The BMW M51 is an inline-6 cylinder Diesel engine produced by the Upper Austrian BMW plant in Steyr from July 1991 through February 2000. Its predecessor is the BMW M21, the successor is the BMW M57.

The BMW M21 is a straight-six diesel engine developed by the Bavarian engine manufacturer BMW. It has swirl chamber injection and is based on the M20 petrol engine and was produced for BMW by the Upper Austrian Steyr engine plant from 1983 to 1991. It was succeeded by the M51.