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What Is the Use of Urea in Diesel?

By Paul Scott
Updated Feb 16, 2024
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Urea is used in several new diesel powered vehicle models as a post-combustion emission suppressant. The rationale behind the technology is the control of nitrogen oxide (NOx) concentrations in the exhaust gases of compression ignition (CI) engines. NOx is a natural byproduct of CI systems and a major contributor to air pollution. The value of using urea in diesel powered vehicles exhaust systems stems from the chemical's ability to convert the majority of the NOx component in the gases to harmless nitrogen and water. This use of urea in diesel vehicles is typically achieved by injecting a fine spray of urea into the exhaust catalyst, thereby effectively neutralizing a substantial percentage of the harmful NOx content of the exhaust emissions.

Compressive ignition systems produce temperatures and pressures far higher than those in spark ignition gasoline engines. These conditions produce high levels of nitrogen oxide in the exhaust gases of diesel engines. Global concerns regarding the growing negative impact NOx emissions have as air pollutants have lead many countries to introduce stringent NOx control measures pertaining to new diesel powered vehicles. These controls have seen an increasing use of urea in diesel engine systems; several top automobile manufacturers include urea injection as a standard feature in their new diesel models.

The use of urea in diesel engines centers around the chemical's natural breakdown of hazardous NOx into harmless water and nitrogen. Most new diesel burning cars feature multistage exhaust emission management systems which separate soot and NOx from exhaust gas progressively and with the urea injection phase being one of the last steps. The urea introduction takes place in the system's selective catalytic converter (SCR) section where a thin jet linked to a separate urea tank sprays a fine mist of the liquid into the NOx rich exhaust gas. The exhaust gas then released into the atmosphere is almost completely soot and NOx free.

At present, several automobile manufacturers are claiming NOx conversion rates of 80% or more using urea in diesel vehicles. These reductions will certainly have a positive environmental impact but may add significantly to the cost of driving a diesel vehicle. These systems may also add inconvenience to the diesel driving experience because many new diesel vehicles only allow a limited number of starts if the urea tank is depleted before cutting out completely and stranding the motorist. Fortunately, most new urea injection diesel models feature notification and warning systems which flag the driver if the urea level becomes low.

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Discussion Comments

By anon287579 — On Aug 26, 2012

@anon287406: You must have visited your friendly German auto dealer. They are notorious for unearthly overcharging customers for simple maintenance items. I once got charged $75 for rotating the tires when it was in their shop under the frequent warranty work required.

By anon287406 — On Aug 25, 2012

The total bill just for adding AdBlue? A stunning $316.99. We were down to 18 percent full on the additive at 16,566 miles. It took 7.5 gallons to fill the tank, costing an eye-opening $241.50 for the fluid alone. The labor to add the fluid plus tax accounted for the rest. None of this was covered by the warranty… At the current rate and cost of consumption, just the AdBlue itself (without the labor, which would probably be included as part of the routine service) would cost $1,457.80 for 100,000 miles of driving. That's a lot of money, knocking about a third off of your fuel savings vs. buying a GL450 V8.

By anon205088 — On Aug 11, 2011

I've been driving a new diesel that uses Urea and it does not add significantly to the cost of driving. It uses about 2 gallons per 5,000 miles, and costs about $3.00 per gallon.

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