TECH ED’S CHOICE

DIESEL QUALITY AND DPF OPERATION

I drive a 2016 Nissan X-trail 1,6 dci. My understanding is the vehicle is fitted with a diesel particulate filter [DPF], which is apparently costly to replace should it become clogged. The vehicle was purchased as a demo with 12 000 km on the clock. I’ve been filling up with 10 ppm diesel and my question is: if I switch to 50 ppm diesel, would that shorten the expected life of the diesel particulate filter? Also, is it true that a lower sulphur content implies less lubrication? WERNER BRITZ Roodepoort

The diesel particulate filter is a device fitted in the exhaust system to catch particulate matter (mostly soot). This technology was developed to help clean up exhaust emissions and it became compulsory in Europe when EU06 emission standards were adopted. South Africa is still only at Eu02-equivalent emissions standards, which do not require a DPF to be fitted to locally sold diesel vehicles. Therefore, many manufactures have a “DPF deletion” option when importing vehicles.

The Technology

The DPF consists of a porous, ceramictype brick in a can that looks very much like a catalyst in the exhaust. It is designed to last the lifetime of the vehicle and not generally a serviceable item. A pressure-differential sensor is connected across the filter to measure the pressure drop with exhaust flow to determine the soot level in the filter. When it reaches a certain level, the ECU triggers a DPF regeneration (cleaning) action. Depending on the drive cycle of the vehicle, this happens every 6001 000 km. The regeneration process adds extra diesel late in the combustion process (post-injection) to raise the temperature inside the DPF (and exhaust) to around 600 degrees Celsius, which is optimal for burning soot. Remember a diesel engine’s exhaust-gas temperature is generally much lower during normal driving. It should take only five minutes or so for the filter to regenerate during optimum conditions.

Shortcomings

The problem, especially with early systems, comes in stop/start driving, where the conditions aren’t feasible to regenerate the filter, or the engine is regularly stopped before regeneration can complete (during highway driving, the higher exhaust temperatures help the filter to passively regenerate, even if the full action is not triggered). In this scenario, the filter will continue to fill and the ECU may prompt the driver to go drive on the motorway where conditions are more favourable for regeneration. If this command is ignored, the filter will continue to fill to a dangerous level where a thermal event (i.e. fire) is possible. In this scenario, the ECU invokes limp mode to limit engine power and force the owner to visit a dealership. It is possible for a dealer to then conduct a controlled service regeneration triggered by service tools. If, however, the filter is completely blocked, the only option is to replace the filter at great cost.

10 vs. 50 ppm

The X-trail can run on 50 ppm, so you should not encounter problems. Sulphur does lead to increased particulate production so it is always better to opt for the lower-sulphur-content diesel. There is no truth to the rumour 10 ppm offers less lubrication because the oil companies specifically add lubricity additives to their fuel products to meet (and exceed) the SANS 342 fuel-requirement spec.