Renault technical director James Allison has revealed the cause of Nick Heidfeld's exit from last weekend's race at the Hungaroring.

"As with most accidents, several incidents combined to cause the fire that Nick suffered in Hungary," said the Englishman, who was also recently elected to the role of chairman of FOTA's Technical Regulations Working Group. "First of all, we ran a slightly different engine mapping strategy in qualifying, which produced hotter than normal exhausts. We believe that this elevated temperature and caused a preliminary crack in the exhaust pipe.

"We presume that the crack then propagated during the laps to the pitstop," he continued, "this was not evident to us as we believe that the failure occurred upstream of the place where we have a temperature sensor. We believe that Nick then came in with a partially failed exhaust. This pitstop took longer than normal, the engine was left at high rpm for 6.3 sec, waiting for the tyre change to be completed. Under these conditions, a lot of excess fuel always ends up in the exhausts and their temperature rises at around 100°C/sec. This temperature rise was enough to finish off the partially failed pipe and to start a moderate fire under the bodywork."

Asked about the explosion witness moments after the German had abandoned his car, Allison revealed: "This was caused by the air bottle which supplies the air valves in the engine. It has overheated in the fire and failed.

"The incident was highly undesirable, as it has caused us to write off a chassis," he added. "We will take steps prior to the next race to reduce the likelihood of a further fire and to ensure that the air bottle cannot overheat. We are in touch with the FIA both to provide them with a full report of the incident and also to explain to them the actions we are taking to prevent a reoccurrence."

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