ADDITIONAL RISK ASSESSMENT OF ALTERNATIVE

REFRIGERANT R‐1234yf

Prepared for

SAE INTERNATIONAL

Cooperative Research Program CRP1234‐4

400 Commonwealth Dr   

Warrendale, PA 15086

PREPARED BY:

Thomas A. Lewandowski, Ph.D., DABT, ERT

GRADIENT

600 Stewart Street

Seattle, WA 98101

Public Release Date: July 24, 2013
i 

Table of Contents

Executive Summary......................................................................................................................... 1

1 Background ......................................................................................................................... 3

1.1 SAE CRP1234 ........................................................................................................... 3

1.2 Testing by Daimler................................................................................................... 6

2 CRP1234‐4........................................................................................................................... 7

3 Scenarios Evaluated ............................................................................................................ 8

4 FTA Structure and Inputs.................................................................................................. 10

4.1 Refrigerant Ignition............................................................................................... 11

4.2 Propagation........................................................................................................... 14

4.3 Exposure to Vehicle Fires...................................................................................... 15

4.4 Fault Tree Inputs................................................................................................... 16

4.4.1 Probability Hot Surface Exceeds 700°C..................................................... 16

4.4.2 Other Ignition‐Related Probabilities......................................................... 18

4.4.3 Propagation and Exposure‐Related Probabilities..................................... 19

5 Results............................................................................................................................... 20

5.1 Results of Current Analysis................................................................................... 20

5.2 Comparison to Results of CRP1234 ...................................................................... 20

6 Sensitivity Analysis............................................................................................................ 22

7 Conclusions....................................................................................................................... 25

8 References......................................................................................................................... 26

   
ii 

List of Tables

Table ES.1 Probabilities of Various Adverse events Compared to Estimated Probabilities of

Events Associated with Vehicle Operation‐Related Leaks of R‐1234yf

Table 1.1 Probabilities of Various Adverse events Compared to Estimated Probabilities of

Events Associated with Leaks of HFO‐1234yf in Prior CRP 1234 Study

Table 3.1 Ignition Scenarios Evaluated by Prior CRP 1234 and CRP 1234‐4

Table 5.1 Probabilities of Various Adverse events Compared to Estimated Probabilities of

Events Associated with Vehicle Operation‐Related Leaks of R‐1234yf

Table 6.1 Results of Sensitivity Analysis

List of Figures

Figure 4.1 Overview of General Structure for New CRP1234‐4 Fault Trees

Figure 4.2 Example Results of OEM Lab and Vehicle Testing for R‐1234yf Ignition

Temperatures

Figure 4.3 CAE Modeling of the Possible Impact of Collision‐related Crushing on Available

Airspace Around the Hot Surface

Figure 4.4 Inclusion of Input Data into FTA

Figure 4.5 Mitigating Factors for Refrigerant Ignition by Crash Type

Figure 6.1 Effect of Release Orientation on Ignition Potential

List of Attachments

Attachment A OEM Data on Refrigerant Distribution in AC System Components (Example 1)

Attachment B OEM Data on Refrigerant Distribution in AC System Components (Example 2)

Attachment C OEM Data on the Impact of Steam Release During Collision on Refrigerant

Ignition and Fire Propagation (Example 1)

Attachment D OEM Data on the Impact of Steam Release During Collision on Refrigerant

Ignition and Fire Propagation (Example 2)

Attachment E OEM Test Data on Effect of Surface Temperature on Refrigerant Ignition

(Example 1)

Attachment F OEM Test Data on Effect of Surface Temperature on Refrigerant Ignition

(Example 2)

Attachment G OEM Data from Vehicle Thermal Testing (Example 1)

Attachment H OEM Data from Vehicle Thermal Testing (Example 2)

Attachment I Analysis of GIDAS Database Regarding Fires and Fuel Releases

Attachment J Supporting Data for the Probability Vehicle Occupants are Unable to Leave the

Vehicle Post‐Collision

Attachment K OEM Test Data on Vehicle Operating Temperatures
   

   1

Executive Summary

From 2006 to 2009 SAE International administered a cooperative research project (CRP1234) which 

evaluated R-1234yf, a new low global warming potential (GWP) automotive refrigerant. Using fault tree 

analysis (FTA) the results of the CRP1234 assessment showed that the risk associated with the use of R1234yf in automotive vehicles is well below those commonly considered acceptable by the general public 

and regulatory agencies. The risk assessment was submitted to the U.S. Environmental Protection 

Agency (US EPA) as part of the Significant New Alternatives Policy (SNAP) approval process, and the 

US EPA subsequently approved R-1234yf for use in US vehicles. 

In the EU, manufacturers are required to comply with the Mobile Air Conditioning (MAC) Directive 

(2006/40/EC) for new vehicle types effective 1 January 2011 with enforcement deferred to vehicles built 

on or after 1 January 2013. The EU commission has supported the introduction of R-1234yf as this meets 

the requirements of the MAC Directive. Manufacturers who have certified new vehicle types to this 

directive are obliged to build vehicles from 1 January 2013 with a low global warming potential (GWP) 

refrigerant. 

On September 25, 2012 the German automotive manufacturer Daimler issued a press release suggesting 

that new testing conducted by the company had shown R-1234yf to pose a greater risk of vehicle fire than 

was estimated by the prior CRP1234 analysis. To address the Daimler claims, a new CRP (CRP1234-4) 

was organized in October of 2012. All Original Equipment Manufacturers (OEMs) were invited to 

attend. 

After extensive testing and analysis, the new CRP concluded that the refrigerant release testing completed 

by Daimler was unrealistic. Their testing created extreme conditions that favored ignition while ignoring 

many mitigating factors that would be present in an actual real-world collision. 

At the same time, the new CRP, with input from Daimler, initiated two new fault tree scenarios to 

realistically address these claims. The new CRP also reviewed and analyzed extensive new OEM test 

data which was used to complete the new FTA. The two new fault tree scenarios consider the possibility 

of an individual being unable to exit the vehicle due to a collision or a non-collision event that involves a 

refrigerant/oil release, the refrigerant/oil being ignited and the fire propagating. The FTA examined 

average risks across the entire global fleet and used a number of conservative assumptions to ensure that 

the final risk estimate would be more likely to overestimate rather than underestimate actual risks. 

Based on the updated analysis, the estimated overall risk of vehicle fire exposure attributed to use of R1234yf is conservatively estimated at 3 x 10-12 events per vehicle operating hour. This is nearly six orders 

of magnitude less than the current risk of vehicle fires due to all causes (approximately 1 x 10-6 per 

vehicle operating hour) and also well below other risks accepted by the general public. The table below 

shows the current overall risk of occupant exposure to adverse events based on R-1234yf usage is on the 

same order of magnitude as that estimated in the prior work of CRP1234. Therefore, the conclusions of 

the former CRP risk assessment are still valid: risks are still very small compared to the risks of a vehicle 

fire from all causes and well below risks that are commonly viewed as acceptable by the general public. 

All OEMs in the new CRP have indicated agreement with these conclusions. The members1

 are 

European, North American and Asian OEMs: Chrysler/Fiat, Ford, General Motors, Honda, Hyundai, 

Jaguar Land Rover, Mazda, PSA, Renault and Toyota. 

 

1

 Daimler, BMW and Audi initially participated in the new CRP but eventually chose to withdraw. 
   

   2

Table ES.1 Probability of Various Adverse Events Compared to Estimated Probability of Events 

Associated with Vehicle Operation-Related Leaks of R-1234yf 

Event Probability per 

vehicle per 

operating hour 

Citation 

Probability of being in a police reported vehicle collision 4 x 10-5 NHTSA, 2013 

Probability of automotive vehicle fire (any cause) 1 x 10-6 Ahrens, 2013; FHA, 

2009 

Probability of vehicle collision due to vehicle brake failure 3 x 10-7 New York State DMV, 

2008 

Probability of dying in a regularly scheduled plane trip in a developed 

nation 7 x 10-8 Barnett, 2011 

Estimated probability of vehicle occupant/former occupant 

experiencing HF exposure above health based limits associated with 

an R-1234yf ignition event 

5 x 10-12 CRP1234, 2009 

Estimated probability of vehicle occupant being exposed to a 

vehicle fire due to R-1234yf ignition (due to leak and ignition in 

engine compartment) 

3 x 10-12 Current analysis 

Estimated probability of vehicle occupant being exposed to an open 

flame due to R-1234yf ignition (primarily due to leak and ignition in 

cabin) 

9 x 10-14 CRP1234, 2009 
   

   3

1 Background

1.1 SAE CRP1234

Due to concerns about global warming, the use of refrigerant R-134a (i.e., 1,1,1,2-tetrafluoroethane), has 

been prohibited in the EU for use in new type approved vehicles as of January 1, 2013 (European 

Fluorocarbon Technical Committee, 2007). Regulations developed by the European Union require that 

any refrigerant(s) intended to replace R-134a have a global warming potential (GWP) less than or equal to 

150 based on a 100-year time horizon. 

