user: Question: What is the average grid number for cars that retired due to engine failure before 18 laps?
Columns: ['Driver', 'Constructor', 'Laps', 'Time/Retired', 'Grid']
Types: ['text', 'text', 'real', 'text', 'real']
Sample row: ['Jackie Stewart', 'Matra - Ford', 90.0, '2:16:54.0', 4.0]
SQL:

assistant: SELECT AVG("Grid") FROM "Table" WHERE "Time/Retired" = 'engine' AND "Laps" < 18.0;