A pioneering diesel engine for BMW and one of the few diesels to be exported to the USA.

The BMW M21 engine is an inline 6 cylinder 2.4-liter diesel. Like the base model BMW M20, the M21 engine has a single overhead camshaft, 2 valves per cylinder and a timing belt. The M21 was available naturally aspirated or turbocharged. Turbo models carried the "td" moniker. More on BMW codes...

BMW fitted the M21 engine in the E28 and E34 5 Series 524td and E30 and E36 3 Series 324td models. The 5 Series 524td models were some of the few diesels to be exported to the USA. The first E28 cars with the new M21 engine were produced starting in May 1983. It was also offered in the smaller 3 Series vehicle range from 1987.

The engine's high specific output meant high peak pressures in the combustion chamber, and thus required stronger cylinder-head bolts. For this reason, the top deck of the block was strengthened as well.

To adapt the engine to mechanical stresses that were higher than those of the base Otto engine (M20), the connecting rods were strengthened, a forged crankshaft was used and the effective width of the bearings in the lower inserts was increased by doing away with their oil grooves.

Inside the pistons a cooling channel was provided for heat extraction; it was supplied with oil through jets. To accommodate the taller pistons required by the diesel engine its block was made 10 mm higher.

Injection was carried out by a distributor pump, which was significantly smaller than the in-line pump that was customarily used for diesel engines.

From 1986 a naturally-aspirated version of the diesel engine was also offered, developing 86 bhp in the BMW 324d. Among the cost-saving measures in this engine were the removal of the turbocharger as well as the internal piston cooling and the oil cooler. The camshaft was redesigned to give longer duration and higher valve lift. The inlet and exhaust manifolds were adapted to the changed installation conditions and reduced stresses.

In 1987 digital diesel electronics (DDE) were introduced in the turbodiesel engine. This electronic regulation and timing system replaced mechanical control of the otherwise unchanged distributor-type injection pump. Better control by the DDE further improved fuel consumption, exhaust emissions and torque. Moreover the accelerator pedal was no longer connected by a a cable to the injector pump but rather by an electronic link.

In 1989 the non-turbocharged engine was also given DDE. This version was offered in BMW's small vehicle range until the end of 1990.

source: Geschichte des Motors, Dr. Karlheinz Lange