Heat oven to 325 degree F. Sift flour, granulated sugar, baking powder and salt into a large bowl. Make a wide well in center. Add oil, then egg yolks, orange peel, orange juice, and lemon peel. Whisk until smooth; set aside.
Beat all the egg whites in a large bowl on medium speed of electric mixer until frothy. Add cream of tartar. Gradually increasing speed, continue beating to stiff peaks. Gradually pour yolk mixture in a thin, steady stream into beaten egg whites, folding gently with a rubber spatula.
Pour batter into an ungreased 10-inch tube pan. Bake 55 to 60 minutes or until toothpick inserted halfway between tube and side of pan comes out clean and cake springs back when touched gently. Immediately invert pan onto neck of funnel or bottle; let hang until completely cool, about 3 hours.
To loosen, run a metal spatula or thin knife around side of pan and tube. Invert pan again and remove cake. Turn cake upright onto serving plate. Sprinkle top with sifted confectioners sugar. Makes 16 servings.