To assist member companies with selection of an R-134a replacement, SAE International administered a 

cooperative research project (CRP1234) to evaluate a proposed replacement candidate for R-134a: 

2,3,3,3-Tetrafluoropropene (R-1234yf). The intended goal of the SAE CRP process is to bring together 

technical experts from different segments of the automotive industry to capitalize on individual member's 

expertise and to develop solutions which benefit the automotive industry as a whole. Fifteen different 

automakers (OEMs) joined the CRP, along with sixteen HVAC component suppliers including DuPont 

and Honeywell International, the developers and suppliers of R-1234yf. The CRP1234 evaluation of R1234yf addressed the risks to vehicle occupants as well as first responders, vehicle assembly and service 

workers resulting from accidental releases of these refrigerants such as might result from damage to 

vehicle air conditioning systems or improper use of vehicle service equipment. Because R-1234yf is 

mildly flammable (ASHRAE Class 2L) the primary concern was refrigerant ignition. The evaluation also 

addressed possible exposures to hydrogen fluoride (HF) formed during combustion or thermal 

decomposition of this refrigerant. The CRP1234 risk assessment followed the general approach of the 

U.S. EPA-sponsored risk assessment of R-152a and CO2 (Blackwell et al., 2006) so as to employ a 

consistent methodology and generate an analysis familiar to regulatory agencies. This involved the 

development of potential exposure scenarios for different endpoints of concern and estimation of the risks 

for those considered plausible using fault tree analysis (FTA). FTA is a methodology that analyzes the 

sequence and combinations of failures that lead to a particular outcome. FTA typically uses conservative 

inputs where values are known with less than ideal certainty so the overall risk estimates are believed to 

overestimate the true risk of the adverse events of concern actually occurring. 

The risk assessment incorporated various types of data including toxicity data, bench scale testing results 

(i.e., flammability and HF generation studies), modeled air concentrations, and vehicle measurements 

(i.e., measurements of passenger compartment and engine compartment concentrations of chemicals in air 

and, where applicable, ignition tests). These data were collected from various sources including 

government databases as well as publicly available and confidential reports. Data not found in existing 

reports were generated by consulting groups and/or CRP1234 members. 

The results of the risk assessment indicated that R-1234yf is comparable to the current automotive 

refrigerant (R-134a) in terms of both human health effects and ecological effects. Results of extensive 

toxicity studies indicate that potential R-1234yf exposures of vehicle passengers in the event of AC 

system leaks into the passenger compartment would be below levels associated with potential health 

effects. Similarly, R-1234yf exposures of AC service technicians would be well below levels associated 

with potential health effects. Potential exposures of automotive assembly workers, which could occur due 

to small losses of refrigerant during vehicle assembly and/or service, were also estimated to be below 
   

   4

levels of health concern based on exposure data collected for R-134a. Thus, potential health effects from 

exposures to R-1234yf were not judged to be a source of concern in the risk assessment. 

With respect to the potential ignition of R-1234yf, the CRP1234 risk assessment considered several 

scenarios: 

 Rupture of evaporator tubing in the event of a vehicle collision, refrigerant release in the 

passenger compartment, and ignition of the refrigerant inside the passenger compartment, 

 Rupture of the AC system under hood in the event of a collision and ignition under hood 

when the vehicle occupant looks under hood to investigate the problem, 

 Rupture of the AC system under hood due to a part failure (i.e., non-collision) and 

ignition under hood when the vehicle occupant looks under hood to investigate the 

problem. 

When the risks for the three potential ignition scenarios were combined, the FTA indicated that the risk of 

an occupant (or former occupant) being exposed to an open flame due to R-1234yf ignition were 

extremely low, on the order of 9 x 10-14 events per hour of vehicle operation. Nearly all of this risk would 

be attributable to release and ignition of refrigerant in the passenger cabin as a result of a vehicle 

collision. Risks for certified AC service technicians were also found to be inconsequential (1 x 10-26 per 

vehicle service hour) because modeling studies indicated air concentrations of R-1234yf in service areas 

would only reach the LFL at a distance of 10 cm or less from the source of the AC system leak. The 

CRP1234 members judged that ignition sources would not be located this close to the AC system given 

the presence of other flammable materials in the engine compartment. 

CRP1234 also evaluated potential exposures to hydrogen fluoride (HF) that might occur in the event of 

R-1234yf ignition or thermal decomposition.2

 The risk assessment considered potential HF exposures in 

the following scenarios: 

 Exposure in the passenger compartment due to refrigerant entering the compartment after 

a collision and being ignited in the passenger compartment, 

 Exposure in the passenger compartment due to HF entering the compartment after being 

produced in the engine compartment should the refrigerant burn or undergo thermal 

decomposition, 

 Exposure under hood for various receptors (former occupants, good Samaritans) when 

refrigerant is released during a collision and burns or decomposes, 

 Exposure under hood for various receptors (former occupants, good Samaritans) when 

refrigerant is released due to a non-collision related AC component failure and the 

refrigerant burns or decomposes, 

 Exposure under hood when refrigerant burns due to a vehicle fire that is caused by 

another, non-AC system related event (e.g., vandalism, fires due to failure of other parts). 

When the risks for these potential HF exposure scenarios are combined, the FTA indicated that the risk of 

an individual being exposed to HF above the relevant health-based limit was extremely low, on the order 

of 5 x 10-12 events per hour of vehicle operation. The risk was almost entirely and equally attributable to 

 

2

 HF is produced during the burning or thermal decomposition of all fluorocarbons, including the current automotive refrigerant 

R-134a. 
   

   5

two scenarios: HF generation by vehicle fires arising from failure of non-AC vehicle components and 

decomposition of refrigerant in the engine compartment after a vehicle collision with subsequent 

aspiration of HF into the vehicle cabin. Because the first scenario also exists at the current time with R134a (which produces HF when present in a fire), the CRP determined that the HF exposure risks 

associated with use of R-1234yf were not significantly greater than those currently present. 

To place the risk estimates developed in the FTA into perspective they were compared to risks of other 

analogous events that are generally considered acceptable by the public (Table 1.1). This analysis 

showed that the risks associated with the use of R-1234yf in automotive AC systems were well below 

those commonly considered acceptable by the public and regulatory agencies. Overall, the CRP1234 risk 

assessment concluded that R-1234yf is suitable for use in direct-expansion air-conditioning systems. 

The CRP1234 risk assessment report was submitted to the U.S. EPA Significant New Alternatives Policy 

(SNAP) office which subsequently approved R-1234yf for use in mobile air conditioning systems 

(MAC).3

 This approval became official on March 29, 2011. The U.S. EPA did not impose any use or 

design restrictions on the use of R-1234yf beyond the requirement that OEMs conduct their own Failure 

Mode Effects Analysis (FMEA)-type analysis and follow SAE J639 (Safety Standards for Motor Vehicle 

Refrigerant Vapor Compression Systems) for their specific vehicle models. 

Table 1.1 Probabilities of Various Adverse events Compared to Estimated Probabilities of Events 

Associated with Leaks of R-1234yf in Prior CRP 1234 Study 

Vehicle-Related Event* 

Probability per 

vehicle per 

operating hour 

Basis 

Probability of being in a police reported vehicle collision 4 x 10-5 NHTSA, 2013 

Probability of automotive vehicle fire (any cause) 1 x 10-6 Ahrens, 2013; FHA, 

2009 

Probability of vehicle collision due to vehicle brake failure 3 x 10-7 New York State DMV, 

2008 

Probability of dying during a commercial plane flight in a developed nation 7 x 10-8 Barnett, 2010 

Estimated probability of vehicle occupant/former occupant experiencing HF 

exposure above health based limits associated with an R-1234yf ignition 

event 

5 x 10-12 CRP1234, 2009 

Estimated probability of vehicle occupant being exposed to an open flame 

due to R-1234yf ignition (primarily due to leak and ignition in cabin) 9 x 10-14 CRP1234, 2009 

Service-Related Event* Probability per 

service hour Basis 

Non-fatal recordable Injury at work (all occupations, per working hour)1 2 x 10-5 NSC, 2004 

Estimated probability of AC service technician exposure to an open flame 

due to R-1234yf ignition (per hour of vehicle service) 1 x 10-26 CRP1234 

* The basis for calculating the comparison risk values are provided in Table 5-1 (vehicle-related events) and the SAE CRP1234 (Phase III) risk 

assessment report (service related events).

