A device for conditioning the atmosphere when testing reciprocating internal combustion engines, wherein the device comprises: - an inlet duct arranged for being connected at a first end to an intake of a combustion engine to be tested and which draws in air from the external atmosphere through a second end; - an outlet duct arranged for being connected at a first end to the exhaust of the combustion engine and which expels exhaust gases into the atmosphere through a second end; - a first communication duct communicating the inlet duct with the outlet duct close to its respective first ends (1a, 3a), such that the intake of the device is in communication with the exhaust thereof; - a bypass valve arranged in the inlet duct; - a super-charging turbo assembly which diverts the airflow in the inlet duct circulating towards the bypass valve, the turbo assembly comprising a turbine coupled to a dissipation system for dissipating the energy generated during expansion, the bypass valve and the turbine being able to be controlled to obtain the desired pressure and airflow value in the inlet duct; - a heat exchanger in the outlet duct for cooling the exhaust gases to a safe temperature; and - a turbocharger in the outlet duct downstream of the heat exchanger, charged by turbocharger charging means, to control, together with the turbine and the bypass valve, the pressure of the intake air and the intake airflow; characterized in that it further comprises - a first three-way valve that connects the second end of the inlet duct upstream of the bypass valve with a section of the outlet duct upstream of the turbocharger through a second communication duct; - an on/off valve in a section of the outlet duct between the heat exchanger and the point of connection of the section of the outlet duct with the second communication duct; - a second three-way valve that connects a section of the outlet duct downstream of the turbocharger and the section of the outlet duct, between the heat exchanger and the on/off valve; - a control valve in a duct that connects the first end of the outlet duct with the atmosphere; and - a section of the outlet duct that connects the heat exchanger with the first communication duct between the connection of the inlet duct and the connection of the duct; such that the configuration of the set of the three-way valves (5, 19), the bypass valves (6, 23) and the on/off valve allows use of the device in an inlet air pressure increase mode or in an inlet air pressure reduction mode. - an inlet duct arranged for being connected at a first end to an intake of a combustion engine to be tested and which draws in air from the external atmosphere through a second end; - an outlet duct arranged for being connected at a first end to the exhaust of the combustion engine and which expels exhaust gases into the atmosphere through a second end; - a first communication duct communicating the inlet duct with the outlet duct close to its respective first ends (1a, 3a), such that the intake of the device is in communication with the exhaust thereof; - a bypass valve arranged in the inlet duct; - a super-charging turbo assembly which diverts the airflow in the inlet duct circulating towards the bypass valve, the turbo assembly comprising a turbine coupled to a dissipation system for dissipating the energy generated during expansion, the bypass valve and the turbine being able to be controlled to obtain the desired pressure and airflow value in the inlet duct; - a heat exchanger in the outlet duct for cooling the exhaust gases to a safe temperature; and - a turbocharger in the outlet duct downstream of the heat exchanger, charged by turbocharger charging means, to control, together with the turbine and the bypass valve, the pressure of the intake air and the intake airflow; - a first three-way valve that connects the second end of the inlet duct upstream of the bypass valve with a section of the outlet duct upstream of the turbocharger through a second communication duct; - an on/off valve in a section of the outlet duct between the heat exchanger and the point of connection of the section of the outlet duct with the second communication duct; - a second three-way valve that connects a section of the outlet duct downstream of the turbocharger and the section of the outlet duct, between the heat exchanger and the on/off valve; - a control valve in a duct that connects the first end of the outlet duct with the atmosphere; and - a section of the outlet duct that connects the heat exchanger with the first communication duct between the connection of the inlet duct and the connection of the duct;