A phase-shifting device, a so-called cam phaser, is arranged between a crankshaft and at least one camshaft to change the at least one camshaft's rotational position in relation to the crankshaft and thus push forward or defer at least one inlet valve's and/or at least one exhaust valve's opening and closing time. The phase-shifting device is connected to an accumulator that can be charged by an oil pump. The oil pressure may be increased with the help of a pressure medium controlled cylinder before or during a phase-shifting process.