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DPFs and GPFs are necessary evils behind today's engines.


Australian consumer law says that goods must be suitable for any particular purpose the buyer makes known to the seller. If you make it clear that you want to drive your intended new 4WD in long grass and do lots of stop-start and low speed work the salesman should tell you that a new turbo-diesel Euro 5 wagon or ute is not for you.



When the DPF pressure sensor indicates
that the DPF is starting to block up, the engine electronic control unit (ECU) programs an ‘active’ regeneration. That process involves increasing EGT, by either post-injection in the engine combustion chambers that adds a fuel burst to the departing exhaust gases, or by using a dedicated injector in the exhaust plumbing.

Post-injection runs the risk of engine oil dilution, caused by unburnt fuel passing by the piston rings and a dedicated exhaust-system injector can block with soot.

In some cases, this active regeneration doesn’t work well enough to burn soot out of the DPF material, so a ‘manual’ regeneration is signalled to the driver, who instigates it via a dashboard button. This process requires the vehicle to be parked and let run through a computer-controlled burn process that can take up to 45 minutes.

Active and manual regeneration cycles cause EGTs to rise and there have been dozens of vehicles destroyed by fire, as a result. Vehicles parked on dry grass, or that have grass build-up around their DPFs are the most likely victims.

Cutting short regeneration or ignoring a DPF regeneration warning light will eventually lead to a plugged DPF, at which point the engine may derate and go into limp-home mode. (If there’s a fault in the DPF pressure sensor this plugging may not register and turbo failure from excessive back pressure is possible.)

If you believe most manufacturers’ claims, the regeneration process keeps the filter clear of soot and everything should be OK.

What they don’t tell you is that, even if regeneration is successful, the filter medium in the DPF is still left with some grey ash residue. Over time this residue builds up and requires professional removal, either on the vehicle, or, after removal from the vehicle, in a dedicated workshop machine.


Ash build-up in a DPF cannot be burnt out by regeneration, so the DPF service life is finite. Vehicle makes make estimates up to 10 years and 200,000km for DPF life, but we’ve heard of many people who have achieved less than 30,000km.

Life-shortening factors are myriad, because the DPF is at the end of the ‘food chain’ and, consequently, what is swallowed upstream of the DPF finishes up in its bowels: an unpleasant analogy, but a clear one!

Let’s say your engine allows microscopic dust particles through its air filter – like many plastic-housing air filters do – and although these grains aren’t large enough to cause piston ring or valve-seat wear, they are large enough to coat the airflow mass sensor in the inlet manifold.

This sensor then sends incorrect airflow data to the ECU, which promptly instructs the fuel pump to alter the injection amount. If fuel delivery is reduced the driver responds with a heavy right foot and if it’s increased the engine gets excess fuel it can’t burn cleanly.

In another scenario a slight air leak downstream of the airflow sensor reduces airflow into the engine, so it’s effectively being over-fuelled.

Any over-supply of fuel causes heaps of engine soot to be dumped in the DPF and it blocks rapidly. Swap out the DPF – at a cost of thousands of dollars – without determining the root cause and the brand new DPF will also block up in short order.

We’ve heard of many people getting two, three and four DPFs before dealer staff woke up to an upstream issue.

Other engine complexities that can cause DPF failure include: dirty fuel that gets past the tiny fuel filters fitted to modern small diesels; engine oil diluted by fuel from post-injection; leaking injectors; air leaks in intercooler and manifold plumbing; exhaust gas recirculation (EGR) valve blockage; minor coolant leaks inside the engine; incorrect engine oil; sensor malfunction anywhere in the exhaust system (there are several sensors); positive crankcase ventilation (PCV) oiling up the inlet manifold and oil leaks in the turbo housing.

There was a well-publicised class action against Toyota, over DPF troubles with its 2.8-litre diesel.


If we liken all these DPF issues to holes in layered slices of Swiss cheese it’s easy to see why there are so many DPF dramas these days. In this famous accident causation model there’s a requirement for several weaknesses to align before there’s system failure.

The more holes you have, the more likely they’ll align and today’s diesel emissions system has plenty of ‘em.


Heated DPFs

There’s a new generation of heated DPFs available, with the DOC incorporated in the DPF and, at the front of the chamber, an electric heating element or a diesel burner.

These seem to be more elegant methods of raising EGT than post-injection or a dedicated diesel injector, squirting fuel into the departing exhaust gases.

EGT is boosted in response to sensor feedback and the electric element or fuel burner heats up the exhaust gas to the required temperature. It’s a far less hit-and-miss operation than hoping raw fuel will ignite in the DOC.

General Motors developed an electrically heated
DPF for its turbocharged 1.9-litre CIDI engine, using a 24V, 1500W power supply, feeding a heating element in its DPF, behind the DOC.

Deutz developed a diesel-fuelled burner for its DPF, eliminating the DOC in the process. Diesel fuel is introduced into a combustion chamber, where it’s ignited by a spark plug.

Automotive industry supplier, Tenneco, has come up with a similar design.



Enter the GPF


Particulate filters were once the sole property of diesel-fuelled vehicles, but not anymore. Since 2014 we now have the gasoline particulate filter (GPF). This is emissions controlling, after-treatment technology, based on the DPF that was developed to control particulate emissions from gasoline (petrol) direct injection (GDI) engines.

In the last few years the development of direct-injection petrol (GDI) engines has sped up, to the point where virtually all new petrol engine designs have this feature.

Previous petrol injection was port-injection, where atomised petrol was sprayed into the inlet manifold, upstream of the inlet valves, pre-mixing the fuel with air. In direct-injection the fuel is sprayed into the combustion chamber, similarly to direct diesel injection.

This stratified-charge induction makes a lean mixture work in a high-compression engine, for better compatibility with turbocharging, giving improved output and economy.

After the VW-led ‘dieselgate’ scandal in 2015, European makers diverted resources from small diesel development to small GDI engines. Diesel car sales in Europe have crashed, although they’re hanging on in light commercial vehicles – temporarily, we think.

The unfortunate downside of GDI technology is an increase in particulate emissions – almost entirely absent in port-injection petrol engines – and possible carbon build-up on the inlet valve stems.

To comply with Euro 6 standards, most GDI engines
have to be fitted with GPFs, although US standards may be met without them.

The only bright note in this doleful tune is the fact that petrol engine EGTs are much higher than those in diesel engines, so a GPF doesn’t have heat-input issues for passive regeneration.

It’s early days for GPFs, but they don’t seem to be having the same degree of trouble that DPFs do. However, they have a finite life and will require ash-removal maintenance.

Many people opt for removal of DPFs from their vehicles, but that’s illegal and will void warranty, so there’s no point pursuing that course any further.

All current-model diesel 4WD wagons, vans and utes have Euro 5 emissions kit that includes EGR, DOC and DPF and some even have selective catalytic reduction (SCR) as well, requiring an AdBlue tank.

There used to be many petrol alternatives, but not in 2020. In real-4WD wagons there are petrol Jimnys, Jeeps, LandCruiser 200s, Haval H9s and Patrols, and that’s about it. The 4WD ute market has far less choice, with only RHD-converted North American products offering petrol power.

Until we get production fuel cell electric or turbine range-extended electric vehicles you’ll have to get used to living with GPFs and DPFs – they’re now necessary on virtually all new internal combustion, reciprocating engines.

































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