What is DEF and how is it used in Canada?
What is Diesel Exhaust Fluid (DEF)?
Diesel Exhaust Fluid (DEF) is a simple aqueous urea solution comprimised of 32.5 % high‑purity synthetic urea dissolved in 67.5 % ultra‑pure, deionized water that meets the strict purity and concentration criteria set out in ISO 22241‑1. These requirements ensure freeze‑thaw stability (DEF freezes at around –11 °C but thaws reliably) and prevent impurities that could damage SCR catalysts or injection systems.
How is Diesel Exhaust Fluid (DEF) used?
In modern diesel powertrains from heavy‑duty trucks and buses to off‑road machinery and stationary generators, DEF is injected into the hot exhaust ahead of a Selective Catalytic Reduction (SCR) catalyst. When heated, the urea thermally decomposes into ammonia (NH₃) and carbon dioxide; the ammonia then reacts with nitrogen oxides (NOₓ) over the SCR catalyst, converting them into harmless nitrogen (N₂) and water vapor (H₂O). This process can reduce NOₓ emissions by up to 90 %, helping engines meet stringent emissions targets without sacrificing fuel efficiency.
DEF is now used across virtually every diesel‑powered sector in Canada. On‑road applications include light and heavy‑duty trucks, transit buses, and vans equipped to meet Euro VI (US 2010) standards. In construction, agriculture, mining, and material‑handling (e.g. loaders, tractors, skid steers, and airport ground‑service vehicles), SCR + DEF has become standard. Stationary compression‑ignition engines, such as prime and standby generators regulated under RICE NSPS/NESHAP, also rely on DEF fed SCR to control NOₓ under environmental regulations. And in the marine sector, where engines burn heavy fuel oil or marine diesel, SCR systems use a higher‑strength urea solution to tackle the vast NOₓ loads produced by large engine blocks.
Diesel Exhaust fluid (DEF) in Marine application
Although the core chemistry is the same, marine applications generally use Marine DEF (often called AUS 40) a 40 % urea solution standardized under ISO 18611, because the higher urea concentration improves NOₓ conversion efficiency in very large, slow‑speed engines (e.g. container‑ship main engines or rail locomotives). By contrast, virtually all on‑road, off‑road, and stationary units use the 32.5 % urea blend (AUS 32) specified by ISO 22241‑1. Both blends are non‑toxic and handled similarly, but AUS 40’s extra 6.5 % urea offers better performance for the massive exhaust volumes of marine and some rail engines.
Why Is Diesel Exhust Fluid (DEF) Mandetory In Canada?
In Canada, DEF is mandatory wherever SCR systems are fitted, because the federal On‑Road Vehicle and Engine Emission Regulations (SOR/2003‑2) harmonize with U.S. EPA Tier 2–4 and Euro VI standards that require after treatment for NOₓ compliance. Under the Canadian Environmental Protection Act (CEPA 1999), both Environment and Climate Change Canada (ECCC) and Transport Canada enforce these limits, and tampering with or bypassing a DEF‑SCR system is illegal and subject to penalties. Similarly, Canadian Arctic and coastal waters are being designated under MARPOL Annex VI as Emission Control Areas (ECAs), where marine DEF use in SCR is critical to meet NOₓ thresholds. These combined regulations make DEF not just an option, but a legal necessity for diesel engines in Canada.
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