CEI CEI Fuel Systems & Engine Combustion 2 — Questions and Answers
Question 1: Which sensor provides feedback to the engine control unit (ECU) to maintain the air-fuel ratio near stoichiometry and is critical for three-way catalytic converter efficiency?
- Oxygen (O2) sensor / lambda sensor (Correct answer)
- Crankshaft position sensor
- Intake air temperature sensor
- Manifold absolute pressure sensor
Correct answer: Oxygen (O2) sensor / lambda sensor
The oxygen sensor (lambda sensor) measures oxygen content in the exhaust, allowing the ECU to perform closed-loop fuel corrections to keep the A/F ratio at stoichiometry for optimal catalytic converter performance.
Question 2: A vehicle shows high positive fuel trim values (e.g., +25%) at idle. What does this most likely indicate?
- An unmetered air leak (vacuum leak) causing the ECU to add extra fuel to compensate for a lean condition (Correct answer)
- A rich running condition caused by a stuck-open fuel injector
- A faulty catalytic converter causing exhaust backpressure
- A failed oxygen sensor providing a false rich signal
Correct answer: An unmetered air leak (vacuum leak) causing the ECU to add extra fuel to compensate for a lean condition
High positive fuel trim means the ECU is adding significantly more fuel than normal, typically because unmetered air (from a vacuum leak) is leaning out the mixture and the ECU compensates.
Question 3: What type of fuel system does a 'direct injection' gasoline engine use, and what emission challenge is uniquely associated with it?
- Fuel is injected directly into the combustion chamber, which produces higher particulate matter (PM) emissions compared to port injection (Correct answer)
- Fuel is injected into the intake manifold, which produces higher NOx due to leaner combustion
- Fuel is mixed with air before the throttle body, which produces higher EVAP emissions
- Fuel is carbureted at idle and injected at high loads, which causes high CO at part throttle
Correct answer: Fuel is injected directly into the combustion chamber, which produces higher particulate matter (PM) emissions compared to port injection
Gasoline direct injection (GDI) injects fuel directly into the cylinder, improving efficiency but producing more ultrafine particulate matter due to incomplete fuel-air mixing before ignition.
Question 4: What is 'engine knock' (detonation), and how does it affect exhaust emissions?
- Uncontrolled auto-ignition of the air-fuel mixture before the spark plug fires, causing incomplete combustion and increased HC and CO emissions (Correct answer)
- Excessive lubrication oil burning in the combustion chamber, increasing PM and HC emissions
- Misfiring caused by a weak ignition spark, producing elevated NOx emissions
- Exhaust valve flutter at high RPM causing HC spikes in the exhaust
Correct answer: Uncontrolled auto-ignition of the air-fuel mixture before the spark plug fires, causing incomplete combustion and increased HC and CO emissions
Engine knock (detonation) occurs when the air-fuel mixture ignites spontaneously before the spark event, causing uneven combustion that increases unburned HC and CO emissions.
Question 5: In a diesel engine, what is the primary pollutant of concern that is significantly higher than in gasoline engines due to the nature of diesel combustion?
- Particulate matter (PM) and NOx (Correct answer)
- Carbon monoxide (CO) and hydrocarbons (HC)
- Carbon dioxide (CO2) and sulfur dioxide (SO2)
- Ammonia (NH3) and nitrous oxide (N2O)
Correct answer: Particulate matter (PM) and NOx
Diesel engines produce significantly more particulate matter (soot/PM) and NOx than gasoline engines due to their high-temperature, lean, heterogeneous combustion process with fuel-rich zones.
Question 6: What is the function of a diesel particulate filter (DPF) in reducing vehicle emissions?
- It traps and periodically burns off soot particles from diesel exhaust to prevent PM from being released into the atmosphere (Correct answer)
- It absorbs NOx from the exhaust stream using a urea-based chemical reaction
- It oxidizes unburned hydrocarbons and CO in the exhaust using a platinum catalyst
- It recirculates exhaust gases back into the intake manifold to lower combustion temperatures
Correct answer: It traps and periodically burns off soot particles from diesel exhaust to prevent PM from being released into the atmosphere
The DPF physically traps soot particles from diesel exhaust in a ceramic filter substrate and periodically regenerates by burning off accumulated soot at high temperatures.
CEI CEI Fuel Systems & Engine Combustion 2
Which sensor provides feedback to the engine control unit (ECU) to maintain the air-fuel ratio near stoichiometry and is critical for three-way catalytic converter efficiency?