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Diesel Exhaust Fluid (DEF)

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Image for Diesel Exhaust Fluid (DEF) from Cat® Parts Store

What is Diesel Exhaust Fluid (DEF)?

Diesel Exhaust Fluid (also known as AUS 32, or generically as urea) is a solution of solid urea dissolved in deionized water to produce a concentration that is 32.5% urea and 67.5% deionized water.

How DEF Works

Controlled amounts of the DEF solution are injected into the engine exhaust stream prior to the SCR catalyst. The temperature of the exhaust converts the urea in the DEF to ammonia. The ammonia and NOx in diesel exhaust react in the presence of the SCR catalyst to convert the NOx into nitrogen (N2) and water (H2O).

Typical properties of a DEF solution with 32.5% urea

Property Typical Value
pH 9.5
Kinetic Viscosity at 25°C 1.28 cSt
Surface Tension at 25°C 65 mN/m, min.
Heat Capacity 0.82 BTU/lb (°F)
Vapor Pressure Factor (relative to water) 0.88
  • Caterpillar offers DEF in 2.5 gallon (9.5 liter) bottles for single use applications and in semi-bulk quantities 264 gallon (1000 liter) totes.
  • Any commercial DEF that meets the ISO 22241-1 specification is allowed for use in Cat® SCR equipped vehicles.


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