What is the operating limit for engine oil pressure below 18% Np?

The operating limit for engine oil pressure below 18% Np is indeed 90 to 135 PSI. This range is crucial for ensuring the engine operates within safe parameters during low speed conditions. Oil pressure is vital for the lubrication and cooling of engine components, and maintaining the correct pressure range helps prevent engine damage and ensures optimal performance.

At low Np, the engine demands specific oil pressure to maintain sufficient lubrication, so adhering to the 90 to 135 PSI range guarantees that the oil viscosity is adequate for protecting engine parts under lower rotational speeds. This avoids scenarios where inadequate lubrication can lead to increased wear or potential engine failure, thus validating why this specific range is established as a limit in operational procedures.