How Pee Could Have Kept Volkswagen’s Nose Clean

Explaining the liquid that—so far—is keeping other carmakers out of VW’s downward spiral.

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Have you ever met one of those guys who liked to brag about the shady things he did to pass the state emissions test? "Yeah, so I reconnect all the bullshit smog hoses I'd pulled off for better performance, then I drive the car real fast all over town right before I get it tested." Or, "I know a dude who'll plug the electronic sniffer into his shop truck so my modified street machine will pass."

You might expect that sort of puerile move from the owner of an unmuffled ’95 Mustang, but from one of the world's largest auto manufacturers? Probably not. That's why it seemed believable when Volkswagen said that it could meet US Environmental Protection Agency emissions standards without using the urea injection system that most other automakers include on their diesel-powered models.

Did you just say “urethra?”

No, urea. For the chemists among you, urea is an organic chemical compound composed of two amidogen groups joined by a carbonyl functional group. For the rest of us, there's a reason the word urea sounds like urine—it's the basis of mammal piss, including yours. Urea also takes center stage in diesel exhaust fluid (DEF), which on diesel-powered cars and trucks is used to remove oxides of nitrogen (NOx, which is bad) from exhaust fumes. Scientist discovered in the early 19th century how to make urea without having the use the renal systems of living animals. It was a genuine leap to not have to wait until something had to pee to get the desired chemical.

Does that mean I can micturate in my diesel fuel tank and save the environment?

Sadly, no. Although DEF has water in it, human urine is almost all water, with only a little bit of urea mixed in (along with proteins, hormones and other substances diesel engines probably wouldn't like). According to Cummins, which manufactures the diesel engines found in Ram pickup trucks, among other vehicles, DEF "is a carefully blended aqueous urea solution of 32.5 percent high purity urea and 67.5 percent deionized water." In other words, your piss won't cut it.

Why do diesel engines require it, but gasoline engines don’t?

Diesel engines run differently from gasoline engines, relying on high pressure rather than volatility (or the ability of a substance to go boom) to ignite fuel. But high pressure means high temperature and a lower concentration of fuel in the air-fuel mixture needed for combustion than in gas engines. The result is a tendency to create sooty exhaust containing more NOx. Have you ever been stuck behind an old dump truck and watched as your world disappeared in a huge black cloud? Those vehicles used neither particulate filters nor urea injection systems to scrub exhaust gasses of harmful pollutants.

How does urea remove NOx from diesel exhaust?

The engine computer decides when urea is needed, and an injector squirts it into hot exhaust gases that then pass into a special catalytic converter, transforming harmful NOx emissions into nitrogen and water. You know how those "Rolling Coal" jerks squirt extra fuel into their tail pipes to create a big, messy soot cloud when a Prius drives by? It's sort of the opposite of that. The urea-injection system works with a particulate filter and other systems to keep the gases coming out of the tailpipe as clean as possible.

Where do I put the urea?

For the better part of a decade, most new diesel-powered vehicles have come equipped with small tanks for storing urea. If you've ever opened the fuel-tank flap on a diesel car or truck, you may have seen—next to the fuel cap—a smaller tube with a plastic cap on it. Sometimes the tank is mounted in the trunk where the spare tire should be. That's where the urea goes in. In passenger cars, a DEF tank holds a couple of gallons and can last for months. Replacing it is part of the car's regular maintenance, and the car's computer will not allow you to run the engine without it.

How much does it cost to refill a urea tank?

The price of a refill varies upon the size of the tank, where you buy it and how the market for urea is doing at the time. Genuine BMW DEF, for example, can cost more than $20 for a 2.5-gallon jug. But why do that when you could buy a 55-gallon vat of the stuff on the Internet for $170?

Why even bother with urea? Is NOx all that bad?

Don't take my word; here's what the EPA has to say about it:

"NOx reacts with ammonia, moisture, and other compounds to form small particles. These small particles penetrate deeply into sensitive parts of the lungs and can cause or worsen respiratory disease, such as emphysema and bronchitis, and can aggravate existing heart disease, leading to increased hospital admissions and premature death."

That sounds pretty ominous. Bearing that in mind, expect to see a lot of finger-wagging aimed at Volkswagen in the coming months and years.

Ah, yes. Volkswagen. How did they manage without urea injection?

All other passenger-car diesel engines on the market today feature a urea-injection system to help meet EPA emissions standards. Volkswagen's 2-liter 4-cylinder diesel was always a capable little thing—the fuel economy numbers are legendary—but the fact that VW didn't use urea to scrub NOx from the exhaust set its engines apart from all the rest. How did VW do it? The short answer is, it didn't. The manufacturer tuned vehicle computers to recognize emissions test conditions and essentially de-tune the engine to pass.

As it turns out, all of today's diesel engines need urea to avoid polluting the air with NOx under normal operating conditions.