 

3

 The regulatory situations in the U.S. and EU are somewhat different. The EU requires the phased in use of an automotive 

refrigerant with a GWP less than 150 but does not grant approvals for individual refrigerants. In the U.S., all new refrigerants 

must be approved by the US EPA SNAP program prior to use in the market. The U.S. Environmental Protection Agency (EPA) 

encourages the use of low GWP refrigerants through the use of emissions credits. 
   

   6

1.2 Testing by Daimler

On September 25, 2012 the German automotive manufacturer Daimler issued a press release suggesting 

that new testing conducted by the company (independent of the SAE CRP) had shown R-1234yf to pose a 

greater flammability hazard than previously understood. Specifically, Daimler stated: “In the new real-life 

test scenario, the refrigerant is dynamically dispersed at high pressure near to hot components of the test 

vehicle’s exhaust system. This corresponds to a serious head-on collision in which the refrigerant line is 

severed and the reproducible results demonstrate that refrigerant which is otherwise difficult to ignite 

under laboratory conditions can indeed prove to be flammable in a hot engine compartment. Similar tests 

of the current R-134a refrigerant did not result in ignition.” While the press release indicated that the 

refrigerant was flammable (a fact already known), statements by Daimler representatives indicated that 

their testing revealed a greater risk of vehicle fire than was estimated by the CRP1234 FTA. 

Details of the exact nature of the Daimler testing were not readily available at the time of the press 

release. Subsequently obtained information clarified that Daimler did not conduct or simulate an actual 

vehicle crash test but rather simulated an engine compartment release, for example due to a part failure. 

Initial reports that the Daimler test corresponded to a serious head-on collision are therefore erroneous 

because, unlike a collision scenario, the Daimler test involved no damage to other vehicle components 

which could serve as mitigating factors for refrigerant ignition. Efforts by one OEM to reconstruct the 

Daimler tests also indicated Daimler used a specific type of release nozzle that is not consistent with a 

broken or crushed AC line. This specialized nozzle produced a fairly slow rate of release and was 

apparently pointed directly under the shielding surface that covered the turbocharger. This configuration 

may have allowed the refrigerant to accumulate at sufficient concentrations near the hot surface. 

Additionally, a long metal tube connected the production refrigerant line to the nozzle and was routed in 

close proximity to the turbocharger surface. During the reconstructed tests it was found that such a 

configuration pre-heated the refrigerant passing through the tube by approximately 20°C. Daimler 

apparently also added valves to the production-level vehicle which could be used to throttle refrigerant 

flow such that gas velocity was reduced. The combination of the specialized nozzle, the pre-heating tube, 

and the refrigerant release throttling are all factors that artificially promote ignition. The reconstructed 

tests show that inclusion of the Daimler setup resulted in refrigerant ignition with a particular vehicle that 

was not observed when a realistic nozzle configuration was used on that same vehicle. Refrigerant 

ignition was typically only achieved when the engine cooling fan was manually overridden to be switched 

off (a situation which would normally not exist when the engine is hot). Thus Daimler's test results 

indicate that in the event of an isolated line break, where refrigerant is released at a slow rate towards a 

hot surface achieved during extreme driving conditions, and where no mitigating factors (hood buckling, 

engine cooling fan operating, release of coolant/steam under hood) are present, the refrigerant can be 

ignited and lead to a vehicle fire. But, because multiple mitigating factors are in fact expected to co-occur 

during a vehicle collision, the proper interpretation of the Daimler results was unclear. 

 
   

   7

2 CRP1234‐4

To study and confirm the results of the Daimler tests, the automotive members of the original CRP1234 

organized as a new CRP (CRP1234-4) starting in October 2012.4

 All OEMs were invited to attend, and 

most did join the new group, including Daimler and other German OEMs.5

The purpose of CRP1234-4 was as follows: 

(1) to evaluate the testing conducted by Daimler to determine its relevance to real-world vehicle 

situations, 

(2) to determine whether the existing CRP1234 fault tree structure should be expanded to 

address the new Daimler information, and if so, to guide that process, 

(3) to provide a forum whereby the OEM members could coordinate their own in-house testing 

and share the results. 

The CRP1234-4 members agreed that the scope of the new CRP would not involve a complete 

reevaluation of the prior CRP1234 FTA. The prior fault trees addressed scenarios not related to the 

Daimler tests (e.g., ignition of refrigerant in the vehicle cabin) and were developed after several years of 

study as a consensus product of all the prior CRP members. The CRP1234-4 members therefore had 

confidence that the prior fault trees fully addressed their relevant scenarios and that reinvestigation of the 

earlier FTA work was not required. 

The members of the new CRP held an initial organization meeting in October 2012. A number of face-toface meetings were subsequently held in Europe and the U.S. Work was also conducted via a series of 

twice weekly conference calls. The intent was to respond quickly to the Daimler test results and arrive at 

a final conclusion regarding the suitability of R-1234yf for MAC system as soon as possible, consistent 

with sound engineering and vehicle safety. 

 

 

4

 To avoid any bias in the new CRP, it was decided that only OEMs would be allowed to participate in the new study. 

Refrigerant manufacturers were also excluded because vehicle application issues are the responsibility and expertise of the 

OEM’s. Under SAE policies, because the membership was effectively changed, it was necessary to organize a new CRP. 5

 The members of CRP1234-4 were: Audi, BMW, Chrysler/Fiat, Daimler, Ford, General Motors, Honda, Hyundai, Jaguar Land 

Rover, Mazda, PSA, Renault, and Toyota. Daimler, BMW and Audi (a part of Volkswagen) subsequently withdrew from the 

CRP in early February 2013. 
   

   8

3 Scenarios Evaluated

Based on a review of the results of the Daimler testing, the CRP members decided to add two new fault 

trees into the analysis of refrigerant flammability. As noted above, the original CRP1234 fault trees 

considered risks of refrigerant ignition when the vehicle occupant leaves the vehicle and investigates in 

the engine compartment. The original FTA did not consider the possibility of individuals being unable to 

leave the passenger cabin in the event of a vehicle fire starting in the engine compartment. This was due 

to test data indicating the low burning velocity and heat of combustion of R-1234yf, which suggested a 

minimal risk of rapidly moving vehicle fire even if the refrigerant was ignited. Based on the additional 

concerns raised by Daimler, the CRP members decided to revisit this decision and include two remaining 

passenger scenarios in the FTA (Table 3.1). The new scenarios are as follows: 

 A scenario where there is a refrigerant/oil release during vehicle operation due to a system part 

failure, such as a broken refrigerant tube (i.e., non-collision). The released refrigerant/oil could 

then be ignited assuming a sufficiently hot surface is present in the engine compartment and the 

ignition event could propagate to other engine compartment materials such that a vehicle fire 

affects the passenger compartment. The scenario would require a simultaneous failure prohibiting 

the individual from leaving the vehicle (e.g., a failure of door locks). It also includes the 

possibility that an individual was able to enter the vehicle and able to drive but unable to exit the 

vehicle in a case of a vehicle fire. Note that the refrigerant release described in this scenario is 

most similar to the conditions of the Daimler test. This was analyzed as fault tree I6. 

 A scenario where a refrigerant/oil release occurs in the vehicle engine compartment due to a 

collision. The released refrigerant/oil could be ignited by a hot surface, spark or glowing wire 

and the ignition event could propagate to other engine compartment materials such that a vehicle 

fire affects the passenger compartment. This scenario also requires that the collision produces 

conditions where the vehicle occupants are unable to exit the vehicle, for example by damaging 

the doors on the side of the vehicle. This was analyzed as fault tree I7. 

The CRP 1234-4 members also considered evaluating the effects of HF generation and reached a 

conclusion based on the following information: 

 The prior CRP1234 conducted an extensive evaluation of potential exposures to HF that might 

occur in the event of R-1234yf ignition or thermal decomposition. That analysis indicated that 

the risk of an individual being exposed to HF above the relevant health-based limit was extremely 

low (5 x10-12) and was not significantly greater than the risk present with R-134a. 

