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Mark Polk, motorhome  and motor coach maintenance expert

By Mark Polk
RV Education 101  

I just returned from the Pennsylvania RV & Camping Show in Hershey, where I saw lot of new motorhomes on display. While looking over a 2010 model built on a Freightliner Custom Chassis, I noticed what looked to be a small fuel tank located under one of the outside storage compartment doors. The cap on the tank had the letters DEF, which I later discovered stands for diesel exhaust fluid.

I had no idea what diesel exhaust fluid was, so I headed over to the Freightliner display to find out. The Freightliner representative was extremely knowledgeable and helpful in explaining what DEF was, and after returning home I decided to research it a bit further.

It turns out DEF was a result of the latest rounds of EPA mandates on diesel engine emissions standards. These mandates have a long history.

For the past 25 years the EPA has aggressively increased diesel exhaust emissions standards on manufacturers, most notably in the areas of Particulate Matter (PM) and oxides of nitrogen (NOx). These diesel exhaust emissions are byproducts of the combustion (or incomplete combustion) of diesel fuel.

Think of the PM as the soot or smoke you see emitted from the exhaust system. Oxides of nitrogen result from a bad reaction between nitrogen and oxygen in high temperatures created by the combustion process.

In 2000 the EPA signed emissions standards aimed at PM emissions, designed to take full effect by the release of 2007 model-year vehicles. That brings us back to DEF and the 2010 EPA mandates, designed to reduce the amounts of nitrous oxide emissions released into the atmosphere.

All but one motorhome manufacturer chose a process called selective catalytic reduction (SCR) to meet this new EPA standard. Simply put, SCR adds a Diesel Exhaust Fluid into the exhaust gas and filters it through a catalyst in the system to convert NOx into nitrogen and water, which is then released into the air.

So, what does all of this mean to the new diesel motorhome owner? Here are my top 7 questions about how SCR may affect the RV owner.

1. Does SCR meet the 2010 emissions standards?
Yes, SCR actually exceeds the 2010 emissions standards, and in addition is said to increase fuel economy by five to eight percent
2. Does SCR provide any benefits over a similar 2007 model engine with Exhaust Gas Recirculation (EGR) technology?
Yes. It doesn’t run as hot. In addition, a comparable engine using SCR will provide more torque and more horsepower than an engine using EGR technology.

3. Will it be difficult to find Diesel Exhaust Fluid?
It is said that DEF will be abundantly available at travel centers and auto parts stores across the country.

4. Is it possible for the DEF solution, made up of two-third parts water, to freeze?
Yes it is possible, but the DEF tank is insulated and the system uses heated lines to deliver the fluid to help avoid freezing during start-up and normal operation.

5. Can DEF degrade?
DEF will degrade, but it would take 125 degrees of direct heat over a long period of time, which is unlikely to happen.

6. If you do not refill the DEF tank and it runs out, will there be a problem?
Yes, the engine will decrease speed to approximately 30 mph until DEF is added to the system again. There are gauges in the coach that will alert you in plenty of time to refill the tank before it runs dry.

7. Will SRC technology cause an increase in cost of the unit?
As with everything else, new technology is sure to affect pricing, but the jury is out on whether the projected increased fuel economy will offset the increase in price over a period of time.
What’s next with diesel emissions standards?

Get ready for the next round of EPA mandates. It looks like the next EPA and DOT mandates are aimed at increased fuel economy and Greenhouse Gas (GHG) emissions. These new diesel emissions standards will be phased in between 2014 and 2018.

The new standards being imposed are sure to dramatically increase the cost of a diesel-powered motorhome again, and the question begs, Will the ends justify the means?

I guess we’ll have to wait and see.

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