Green Technology - SCR - FAQ

To help you better understand how SCR works, we've included a list of frequently asked questions. For more information, visit this educational site, www.factsaboutscr.com.

Download a full list of questions and answers from the Thomas Built Bus and School Bus Fleet webinar ‘2010 EPA Emissions–The SCR Solution’.

Question: Why SCR with DEF?

Answer: The advantage of using DEF is that it enables the engine to use less EGR and higher oxygen levels, offering more efficient combustion. DEF allows us to avoid the disadvantages of increasing EGR to extreme levels and meets the EPA’s requirements of near-zero NOx emissions (0.2g/bhp-hr). The results are maximum fuel efficiency and an emission of nitrogen and water, which are elements in the air we breathe every day. Posted:11-24-2008

Question: What is DEF?

Answer: DEF stands for diesel exhaust fluid, which is an aqueous solution of 32.5% urea and 67.5% deionized water. It is clear, non-toxic, biodegradable and non-flammable. Posted:11-24-2008

Question: How do I know when my vehicle is low on DEF?

Answer: A dash-mounted gauge will notify drivers via an escalating driver-warning system as DEF tank levels reduce. Ample warning will be given before the DEF tank empties. An empty tank will not cause the bus to shut down while driving. A five mph de-rate will only occur after the bus has been turned off and restarted. Full power will resume after DEF tank is filled. Updated:05-27-2009

Question: Will the 2010 Emissions Cummins engines be compatible with BioDiesel?

Answer: Cummins products are compatible with up to B20. 2010 products will also maintain the ability to use up to B20. Posted:06-10-2009

Question: Is there a fuel economy difference from the 2007 Emissions Cummins engine and the new 2010 Emissions Cummins engine?

Answer: 2007 and 2010 midrange Cummins products will be comparable in fuel economy, however if Cummins would have gone to 'in-cylinder' we would have lost 5-9% in fuel economy. Posted:06-10-2009

Question: Will the amount of Nitrogen exhaust be enough to ignite to flame?

Answer: No. Nitrogen is not flammable. Posted:06-10-2009

Question: What is the life of the SCR system and its components?

Answer: The SCR system is designed to last the life of the engine. Posted:06-10-2009

Question: What are the preventive maintenance requirements on the SCR system on the 2010 Emissions Cummins engine?

Answer: There are no preventive maintenance requirements other than a DEF filter that will need to be serviced at 200,000 miles. Posted:06-10-2009

Question: How many sensors are there on the 2010 Emissions SCR System for the Cummins Engine and a Thomas Built Bus?

Answer: 2 NOx sensors, 2 temperature sensors and a diesel exhaust fluid level sensor. Posted:06-10-2009

Question: Will diesel exhaust fluid degrade at higher temperatures (above 85° F) if it is stored in a sealed container?

Answer: In order for diesel exhaust fluid to degrade while stored at higher temperatures it must be stored at over 85°F for over 6 months. If the temperature decreases during that time of storage, then the storing timeline is increased. DEF stored at below 85°F will provide for a 12 to 18 month shelf life. Posted:06-10-2009

Question: Does diesel exhaust fluid freeze and at what temperature?

Answer: Diesel exhaust fluid freezes at 11°F/-11°C. It does not freeze as a solid block at that temperature, but it has a 'slushy' consistency. To freeze to a solid mass requires temperatures below -20°F/-25°C for 48 hours. From a solid frozen mass, the system will take approximately 40 minutes to thaw the diesel exhaust fluid, which is faster than required by the EPA. Posted:06-10-2009

Question: Are there any biohazard requirements of concerns for diesel exhaust fluid? Any clean up requirements?

Answer: No. Diesel exhaust fluid does not have a bio-hazard clean up requirement. Posted:06-10-2009