* Controversial refrigerant does not start fire in Opel SUV
* Testing agency says test done under extreme conditions
* Honeywell/DuPont's 1234yf is only EU-approved refrigerant
* Daimler says coolant can cause fire, but test controversial
FRANKFURT, April 3 (Reuters) - General Motors' German brand Opel said on Wednesday that it had found no evidence in a crash test on its new Mokka SUV that a controversial air conditioning refrigerant could catch fire in a collision and release toxic fumes.
Opel's domestic rivals Daimler and Volkswagen are both developing expensive carbon dioxide-based air conditioning systems in order to avoid what they say is a fire hazard posed by Honeywell and DuPont's new refrigerant HFO-1234yf, which emits poisonous hydrogen fluoride gas when it burns.
Opel said a realistic test conducted together with the independent testing agency TÜV Rheinland had failed to ignite refrigerant that leaked from the system after impact.
The test is the first to be published since Daimler said in September that 1234yf, the only air conditioning coolant on the market that conforms to a new European Union directive on greenhouse gases, could be the primary source for a vehicle fire.
Many in the industry have questioned the relevance of Daimler's test procedure, but its findings have nevertheless rattled global carmakers, which had so far universally agreed to employ the chemical. It could also cost the two producers of the patented HFO-1234yf billions of dollars in lost investments and revenue.
The EU has ordered a phase-out of the widely used refrigerant R134a because of its potency as a greenhouse gas.
Opel began installing 1234yf-based systems in its Mokka model at the start of the year but Daimler is violating the EU directive by continuing exclusively to use the non-flammable R134a.
Opel research and development chief Michael Ableson said there was "no alternative in the near future to refrigerant 1234yf".
"Other possibilities such as CO2-based refrigerants are still in the development stage and are years away from entering the market," he said in the statement.
Daimler says frontal crash simulations that it conducted internally last August showed that a fire could ignite under the hood of a car when the new Honeywell refrigerant, mixed with air conditioning lubricant, came into contact with the manifold of a turbo-charged petrol engine at around 650 degrees Celsius (1,200 Fahrenheit).
Opel said on Wednesday it had crash-tested a 1.4 litre turbo-charged Mokka at a speed of 50 km/h (31 mph) against a movable deformable barrier, meant to simulate driving headlong into the last car in a tailback.
It said the impact had damaged the air conditioning system, causing a leak near the Mokka's hot engine manifold, but without causing a fire.
Gunnar Pflug, head of the traffic safety centre at TÜV Rheinland, said the test had been designed and conducted under TÜV Rheinland's supervision to recreate the extreme conditions that Daimler says are necessary to induce a fire. It included reaching an engine temperature more than 100 degrees Celsius higher than that recorded by Daimler.
"Much faster speeds and there would have been nothing left of the engine compartment, basically," he said. "What we did was no standard test, this was the first of its kind."
Manufacturer Honeywell concedes that the mixture is indeed flammable and releases toxic gases when burning, but argues that Daimler's simulations employed 'ideal' conditions expressly designed to create a desired effect.
Daimler, Mercedes' parent, says simulations as opposed to real crash tests were necessary because each car deforms differently during impact, even if the same test parameters are always applied.
Pflug said this was only partly true.
"The same parts are not always damaged exactly the same way down to the millimetre, but if it isn't the third air conditioning fin that breaks, then it's the second or seventh," he said.