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.