Selecting the right lubricant for a cars AC system is a tricky business, especially when factoring in refrigerants and electric compressors. Liz Dixon compares the performance characteristics of two synthetic base oils.
By 2030, there will be a projected 125 million electric-powered vehicles on the worlds roads, according to the International Energy Agency. Many of them will use electric compressors in their air conditioning systems. The complexity of choosing the right oil for these systems means that original equipment manufacturers and engineers are often not making the most effective choice in order to keep them running at the optimum level.
Meanwhile, evermore-stringent environmental legislation, especially in the European Union, and the rapid growth of the hybrid and electric vehicle market are pushing OEMs of AC units toward more environmentally friendly refrigerants.
Among this new legislation is the EUs Directive 2006/40/EC, which fully came into effect in 2017. This legislation applies to mobile air conditioning, or MAC, systems and according to its stipulations, AC systems in motor vehicles that have been type-approved after Jan. 1, 2011, may not be filled with fluorinated greenhouse gases with a global warming potential, or GWP, higher than 150. In practice, it means that any new car from 2017 onward in the European Union cannot include the previous standard hydrofluorocarbon refrigerant, R134a, which exhibits a high GWP of over 1,400.
GWP is a measure of how much energy the emission of 1 metric ton of a gas will absorb over a given period of time, relative to the emissions of 1 ton of carbon dioxide, according to the United States Environmental Protection Agency.
From R134a to R1234yf
Compliance with this directive led to the development and adoption of R1234yf, a class of hydrofluoroolefin refrigerant that the UN Intergovernmental Panel on Climate Change confirmed has a GWP of 1 (equal to that of CO2). In addition, R1234yf has low ozone depletion potential. Developed to be a drop-in replacement for the banned R134a refrigerants, R1234yf is now the industry standard for new vehicles and R134a is being phased out.
No matter how well designed MAC systems might be, over the years they will eventually suffer issues such as seal shrinkage and release their refrigerant into the atmosphere. R1234yf has been chosen as the latest generation of refrigerant because it degrades readily under standard environmental conditions.
This high level of reactivity poses some problems for AC systems, as it causes the refrigerant to destabilise over time, particularly compared with R134a chemistries. To counter this effect and support the refrigerant through its lifespan, the right lubricant chemistry is vital for long-term operation.
How is this lubricant selected? Fundamentally, it 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 R1234yfs molecular structure causing a high level of chemical reactivity, the lubricant must have the correct stability properties to counteract the refrigerants inherent instability, in addition to appropriate miscibility properties with this new refrigerant type. In this regard, lubricants formulated with polyalkene glycol, or PAG, an API Group V base oil made by reacting ethylene or propylene oxides with alcohol, have the most preferential properties.
Yet, automotive OEMs have tended to shy away from PAGs for electrical systems, instead preferring to use polyolester, or POE, produced by reacting monobasic acid with polyhydric alcohol. This is likely a throwback to stationary refrigeration, when MACs were in development and where refrigerant gas and oil come into direct contact with electrical components. In these systems, electrical current leaking into the refrigerant is unacceptable for a variety of reasons, with the risk of electric shock being the foremost.
PAG was developed during the Second World War by the U.S. Navy as a fire-resistant hydraulic fluid. It is used in specialized applications, such as worm gears, compressors and turbines.
Belt-driven and Braces
In most MAC systems in internal combustion engine vehicles, where the compressor is belt-driven from the driveshaft, electrical resitivity is not a consideration. This is quickly changing, however, as EVs with electrical MAC systems make headway.
These systems require further considerations of the lubricants electrical properties. Historically, PAGs have exhibited higher levels of electrical conductivity than the industry considers acceptable, and these levels are largely the result of systematic manufacturing imperfections, 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 historical and contemporary PAG-based solutions have exhibited such electrical properties is because of how they are formulated and processed. Only a handful of PAG producers worldwide have attempted to address this problem, with others continuing to perpetuate the PAG misconception by producing materials to a inferior standard than required for applications where electrical insulation is a consideration.
When PAGs are processed under more stringent conditions to achieve higher levels of purity, the results are less contaminants and a lubricant that is perfectly safe for use in hybrid and EV compressor systems.
POEs a Question
As the use of electric compressors has increased, there have been moves toward POEs being used as MAC lubricants. The problem with this is that POEs have inherently inferior chemical stability compared to stable PAG. As a result, they are inadequate at stabilising R1234yf refrigerants.
There are also long-term impacts, which can be seen when looking at how POEs and PAGs react to water ingress. PAGs are hygroscopic and therefore absorb water from their environment, and they have high water saturation points. Hence this ingressed water hydrogen bonds directly to the PAG molecules without causing a chemical reaction.
This hydrogen-bonding prevents water from freely existing in the system and reacting with system components, so the bonded water molecules will not contribute to problems such as metal corrosion.
The same cannot be said of POEs. As water inevitably ingresses into the system, POEs are likely to undergo a reverse esterification reaction, like any other ester. This reverse esterification reduces the POE back into its constituent acidic and alcoholic components, which then go on to attack metallic and rubber components, causing corrosion.
Furthermore, these contaminants are particularly disadvantageous in new R1234yf systems because of the instability of the refrigerant. The alcoholic and, most notably, the acidic contaminants further chemically destabilise the R1234yf in the system, with predictable consequences to system stability and lifespan.
When we consider these factors, PAGs emerge as the clear lubricant of choice. However, this is only the case if the material has a higher level of purity. Research has suggested that many aftermarket products sold as PAGs only loosely fit what physical and chemical criteria characterize this type of base oil. For example, some contain esters as the main components, whilst being described as PAG based. Many other aftermarket PAG products exhibit total acid number values that are unacceptable for PAG lubricants.
It is clear that good-quality PAG chemistries outperform POEs in R1234yf electric MAC systems in almost every case, which is why compressor OEMs and mechanical engineers should consider 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.
Liz Dixon is the global technology director of Shrieve Group, a supplier of synthetic speciality refrigeration lubricants. Dixon has more than 33 years of experience in synthetic lubricant chemistry, having previously served in several technical and business roles at Cognis (now part of GEO Speciality Chemicals) for 11 years and Shrieve for 12 years. She has a PhD in electrochemistry and a degree in applied chemistry, both from the University of Portsmouth.