DRIVING FROM VA CLINIC BONHAM, TX TO HOME IN MCKINNEY, TX. AUTO FAILED TO START ON FIRST TRY, STARTED ON SECOND TRY (NEVER HAPPENED BEFORE). WENT TO LUNCH IN BONHAM. RESTARTED CAR FIRST TRY. DROVE HIGHWAY TX121 APPROXIMATELY FIFTEEN MILES AND ENGINE DIED AT HIGHWAY SPEED. TRIED A ROLLING RESTART WITHOUT LUCK. PARKED AND TRIED RESTART TO NO AVAIL. SITTING IN CAR CALLING TOW TRUCK AND NOTICED SMOKE FROM ENGINE COMPARTMENT. SMOKE CONTINUED TO BE THICKER SO GOT OUT AND WAVED DOWN TRAFFIC TO GET A FIRE EXTINGUISHER (NONE AVAILABLE) AND CALLED 911; EMPTIED BELONGINGS FROM CAR AND WATCHED IT BURN. FIRE DEPARTMENT CAME AND PUT IT OUT BUT COMPLETELY BURNED.