NOVELTY - The system (10) has a refrigeration unit (14) comprising a refrigerant circuit (15) through which refrigerant is circulated in heat exchange relationship with air (100) drawn from a mobile refrigerated cargo box (12). A fuel-fired engine (16) i.e. diesel engine, powers the refrigeration unit. The engine comprises an exhaust system through which exhaust gases (200) generated by the engine are discharged. Engine exhaust gases to an engine coolant heat exchanger (18) comprises a heat exchanger with engine coolant pass (17A) interplaced in an engine coolant circuit (17) and gas-side pass.    USE - Transport refrigeration system for controlling temperature associated with a mobile refrigerated cargo box of a trailer, a lorry and an intermodal container to transport a fresh/frozen perishable product. Uses include but are not limited to a produce, meat, poultry, fish, dairy product and a cut flower.    ADVANTAGE - The system selectively heats cargo box air and defrosts an evaporator coil without increasing shaft load on the engine. The system is designed such that waste heat from the engine replaces electric resistance heat and hot refrigerant vapor as a source of heat when the refrigeration unit is operated in a box air heating mode, thus reducing engine shaft load when heat is required by the refrigeration unit, and hence increasing engine efficiency. The system is designed such that utilizing engine waste heat as a heat source for the refrigeration unit boosts available heating capacity for operation in the heating mode. The system absorbs heat from the engine exhaust gases so as to reduce temperature of engine exhaust gases downstream of engine exhaust to the engine coolant heat exchanger, thus enabling safe use of effective diesel particulate filters to remove particulates from cooler exhaust gases. The system comprises a check valve interplaced in a downstream leg of branch engine coolant circuit downstream of the engine coolant pass and upstream of intersection with the engine coolant circuit when an engine coolant flow diversion valve is positioned in a position, thus preventing back flow of engine coolant into the engine coolant pass.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a method for operating a transport refrigeration system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a transport refrigeration system including an engine coolant to a box air heat exchanger.    Transport refrigeration system (10)    Mobile refrigerated cargo box (12)    Refrigeration unit (14)    Refrigerant circuit (15)    Fuel-fired engine (16)    Engine coolant circuit (17)    Engine coolant pass (17A)    Engine exhaust gases to engine coolant heat exchanger (18)    Air (100)    Exhaust gases (200)