A definition of PTO is a power take-off from propulsion system to power pumps, compressors, generators, propellers in case of direct acting propulsion system and other rotating machinery.
A definition of PTI is a power take-in. This system is not so wildly used in main propulsion system in comparison with PTO as a main purpose of it is tow operations. If you sail on AHTS (anchor handlers) you should be familiar with this system. On our AHTS we have used PTI function in towing mode where high power demand was required. Propulsion system layout was next:
1 Main propulsion engine;
2 Diesel generator;
3 Electric motor;
4 Fixed pitch propeller.
In normal sailing mode we have utilized main engine alone. In towing mode, diesel generator transmitted its power via high-voltage generator to an electric motor 3, which in turn supplied this power into the shaft line via fitted PTI gear – to fulfil higher power demands.