to meet climate targets.

But new research suggests direct injection is not a perfect solution for reducing our cars’ contribution to climate change. Carnegie Mellon air quality researcher Naomi Zimmerman shares the good and bad of this technology.

Segment Guests

Naomi Zimmerman

Naomi Zimmerman is an air quality researcher at Carnegie Mellon’s Center for Atmospheric Particle Studies in Pittsburgh, Pennsylvania.

Segment Transcript

IRA FLATOW: And now it’s time to play Good Thing, Bad Thing. Because every story has a flip side.

American fuel economy keeps on rising. In 2014, light-duty vehicles were nearly 70% more fuel efficient on average than in 1980. And one of the newest drivers in improved fuel economy, it’s not a battery, it’s not a renewable energy technology. It’s an engine innovation called direct injection.

In 2015, nearly half of the new vehicles had direct injections, like fuel injection. And that percentage is expected to go up.

Sounds great, right? Well maybe not so fast. Not entirely.

Here to discuss the good and the bad of direct injection is my guest Dr. Naomi Zimmerman, a post-doctoral researcher at Carnegie Mellon University Center for Atmospheric Particle Studies. Author on a new research paper on the climate impacts of direct engine injection engines.

Welcome, Dr. Zimmerman.


IRA FLATOW: So what is the difference between– I know about fuel injection. You take the gasoline. You inject it into the engine. But so there is direct injection, and what’s the other one? What’s the–

DR. NAOMI ZIMMERMAN: Port fuel injection.


DR. NAOMI ZIMMERMAN: So in that case, you inject the fuel into the air intake. And it mixes really well before it gets into the combustion cylinder.

IRA FLATOW: So what’s the good thing and bad thing about this whole thing?

DR. NAOMI ZIMMERMAN: Sure. So the good thing as you sort of implied it is that the new gasoline direct injection engines have improved fuel economy compared to the port fuel injection strategy. But the bad thing is that with the direct injection of the fuel into the cylinder, you can actually produce a lot more black carbon, which is a climate warming agent.

IRA FLATOW: That is not a good thing. So black carbon, so you get more black carbon out of this?

DR. NAOMI ZIMMERMAN: Yes. That’s correct.

IRA FLATOW: Is there any way– is there any way to combat that?

DR. NAOMI ZIMMERMAN: Yeah. Absolutely. And there’s a whole sort of string of things that can happen.

The first thing you can do if you think about the pipeline of a car is that you can deal with the fuels. It’s been shown that some components of the fuels may influence how much black carbon forms in these engines.

And then when you get to the engine itself, there’s a lot of work in the auto industry right now to try to improve the injection design to minimize black carbon emissions.

And then after the engine, you can also put some after treatment on the exhaust line. So things like gasoline particulate filters, which may induce a small fuel penalty, could still be a means of balancing out this issue and making it sort of climate neutral or climate beneficial.

IRA FLATOW: So how do you balance off the trade-offs? If you get a little more fuel efficiency but you get more carbon, how do you balance out that trade-off?

DR. NAOMI ZIMMERMAN: Yeah. So that’s what I was looking at in the study was just trying to figure out exactly how much improvement in fuel economy you might need to offset those black carbon emissions. And so if you are achieving those fuel economies that you need to offset the black carbon, then you’re great right off the bat. And if not, then again, like I said, you can try to target fuels, or revisit the way we inject fuel into the cylinders, or put on gasoline particulate filters.

IRA FLATOW: Can you actually shop for a car by asking, hey, is it is it a direct injection, or what?

DR. NAOMI ZIMMERMAN: I think it’s not– something that’s not very well advertised right now. So most people, when they go to shop for a car, they’re looking sort of just explicitly at the fuel economy. Is it good fuel economy compared to the car we’re replacing it with? And it’s not really well-advertised whether or not it’s a port fuel injection or a gasoline direct injection.

Certainly, it’s something that you might want to ask the next time you’re buying a new car. And hopefully, the person that is selling you the car would know. Or if not, do some research online. You can normally dig up what kind of technology your engine has.

IRA FLATOW: So your bottom line is it may save you a little bit of fuel efficiency with a direct injection, but you’re going to pay for it by putting out soot basically.

DR. NAOMI ZIMMERMAN: Yeah. That’s possible, yeah.

IRA FLATOW: All right, Dr. Zimmerman. Thank you for taking time to be with us.

DR. NAOMI ZIMMERMAN: Oh, thank you so much for having me.

IRA FLATOW: You’re welcome. Dr. Naomi Zimmerman, post-doctoral researcher studying air quality at Carnegie Mellon University.

Do you know if milk cartons fall under paper or plastic when you recycle? And can you recycle a plastic bag? This is not a simple answer.

We’re going to answer all of these questions when we ask a recycler. Our number, 844-724-8255.

It gets pretty tricky, especially those little triangles at the bottom of the– You’ll find out they’re not what they should be. We’ll tell you why after this break. Stay with us.

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