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Jul 25, 2023

What to do when a diesel particulate filter needs to be burned off and you're offroad...

FIRST, AN EXPLANATION for readers not familiar with a DPF, which is a Diesel

FIRST, AN EXPLANATION for readers not familiar with a DPF, which is a Diesel Particulate Filter.

A ‘particulate’ is a tiny fragment of matter, solid or liquid, suspended in the air. These are highly carcinogenic, which means they contribute to cancer. Diesel engines produce significant amounts of particulates in their soot caused by incomplete combustion of fuel, although this has been steadily improved over the years by technology such as common-rail fuel injection. Nevertheless, diesel particulates are still a problem and that is why there is a special filter fitted to modern diesels to catch them – the DPF.

Like any filter, the DPF fills up with particulate matter (PM) and then it needs to be cleared. How does the vehicle know the DPF is full? It doesn't measure it directly, but looks at various factors which indicate the DPF is filling; for example travel distance, exhaust back-pressure and general operating conditions.

The clearing of the DPF is done by burning off the PM when the exhaust temperature gets high enough, as it would on a freeway cruise. This is known as passive regeneration. There's also active regeneration, which is when the filter fills up but the vehicle never has a chance to clear it using the passive system because the exhaust doesn't get hot enough, for example when it's used for lots of short trips. In that case, the car needs to artificially increase the exhaust temperate for the burn-off to happen – this is either a workshop procedure, or some vehicles have a special button so the user can do it.

Regeneration takes anywhere from 5 to 25 minutes, depending on lots of different factors such as the amount of soot built up, type of engine and vehicle operating conditions.

When the PM is burned off it becomes ash, and therefore isn't particulate matter. During the regeneration process fuel consumption is increased a little, but not enough to make a significant difference.

The DPF can only survive for a certain number of cycles before it needs replacing. Exactly when is dependent on the vehicle and its use, but for higher-mileage vehicles it will be a reasonably significant cost that prospective owners should bear in mind.

OK, so that's the DPF and what it does – now onto the actual reader question. The apparent problem is that on a long offroad desert trip then speeds will be very low, and therefore the DPF won't get a chance to burn off and may fill up. At that point the vehicle will at the very least flash up a warning saying the DPF is full. You’re also in danger of filling up the DPF beyond the point at which it can clean itself, so you’ll be up for a new one. And you can't go for a high-speed run when you’re in the middle of the desert.

We are aware of problems with DPFs on long trips, but they are infrequent. Firstly, in order to get to a remote location you’ve typically driven many hundreds of kilometers, so the DPF should be pretty much empty thanks to passive regeneration by the time you start the low-speed offroad part of the trip. When you’re offroad even spending a week or two doing low-speed work shouldn't fill it up either, as it takes a while to fill. However, we have reports of diesel 4WDs that spend a solid day doing low-speed work – manoeuvring trailers for example – and after that they need a DPF clean.

Now you may think that while speeds are slow the exhaust temperate is high so passive cleaning still works. Not the case for the Triton, as Mitsubishi tell us the minimum speed for a DPF burn to occur is 40km/h, and that "there is no indication on engine load dependancy and the vehicle must be driven at 40km/h or above for the burn-off to occur." And that's not unusual. For the Holdens it's 50km/h minimum.

For the Ranger, Ford say that "vehicle speed and exhaust temp are both inputs into the activation of DPF regeneration. Low speed regeneration is achievable including driving in Low Range. The typical minimum vehicle speed to enable regeneration is 30-40km/h and the higher engine load will assist reaching the minimum exhaust temperatures to enable regeneration of the DPF. Once activated while driving, the vehicle can still regenerate at idle for a short periods."

Volkswagen tell us that "even when driving at slow engine speeds the ECU will try to regenerate the DPF to keep levels low. If the DPF warning light is illuminated the recommendation is to drive as per the attached guidelines [ which involve higher speed driving". They also said that if the engine was hot enough the "DPF could regenerate even when the car was sitting at the lights, for example." And Isuzu? They don't use DPFs in their current models. Toyota do, but tend to have a manual cleaning cycle that can be done at idle – see photo above.

What happens if a DPF fills up? Ford tell us that "there are several warnings designed to alert the customer to soot overload conditions, the very last being a torque limitation. The vehicle can still be driven, however it is advised that the customer should visit a dealer where they can perform a stationary service regeneration using a service tool." That's pretty typical.

It is also worth mentioning the fire risk. Any 4WD operated in long grass that is dry runs the risk of fire, not so much from setting the vegetation alight as it goes by but from a buildup of grasses that get caught in and around the exhaust. This has always been a risk, but with the higher temperature as a result of a DPF burnoff it is even more of a risk. The solution is to avoid long grass, or if you must drive through it constantly check and remove any buildup of debris.

So in summary; you probably don't need to worry about your DPF with any 4WD like the Triton provided it is serviced correctly and checked before your long trip, but you should know how yours operates.

A DPF is a necessary evil these days. It is only there for emissions, but adds complexity and isn't essential to a vehicle's fundamental operation, yet the vehicle can be immobilised or put into limp mode because of it. That is a problem for those of us that rely on vehicles in remote locations, but it is not one that manufacturers seem inclined to address. Kudos has to go to Toyota for including a manual DPF system, and to some extent Ford for making their system work at low speeds. But a minimum speed for DPF re-gen? That's not good, robust bush design.

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