Dr. Liz Dixon, Global Technology Director of the Shrieve Group, explains why using polyalkylene glycol (PAG) lubricants will enhance the performance of future electric commercial vehicle fleets.
The recent announcement from the UK Government that the ban on petrol and diesel vehicles is being brought forward by five years puts further pressure on commercial vehicles. An increasing number of electric commercial vehicles have entered the market in recent years, but the number remains relatively low. According to data from the European Environment Agency, registrations of electric and plug-in hybrid vans only account for 0.8% of van sales in the EU.
However, these numbers are likely to change in the years ahead, and mechanical engineers must be prepared. HEVs have increased the use of electric air-conditioning compressors, which poses an interesting challenge; not only because of environmental policies, but also due to the changing nature of refrigerants.
From R134a to R1234yf
Among this new legislation is European directive 2006/40/EC, which fully came into effect in 2017 and applies to mobile air conditioning (MAC) systems. 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 global warming potential (GWP) of 1.0.
Developed to be a drop-in replacement for previously-used 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? 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. 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 CV 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 double end-capped PAG-based lubricant specifically designed to meet the long-term needs of electric compressors that use R1234yf refrigerants.
Crucially, Zerol HD is 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 polyolesters (POEs).
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 and PAG.
As a base fluid choice they are inadequate at stabilising R1234yf refrigerants as a result. 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 for commercial vehicles.