US researchers are developing engine oil additives to reduce harmful emissions, boost fuel efficiency and improve the durability of catalytic converters.
ZDDP is a phosphorus-based additive that has been used in engine oil since the 1930s. Phosphorus is an integral part of its chemistry, but some of this vaporises and deposits on catalytic converters. This renders the catalytic surface less effective and the tailpipe emissions of greenhouse gases increase the more phosphorus solidifies on the surface. If we use less ZDDP, then less phosphorus will deposit on the catalytic converter, says materials science engineer Pranesh Aswath at the University of Texas.
Engine oil acts as a lubricant that forms a very thin protective film on the surface of the engine. Newer engine oils are thinner and less viscous, which means less energy is wasted moving oil around the engine, improving efficiency, ‘but the downside is that when your oil becomes very thin, you don’t form enough of the boundary layer between contacting surfaces, so your additive chemistry becomes even more important so that you can protect the surfaces,’ Aswath explains.
He is improving the chemistry of ZDDP, and reducing the amount required, by developing nanoparticles that increase the efficiency of tribofilms. ‘If you can do that while lowering the phosphorus content, with the use of nanoparticles that delivers specific chemistries like either fluorine or boron, then you can potentially get the same or better outcome with lower amounts of additive chemistry in the engine oil,’ Aswath says.
His team recently reported fluorinated zinc oxide nanoparticles engineered to deliver anti-wear and anti-friction chemistries at engine interfaces, with a focus on reducing ZDDP additive in automotive lubricants without compromising performance (Wear, doi: 10.1016/j.wear.2021.203717). The nanoparticles were coated with fluorine-rich polymer films, followed by a methacrylatebased coating to improve dispersion in the oil.
‘We functionalised a very small amount of fluorine, in parts per million, but this allowed us to deliver the cationic species [zinc] to the contacting surface much more efficiently,’ says Aswath, effectively creating metallic glass films to protect the engine.
In related research, the researchers in Texas used borate-coated zinc oxide nanoparticles to replace ZDDP in automotive lubricants (Langmuir, 2021, 37, 1743). They also tested it in combination with ZDDP and achieved a reduction in phosphorous concentration to 350ppm without compromising the tribological protection of the sliding contacts.
The team in Texas has spent time working on compatibility with other engine oil products. ‘There is more work to be done to ensure that, irrespective of where you put the nanoparticles, irrespective of what brand of engine oil you are working with, it should work,’ says Aswath.
The chemists have now filed patents and believe their formulation is economical and of interest to industry.