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The BMW M21 is a six-cylinder-inline-diesel engine developed by the Bavarian engine manufacturer BMW. It has swirl chamber injection and is based on the M20 petrol engine[1] and was produced for BMW by the Upper Austrian Steyr engine plant from 1983 to 1991. It was succeeded by the M51.[2]

BMW E28 engine 2 16.09 JM.jpg
Displacement2.4 L; 149.1 cu in (2,443 cc)
Cylinder bore80 mm (3.15 in)
Piston stroke81 mm (3.19 in)
Block materialCast iron
ValvetrainSOHC 2 valves x cyl.
Compression ratio22.0:1
RPM range
TurbochargerGarrett (85 kW (116 PS) units only)
Fuel systemSwirl chamber injection, distributor fuel injection pump
ManagementBosch DDE (from 1987)
Fuel typeDiesel fuel (DIN 51601)
Cooling systemWater-cooled
Power output63–85 kW (86–116 PS)
Torque output152–220 N⋅m (112–162 lb⋅ft)



In the 1970s BMW decided to develop an engine, which would both be powerful and have a good fuel economy. This was caused by the oil crisis in 1973. In 1975 a group of BMW engineers started working on the M21 diesel engine, the M20 petrol engine was used as the basis. In 1983 at the IAA, the first passenger car was shown to the public which made use of the 85 kW (116 PS) M21. It was the E28 524td, which has a top speed of 180 km/h (110 mph) and reaches 100 km/h (62 mph) in 12,9 s. This 5er-BMW was the fastest series production diesel car in the world back in 1983. It has a fuel economy of 7.1 L/100 km (40 mpg‑imp; 33 mpg‑US).[2]

Technical descriptionEdit

As per the M20, the M21 is water-cooled,[3] has a cast iron block and a SOHC valvetrain. The camshaft is driven by a belt,[2] each cylinder has one inlet and one outlet valve.[1] Compared to the M20, the M21 has reinforced connecting rods, cylinder heads, pistons, valves[2] and a reinforced crankshaft[1] with seven[3] bearings.

For faster engine startup the M21 has a glowplug system called instant start, which reduces the time to reach starting temperature compared to similar diesel engines.[2] The fuel is injected into swirl chambers.[1]

A Garrett turbocharger[4] is used (without an intercooler). Initially, the M21 was only available as a turbocharged engine. In 1985, BMW introduced a naturally aspirated version of the M21, which was popular in countries with a high motor vehicle tax.[2]

Initially, the fuel pump was controlled mechanically. From 1987, an electronically controlled fuel pump was used,[5][6] which increased the torque output by 10 N⋅m (7 lb⋅ft). The updated engine has a smaller turbocharger, which improves response.[1]


Engine Aspiration Power Torque Redline Year
M21D24[3] Turbo-diesel 85 kW (116 PS; 114 hp)
@ 4800 rpm
210 N⋅m (155 lb⋅ft)
@ 2400 rpm
5350 rpm 1983
220 N⋅m (162 lb⋅ft)
@ 2400 rpm
Natural 63 kW (86 PS; 84 hp)
@ 4600 rpm
152 N⋅m (112 lb⋅ft)
@ 2500 rpm
5150 rpm 1985


See alsoEdit


  1. ^ a b c d e Andreas in Bimmer: Die BMW-Sportdiesel: Von 524td E28 über 530d E39 bis BMW M550d F10, published at the 21st of January 2012, retrieved at the 9th of April 2015. (german)
  2. ^ a b c d e f Ronan Glon in 30 years ago: BMW introduces its first diesel engine, published at the 22nd of Juli 2013, retrieved at the 9th of April 2015.
  3. ^ a b c Meyer Motoren: BMW M21. Retrieved at the 9th of May 2016. (german)
  4. ^ Auto Motor und Sport: Der zweite 5er von BMW: Die Baureihe E28 Der unscheinbare Revoluzzer, retrieved at the 9th of April 2015. (german)
  5. ^ BMW M21 engine page. Retrieved at the 9th of May 2016.
  6. ^ E30 Zone- M21. Retrieved at the 3rd of January 2017.
  7. ^ a b Turbo Diesel Lincoln Continental M21. Retrieved at the 3rd of January 2017.
  8. ^ Meet the Vixen RV, BMW 2 Series Active Tourer’s American Ancestor. Retrieved at the 3rd of January 2017.

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