Addressing the use of PAG in electric compressors

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There will be 125 million electric cars on roads around the world by 2030, many of which will use electric compressors in their AC systems. However, the complexity of choosing the right oil for their AC system means OEMs and mechanical engineers are often not making the most effective choice.

 Here, Dr Liz Dixon, global technology director of the Shrieve Group, a major supplier of synthetic speciality refrigeration lubricants, explains why polyalkylene glycols (PAGs) are the lubricant of choice for hybrid and electric vehicle ACs.

The hybrid and electric vehicle (HEV) market is growing rapidly and driving up use of electric air-conditioning compressors, and environmental legislation is leading AC original equipment manufacturers (OEMs) to use more environmentally friendly refrigerants.

Among this new legislation is European directive 2006/40/EC, which fully came into effect in 2017. This legislation applies to mobile air conditioning (MAC) systems and, in the words of the European Commission, “stipulates that air conditioning systems in motor vehicles type-approved after 1 January 2011 may not be filled with fluorinated greenhouse gases with a global warming potential (GWP) higher than 150”.

From R134a to R1234yf

Compliance with this directive led to the development and adoption of R1234yf, a class of hydrofluoroolefin (HFO) refrigerant that the UN Intergovernmental Panel on Climate Change (IPCC) confirmed has a GWP of 1.0. In addition to this, R1234yf has a low ozone depletion potential (ODP). Developed to be a drop-in replacement for R134a refrigerants, R1234yf is now the industry standard for new vehicles and R134a is being phased out.

Unfortunately, the R1234yf chemical structure that ensures a low GWP can also cause issues with refrigerant stability, as the HFO R1234yf molecule is more chemically reactive than R134a. To counter this, the right lubricant is vital for long-term operation. So, how do you select this lubricant? It fundamentally boils down to chemistry.

Of course, the core properties of a good lubricant — viscosity, lubricity and thermal stability — have remained central to selection for many years. But with R1234yf’s molecular structure causing a high level of chemical reactivity, the lubricant must have the correct stability properties to counteract the refrigerant’s inherent reactivity, in addition to appropriate miscibility properties with this new refrigerant type. In this regard, PAG lubricants have the most preferential properties.

The preferred chemistry

Electrical systems require further considerations of the lubricant’s electrical properties. Historically, PAGs have exhibited higher levels of electrical conductivity than the industry considers acceptable, and these levels are largely the result of factors such as residual catalyst, acidity and water in the lubricant. This has created a perception of PAGs as unsuitable for use in semi-hermetic and hermetic systems.

The reason many PAG-based solutions have exhibited such electrical properties is because of how they are formulated and processed. If these PAGs are processed under more stringent conditions to achieve higher levels of purity, you get less contaminants, and a resultant lubricant that is perfectly safe for use in hybrid and electric compressor systems. This is something that the chemistry specialists at Shrieve considered when developing Zerol HD, which overcomes the concerns associated with previous PAG chemistries.

Zerol HD is a sophisticated double end-capped PAG-based lubricant that is specifically designed to meet the long-term needs of electric compressors that use R1234yf refrigerants. It is also suitable for use in belt driven systems that employ R1234yf and backwardly compatible with R134a, providing less chance of cross-contamination. The lubricant is available in several viscosity grades and has been optimized with specifically developed additive technology to enhance system performance, efficiency, stability and longevity.

Crucially, Shrieve brings its technology and expertise to the development of Zerol HD, ensuring that it is effectively manufactured to remove the residual contaminants that affect the lubricant’s conductivity. Because of this, ZEROL HD demonstrates 35 kV dielectric strength and 1010 Ohm-cm electrical resistance, making it more than suitable for use in electrically driven systems. With these properties, the product is an example of a PAG that is a universal fit for both mechanical and electrical MACs, and a preferential alternative to polyol esters (POEs).

POE shortcomings

As the use of electric compressors has increased, there have been moves towards POEs being used as MAC lubricants. The problem with this is that POEs have inherently inferior chemical stability compared to the ZEROL HD, PAG, and as a basefluid choice they are inadequate at stabilizing R1234yf refrigerants as a result.

Then there are the long-term impacts, and we can see this when we look at how POEs and PAGs react to water ingress. Although PAGs are hygroscopic and therefore they do absorb water from their environment, this ingressed water hydrogen-bonds directly to the PAG molecules without causing a chemical reaction.

This hydrogen bonding prevents the water from freely existing in the system and reacting with system components, so the bonded water molecules won’t contribute to problems such as metal corrosion.

The same cannot be said of POEs, which undergo a chemical reaction when exposed to water. The water can cause the esters to break down into their constituent parts, which produces acidic by-products that cause further chemical instability in the system.

Conclusion

It’s clear that good quality PAG chemistries outperform POEs in R1234yf electric MAC systems in almost every case, which is why we urge compressor OEMs and mechanical engineers to use them as the lubricant of choice. The challenge ahead for the industry is to stem the flow of ineffective aftermarket lubricants based on inferior PAG and POE chemistries from undermining long-term compressor performance.

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