Tihe fresh water preheater, code P51635-14, is an essential thermal-management component in the MAN L21/31 engine, designed to warm the coolant before startup or during low-temperature operation. By raising the temperature of the fresh water circulating through the engine block, the preheater helps reduce cold-start wear, improves combustion efficiency, and enables smoother, faster engine start-up. Particularly valuable in cold climates and marine environments, this preheating system enhances operational readiness and contributes to overall engine longevity.