 In a presentation made on September 17, 2012, the VDA members Daimler, Audi, BMW, and 

Opel stated that thermal decomposition will not lead to relevant concentrations of HF in or 

around the vehicle. In the event of a refrigerant fire, the VDA confirmed that similar behavior, 

regarding HF generation, exists between R-134a and R-1234yf. Therefore, the VDA concluded 

that there is no additional HF-exposure risk to occupants, first emergency responders, and 

firefighters associated with the usage of R-1234yf. 
   

   9

 Due to the irritancy of HF, individuals will leave the area unless unable to do so. The idea of 

individuals being unable to leave the vehicle is already reflected in the new scenarios described 

above. 

 Due to some HF-specific mitigation factors (e.g., convection of HF away from the vehicle by the 

heat generated from the fire), the risk of HF being present above the health based limit should be 

even lower than the risk of vehicle fire, i.e. the conclusions in the above bullets are conservative. 

 The CRP1234-4 members are also aware that studies conducted by another SAE CRP (the MRB 

CRP) have shown that even in the event of R-1234yf ignition, HF concentrations at the vehicle 

exterior (i.e., along the side of the vehicle) do not exceed the health based HF limit of 95 ppm. 

The current understanding of HF generation indicates that significant HF exposure is only relevant in the 

event of sustained refrigerant fire and not because of thermal decomposition. In the two new scenarios 

described above, the risk of having HF present above the health based limit would therefore be no higher 

than the risk of exposure to a vehicle fire. Given the current understanding of HF generation, CRP1234-4 

concluded that the prior analysis of risks associated with HF is still valid. For the new scenarios 

considered, the risk that could be estimated for HF exposure would be equal to or less than the risk of 

exposure to a vehicle fire, which is assessed by the new trees. 

Table 3.1 Ignition Scenarios Evaluated by CRP 1234 and CRP 1234-4 

Triggering Event

Location of 

Refrigerant 

Release 

Location of 

Potential Exposure

Exposed 

Individual

Exposure to open 

flame (ignition) 

Fault 

Tree ID 

Collision Passenger 

compartment 

Passenger 

compartment Vehicle occupant Evaluate via FTA I3 

AC system leak 

(non-collision) 

Engine 

compartment Engine compartment Former occupant investigating engine Evaluate via FTA I4 

Collision Engine 

compartment Engine compartment 

Former occupant 

investigating engine 

or good Samaritan 

assisting remaining 

occupants 

Evaluate via FTA I5 

AC system leak 

(non-collision) 

Engine 

compartment 

Passenger 

compartment 

Remaining 

occupants Evaluate via FTA I6* 

Collision Engine 

compartment 

Passenger 

compartment 

Remaining 

occupants Evaluate via FTA I7* 

*New fault trees developed in this study

 
   

   10

4 FTA Structure and Inputs

The original CRP1234 trees took a highly conservative view concerning refrigerant ignition and vehicle 

fires, using a single parameter (i.e., air currents do not extinguish ignited refrigerant) to differentiate brief 

isolated ignition events which pose no risk from those which could lead to ignition of other vehicle 

components. Because the reported Daimler tests suggested that refrigerant ignition events might be more 

sustained and likely to involve other vehicle components than originally believed, CRP1234-4 adopted an 

expanded structure for fault trees I6 and I7 which examined in more detail the factors that allow a 

refrigerant ignition event to propagate into a vehicle fire.6

 This is shown in simplified form in Figure 4.1. 

Each of the new trees therefore contains a refrigerant ignition section which feeds into a higher level 

portion of the tree which evaluates the probability of propagation of ignition to other vehicle components. 

The resulting risk of vehicle fire in turn feeds into the top level portion of the FTA which considers the 

possibility of an individual being present and involved in the fire (i.e., occupant unable to leave). Each of 

these levels is discussed below. 

It should also be noted that CRP1234-4 opted to use a new terminology to describe the categories of 

vehicle collisions being studied. The prior CRP referred to high, medium and low speed collisions. 

These were tied to speeds of > 50 kilometers per hour (kph), 16 to 50 kph, and < 16 kph. However, these 

speed ranges actually refer to the closing speed of the vehicles on impact rather than the speed of the 

vehicles while under operation prior to the accident. To avoid confusion and be more consistent with 

OEM vehicle accident terminology, the current fault trees refer to high, mid and low severity collisions. 

The new terms refer to the same impact speed categories as before and are similarly broken down by side 

or front impact location. It is thus a matter of clearer terminology rather than a change in the actual 

conditions being assessed. The original CRP fault trees also used these collision speeds to determine the 

likelihood of having extremely hot exhaust surface temperatures prior to collision. Using this concept, 

high speed collisions were more likely to have hot exhaust surface temperatures and low speed collisions 

were less likely to have similar temperatures. CRP1234-4 noted that even low speed collisions could 

occur after high speed driving given the amount of braking that occurs before the collision. The new fault 

trees therefore use a factor for preconditioning (% of vehicle drive time that vehicles would have 

extremely hot exhaust surface temperatures) that is independent of collision speed. 

 

6

 This expanded structure was not used for the previously evaluated scenarios but because the original analysis took a more 

conservative approach, the risks for the earlier fault tree scenarios would also be expected to be decreased. 
   

   11

Figure 4.1  Overview of General Structure for New CRP1234‐4 Fault Trees

4.1 Refrigerant Ignition

The portions of fault trees I6 and I7 addressing refrigerant ignition consist of several elements. For the 

collision case, three potential ignition sources are considered: a hot surface such as a manifold or catalytic 

converter (above a temperature of 700°C), an electrical short of sufficient energy, and a hot glowing wire 

such as an exposed and shorted battery cable. For each ignition source, conditions are also considered 

which must be present in order to have a flammable refrigerant concentration occur in the vicinity of the 

ignition source. These include the following: 

 the frequency of a collision of a certain severity (high severity front, high severity side, midseverity front, etc.), 

 the probability that such a collision causes a breach in the AC system, 

 the probability an AC system breach releases refrigerant in the vicinity of the hot surface, 

 the probability the engine cooling fan is not operational, 

 the probability that crushing of the engine compartment does not reduce the air volume near the 

hot surface to preclude ignition, and 

 the probability that mitigation does not occur due to the release of coolant and steam during the 

collision. 

Many of the inputs for the mitigating factors are shared among the three potential ignition sources. For 

the non-collision case the structure is simpler; the only ignition source is the hot surface (because glowing 

wires or significant sparks require a collision to occur) and the list of mitigating effects is reduced to those 

that do not involve a collision (the direction of the release and the operation of the cooling fan). 
   

   12

The identification of potential mitigating factors for refrigerant ignition was one of the major topics of 

discussion by the CRP and was informed by extensive data gathering. For example, the mitigating impact 

of the vehicle cooling fan in dispersing the refrigerant and creating air current that limit the degree of 

refrigerant contact with the hot surface is borne out by multiple OEM vehicle tests, including those 

conducted by Daimler. Similarly, if refrigerant is released in a direction away from the hot surface (e.g., 

out towards the front of the vehicle) then ignition is extremely unlikely. This is particularly relevant for 

frontal collision scenarios where damage to portions of the AC system at the front of the vehicle (i.e., the 

condenser) is likely in cases where more rear-ward portions (i.e., nearer the hot surface) are also 

damaged. For example, one OEM examined vehicle crash data for one vehicle model and observed that 

in all cases when a crash was severe enough to damage AC lines inside the engine compartment, the 

condenser at the front of the vehicle was also damaged. Studies conducted by another OEM indicated 

that approximately 20-70% of the refrigerant is contained in the condenser (depending on operating 

conditions) and an additional 3 to 9 percent is contained in the evaporator (i.e., inside the cabin). Similar 

data were obtained by another OEM which examined refrigerant distribution in vehicles operating at midambient and high demand (i.e. high load) conditions. In this case, it was found that 70 to 78 percent of 

the system charge was contained in the condenser. More detail on these OEM studies is contained in 

Attachments A and B. These data support the concept that in some types of vehicle collisions, most of 

the refrigerant will be released out the front of the vehicle and in the event of an underhood line breach 

only a small amount of refrigerant will be released underhood. 

With regards to release of steam or engine coolant as a mitigating factor, the OEMs considered that 

crashes severe enough to cause refrigerant releases inside the engine compartment are also highly likely 

to damage the radiator and/or coolant tanks. Because the hot exhaust surface condition required for 

possible refrigerant/oil ignition implies hot coolant as well, the release of coolant which is hot and under 

pressure will create a steam cloud that will both drive refrigerant out of the engine compartment and 

displace the oxygen needed to initiate refrigerant ignition. This was evident when reviewing frontal 

impact crash test results obtained by several of the OEMs. These vehicles were crashed while containing 

production level fluids like coolant, engine oil, and R-1234yf/PAG oil. While the OEMs did recognize 

that the glycol portion of coolant is flammable, they also considered that a period of time is required for 

sufficient water to be boiled off from the released coolant for the material to become flammable. The 

time period required for this to occur will be longer than the time frame for a refrigerant release. 

