Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_15716 ( "Year" real, "Entrant" text, "Chassis" text, "Engine" text, "Points" real )

### Question ###
What is the average Year when there was a cosworth straight-4 engine, and the Entrant was ron harris / team lotus?

### Accurate SQL ###

SELECT AVG("Year") FROM table_15716 WHERE "Engine" = 'cosworth straight-4' AND "Entrant" = 'ron harris / team lotus'