First things first: It is illegal to use a flammable refrigerant to recharge an automotive air conditioning system. There are some exceptions, such as the new HFO-1234YF refrigerant, which is mildly flammable under certain conditions. Flammable refrigerants may also be used in the trailer refrigeration units on heavy duty trucks. But other than these, no flammable refrigerant should ever be used in a car or light truck A/C system.
Propane, butane and a number of other hydrocarbon mixtures and blends actually work quite well as refrigerants, with cooling characteristics similar to R-12, R-22 and R-134a. But if the evaporator inside the passenger compartment develops a leak, the flammable vapor may create a potential for a fire or explosion. Do you really want to risk being cremated while driving your vehicle?
Another thought to keep in mind is what happens if your vehicle is involved in an accident. The A/C condenser sits right in front of the radiator and contains high pressure refrigerant vapor and liquid. If the condenser is ruptured in a frontal collision (which it often is), high pressure flammable vapor will be released, almost guaranteeing an underhood fire!
Do you really want to risk your life or the lives of your passengers by using a cheap flammable refrigerant in your vehicle’s air conditioning system? R-134a is the only refrigerant that should be used when recharging a late model automotive A/C system.
EPA SNAP Rules
The US EPA SNAP regulations do NOT allow the use of flammable refrigerants in vehicles.
The only exception is a new refrigerant called HFO-1234yf which is only mildly flammable under certain conditions and is not even in production yet. For info on EPA SNAP regulations, see this link:
EPA SNAP Rules Regarding Flammable Refrigerants
The Mobile Air Conditioning Society (MACS) also does NOT approve of using flammable refrigerants. Nor does the Society of Automotive Engineers (SAE). Like the EPA SNAP rules, the only exception is the new HFO-1234yf refrigerant that will start appearing in some new cars in 2013 or beyond as a replacement for R-134a.
Use R-134a Only
The EPA, MACS and SAE all say that the only acceptable replacement refrigerant for newer R-134a system is R-134a. Nothing else should be used to avoid cross-contamination problems and cooling performance issues. Older vehicles with R-12 can be retrofitted with R-134a or other approved alternative refrigerants (which also must NOT be flammable).
Here is document that contains a lot of good information about alternative refrigerants, though it is dated 1999. The basic information is unchanged, and the only update would be the approval of HFO-1234YF for future A/C systems.
Flammable Refrigerants are Against the Law!
Many city municipalities and states have laws prohibiting the use of flammable refrigerants in vehicles.
In Europe, propane is allowed in some home refrigerators, but the amount is so small that it poses no explosion or fire hazard. Refrigerators must also pass safety tests that show any refrigerant leak cannot create a fire or explosion hazard.
OZ-12 and HC-12A
Alternative refrigerants that do not meet the EPA's criteria for environmental acceptability or safety include OZ-12, HC-12a, R-176 and R-405a. Flammable refrigerants such as OZ-12 and HC-12a contain large quantities of hydrocarbons (propane, butane, isobutane, etc.) and have been declared illegal for use in mobile A/C applications but are still finding their way into A/C systems because of their cheap price.
The EPA does not approve of flammable refrigerants because the EPA says they pose a significant danger to a vehicle's occupants should a leak occur. The EPA says a spark from a cigarette or a switch could ignite leaking refrigerant causing an explosion and fire.
Counterpoint: The makers of HC-12a contend that the flammability danger is highly exaggerated and there is "no evidence" to support claims that a cigarette or spark can ignite their product. They also contend that the EPA's position on their product is unfair, and that HC-12a has been successfully used in many vehicle A/C systems worldwide with no accidents or injuries due to ignition.
Another alternative refrigerant that has been proposed for automotive use is HFC-152a. It has a GWP rating of 140, which meets the European requirements for combating global warming. HFC-152a has cooling characteristics very close to that of R-134a, and could probably be used as a direct substitute for R-134a with few if any modifications. But HFC-152a is flammable, so it is currently illegal to use.
One possible solution to the flammability issue HFC-152a (or other flammable refrigerants) is to add a leak sensor inside the vehicle that warns the passengers if a leak occurs, and automatically opens the power windows to vent the vapors (thus, reducing the fire/explosion risk).
Another solution is to redesign the A/C system so that it uses a "secondary loop" to keep the flammable refrigerant in the engine compartment and out of the passenger compartment. With this approach, the refrigerant circulates through an intermediate heat exchanger and chills a liquid (probably a water/antifreeze mixture) that then flows through the HVAC unit inside the vehicle. A recent report from the U.S. EPA says this approach meets its safety criteria, while also being energy efficient. But it does not reduce the risk of fire in a frontal collision, and it still poses a risk to technicians and do-it-yourselfers while recharging or servicing the A/C system.
The newest alternative refrigerant is HFO-1234yf. The refrigerant will eventually replace R-134a, starting in model year 2013 on some vehicles. It has thermal characteristics that are similar to R-134a, so no major modifications to the A/C system are necessary. It has a low GWP rating of less than 150, which meets the European requirements for reducing global warming. But it is mildly flammable (though much less so than HFC-152a) and will only burn under certain conditions (which should NOT occur in normal automotive usage).
If you want more information about HFO-1234YF, see 1234facts.com.
Citing concerns that the alternative refrigerant HFO-1234yf can burn in real life conditions inside a vehicle, Volkswagen said that it will NOT switch its vehicles to HFO-1234yf. At this time, it appears that CO2 is the best alternative to R-134a, according to VW. This decision follows a similar announcement late last month by Daimler (Mercedes Benz) to abandon work on HFO-1234yf in favor of CO2 or other alternative refrigerants.
HFO-1234yf can by ignited by a spark within seconds of its release, and emits highly toxic fumes and acids as it burns according to tests conducted by Daimler. CO2 is nonflammable and is carbon neutral, so it appears for now that HFO-1234yf is on hold as far as two of the leading German auto makers are concerned.