My recent post on the 1953 Chrysler Airtemp air conditioning system and its rather unusual R-22 refrigerant spawned a little confusion over various A/C refrigerants, so I pulled together this post to explain this topic more fully than I was able to in my Chrysler post.
All air conditioning systems rely upon some sort of refrigerant to undergo a phase change between gas and liquid (and back again) to remove heat from the area being cooled. The earliest A/C systems used a wide variety of refrigerants like ammonia, chloromethane, propane, and sulfur dioxide. None of these compounds were really suitable for automotive use, as they are all either flammable or toxic (or both). Over the years, various refrigerants have been used in automotive air conditioning. Let’s take a look.
In the 1930s, GM and DuPont partnered to develop safer refrigerants and came up with Dichlorodifluoromethane, better known by its trade name Freon, or today known simply as R-12. Here was a refrigerant that seemed (at the time, anyway) to be completely harmless: R-12 was odorless, colorless, non-flammable, and non-toxic. Indeed, a common way to check for leaks in R-12 systems in the early days was to hold an open flame up to the system around areas with potential leaks.
This was a particularly productive time for the DuPont team: Between 1930 and 1935, this team not only discovered R-12, but also Chlorodifluoromethane (R-22) and a number of other less commonly used refrigerants and propellants (R-11, R-113, and R-114).
Now might be a good time for a quick sidebar to explain the conventions of naming refrigerants. ASHRAE (the American Society of Heating, Refrigeration, and Air-Conditioning Engineers) has long been responsible for keeping the “master list” of refrigerant identifiers, all of which start with the letter “R” (for refrigerant, of course) followed by a numerical designation. Methane-based CFC refrigerants are identified with two-digit numbers, indicating the number of hydrogen atoms (plus one) and the number of fluorine atoms, respectively. Following this convention, we can infer that R-12 is considered to be a methane-based CFC with no hydrogen atoms and two fluorine atoms, while R-22 has one hydrogen atom and two fluorine atoms.
Three-digit numbers starting with “1” (like R-113) are ethane-based, and those that start with “2” are based on propane. Numbers starting with “7” are inorganic compounds, many of which are not commonly used (or even thought of) as refrigerants: For example, pure hydrogen (H2) is R-702, ammonia (NH3) is R-717, and water (good old H2O) is called R-718 when employed as a refrigerant. Most CFCs are fully saturated (meaning that all bonds between atoms are single bonds to maximize the number of hydrogen atoms). If a CFC is unsaturated (meaning that there are one or more double-bonds between carbon or fluorine atoms), an extra “1” is prepended to the identifying number, as in the case of R-1234yf.
Lastly, there are sometimes isomeric variations of some refrigerants (isomers, as you recall from your high school chemistry, are compounds that have the same molecular formula but different arrangements of the constituent atoms). These are indicated with a trailing letter (as in the case of R-134a). You can view the full list at ASHRAE’s website, if you are interested.
Now back to automotive refrigerants:
All early automotive air conditioning systems used R-12 (with the previously noted exception of the 1953-55 Chrysler Airtemp system, which used R-22).
R-12 would go on to become the standard for automotive air conditioning for many decades, while R-22 would in turn become the standard for residential and commercial cooling applications. There are several reasons for this. R-22 has a lower boiling point, a higher heat of vaporization, and a higher specific heat than R-12, which makes it a slightly more efficient refrigerant than R-12. However, R-22 has several disadvantages that have precluded its use in automotive applications. The pressures that R-22 operates are roughly double those of R-12, which require heavier components that would, in turn, add weight and draw more engine power (recall the monster V4 R-22 compressor in the 1953 Chrysler). While size and weight are of little concern to building-installed A/C systems, these are critical parameters for vehicle applications. R-22 also has the unfortunate habit of turning corrosive when overheated or exposed to moisture, both of which are more likely to occur in a car than in a building. This corrosiveness also means that you can’t use any rubber components (like O-rings) in R-22 systems, and instead have to use more expensive flared fittings. All of these factors conspired to make R-12 the de facto standard refrigerant for automotive air conditioning, even if it is slightly less efficient.
While early CFCs like R-12 and R-22 were at one time thought to be completely harmless, starting in the 1970s a growing body of scientific evidence demonstrated a link between some CFCs and depletion of the ozone layer. The Montreal Protocol of 1987 mandated a phaseout of the most ozone-depleting chemicals, including R-12. Auto manufacturers started switching their A/C systems over to R-134a in the early 90s, which has only a tiny fraction of the ozone depletion potential of R-12.
R-134a is not a drop-in replacement for R-12: R-134a is not compatible with the mineral oil lubricants commonly used in R-12 systems, and is also slightly less efficient than R-12 (which I’ve personally observed in systems I’ve converted over from R-12 to R-134a). For this reason, R-134a systems use different fittings than R-12 to avoid any mixup. Converting a system to R-134a may also require replacing the receiver/dryer (and possibly the compressor as well) to remove any traces of mineral oil from the R-12 system.
R-134a might have been a win for the ozone layer, but it has had a negative impact on global warming: R-134a is a global warming agent over 1,000 times more potent than CO2. After years of research, R-1234yf was developed as a replacement. It has similar thermal properties to R-134a, but with far less global warming potential than R-134a (less even than that of CO2). The only downside to R-1234yf is that unlike previous CFCs, R-1234yf is slightly flammable. For this reason, retrofits of earlier systems (either R-12 or R-134a) to R-1234yf are not possible and should never be performed.
Unlike the previous R-12 phaseout, the phaseout of R-134a does not include an outright ban, in part due to the above-mentioned inability to retrofit. Auto manufacturers started switching over to R-1234yf in the early 2010s, and by 2018 (the latest year I could find data for), roughly 50% of all new cars and trucks were shipping with R-1234yf-based air conditioning systems. Some, like BMW, have switched their entire product lines over to R-1234yf, while others, like Volvo, have not yet begun the changeover and are still 100% on R-134a. Most manufacturers are somewhere in between, usually switching to R-1234yf when performing a major model refresh.