NOVELTY - The arrangement (1) has a waste heat recovery system (10) comprising a main line (21) in which working fluid e.g. water, is pumped and heated in a heat exchanger (11) using a heat source produced by operation of an internal combustion engine (2) and expanded in an expander (12) and a derivation line (22) in which the fluid flows. The derivation line includes an inlet and an outlet that are connected to the main line downstream from the exchanger and upstream from the expander and a heat storage device (20) that stores heat from the source and releases previously stored heat to heat the fluid.    USE - Internal combustion engine arrangement e.g. diesel engine arrangement and spark ignition engine arrangement, for use in industrial vehicles.    ADVANTAGE - The waste heat recovery system can store the heat from the heat source and release the previously stored heat with respect to heat capacity to heat the working fluid in an effective manner, while protecting the working fluid from overheating to increase service life of the waste heat recovery system and avoiding a need to change the working fluid, thus reducing maintenance cost of the arrangement. The arrangement enables more efficient use of thermal energy from the heat source without impairing overall efficiency by avoiding fan activation and without damaging elements of the working fluid in wide range of operating conditions, thus increasing efficiency of waste energy recovery and usage, while providing high thermal efficiency and high heat transfer rate during phase change. The arrangement utilizes heat storage material with high heat storage capacity and longer storage period, thus improving performance of the arrangement in a steady state condition. The arrangement is designed such that the working fluid coming out of the heat storage device has enthalpy lower than the working fluid coming into the heat storage device when a cooling package restricts rejection of available energy from the heat source, thus reducing load on a condenser. The arrangement uses sensible, latent or chemical heat storage units in an operating phase in which working fluid is cooled down if working fluid temperature at a heat exchanger outlet is high to ensure efficient operation of the expander.    DETAILED DESCRIPTION - The heat storage device is a downstream heat storage device. The waste heat recovery system is a rankine type waste heat recovery system, kalinka type waste heat recovery system, brayton type waste heat recovery system, stirling type waste heat recovery system or ericsson type waste heat recovery system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of an engine arrangement comprising a waste heat recovery system.    Engine arrangement (1)    Internal combustion engine (2)    Waste heat recovery system (10)    Heat exchanger (11)    Expander (12)    Downstream heat storage device (20)    Main line (21)    Derivation line (22)