Renault technical director James Allison has revealed that the fire that forced Nick Heidfeld out of the Hungarian Grand Prix is believed to have been caused by a crack in the exhaust of his car.

Heidfeld's car burst into flames shortly after leaving the pits in Budapest, with the German bailing out once he had cleared the end of the pit lane. As marshals rushed to put the flames out, an air bottle in the sidepod of the car exploded, showering the side of the circuit in debris.

The incident was the second this season in which Heidfeld's car has caught fire and has caused his R31 to be written off, with Allison saying the suspected exhaust failure was the catalyst for what followed.

"As with most accidents, several incidents combined to cause the fire that Nick suffered in Hungary," he said. "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 - 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."

Allison added that the team was in contact with the FIA over the incident and was working to ensure there was no repeat of either the initial fire, or the explosion that followed.

"The incident was highly undesirable, as it has caused us to write off a chassis," he said. "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 re-occurrence."