what is the engine that saw 0 points, and a Chassis of porsche 718, in 1961?
CREATE TABLE "world_championship_results" (
  "year" real,
  "entrant" text,
  "chassis" text,
  "engine" text,
  "points" real
);
SELECT "engine" FROM "world_championship_results" WHERE "points"=0 AND "chassis"='porsche 718' AND "year"=1961;