Excerpted from Take the Wheel: A Woman’s Guide to Buying a Car Her Own Damn Self by Kristen Hall-Geislernew and improved and available November 2017!

Back in the early days of the automobile, everybody and his brother (never his sister) thought they could build a better engine. The internal combustion engine we know and love and find under most hoods is called an Otto cycle engine. But not long after Nikolaus Otto built his engine, Rudolf Diesel built his.

The differences are slight—the Otto engine uses spark plugs to ignite the fuel, while the Diesel engine uses glow plugs—but they’re significant enough to require slightly different forms of fuel. For decades, diesel fuel caused that smelly, black, horrid stuff that streamed out of the exhaust pipes on semitrucks. But over the years, the federal government has passed stricter regulations for emissions from diesel engines for both semitrucks and passenger vehicles. The end result is what we now call “clean diesel.” It’s an energy-dense fuel, so you can really rack up the miles per gallon.

Another benefit, for those with access to it, is that you can run biodiesel in most modern diesel engines. This stuff is made from old french-fry grease or other industrial veggie-oil waste. Where I live in Portland, Oregon, this caught on pretty early, and you could tell when you were following a biodiesel car not by the loud-and-proud eco-stickers all over the trunk, though those were certainly there, but by your sudden and unexplained cravings for french fries, or doughnuts, or whatever the source was for the fuel.

Who Gets the Biggest Bang for This Buck?

Diesel vehicles excel on the highway, turning in hybrid-like numbers or better in some cases. These cars work best for road warriors who travel for their jobs or take as many four-day weekends at the coast as they can squeeze out of their paid time-off days.

Diesel has gotten hard to find in the United States in the wake of the Volkswagen scandal, and it’s not looking good for this fuel in the future either. A quick refresher: in the summer of 2015, researchers found that VW had used a few lines of code to cheat emissions testing. The tests are done on machines that run the engine through a pattern—fast, then slow, then accelerating hard, etc. The engineers knew the pattern, so when the car’s computer recognized it, it changed the way the engine ran to reduce the emissions coming out of the tailpipe. When the test was over and normal driving resumed (turning the steering wheel was a dead giveaway to the software), the engine mapping reverted to a sportier, more fun, and dirtier profile. This was done for years in Volkswagens and Audis, and in the wake of stricter, more thorough emissions tests, several other manufacturers got caught. Not to the tune of $15 billion to right the wrongs in 500 million cars like VW, but caught cheating nonetheless. That spurred VW (and likely other manufacturers) to invest more heavily in electrified vehicles.

But that’s not all. In addition to France banning the sale of gasoline- and diesel-powered vehicles in 2040, four major global cities are banning diesel by 2025. Paris, Mexico City, Madrid, and Athens decided that diesel, with its particulates and nitrogen oxides, was contributing massively to the air quality issues in those cities. Not even clean diesel is clean enough.

Is It Really Green?

Diesel is still drilled out of the ground, just like gasoline. They’re both products of crude oil, along with jet fuel, kerosene, and motor oil. A little more bad news: since diesel is so heavy and oily, it emits far more soot out the tailpipe, which you’ve surely noticed coming from older diesel engines. This leads to smog and crap in your lungs. Some manufacturers have figured out ways to counteract the soot without cheating on tests. Mercedes-Benz claims that with its BlueTEC system, you can stand directly behind the tailpipe in a white suit and not get dirty. I would probably still get dirty because I seem to be like Pigpen from Peanuts, but I get what they mean.

Biodiesel is gaining traction beyond the old Volvos converted by hippies to run on french fry grease. It’s an EPA-approved fuel now made from recycled cooking oil, soybean oil, and animal fats. It can be used in any diesel engine without making any modifications, and it does not void any manufacturer’s warranty, especially if it’s a blend of biodiesel and petroleum-based diesel. The petroleum diesel—diesel classic, let’s call it—helps clean out greasy deposits from the biodiesel. Diesel in any form is pretty oily, so the engines are capable of handling most of it anyway. Like ethanol blends, you’ll know it by its letter and number: a 2 percent blend of biodiesel is B2; a 20 percent blend is B20.

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