user: Question: What Valvetrain has a fuel system made up of Petrol engines?
Columns: ['Model', 'Engine', 'Displacement', 'Valvetrain', 'Fuel system', 'Max. power at rpm', 'Max. torque at rpm']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Petrol engines', 'Petrol engines', 'Petrol engines', 'Petrol engines', 'Petrol engines', 'Petrol engines', 'Petrol engines']
SQL:

assistant: SELECT "Valvetrain" FROM "Table" WHERE "Fuel system" = 'Petrol engines';