Apfelbeck worked, amongst other venues, for BMW and Horex Maico and KTM. He patentet several cylinder head and valve operating designs (around 1960), all related to overhead camshafts.

His most famous patent was the radially orientated 4-valve OHC cylinderhead design, where the two inlet valves (and outlet valves ditto) are positioned opposed to one another. This concept originally seemed most useful in single cylinder motorcycle engines and the patent was already registered in 1937 (!). Apfelbeck used it later for BMW when he was asked to design a Formula 2 engine. He designed an in-line 4 cylinder 2 liter 16 valve (radial) OHC engine, which developed 280hp at 8500 RPM, remarkable at that time (early nineteen sixties)