Additional detail on the potential effects of coolant/steam release in a collision is provided as 

Attachments C and D. 

A critical consideration for the ignition portion of the FTA is the temperature of the hot surface that is 

necessary to ignite the refrigerant after it is released. Various studies conducted by multiple CRP1234-4 

members have indicated that temperatures of 750°C or higher are generally required to ignite R-1234yf in 

an engine compartment (See for example Figure 4.3). Daimler reported that refrigerant ignition was 

observed at temperatures below 700°C although additional testing has indicated that the temperature 

being measured was not at the hottest location on the exhaust surface in the Daimler studies.7

 To be 

certain of the temperature of the exhaust surface required to ignite a flammable mixture of refrigerant and 

oil, hundreds of new release tests (> 500 individual tests) on multiple vehicles were run. 8 No ignitions 

were ever observed below 700°C and many releases showed no ignition at temperatures as high as 850°C 

(see Attachments E and F for more detail). To be conservative, the CRP risk assessment assumed that a 

 

7

 One of the OEMs did extensive testing with Daimler’s B class vehicle and noted that the hottest portion of the exhaust system 

was on the back surface of the exhaust system (closer to the engine). Daimler reportedly measured exhaust surface temperatures

on the front side of the exhaust surface which was found to be 25 to 30 degrees lower in temperature. 8

 Refrigerant release testing on hot surfaces to simulate the damage caused by collisions fails to consider numerous mitigating 

factors of a collision. The OEM refrigerant release testing was done to better understand the exhaust surface temperatures and

shield parameters required to ignite a refrigerant and oil mixture. 
   

   13

refrigerant temperature of 700°C would be sufficient to always ignite a flammable concentration of the 

refrigerant and oil.9

Figure 4.2  Example Results of OEM Lab and Vehicle Testing for R‐1234yf Ignition Temperatures

For vehicle testing each data point represents a separate release for a particular vehicle.  Open

symbols indicate no refrigerant ignition, filled symbols indicate ignition.

To account for the differences noted in ignition characteristics between 700°C and 850°C, during initial 

discussions the CRP had considered the possibility of a distinction between exhaust and heat shield 

designs that tend to create a stagnant zone for the refrigerant and oil mixture to be heated and those that 

tend to allow the refrigerant and oil mixture to flow up and away from the hot surface creating a nonstagnant zone case. Studies conducted by CRP members suggested that refrigerant ignition in the range 

of 700°C requires a static pocket of refrigerant and oil in close proximity to the hot surface such as might 

occur if the hot surface were covered with a heat shield with limited ventilation. This would allow the 

refrigerant to rapidly acquire enough heat from the hot surface in order to become ignited (API, 2003). 

The stagnant/non-stagnant zone approach was proposed as a way of reconciling the Daimler results 

(which reported ignition) with those of other OEMs (which did not observe ignition). Several OEMs 

observed that Daimler tested a vehicle with a highly restrictive heat shield. After considerable testing and 

discussion, the CRP decided that, although information was consistent with the idea of a distinction 

between stagnant and non-stagnant conditions, there were insufficient exhaust and shield configuration 

data to fully parameterize this in the FTA. It was therefore decided to conservatively assume that a 

stagnant zone condition could exist in any vehicle which could result in refrigerant ignition if the 

refrigerant, oil and air mixture came into close proximity with a surface at or above 700°C. The 

implications of this decision are explored via sensitivity analysis in Section 6. 

 

9

 The earlier CRP1234 FTA assumed a value of 550°C would be sufficient to cause ignition, although testing conducted at 

INERIS and Hughes indicated much higher temperatures in the range of 750°C. The higher temperature is now supported by 

results multiple tests conducted by the OEMs as part of the current effort. CRP1234-4 is also aware of testing conducted by 

another CRP (SAE CRP MRB) which included R-1234yf as a comparison material. In those tests as well, R-1234yf ignition also 

occurred at approximately 750°C (Peral-Antunez, 2011) 
   

   14

4.2 Propagation

After reviewing many of the release tests, the CRP determined that refrigerant and oil ignition was 

frequently brief and represented little more than a flash under hood. Some ignition events lasted longer 

and were generally accompanied by yellow/orange flames which were understood to represent oil 

ignitions that may have lead to propagation of an under hood fire. To consider possible propagation of 

the refrigerant ignition event, the CRP considered 3 different situations: propagation by ignition of 

combustible solids, propagation by ignition of combustible fluids, and propagation by ignition of 

flammable fluids. In the context of the FTA: 

 Combustible solids refers to plastic parts that might be present in the refrigerant ignition zone. 

Each part has its own thermal properties (e.g., depending on material and thickness) and would 

require some period of exposure to the ignited refrigerant/oil mixture to itself become ignited. 

 Combustible fluids are materials such as motor oil, brake fluid, or transmission fluid that have 

relatively low vapor pressures and would also require exposure to a heated exhaust surface (to 

vaporize some of the fluid) or an extended period of exposure to the burning refrigerant-oil 

mixture in order to become ignited. In addition, these fluids are normally contained within 

vehicle systems and would need to be released (e.g., in the event of a collision) in order to be 

exposed to the ignition event. 

 Flammable fluids pertain to gasoline/petrol or diesel fuel. Unlike combustible fluids, these 

materials have a high vapor pressure and would likely ignite upon contact with a burning 

refrigerant-oil mixture. On the other hand, due to their flammable nature, vehicles are 

specifically designed to avoid release of these materials. Note that two other flammable 

materials, the methanol portion of window washer fluid and the ethylene glycol portion of coolant 

do not factor prominently into this analysis. In both cases, the material is typically a mixture with 

water and the water would have to be evaporated in order for the mixture to become flammable. 

It was assumed that the point at which this occurs would be well after the refrigerant ignition has 

occurred. Moreover, as pertains to washer fluid, in many car designs the tanks are in the very 

front of the vehicle or in the wheel well and therefore not in proximity to the hot surface. 

For fault tree I7, the collision case, all three propagation sources were considered possible. For fault tree 

I6, only the combustible solids were considered viable because combustible and flammable fluids require 

significant damage to the vehicle in order to be released. 

In addition to the probability that each type of combustible/flammable material is released and exposed 

for a sufficient time to the ignited refrigerant to itself become ignited, a number of mitigating factors are 

considered. These include: 

 the probability that combustible or flammable materials are released away from the refrigerant 

ignition zone, 

 the probability that air currents due to a collision extinguish the ignited refrigerant before the 

flame can propagate to other materials. 

These factors are not included in the flammable fluids case because the high vapor pressure of these 

materials was believed to exclude much of the effect of mitigation. Each propagation branch also has an 

input that excludes the possibility of direct production of a vehicle fire due to ignition of the 

combustible/flammable material (without any role for the refrigerant). The rationale is to exclude vehicle 
   

   15

fires where the refrigerant is not an initiating factor because the goal of the FTA is to determine the added 

risk of using this refrigerant, not the overall risk due to all vehicle systems. 

With regards to the effect of crushing of the engine compartment due to a collision, the CRP members 

considered that in some collisions (primarily severe frontal collisions), crushing of the engine 

compartment could push parts close enough to the hot surface so as to nearly eliminate any air gap around 

the hot surface where refrigerant could be present (Figure 4.2). Crushing could create conditions similar 

to those found in a flame arrestor, where the available air gap, between the hot surface and cooler 

components, is so small as to quench the ignition kernel and prevent propagation to the surrounding 

flammable gas (Edwards, 1991). Essentially, crushing the front-end components (condensers, radiators 

and fans) would have a quenching effect as these relatively cool components tend to quench the flame as 

it expands away from the space around the hot surface, reducing the potential for propagation (Zabetakis 

et al., 1965). 

