Any help with these last few would be appreciated. 6dn: Word from French couteau for a large knife C-T-O; 19ac: Person reading carefully --R---R; 23ac: Confined gas at constant pressure varies inversely with pressure B---E- --W; 19dn: Act of preparing oneself for a challenge: ----H --; 20dn: Natural fizzey mineral water ----S(orZ)--; 27ac: It opens valve in internal combustion engine --S---D.