Sep 14, 2020
Posted by Keith Howard, Strategic Technology Manager, Paul Kirkman, Research Chemist, Strategic Research
This article highlights the key takeaways from our recent webinar, Gasoline Particulate Filters – Insights into Engine Lubricant Effects, from our series presented by Lubrizol experts on important industry trends. You can view the full webinar and register for upcoming webinars here.
Gasoline particulate filters (GPFs) are a relatively recent phenomenon in the automotive industry. The first mass-produced car with a GPF installed was the Mercedes-Benz S500 (introduced in 2014) and, since then, GPFs have been increasingly common in makes and models of automobiles around the world.
There were three major drivers of the adoption of GPFs worldwide.
- The first was the widespread introduction of gasoline direct injection (GDI) engines. GDI engines provide fuel efficiency and power benefits by injecting fuel directly into the cylinders, but the downside is that they generate higher levels of carbonaceous particulates than port fuel injected (PFI) engines.
- The second factor was the introduction of a particulate number (PN) limit in addition to particulate mass (PM) as a measure of the particulate emissions from automobiles. It currently applies to light duty diesel and GDI engines, but not PFI ones. The impact on health is more closely related to the number of particles rather than the mass.
- The final driver was the application of real-driving emissions (RDE) standards implemented through Euro 6. Once those requirements were put in place, the majority of GDI engines could no longer meet the PN limit without GPFs which became standard for GDI engines in Europe
Future regulatory changes in Europe and China will soon make GPFs standard equipment for all gasoline-powered vehicles, including PFI and hybrids.
GPFs are constructed of a ceramic material and contain longitudinal channels, which run the entire length of the filter. The channels have porous walls and alternately blocked channel ends. This structure forces exhaust gases arriving at the filter inlet face through a porous wall to reach the outlet. As they do so particles being carried by the exhaust gases are trapped by the channel walls in one of three ways:
- Brownian diffusion
As well as capturing combustion derived particles, lubricant-derived particles within the exhaust stream are also trapped by the GPF, creating an ash layer on the channel walls, which can affect the engine performance as described later.
The next generation of GPFs will have to contend with a reduction in the lower limit for the size of particles, which will go down from 23 nanometers to 10 nanometers. As a result, next-generation GPFs will need further improved filtration efficiencies. Unfortunately, this could leave them more vulnerable to lubricant ash-related blockages as lubricant-related particulates are more significant in the 10 nanometer to 23 nanometer range.
Engine lubricants and GPFs. Initially, the lubricant derived ash layer on the channel walls improves filtration efficiency. Only a small amount is required to observe the improvement. Once larger amounts build up, they can produce significant long-term blockage, which affects performance and efficiency (during studies by Lubrizol in coordination with Corning, 5% losses were measured). In addition, ash composition also affects blockage, as its structure may be porous or non-porous. Reduced SAPS (Sulfated Ash, Phosphorous, Sulfur) engine oils are essential to long-term emissions compliance.
Real-world implications. To gather data about the effects of long-term ash buildup in GPF equipped vehicles Lubrizol, in collaboration with Corning Environmental Technologies, conducted a real-world test, consisting of nine taxis in Shanghai, China, which were driven until they each reached 160,000 to 200,000 kilometers over the course of the test. Each vehicle was retrofitted with a GPF and sported a 1.4-liter turbo GDI engine.
The taxis were split into three groups of three vehicles each and were given one of three lubricants. The lubricants were all the same viscosity, 0W-20, but varied in ash quantities from 0.79 weight-percentage of ash to 1.32 weight-percentage of ash. The ash levels were altered by changing the detergent system, specifically the levels of calcium and magnesium within the lubricants. Throughout the testing, GPF blockage was continually monitored (via backpressure), along with oil consumption every 10,000 kilometers and GPF ash collection at 50,000 kilometer intervals. Finally, a range of complementary analytical techniques were applied to understand the locations of filtered ash, providing a detailed and holistic understanding of ash collection within GPFs.
What the tests showed was what one would expect to see: higher ash lubricants caused more ash to be collected in the GPF, while lower ash lubricants produced lower ash volumes in the GPFs. The increased levels of collected ash resulted in a higher exhaust backpressure. This increase in backpressure was a result of thicker ash layers on the GPF channel walls, and longer ash plugs at the GPF channel ends.
At the end of the trial, the relative power drops and fuel efficiencies were measured, demonstrating that the lower-ash lubricant gave a 4.5% advantage in power, and used 5.5% less fuel compared to the higher-ash counterpart.
Lower SAPS oils were found to give significant benefits in reducing long-term GPF blocking, helping to maintain performance and efficiency; high performance, aftertreatment compatible oils are essential in keeping engines and aftertreatment, including GPFs, operating efficiently for life.
For more information about GPFs, contact your Lubrizol representative and watch the full in-depth webinar here.