In a turbocharged diesel engine (10), the charging pressure P.sub.La is adjusted to run the engine (10) with an empirically determined optimum fuel consumption. A charging pressure feedback signal P.sub.La is fed to a regulating device (40) which receives various operating parameters, including ME.sub.d which represents the power output. A characteristic field in a memory zone (50) in the regulating device (40) determines a control variable (TV) which operates a turbine by-pass valve (22) via an electro-pneumatic converter (28) and a penumatic (vacuum) actuator (26). The characteristic field is such that the charging presssure P.sub.La is reduced in the part load range (cruising speed) to improve fuel consumption.