The BMW M21 was a 2.4 L diesel straight-6 engine.
It was introduced in 1983 with the BMW M20 design as its basis. It was available with a turbocharger. It was also the last diesel sold in the North American market in the E28 before the reintroduction of a diesel by BMW in the North American market E90 in 2009, the BMW M57.
Applications:
The UnixNerd's BMW M21 engine page with photos, history and common problems.