Figure 4.3  Computer Aided Engineering (CAE) Modeling of the Possible Impact of Collision‐related

Crushing on Available Airspace Around the Hot Surface

Numerous factors that would provide a mitigating effect in preventing ignition also can be expected to 

have a mitigating effect in preventing propagation. For example, hood buckling has been shown to 

provide a mitigating effect in preventing ignition by allowing refrigerant concentration to flow out of the 

engine compartment. If the refrigerant were to still ignite in such a scenario, the buckled hood aids in 

preventing propagation by creating an air space above the ignition zone. This allows the ignition to 

consume the refrigerant in a region free of combustible solids. Once the refrigerant is consumed, the 

ignition kernel extinguishes with a reduced risk of propagation to other materials. Other factors (e.g., 

release of steam) likely play a similar role in providing mitigation for both ignition and propagation 

scenarios. Nonetheless, the CRP decided in the current analysis to only consider the mitigating aspects of 

these factors at one level of the FTA. This generally meant elimination of several mitigation factors 

during consideration of propagation. There was a concern that including the effects of these factors in the 

ignition and propagation trees could be perceived as diluting the overall risk ("double counting"). As a 

result, the risk associated with propagation is likely overstated in the fault trees. However, this leads to a 

more conservative assessment of the refrigerant and the CRP1234-4 members decided to move forward 

with this more conservative approach. 

4.3 Exposure to Vehicle Fires

A vehicle fire will not necessarily result in injury of the vehicle occupants. In most instances, the 

occupants will be able to leave the vehicle and avoid the potentially dangerous situation. It is only in 

cases where the occupants are unable to exit that exposure to the vehicle fire could occur. In the 
   

   16

CRP1234-4 trees, the risk of vehicle fire therefore feeds into a higher level of analysis where the 

likelihood of a passenger remaining in the vehicle is considered. For the non-collision scenario, the only 

input at this level is the likelihood that a vehicle fault prevents the occupants from existing. For the 

collision scenario, the relevant inputs are the probability that the collision causes vehicle occupants to 

remain in the vehicle and the probability that first responders or other individuals do not help the 

occupants to leave before the passenger cabin becomes involved in the vehicle fire. 

4.4 Fault Tree Inputs

Once the structure of the FTs was established, values for each of the inputs were assigned as probabilities 

based on information obtained from a range of sources (Figure 4.4). 

Figure 4.4  Inclusion of Input Data into FTA

4.4.1 Probability Hot Surface Exceeds 700°C

The likelihood of exhaust-surface temperatures exceeding 700°C is a key input for the FTA and is 

linearly related to the top event probability. The CRP1234-4 members agreed on a value of one percent 

(0.01) as indicative of the likelihood that a typical vehicle has a hot surface exceeding 700°C at any given 

time. This is supported by an analysis conducted by one OEM which evaluated 57 customer vehicles 

with 4 cylinder, V6, and V8 gasoline powered engines in North America and Europe. The vehicles were 

equipped with sensors that recorded both engine temperature and operating conditions. Based on prior 

thermal testing, the operating conditions which could allow a vehicle to reach temperatures above 700°C 
   

   17

were (1) operation at wide-open throttle (WOT), (2) continuous climbing at greater than 4% grade for 

more than 90 seconds, and (3) operation at maximum velocity (Vmax). Vmax was defined as the 

maximum possible speed of vehicle operation on a level road as well as speeds 50 kph below this level. 

Each vehicle was analyzed to identify the percentage of time spent at the three conditions. The results 

were then combined and processed through a Weibull analysis to generate an aggregate vehicle operation 

profile. According to the results, an average (50th percentile) customer engages in operating conditions 

that could result in hot surface temperatures above 700°C less than 1 percent of the time (0.12% or 

0.0012, nearly 1 order of magnitude below the value used in the FTA). The analysis indicated that a 

customer with a more aggressive driving style (90th percentile) engages in operating conditions that could 

result in hot surface temperatures above 700°C 1.5% of the time (i.e., 0.0153). The methodology 

employed in this analysis was conservative for a number of reasons: 

1) It was assumed that every vehicle will achieve exhaust surface temperatures > 700°C when 

driven at WOT, continuous grade, or Vmax. Such an assumption disregards diesel 

applications, hybrid vehicles, and certain naturally aspirated vehicles that are incapable of 

achieving 700°C. 

2) It was assumed that whenever a vehicle experiences WOT for any period of time that a 

temperature > 700°C is achieved. Thermal characterization data suggests that repeated back-toback WOTs are necessary to achieve such a temperature. 

3) It was assumed that all continuous grade operation when grade > 4% and duration > 90 seconds 

will result in a temperature greater than 700°C. This does not account for grade driving at low 

load conditions. 

4) The analysis neglects the impact of operation at low ambient temperatures. 

5) It was assumed that Vmax operation is relevant for all global regions. When the OEM 

characterized Vmax operation, it found that there were no relevant occurrences of Vmax in 

North America (likely due to speed limits well below Vmax). However, Vmax operation was 

identified in Europe and this occurrence rate was considered for all global usage. Additionally, 

the OEM included speeds 50 kph lower than Vmax and counted their occurrence rate as a 

Vmax occurrence to add an additional level of conservatism. 

6) Data were combined across all three categories and fed into the Weibull analysis. This assumes 

that the same customer is always driving under the three conditions, which is highly unlikely 

(e.g., individuals who frequently drive at Vmax may not also use their vehicles to tow loads up 

steep grades). 

All of these assumptions would tend to overestimate the likelihood that a vehicle will be operating with a 

hot surface above 700°C. More detail on these tests is provided in Attachment G. 

Data was obtained by another OEM which conducted road testing to measure underhood exhaust 

temperatures. The vehicle studied had a turbocharged gas powered engine, and was tested under two load 

conditions; loaded to near its gross vehicle weight rating (GVWR) and towing a trailer at its gross 

combined weight rating (GCWR). Testing was conducted across variable roads and grades in the vicinity 

of Phoenix, AZ. When the vehicle was driven without the trailer, an underhood exhaust temperature 

above 700°C was reached 0.3% to 1% of the time and the longest time exceeding 700°C was less than 2 

minutes. Even during testing with a trailer the longest period of time the vehicle underhood exhaust 

temperature exceeded 700°C was less than 4 minutes. Although with a trailer, 700°C was exceed 6 to 

16% of the total route time, it must be considered that customers tow a trailer less than 1% of the time 

which makes the contribution to the overall average time above 700°C insignificant. More detail on these 

tests is provided in Attachment H. 
   

   18

4.4.2 Other Ignition‐Related Probabilities

A number of input probabilities were retained from the prior CRP1234 analysis, including the frequency 

of different types of vehicle crashes, the probability that the AC system leaks in the event of a collision of 

a given severity/type, the probability of a non-collision related AC system leak, and the probability of an 

electrical spark or hot glowing wire ignition source being present after a collision. Many of these were 

taken from the German GIDAS database. The GIDAS (German In-Depth Accident Study) database is 

built on the analysis of over 22,000 completely documented and reconstructed motor vehicle accidents 

which were extensively investigated as to cause and contributing factors. The work related to GIDAS is 

funded by the German Federal Highway Research Institute (BASt) and the German Association for 

Research in Automobile Technology (FAT). 

Input probabilities concerned with the presence of flammable refrigerant concentrations in the engine 

compartment near the hot surface were determined based on studies conducted by several OEMs which 

described the distribution of refrigerant within the different portions of the AC system during different 

stages of AC system operation. They were also based on knowledge of engine compartment 

configurations and the frequency of vehicle designs which would place the engine compartment portion 

of the AC system (the compressor) on the same side of the engine block as the hot surface. Vehicle crash 

analysis surveys were used to assess the likelihood that various engine compartment components (e.g., 

AC lines, engine cooling fan, radiator and coolant tanks) would be damaged in the event of a crash as 

well as the degree of crushing that occurs in the engine compartment after crashes of different types. 

Finally, a number of inputs were based on the consensus of industry experts with knowledge regarding 

automotive system design and operation. 

As concerns refrigerant ignition, a number of the potential mitigating factors are affected differently by 

collision severity (Figure 4.5). For example, the probability of the engine cooling fan is damaged and 

non-operational is large for a high severity front collision and very small for a low severity side collision. 

On the other hand, the probability that steam is not released is small for a high severity collision and large 

for a low severity collision. Thus the combination of mitigating factors for refrigerant ignition is complex 

and non-linear. 

Figure 4.5  Mitigating Factors for Refrigerant Ignition by Crash Type
   

   19

4.4.3 Propagation and Exposure‐Related Probabilities

Important probabilities for the propagation portion of the FTA are the probabilities of release of 

combustible and flammable fluids and the probabilities these materials would ignite due to the collision 

without involvement of the refrigerant. Input values for these parameters were obtained from the GIDAS 

database and the supporting documentation is included as Attachment I. 

