Exhaust Emissions & Catalytic Converters Flashcards
6 cards from real Exhaust practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
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What is the purpose of Selective Catalytic Reduction (SCR) in modern diesel exhaust systems?
Answer: To reduce nitrogen oxides using a urea solution
SCR systems inject diesel exhaust fluid (DEF, a urea solution) into the exhaust stream to chemically reduce NOx into harmless nitrogen and water.
What is 'diesel exhaust fluid' (DEF) primarily composed of?
Answer: Urea and deionized water
DEF is a 32.5% solution of urea in deionized water, used in SCR systems to convert NOx emissions into nitrogen and water vapor.
Which emission from gasoline engines is most directly associated with the formation of ground-level ozone (smog)?
Answer: Volatile organic compounds (VOCs)
VOCs react with nitrogen oxides in sunlight to form ground-level ozone, which is a major component of smog and a respiratory irritant.
What does an EGR valve do in an emissions control system?
Answer: Recirculates a portion of exhaust gases back into the intake
An Exhaust Gas Recirculation (EGR) valve routes a portion of exhaust gas back into the intake manifold, lowering combustion temperatures and reducing NOx formation.
Which OBD-II readiness monitor must be set before a vehicle can pass an emissions inspection?
Answer: Catalyst monitor
The catalyst monitor is a key OBD-II readiness monitor that must report 'ready' status, confirming the catalytic converter has been evaluated by the vehicle's computer.
What is the term for the process by which a DPF burns off accumulated soot?
Answer: Regeneration
Regeneration is the process of burning accumulated soot from a diesel particulate filter, either passively (while driving at highway speeds) or actively (via injecting extra fuel).