Exhaust Diesel Exhaust & DPF Systems 2 — Questions and Answers
Question 1: What is 'ash loading' in a DPF system, and how is it managed?
- The accumulation of incombustible metallic ash from engine oil additives, removed by periodic DPF cleaning (Correct answer)
- The buildup of carbon soot on the DPF substrate, removed by regeneration
- The coating of the DPF with DEF residue, removed by high-temperature purging
- The accumulation of fuel additives on the DPF walls, removed with chemical cleaning
Correct answer: The accumulation of incombustible metallic ash from engine oil additives, removed by periodic DPF cleaning
Ash from engine oil additives that pass through the combustion process cannot be burned off during regeneration and gradually accumulates in the DPF, requiring periodic professional cleaning or replacement.
Question 2: What sensor is commonly used by the ECU to monitor DPF soot load?
- Mass airflow sensor
- Differential pressure sensor across the DPF (Correct answer)
- Upstream oxygen sensor
- Particulate matter sensor in the exhaust tip
Correct answer: Differential pressure sensor across the DPF
A differential pressure sensor measures the pressure drop across the DPF; as soot accumulates, the filter restricts flow and pressure difference increases, signaling the ECU to initiate regeneration.
Question 3: Why is it important to use the correct specification engine oil in a modern diesel equipped with a DPF?
- Incorrect oil causes excessive fuel dilution of the DPF catalyst
- Low-SAPS oils are required to minimize ash-forming additives that accumulate in the DPF (Correct answer)
- Only synthetic oil is compatible with DPF-equipped engines
- Incorrect oil causes the EGR valve to stick open, overloading the DPF
Correct answer: Low-SAPS oils are required to minimize ash-forming additives that accumulate in the DPF
Low-SAPS (Sulfated Ash, Phosphorus, and Sulfur) engine oils are required for DPF-equipped diesels because high-SAPS oils produce more incombustible ash that prematurely fills and blocks the DPF.
Question 4: What happens if a DPF regeneration cycle is frequently interrupted before completion?
- The DPF automatically resets its soot counter
- Incomplete regeneration causes oil dilution from excess post-injection fuel and continued soot accumulation (Correct answer)
- The engine switches to limp mode permanently
- The EGR valve compensates by recirculating more exhaust
Correct answer: Incomplete regeneration causes oil dilution from excess post-injection fuel and continued soot accumulation
Interrupting regeneration before completion means the extra injected fuel can wash into the oil (causing oil dilution) and soot continues to accumulate, potentially blocking the DPF.
Question 5: What is a 'forced regeneration' or 'stationary regeneration' for a diesel DPF?
- A regeneration triggered only at highway speeds with cruise control
- A dealer-initiated regeneration performed while the vehicle is stationary using diagnostic software (Correct answer)
- An emergency shutdown procedure when DPF is at maximum load
- A driver-initiated button press that opens the DPF bypass valve
Correct answer: A dealer-initiated regeneration performed while the vehicle is stationary using diagnostic software
A forced or stationary regeneration is performed by a technician using diagnostic tools to command the ECU to run a full regeneration cycle while the vehicle is parked, clearing a heavily loaded DPF.
Question 6: In a diesel exhaust aftertreatment system, what is the correct order of components from engine to tailpipe?
- DPF → DOC → SCR → Tailpipe
- SCR → DOC → DPF → Tailpipe
- DOC → DPF → SCR → Tailpipe (Correct answer)
- EGR → SCR → DPF → Tailpipe
Correct answer: DOC → DPF → SCR → Tailpipe
Modern diesel aftertreatment systems arrange the Diesel Oxidation Catalyst (DOC) first to oxidize CO and HC, then the DPF to trap particulates, then the SCR to reduce NOx, before the tailpipe.
Exhaust Diesel Exhaust & DPF Systems 2
What is 'ash loading' in a DPF system, and how is it managed?