Tuesday, April 30, 2013

Technical Corner - What is Diesel Exhaust Fluid?

Phil B here. I know the vast majority of our readers don't have a diesel powered vehicle, and those that do probably don't need Diesel Exhaust Fluid (DEF for short).  You're probably thinking that this is a joke thing that you send the new guy to fetch from the warehouse, like muffler bearings or blinker fluid.  Its a real thing though so I thought it might be interesting for the nerds among us to delve into the science of it.

Diesel emissions - my personal strike zone of expertise.
You guys know that the history of emission controls go back a long way in the US and Europe.  Its been a rocky road for the manufacturers of cars and engines, and sometimes the EPA or CARB (California Air resource Board) have taken measures so severe that the automotive industry in America is only now recovering from them.  The aim of all these emission controls is to keep our air clean, and if you think its all a waste of time you should probably go visit China to get a good idea of what unregulated emissions smells like.  But what is "smog" other than a combination of the words smoke and fog?

Smog in China - photo courtesy of the Guardian.co.uk

Warning: science content follows:

Smog is made of up airborne particulate matter (like a fine dust or soot), low level ozone and Oxides of Nitrogen.  The particulate matter could come as a direct emission from an engine, or it could come from organic compounds (or unburned hydrocarbons) that are then converted (oxidized) into particulate matter in the presence of sunlight.  As a result The EPA and CARB regulate the emission of NOx, particulate matter and unburned hydrocarbons from internal combustion engines.  Gasoline engines typically manage these emissions with a "Three-way Catalyst" along with careful management of air fuel ratio (the electronic fuel injection we have seen for decades).  In the modern era gasoline engine emissions have been reduced significantly and smog levels in cities eased compared with the 1960's and 70's.  The main changes in the last decade or so have been to minimize emissions of evaporated fuel, crankcase emissions or improving catalyst efficiency during cold operation after startup.

Diesels however have been more difficult to improve.  The 3 way catalysts don't work on diesels because they operate with a lot of excess air (lean overall air fuel ratio) and NOx would not be converted.  Diesels also have a great deal more particulate matter to manage.  The diesel engine industry has worked together with the EPA and CARB to write reasonable legislation in stages that set useful but achievable targets, the result is that US Diesel emissions standards are some of the toughest in the world.  Europe by comparison has focused less on Smog and more on greenhouse gasses.  The differences between the European standards and the US standards are subtle but important.  It would be easy to assume this is only a philosophical difference but the primary greenhouse gas is CO2 and its emission tracks closely with fuel economy.  If an engine burns less fuel it will emit less CO2.  Fuel is more expensive in Europe due to taxes so the effort to reduce CO2 fits easily with their strong customer desire for better fuel economy.  While there is sunlight and smog in Europe the controls in placed on NOx have lagged behind the US.  This is the primary reason why those nifty Euro Diesel cars that get a million miles per gallon don't get imported to the US - they don't meet US emission standards.  A few exceptions exist in the luxury market (VW, BMW, Mercedes Benz) where the R&D expense of making those engines compliant can be recouped.  The measures needed to meet US emissions tend to reduce the efficiency of the engines too so those Euro imports don't usually get such good gas mileage in the US.  Don't forget that any UK based mpg figures also us British Gallons - which are larger than US gallons too, so that makes their mpg figures look better.

If you didn't read that wall of text (and who can blame you) you missed the "why", when it comes to Diesel Emission regulations.  All you need to know is that for the US market, Diesel engines since about 2008 have been required to meet some very stringent standards for NOx emissions.  There are a couple of ways of reducing NOx emissions in Diesel engines:

  • EGR.  Dont be fooled into thinking "thats easy we already have that on gasoline engines".  The NOx reduction required and the fact that diesel engines are turbocharged makes this challenging to adopt.  It requires extra heat exchangers, control valves and in some cases pumps, all of which need to operate reliably when dealing with high temperature exhaust gas.
  • SCR - stands for Selective Catalyst Reduction.  This is the method we need to focus on to answer the original question "what is diesel exhaust fluid"
modern diesel engines generally only need one of these technologies to meet emission standards so it's rare to find both of them on an engine.  Combined with a particulate filter, a modern fuel injection system and electronic controls, some very clean Diesel engines are entering operation in the US.

So what is SCR?  In short its a different type of Catalyst specifically for the reduction of NOx.  The Catalyst needs Urea to be injected into the exhaust gas stream for the chemical reaction to convert NOx to harmless gaseous Nitrogen (N2).  The need for Urea is where the Diesel Exhaust Fluid comes in.  DEF is a controlled mix of distilled water and urea.  Urea is produced from Ammonia + CO2 and Ammonia is produced from Hydrogen and atmospheric nitrogen.  So by the end of the SCR process those atmospheric components are returned to the atmosphere.  The actual catalyst can be copper zeolite, or it can be Vanadia based, but they both perform the same function with some subtle differences in efficiency, cost and life.  They both require the same urea based DEF fluid to operate.

And here is what it might look like if you happen to come across it in the real world:

DEF is marketed under many names from different companies.  BlueSky, BlueDef, Terracair, Adblue, Airshield etc... but they all contain the same ratio of Urea and water.  

The DEF is stored in a separate tank on the vehicle, typically a smaller tank than the fuel tank.  Most vehicles use Urea in small quantities compared with the diesel fuel, anywhere between 1% to 5% is typical.  DEF should cost less than Diesel fuel (around $2.79 per gallon) but in some cases automotive manufacturers are recommending their own brand of fluid.  Some Benz dealers are charging over $30/gal for their own "Adblue" fluid which is identical to the DEF fluid at truck stops!

The EPA requires that the engine operates with a feedback control and tank sensors so that the engine will not operate without the appropriate fluid (other than a "limp home mode") and so that the catalyst can't be  removed.  The particulate filters and SCR devices increase exhaust backpressure which does harm the performance and efficiency, the manufacturers attempt to design the systems to minimize the impact.

So there you have it, Diesel Exhaust Fluid, what it is, why its needed and you'll probably never hear about it again.  If someone mentions it you don't have to wonder if they are kidding.  Blinker fluid however is still fictional, but with all these fancy LED's its only a matter of time.  Muffler bearings?  Well I'll leave that to you to figure out.

This LED lamp uses a fluid to keep the ultrabright LED's cool and efficient.

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