Abstract: A hybrid power unit of a work machine includes an engine and an energy storage device. The engine includes a crankshaft for driving a load and the crankshaft is attached to an energy storage device via a clutch. The energy storage device can be charged by the engine to store energy, that may be used to provide, when required, a boost to engine performance. The clutch may engage/disengage the energy storage device from the engine and/or control a clutch pressure. The clutch protects the crankshaft from shock loading and can slip to prevent over-torqueing of the crankshaft.