Probabilities addressing the likelihood that the vehicle occupants cannot exit vehicle after a collision were 

based on a study by Funk et al. (2002) as well as statistics collected by various law enforcement agencies 

in US jurisdictions (Attachment J). 
   

   20

5 Results

5.1 Results of Current Analysis

Due the very low likelihood of a refrigerant tube rupture or similar event while the vehicle is operational 

and mitigating factors are absent, the non-crash scenario (I6) is estimated to pose negligible risk of 

exposure to a vehicle fire (10-19 events per vehicle operating hour). This is not surprising because in the 

prior CRP1234 analysis, the non-collision related leak was also not a significant contributor to overall 

risk. The more important scenario for the present analysis involves fault tree I7, the collision-related 

release in the engine compartment that produces a vehicle fire and where the passenger remains in the 

vehicle. The overall risk of vehicle fire exposure in this situation is 3 x 10-12 events per vehicle operating 

hour. This is several orders of magnitude less than the pre-existing risk of vehicle fires due to all causes 

(approximately 1 x 10-6 per vehicle operating hour) and also well below other risks generally accepted by 

the public. The primary contributors to the overall risk of fire exposure in fault tree I7 include high 

severity crashes (due to greater likelihood of a passenger remaining in the vehicle but also greater 

likelihood of mitigating factors being activated) and mid-severity crashes (due to lower likelihood of 

mitigating factors being activated but also lower likelihood of a passenger remaining in the vehicle). 

5.2 Comparison to Results of CRP1234

Table 5.1 shows the results of current risk assessment in the context of the results obtained by CRP1234. 

As shown in the table, the risk of a vehicle fire which involves the vehicle occupants is on the same order 

of magnitude as the previously estimated risks of HF exposure above a health based limit. The risk of 

vehicle fire involving the occupants is approximately 2 orders of magnitude greater than the ignition risks 

estimated for CRP1234. That assessment had as its most significant scenario refrigerant ignition inside 

the vehicle cabin, a scenario where the hot engine compartment surface is not relevant. The more 

comparable scenario in CRP1234, which involves the occupant investigating in the engine compartment 

and being exposed there to the refrigerant ignition, had a value of 9 x 10-15 and as part of that scenario 

assumed a very low likelihood (10-4) of the occupant being willing or able to investigate inside the engine 

compartment after a collision. Because the probability of a passenger remaining in the vehicle without 

being aided is several orders of magnitude higher than the probability of the occupant investigating, it is 

not surprising that the risks calculated in the present analysis are higher. Nonetheless, as shown in Table 

5.1, the overall conclusions of the former CRP1234 risk assessment are still valid: risks are still very 

small compared to the risks of a vehicle fire from all causes and well below risks that are commonly 

viewed as acceptable by the public. 

 
   

   21

Table 5.1 Probabilities of Various Adverse Events Compared to Estimated Probabilities of Events 

Associated with Vehicle Operation-Related Leaks of R-1234yf 

Event Probability per 

vehicle per 

operating hour* 

Citation 

Probability of being in a police reported vehicle collision1 4 x 10-5 NHTSA, 2013 

Probability of automotive vehicle fire (any cause)2 1 x 10-6 Ahrens, 2013; FHA, 2009 

Probability of vehicle collision due to vehicle brake failure3 3 x 10-7 New York State DMV, 

2008 

Probability of dying during a regularly scheduled plane trip in a developed 

nation4 7 x 10-8 Barnett, 2010 

Estimated probability of vehicle occupant/former occupant experiencing HF 

exposure above health based limits associated with an R-1234yf ignition 

event

5 x 10-12 CRP1234, 2009

Estimated probability of vehicle occupant being exposed to a vehicle 

fire due to R-1234yf ignition (due to leak and ignition in engine 

compartment) 

3 x 10-12 Current analysis 

Estimated probability of vehicle occupant being exposed to an open flame 

due to R-1234yf ignition (primarily due to leak and ignition in cabin) 9 x 10-14 CRP1234, 2009 

1 There were 5,338,000 police reported vehicle collisions in the U.S. in 2011. This is divided by the number of registered vehicles in that year 

(257,120,000) and the average number of hours each vehicle is operated (approximately 500 hours based on SAE J2766, Table 6). 2 Ahrens reports that there were an average of 152,300 vehicle fires per year in the U.S. from 2006 to 2010. This is divided by the size of the US 

vehicle fleet in 2008 as a midpoint (247 million vehicles, FHA, 2009) and the average number of hours a vehicle is operated per year (approximately 

500 hours based on SAE J2766, Table 6). 

3 The New York State Department of Motor Vehicles reports 1753 accidents in 2008 attributed to brake failure (NYSDMV, 2008) and roughly 11 million 

registered vehicles in New York State in 2008. Combining these data with the "operating hours per year" suggested in J2766 (~500 as an average) 

yields the an accident frequency per vehicle operating hour. 

4 Includes both commercial jet and propeller aircraft flights. Risks in developing or underdeveloped nations are higher.

 
   

   22

6 Sensitivity Analysis

In order to better understand the impact of some key assumptions on the results of the FTA, the 

CRP1234-4 members conducted a sensitivity analysis. Parameters that were felt to be uncertain were 

changed to other plausible values and the resulting frequency of the top event (i.e., exposure to a vehicle 

fire) was then compared to the value obtained using the original assumption. The results of the sensitivity 

analysis are summarized in Table 6.1. The parameters evaluated in the sensitivity analysis of the R1234yf fault trees consisted of the following: 

 Probability the hot surface reaches 700°C. As noted above, the CRP members decided to 

conservatively use 700°C as the temperature needed to ignite a mixture of R-1234yf and 

oil. Based on OEM studies concerning the frequency of specific vehicle operating 

temperatures, the CRP decided to use a value of 0.01 to represent the probability that 

vehicles had hot surfaces at or above 700°C (i.e., that as an average over the global 

vehicle fleet and across all vehicle operating times, vehicles might reach or exceed 700°C 

one percent of the time). OEM discussions revealed however, that there was substantial 

variation among OEMs concerning fleet operating temperatures. It was noted that much 

of the variation was due to uncertainty of converting OEM vehicle test data to actual 

customer usage profiles. In general, smaller turbocharged vehicles tend to operate at 

higher average and extreme temperatures while many engines and especially diesel 

engines typically operate at lower average and extreme temperatures. One OEM 

conducted an analysis of customer use profiles on three different types of vehicles and 

evaluated the probability of achieving underhood temperatures exceeding 700°C for the 

50th and 90th percentile customer. The results varied considerably by vehicle type with 

the 50th percentile customer exceeding 700°C approximately 0.03% of the time in a midsize diesel turbo van to 3.1% of the time in a small turbocharged gas powered crossover 

vehicle. Even in the latter vehicle type which is considered the worst case vehicle for this 

OEM, the 90th percentile customer only exceeded 700°C 5.7% of the time despite 

operating the vehicle under extreme conditions. These data are summarized in 

Attachment K. To assess this variability, the probability was increased in the FTA from 

1% to 3% to the extreme of 10%. These changes increased the frequency of the top event 

in both fault tree I6 and I7 and the increase was linear resulting in up to an order of 

magnitude change.

 Direction of refrigerant release. The OEMs were uncertain about the impact the direction 

of the refrigerant release relative to the hot surface has on the ignition potential. It is 

clear that releases pointing directly away from the hot surface (i.e., out the front of the 

vehicle) would have a substantially reduced probability of producing refrigerant ignition 

(Figure 6.1). The extent to which releases to the side or up or down would reduce the 

refrigerant concentration near the hot surface is less clear. To assess this uncertainty, the 

base value for the high severity case (0.5) was increased to 0.75. Alternatively, the base 

value for the mid severity case (0.75) was decreased to 0.5 (the mid severity case has a 

higher base probability because a high severity collision is more likely to buckle the hood 

and allow the refrigerant to move upwards rather than laterally). Neither change had an 

appreciable impact on the risk of the top event (i.e., the risk of the top event did not 

change).
   

