Emission Systems and Engine Oils

What is a DPF (Diesel Particulate Filter)?

Diesel Particulate Filters (DPF), sometimes known as Catalysed Particulate Filters (CPF), form part of the latest generation systems, designed to clean exhaust emissions in many late model diesel vehicles.

DPFs can look like the images below -

 

Diesel Particulate Filters

 

What does a DPF do and how does it work?

DPF's are located in the exhaust system of many post 2006 diesel powered vehicles. Their purpose is to trap diesel particulate matter expelled from the engine that would otherwise be exhausted into the atmosphere. After trapping the particles from the exhaust and at predetermined intervals, the DPF does what is called a regeneration where it burns off the diesel particulate matter. This is controlled by the vehicles electronic computer which is signalled by sensors attached to the DPF. The result is a significant lowering of exhaust emissions (NOx) and particulate matter.

For optimum performance of the DPF, most vehicles will require a Low or Mid SAPS oil.

Why do DPFs need Low or Mid SAPS oils?

A blocked DPF will stop or severely reduce a vehicles performance and replacement or manual regeneration is extremely expensive. Using the correct engine oil, is by far the best option. Different vehicle manufacturers might specify the same viscosity but with different chemical properties depending on their engine and the type of DPF fitted to the vehicle. It is extremely important that the correct engine oil is used as it can have a big impact on the life and performance of the DPF. Hence DPFs generally require a low or mid SAPS oil because -

  • Ash is produced after combustion and is carried, along with soot to be trapped by the DPF. Too much ash and it won’t be able to be removed during regeneration, thus clogging the filter which can then cause exhaust back pressure and potential engine damage.

  • Excessive Phosphorus can poison exhaust system catalysts, reducing their efficiency.

  • Excessive Sulphur can poison devices that remove nitrogen oxides from exhaust emissions.

Below is a picture showing the effects of using the incorrect oil in a vehicle fitted with a DPF.

DPF Oil Importance

 

 

Exhaust Gas Recirculation Systems

In recent years, engine manufacturers have been required to reduce the levels of nitrogen oxides (NOx) in diesel engine exhaust to meet Tier 3 and higher emission standards required by Governments and Environmental Protection Agencies. One of the reasons for this is because NOx gasses released by the exhaust, have been associated with respiratory disease and cancer. This requirement is accomplished by changes in engine designs that include retarded timing, raised piston rings, selective catalytic reduction and the use of Exhaust Gas Recirculating (EGR).

Engine manufacturers use EGR to control NOx emissions by recirculating exhaust gases back into the combustion chamber to be burned a second time, thereby reducing emissions associated with health risks. The amount of exhaust gas introduced into the combustion chamber will displace oxygen, creating cooler combustion. In doing this, many of the exhaust contaminants end up in the engine oil.

Diesel engine oils are exposed to a high level of contamination that can degrade the oil and damage engine parts. Exhaust gas recirculation can have a detrimental effect on engine durability and its effects on the oil. Oils exposed to the EGR environment show an increase in soot content, acid number and viscosity, while the engine and oil are both exposed to corrosive/acidic gases and particle build up.

Cooled EGR occurs when the engine coolant absorbs exhaust gas heat before entering the combustion chamber. Because the engine coolant takes up the heat from exhaust gases, the engine cooling system runs hotter, therefore the oil gets hotter. Oil oxidation rate doubles with every 180 Fahrenheit (40 Celsius) increase. Oil sump temperatures can also be increased.

In addition to the stress that higher temperatures put on the engine oil, exhaust gases can act as a catalyst for oxidation and nitration in the oil. An improperly operating EGR system can severely aggravate this problem. Waste gate components in an EGR system can be particularly susceptible to surface scuffing damage. If an exhaust gas recirculating system is not operating properly, the engine oil can rapidly deteriorate. Sometimes going so far as to turning the oil into an oxidized, acidic sludge. The catalysing effects of the contaminants introduced into the engine and its lubricant make the oil much more prone to oxidation, nitration and sulphation.

Viscosity vs Soot

Production of sulphuric acid due to current sulphur levels in diesel fuel and nitric acid from NOx compounds that are recirculated back into the engine through the EGR require engine oils with a higher base number (TBN) and detergency to counter-act the damaging effects of these acidic contaminants. Diesel engines using EGR systems will also require a higher level of dispersancy because of increased soot loading in the oil. Without increased dispersancy, the higher levels of soot and particulate matter will not stay in suspension, increasing wear to cylinder sleeves, rings and valve train components.

Engine oil manufacturers have formulated oils to protect against effects of the EGR environment so that they can provide the required protection that current engine designs need. This has led to API (American Petroleum Institute) CI-4/CJ-4 oil ratings.

These changes in diesel engine design, which include EGR systems, are pushing the performance requirements of diesel engine oil. Older soot limits of 1.5 percent were normal in most heavy-duty diesel engines. Soot limits of three percent are now generally accepted, and higher levels are expected in the future. Soot levels are definitely expected to increase well beyond the nominally accepted level of three percent.

Oil change intervals will be proven by the lubricant’s ability to handle the added stress by maintaining an acceptable level of alkalinity reserve (base number), proper viscosity limits through dispersancy and antioxidants and wear control.

Viscosity Effects with Soot

Engine Oil