EGR vs. SCR: Making Sense of Interim Tier 4 Myths
by Joe Mastanduno
April 26, 2011
January 2011 marked the start of Interim Tier 4 (IT4)/Stage III B for diesel engines 174 horsepower and above, the fourth stage of the emissions regulation for off-road equipment, set by the Environmental Protection Agency (EPA) for North America.
IT4 calls for a reduction in particulate matter by 90 percent along with a 50 percent drop in NOx (Nitrogen Oxide) compared to Tier 3 regulations. There are two main technologies being implemented across the industry to meet these stringent emissions standards—SCR (selective catalytic reduction) and EGR (exhaust gas recirculation).
Both SCR and EGR play a significant role in lowering nonroad diesel engine emissions, but several misconceptions have surfaced as to which IT4 technology is the best to implement today in order to meet IT4 requirements. Here’s one we commonly hear from John Deere’s customers:
- Myth: A cooled EGR-equipped diesel engine has a higher internal combustion temperature than a SCR-equipped engine.
- Fact: As its name implies, cooled EGR cools and mixes measured amounts of exhaust gas with incoming fresh air to lower the engine’s peak combustion temperature, thereby reducing nitrogen oxide (NOx) emissions to an acceptable level. The exhaust gases are routed through an exhaust filter, which traps the particulate matter (PM).
In contrast, engines equipped with SCR technology usually operate at higher combustion temperatures in order to reduce particulate matter to acceptable emissions levels. This places additional heat on the internal components of the engine and creates more NOx. To reduce NOx, a diesel exhaust fluid (DEF) is injected into the exhaust stream, which combines with the exhaust gases in the SCR catalyst.
In many cases, cooled EGR engines actually have cooler internal engine combustion temperatures than SCR engines. Cooled EGR engines do place more cooling requirements (higher heat rejection) on the cooling system; however, some manufacturers, such as John Deere, have adopted new variable speed fan drives and cooling system designs for IT4 that meet cooling needs in the most efficient method possible.
Keep in mind, the responsibility to comply with IT4 technologies falls on equipment manufacturers, not users. So, what emission requirements do you need to meet? Click here for top strategies to evaluate your local emissions landscape and create a plan for long-term success.
Additional Resources
Unfamiliar with these SCR and EGR? You can go here for an overview of how EGR and SCR work and how they are different.
Joe Mastanduno
Joe Mastanduno, John Deere product marketing manager for engines and drivetrains, is a recognized expert on engine emissions with more than 20 years experience in the power train business. For more about off-road emissions compliance, please visit www.straightforwardblog.com
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