Exhaust components
- DPF (diesel particulate filter)
- A filter in a diesel exhaust that traps soot. Most are wall-flow filters with a ceramic honeycomb inside.
- DOC (diesel oxidation catalyst)
- A catalyst, usually ahead of the DPF, that oxidises carbon monoxide and hydrocarbons and helps the DPF regenerate.
- SCR (selective catalytic reduction)
- A catalyst that reduces nitrogen oxides by reacting them with AdBlue.
- GPF (gasoline particulate filter)
- A particulate filter fitted to petrol direct-injection engines.
- EGR (exhaust gas recirculation)
- A system that routes some exhaust gas back into the intake to reduce nitrogen oxides. A sticking EGR valve or a leaking EGR cooler can raise soot output or put coolant into the exhaust.
- Substrate
- The honeycomb inside a filter or catalyst. Cracks, melting or missing material mean the part needs replacing.
- Wall-flow filter
- A filter whose channels are blocked at alternate ends, so exhaust gas has to pass through the porous walls, leaving particles behind.
What blocks a filter
- Soot
- Carbon particles from combustion. Soot burns off during regeneration.
- Ash
- Non-combustible residue, mostly from engine oil additives. Regeneration can’t remove it, so it builds up until the filter is cleaned off the vehicle.
- Oil contamination
- Engine oil reaching the filter, usually from a worn turbo, seals or another engine fault. The fault needs fixing as well as the filter.
How filters are managed
- Regeneration
- Burning soot out of a DPF. Passive regeneration happens during normal driving; active regeneration is triggered by the engine control unit; a forced regeneration is started with a diagnostic tool.
- Back-pressure
- Resistance to exhaust flow. A blocked filter raises back-pressure, which the vehicle measures with a differential pressure sensor.
- Differential pressure sensor
- The sensor that compares exhaust pressure before and after the DPF. The engine control unit uses it to judge how full the filter is, so a faulty sensor or a split pipe to it can stop regeneration or report a blockage that isn’t there.
- On-vehicle cleaning
- Treating a filter in place with additives, foams or a forced regeneration. It addresses soot and oil contamination, not ash.
- Off-vehicle cleaning
- Removing the filter and cleaning it in a machine, which can flush out ash as well as soot.
Machine terms
- Closed-loop water
- Wash water held in the machine’s tank, filtered and settled after each pass, and reused for many cleans rather than discharged continuously.
- Adaptor
- A fitting that seals a filter to the machine, so water and air pass through the filter rather than around it.
- Pressure test
- Measuring a filter’s resistance to air flow before and after cleaning, to record the change.
- Soft start
- Bringing the wash water flow up gently at the start of a cycle, so a heavily blocked filter isn’t hit at full pressure straight away.
- Hot drying and cold drying
- Drying a cleaned filter with heated air, or with unheated compressed air, in which case drying is finished on the vehicle as the manual sets out.
- LCV and HGV
- Light commercial vehicles, such as vans, and heavy goods vehicles, such as trucks.