DPFs: What you need to know
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Almost one in five vehicles on the road right now are diesel powered, but missing information on used diesel vehicles may cause future buyers a bit of grief.
As diesel vehicles age and are sold on the used vehicle market, it’s important for owners to be aware of systems and information that may not be passed on.
One emission-reducing device is the diesel particulate filter or DPF. Under normal conditions they are designed to be maintenance-free but performance can be affected and warning lights illuminated when these block or fail.
What is a DPF?
The Diesel Particulate Filter (DPF) is a device designed to reduce emissions from diesel fuelled vehicles. They are located within the vehicle’s exhaust system and remove diesel particulate matter (or soot) from the exhaust gases, before they’re emitted into the atmosphere.
How does a DPF work?
● Exhaust gases flow into the filter
● As the exit is blocked, the clean exhaust gases are forced to escape through porous cell walls.
● The holes within the cells walls are not large enough to allow the particulate matter to pass through, so trap the soot inside the filter.
As with any filter, the DPF needs to be emptied and cleaned regularly to maintain performance.
This is achieved through a process called regeneration where the accumulated soot is burnt off at high temperatures (around 600C) to leave only a residue of ash.
This renews or regenerates the filter, so that it’s ready to take on more pollution from the engine.
DPF vehicles can incorporate an active, passive or forced regenerative process.
Active regeneration occurs automatically and generally requires no intervention under normal driving circumstances. There are however, some driving conditions that can prevent active regeneration from occurring.
For instance when a vehicle is not used for long periods of time (less than 10 minutes) or continuous low-speed driving, the exhaust system will simply not reach the backpressure or load sensor limits to allow the regenerative system to activate.
This type of regeneration can also be initiated by the vehicle engine management system periodically depending on vehicle use and can take about 10 minutes.
If the active system fails to operate, forced regeneration may be required. A warning light or message may indicate that forced regeneration is required, or can also indicate a problem.
If the warning is an indication the filter is getting blocked and is ignored, the vehicle could default to a reduced power “limp home” mode and require immediate assistance by a workshop.
Passive regeneration can take place as a part of normal driving at a motorway pace, where exhaust temperatures are higher.
This system can have an integrated oxidising catalytic converter located close to the engine where exhaust gases are hot enough so that regeneration is possible.
How will I know when the DPF is regenerating?
Here are some vehicle characteristics often associated with DPF regeneration:
● Increased idle speed (about 1200rpm)
● Engine note change
● A hot or unusual smell emitting from the exhaust
● Cooling fan operating
● White smoke from the exhaust
● Increased fuel consumption
Do not be alarmed if you notice some of these characteristics every now and then.
It’s important to note that heat and burn hazards can be present due to the immense heat involved in the regeneration process. Always follow the manufacturer’s safety recommendations outlined in the vehicle owner’s manual
What can you do?
Every vehicle type and engine combination can have a different effect on DPF regeneration and has the potential to block the filter.
It’s important to use a low-ash engine oil designed to reduce DPF soot build-up.
Try to do some out-of-town driving every now and again to allow the exhaust system to reach higher temperatures, in turn allowing the DPF system to regenerate.
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