BMW's diesel engine of the '90s was at launch the most powerful diesel on the market, as well as one of the most economical!

The successor to the M21 diesel, the BMW M51 engine is an inline 6 cylinder 2.5-liter diesel. The M51 engine has a single overhead camshaft, 2 valves per cylinder and 2 simplex timing chains. An enhanced Digital Diesel Electronics (DDE) system by Bosch was utilized on this engine. The M51 was only produced as a turbocharged engine, with normal and intercooler models. Turbo models carried the "td" moniker, while intercooler models are coded "tds". More on BMW codes...

BMW fitted the M51 engine in the E34 and E39 5 Series 525tds, the E36 3 Series 325td and the 7 Series 725tds models. No M51 diesel powered cars were officially exported to the USA.

BMW began producing a new generation of six-cylinder diesel engines in September 1991. The normally-aspirated version was dropped from the range and in its place the turbocharged version was offered with two different output levels; 143 bhp with intercooling and 115 bhp without charge cooling.

The aim of the redesign for 1991 was to increase torque and output while reducing fuel consumption. The swirl-chamber design was retained. So was the cylinder spacing, still 91 mm, which made possible the production of various parts in conjunction with those of the counterpart Otto engine. Increasingly, however, this commonality of components was set aside in favor of the optimization of the design for its diesel application. The displacement was increased to 2.5 liters.

Valves were opened by bucket-type tappets which contained hydraulic valve clearance adjustment. The injector pump was driven by a single-row chain from the crankshaft. From there another single-row chain drove the overhead camshaft.

The DDE control system was further refined. Depending on the market, diesel vehicles were fitted with oxidation catalytic converters. Exhaust-gas recirculation was used onl all engines to reduce their emissions of oxides of nitrogen. To suppress engine noise, BMW diesel cars were given completely-enclosed engines. Air flow around the engine was automatically regulated by temperature-controlled louvers behind the radiator.

source: Geschichte des Motors, Dr. Karlheinz Lange