Is Diesel the greenest?
September 24, 2010 2:05 PM   Subscribe

Making it simple, if possible: Is diesel a cleaner for the environment fuel than unleaded for a car that gets similar gas mileage? Basically, deciding whether to get a hybrid or a diesel car.

This topic has been touched upon in AskMefi, yes, but it's been half a decade since and I was not clear from the answers. Perhaps more fuel mileage, I want to emit the least polluting gas I can when I drive. I felt that the diesel proponents were saying their gas is better for the environment than unleaded? Is that true? I appreciate people's help in this situation. Thanks.
posted by skepticallypleased to Travel & Transportation (13 answers total) 2 users marked this as a favorite
Petrol engines are on average 25% efficient, Diesel are about 30%. Take from that what you will, I'm sure a petrochemical engineer will add a more definitive answer.
posted by Biru at 2:08 PM on September 24, 2010

There's more carbon in a gallon of diesel than in a gallon of gasoline. That's why it gets more mileage per gallon, but that also means there's more greenhouse gas emissions from burning from that gallon.
Gasoline carbon content per gallon: 2,421 grams
Diesel carbon content per gallon: 2,778 grams
posted by alms at 2:16 PM on September 24, 2010

I thought one of the benefits of diesel is that it's exhaust output has more heavier particulates that fall down instead of making it into the atmosphere as greenhouse gasses. This makes it "dirty" in the conventional sense of the word but cleaner in the sense of pollution? I have no cite for this, this is just what I remember from the last time I looked into it.
posted by RustyBrooks at 2:22 PM on September 24, 2010

Diesel exhaust is nasty. The micro-particulates are awful. Never mind the greenhouse effects, worry about the cancer and asthma links. They've made a lot of progress with diesels but they last a long time so old ones still roam the streets.
posted by chairface at 2:52 PM on September 24, 2010

Diesels emit more particulates (soot) and more NOx (a major smog component). Gas cars emit more VOCs, more CO, more benzene, and so on. Diesel has about 10% more carbon content per unit volume (but diesel engines are generally more efficient). If two vehicles have the same mileage and one burns diesel and the other burns gas, then I think the gas vehicle is a better choice (slightly less carbon emissions, less pollution).

That said, diesel engines actually become a little more efficient after breaking in, so carbon emissions might be roughly equal in the end. There are also greater environmental costs to manufacture a hybrid than a diesel car. You also need to consider what type of driving you'll be doing. In urban / suburban conditions, with lots of stop and go, a hybrid makes a lot of sense. Driving on the open highway, a hybrid might not be as advantageous.
posted by ssg at 2:57 PM on September 24, 2010 [2 favorites]

Diesel exhaust is pretty bad. But if you get one of the cleaner-burning new diesels you can use bio-diesel (lower carbon footprint), or if you get a converter you can even filter waste oil from nearby restaurants and use grease. I have a coworker who has a grease car, and it makes him happy because he's not supporting the petroleum lobby And he's burning oil that would be burned anyway, so there's not a lot of net carbon impact. It is a fair amount of work filtering and installing the system, though.

If I recall correctly, some of the clearner-burning new diesels don't get Quite the awesome mileage that the older ones did, but they can still be almost as efficient as hybrids while still having engines that are longer-lasting than gasoline engines, thereby saving the significant environmental (and financial) cost of buying a new car that much sooner.
posted by ldthomps at 3:03 PM on September 24, 2010

If you get a diesel engine, you have the option to use biodiesel, or even convert to running on vegetable oil.
posted by expialidocious at 3:04 PM on September 24, 2010

Best answer: Also, the US EPA has a Green Vehicle Guide, which gives scores for greenhouse gas emissions and for pollution to individual models.
posted by ssg at 3:43 PM on September 24, 2010

Response by poster: Thanks for the link narrows my list quite so. Still, I could have thought that the "clean diesel" folks were saying that their diesel was better for the environment.

But, by the EPA list, none of the "elite" certified cars are diesel?!

So, a Jetta "Clean Diesel" is not better for the environment than a Prius, right?....hope so.
posted by skepticallypleased at 5:00 PM on September 24, 2010

Something important to consider is the pollution that's generated during the refining process for each type of fuel. My electric lamp is clean as a whistle compared to a burning a candle, but factor in the coal that was fired to make the electricity, and well...

It might be that your front-end + back-end pollution = the same net emissions, in which case, pick the one that gives you better mileage.
posted by Dr. ShadowMask at 7:19 PM on September 24, 2010

Response by poster: I can see that.....DSM....but what's happened is tough to control. Trying to go green for something existing. Thanks for the responses here, I've appreciated them.
posted by skepticallypleased at 7:54 PM on September 24, 2010

If you get a diesel engine, you have the option to use biodiesel, or even convert to running on vegetable oil.

Not the new ones. Unfortunately the new "clean diesels" can tolerate only 5% biodiesel (or so the manufacturers say). Certainly no veggie oil. To do that you'll need a pre-2007 diesel, but those mostly emit more particulates than an equivalent gasser. These days if you buy a diesel you're stuck running petrodiesel in it.
posted by harkin banks at 9:03 PM on September 24, 2010

If you get a diesel engine, you have the option to use biodiesel, or even convert to running on vegetable oil.

Assuming that using biodiesel or vegetable oil is "green." I wouldn't assume that. Demand for biodiesel outstrips supply of used fryer oil. The gap is met with oil from other sources, like crops, which generally need fresh-water, which is itself an increasingly scarce resource, and arable land, which is also increasingly scarce. People pin a lot of hope on other of sources bio-oils, I wouldn't.
posted by Good Brain at 11:35 PM on September 25, 2010

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