   23

Figure 6.1  Effect of Release Orientation on Ignition Potential

 Inclusion of nonstagnant zone. As noted above, the CRP members did not believe they 

had sufficient data to support including the distinction between a stagnant and nonstagnant zone in the base case FTA and thus they assumed that a stagnant zone could be 

present in all vehicles. However, testing by CRP members has suggested that stagnant 

zones are only likely with specific vehicle conditions relating to air flow, heat shields, 

and under hood configuration. To better understand how this idea affects the FTA result, 

the fault trees were modified to include stagnant and non-stagnant zone branches with 

respect to the hot surface. It was assumed that 20% of vehicles are configured in such a 

way that a stagnant zone is possible and that in such cases, a temperature of 700°C is 

sufficient to ignite the refrigerant. However, in the 80% of vehicles without such 

configurations, a hot surface temperature of 750°C would be required to ignite the 

refrigerant (this value is consistent with prior testing done at INERIS and Hughes as well 

as newer OEM data). It was assumed that vehicles reach this temperature only 0.1% of 

the time. For the non-stagnant case it was also assumed that only 10% of the time was 

the refrigerant release velocity low enough to enable refrigerant ignition (this was a 

critical finding from the OEM studies and also supported by the literature, e.g., API, 

2003). The result of this analysis was a 65% decrease in the risk of exposure to a vehicle 

fire. 

 Combustible solids located where they could be exposed to ignited refrigerant. In the 

base case analysis, the OEMs assumed that combustible solids (e.g., plastic or rubber 

parts) would nearly always be located close enough to the ignited refrigerant in order for 

propagation to occur (0.99). However, it is possible that OEMs could redesign their 

vehicles to eliminate or substantially reduce this possibility (e.g., by switching to more 

fire resistant materials). To explore the potential impact of such a change, the probability 

of this value was reduced from 0.99 to 0.5 or to 0.1. These changes have a minimal 

impact on the risk of the top event because propagation due to release of flammable 

fluids is a sufficient contributor in the high severity cases so that a change concerning 

combustible solids does not have a major effect on the overall risk. Note however, that 

the risk of a vehicle fire (irrespective of whether an individual is present) is reduced to a 

greater degree (by a factor of 6) because the greatest contributor the vehicle fire risk (as 

opposed to exposure) is the mid severity front collision where the role of flammable 

fluids is negligible compared to the role of combustible solids. 
   

   24

Table  6.1  Results of Sensitivity Analysis

Change Estimated Risk of

Exposure to

Vehicle Fire

Impact

Base Case 3E‐12

Increase prob of hot surface

above 700°C  from 0.01 to 0.03

(3x higher)

9E‐12

Risk of top event changes linearly with the change

in probability the hot surface reaches/exceeds

700°C Increase prob of hot surface

above 700°C from 0.01 to 0.1

(10x higher)

3E‐11

Direction of refrigerant release  

towards hot surface for high

severity cases increased from

0.5 to 0.75

3E‐12 Negligible change because the mid severity case

still contributes substantially to the risk

Direction of AC release

towards hot surface for mid

severity cases decreased from

0.75 to 0.5

3E‐12 Negligible change because the high severity case

still contributes substantially to the risk

Inclusion of stagnant zone,

non‐stagnant zone

temperature set at 750°C

7E‐13 Modest reduction in risk (65%).  The stagnant

zone case becomes the risk driver but is present

in limited percentage of the vehicle fleet.

Probability combustible solids

are located where they can be

ignited by the refrigerant

decreased from 0.99 to 0.5

2E‐12 Minimal reduction in risk because propagation

due to release of flammable fluids contributes

enough in the high severity cases to keep risks at

the same order of magnitude Note that risk of

vehicle fire would be changed to a greater degree

[~6 times decrease] because the risk driver there

is the mid severity front collision where the role

of flammable fluids is minor.

Probability combustible solids

are located where they can be

ignited by the refrigerant

decreased from 0.99 to 0.1

1E‐12
   25

7 Conclusions

As noted in the original CRP1234 evaluation, R-1234yf is a non-ozone depleting substance with a GWP 

of 4. It has equivalent or lower toxicity compared to R-134a in terms of both acute human health effects 

and ecological effects. HFO-1234yf is flammable, but extensive testing has indicated that the ignition 

potential of this chemical is far less than that of other flammable refrigerants (e.g., R-152a). The risk 

assessment conducted by the prior CRP indicated that use of R-1234yf in automotive vehicles would 

involve an acceptable level of risk. For example, the FTA indicated that the risk of an occupant (or 

former occupant) being exposed to an open flame due to R-1234yf ignition would be extremely low, on 

the order of 9 x 10-14 events per hour of vehicle operation. The analysis indicated that the risk of an 

individual being exposed to HF above the relevant health-based limit was also extremely low, on the 

order of 5 x 10-12 events per hour of vehicle operation. A comparative analysis showed that these risks 

were well below those commonly considered acceptable by the public and regulatory agencies. The risk 

assessment was submitted to the U.S. Environmental Protection Agency (US EPA) as part of the 

Significant New Alternatives Policy (SNAP) approval process, and the US EPA subsequently approved 

R-1234yf for use in US vehicles. 

In response to a press release issued by Daimler which suggested a greater fire risk for R-1234yf than 

previously estimated, a new CRP was convened (CRP1234-4) to study the Daimler claims and to 

determined whether revision of the prior FTA was necessary. During numerous and frequent face-to-face 

and phone meetings, the CRP members determined that the refrigerant release testing completed by 

Daimler was unrealistic in that it created extreme conditions that favored ignition while ignoring many 

mitigating factors that would be present in an actual real-world collision. These factors include the 

potential dispersing effect of the engine cooling fan, the quenching effect of front end compartment 

deformation, the extinguishing effect of steam released due to radiator breakage, and the diversion of 

refrigerant through a damaged condenser to the exterior of the engine compartment. At the same time, 

the CRP members developed two new fault tree scenarios to realistically address the Daimler claims. 

They also reviewed and analyzed extensive new OEM test data which was used as input for the new FTA. 

The two new fault tree scenarios consider the possibility of an individual being unable to exit the vehicle 

after a refrigerant release (either due to a collision or a non-collision related AC system fault), the 

refrigerant being ignited and the refrigerant fire propagating to other vehicle components. The FTA 

examined average risks across the entire global fleet and used a number of conservative assumptions to 

ensure that the final risk estimate would be more likely to overestimate rather than underestimate actual 

risks. Based on the updated analysis, the estimated overall risk of vehicle fire exposure is on the same 

order of magnitude as that estimated in the prior work of CRP1234. Therefore, the conclusions of the 

former risk assessment are still valid: risks are still very small compared to the risks of a vehicle fire from 

all causes and well below risks that are commonly viewed as acceptable by the public. 
   26

8 References  

Ahrens M. 2013. Local fire department responses to fires involving automobiles, buses and larger trucks: 2006-2010 

estimates. National Fire Protection Association. 22 pp.

American Petroleum Institute (API). 2003. Ignition risk of hydrocarbon liquids and vapors by hot surfaces in the 

open air. API RP 2216-03. December 

Barnett A. 2010. Cross-national differences in aviation safety records. Transportation Science 44(3):322-332 

Blackwell N, Bendixen L, Birgfeld E. 2006. Risk analysis for alternative refrigerant in motor vehicle air 

conditioning. September. 

Edwards, JC.1991. Thermal models of a flame arrester. Bureau of Mines, U.S. Dept. of the Interior, Washington, 

D.C. 23 pp. 

Funk DL, Politis JF, McErlean M, Dickinson ET. 2002. Necessity of fire department response to the scene of motor 

vehicle crashes. Am J Emerg Med. 20(7):580-2 

Gradient Corporation. 2009. Risk assessment for alternative refrigerant HFO-1234yf. (Phase III) Prepared for SAE 

International Cooperative Research Project 1234. December 17. 

National Highway Traffic Safety Administration (NHTSA). 2013. Traffic Safety Facts 2011. National Center for 

Statistics and Analysis. DOT HS 811 753 

New York State [USA] 2008. 2008 Highway safety annual report. Governor's Traffic Safety Committee. 78 pp. 

www.nysgtsc.state.ny.us/GTSC2008AnnualFULL.pdf 

New York State Department of Motor Vehicles (NYSDMV) [USA] 2008. Annual report. 23 pp. 

http://www.nydmv.state.ny.us/broch/ar2008.pdf 

Peral-Antunez, E. 2011. Recent experiences in MAC system development “New alternative refrigerant assessment” 

SAE 2011 Alternative Refrigerant and System Efficiency Symposium, September 27-29. Phoenix AZ. 

Smyth KC, Bryner NP. 1997. Short-duration autoignition temperature measurements for hydrocarbon fuels near 

heated metal surfaces. (2104 K). Combustion Science and Technology. 126: 225-253. 

Zabetakis, M. 1965. Flammability characteristics of combustible gases and vapors. Bulletin No. 627. United States 

Department of the Interior, Bureau of Mines. Washington 121 pp.