Drivers are only allowed to use eight engines per season and the German is believed to have used seven already , further complicating his title hopes .
Vettel withdrew for the second straight race , pulling out due to an engine failure near the halfway mark .
Drivers are only allowed to use eight engines per season and the German is believed to have used seven already , further complicating Vettel 's title hopes .
Vettel withdrew for the second straight race , pulling out due to an engine failure near the halfway mark .