What do you remember from high school chemistry?
I’m not looking for specifics, I just want to make sure you’ve got the basics down, that when various chemicals are mixed they produce different reactions — which can then be instantly changed by added another.
See, to understand the newest diesel exhaust treatments in today’s trucks, that’s about all you need to know. Start with one chemical compound and add another to produce a completely different third one.
It’s dry stuff, I know, except for one interesting piece of trivia: the principal chemical in diesel exhaust fluid (DEF), the stuff that lets modern diesels run clean exhaust, is urea. As in the active ingredient in human urine.
In mammals, urea acts as a vehicle for getting rid of excess nitrogen in the body, and in a way it does the same thing for diesel exhaust.
But urine is not what’s going into your exhaust. No, today urea is commercially produced as a powder and is used widely in the chemical industry, chiefly in making fertilizer and DEF.
(Add water to it and you also get ammonia—there’s a smell we’re all familiar with.)
Why urea? Why now?
Pee jokes aside, 2013 marks the year that all Detroit-built diesel engines (for pickups) will adopt an exhaust after-treatment using what’s generically called “diesel exhaust fluid,” a.k.a. urea.
Chrysler’s just announced its Cummins diesel engines would fall in line this year with Ford’s and General Motors’, who adopted this technology in 2011. Moving forward, which ever diesel you buy, you’ll find a DEF system on it.
The change follows the introduction of a new set of federal emissions standards (they’ve been slowly phased in since 2010) that call for an 80 percent reduction in the nitrogen oxide (NOx) levels found in diesel exhaust.
NOx is harmful to humans, and contributes to acid rain and ozone depletion. It’s obvious why the government wanted to limit it — but the goals set in 2010 were huge. Not only were they asking for an 80 percent reduction in NOx, they were asking for an 80 percent reduction compared to the previous limits – the new goal was virtually zero.
Before 2010 manufacturers had used conventional scrubbers and rich-fuel cylinder burn technology to cut down NOx. But to hit the mandated 80 percent NOx reduction, they basically had to introduce other chemicals into the exhaust gas stream to change (at a molecular level) the composition of the exhaust.
What’s in your diesel exhaust now? Water vapour and inert nitrogen — which is harmless.
The good news is this new technology asks next to nothing of the owner, except that you top up the DEF – which is relatively cheap – at intervals in the 10,000 km range. Most manufacturers have designed tanks that will hold enough DEF to get its trucks from one scheduled service appointment to the next so the dealer can fill it if you don’t.
What is urea exactly?
So, if not urine, what is DEF exactly? Scientifically, it’s a solution that’s approximately 67.5 percent water and 32.5 percent pure urea. Practically, it’s a chemical catalyst that has now forever changed how our diesel engines work.
The whole DEF-adding process takes place in the downstream exhaust – not in the engine – in several chambers ahead of the muffler, and basically takes three steps. And heat. Lots of heat.
(While each manufacturer has its own DEF system design, they generally all work the same way.)
Step one: Exhaust enters the first chamber which converts and oxidizes hydrocarbons into water and carbon dioxide. This requires around 250 C of heat, provided by the engine.
Step two: As the exhaust enters the second chamber, it’s dosed with DEF. As the DEF is heated, it splits into ammonia and carbon dioxide and is atomized. These particles then enter a mixer – like an auger or corkscrew – that swirls it into the exhaust.
The chemical reactions (taking place at a temperature between 200 C and 500 C, remember) between the exhaust and the DEF reduce the mixture to water vapour and inert nitrogen.
Step Three: The last chamber is a soot trap. This scrubber captures whatever soot particles are left over as the exhaust passes through what looks like a ceramic honeycomb. When the trap is full, sensors activate a burn cycle where up to 600 C of heat is introduced, burning away the soot.
The result? Diesel exhaust that has never been cleaner. In fact most new diesel engines are now cleaner than many gasoline-powered engines.
And the best part is that no urine is required — just DEF.