BMW N20 Engine (2011)

The BMW N20 was an inline, four-cylinder turbocharged petrol engine that was available from January 2011. Effectively replacing the N52 and N53 six-cylinder engines, the design of the N20 engine was based on the six-cylinder N55 engine. With its 84.0 mm bores and 90.1 mm stroke, the N20 engine had a capacity of 1997 cc.

To minimise fuel consumption, the N20 engine had ‘EfficientDynamics’ technologies such as start-stop, an on-demand oil pump, an electrically-driven coolant pump and ‘Brake Energy Regeneration’ (a variable output alternator that increased output when the vehicle was coasting or braking).

Block and crankcase

The N20 engine block, crankcase and bedplate were produced from die-cast aluminium (AlSi9Cu3). For the cylinder walls, the N20 engine introduced electric arc wire spraying whereby high voltage was used to ignite an electric arc at both ends of an iron wire. The high temperatures (around 3000 degrees Celsius) melted the wire, which was continually fed by the wire feed unit. Furthermore, the molten iron was blasted onto the cylinder wall surface at pressure via the central and secondary compressed-air supplies. As a result of high kinetic energy and capillary action, the molten iron would mechanically bond into the depressions and undercuts of the aluminium cylinders where they would solidify to form a strong coating.

Crankshaft, connecting rods and pistons

The N20 engine had a forged crankshaft that was made of C38modBY, was supported by five bearings and had four balance weights. For the first time by BMW, the N20 crankcase had a positive crankshaft offset (where the crankshaft axis was offset from the cylinder centre line) and negative pin offset. As a result of this design, piston rocking would occur later and the thrust force and friction of the piston on the cylinder wall were reduced.

Like the N55, the N20 connecting rod had a specially formed hole in the small end – this design evenly distributed the force acting on the wrist pin over the entire surface of the rod bushing and reduced the load at the edges as the piston is forced downward on the power stroke. Furthermore, the N20’s full slipper skirt pistons were produced by FM; oil spray nozzles were used to cool the piston crowns and lubricate the wrist pins.

The N20 engine had two counter-rotating balance shafts that were driven by the crankshaft via a tooth-type chain. The map-controlled, variable volumetric flow oil pump was also driven off the crankshaft.

Cylinder head

The N20 engine had variable intake and exhaust camshafts (BMW’s double VANOS) as per the M73 engine, and a third-generation variable intake valve lift system (Valvetronic III) as per the N55 engine.

The N20 engine had four valves per cylinder that were actuated by roller cam followers, with the intake and exhaust valves carried over from the N55 engine. As such,

• the intake valve seats were induction-hardened; and,
• the exhaust valve stems were hollow and sodium-filled, while the exhaust valve seats were made from hardened material.

The chain drive in the N20 engine was divided into an upper camshaft drive section and a lower oil pump drive section. The timing chain was lubricated by an oil spray nozzle located in the chain tensioner, while the lobes on the camshaft were also lubricated via oil spray nozzles. For the exhaust camshaft, the cylinder head featured an oil pipe which sprayed oil through small holes directly onto the cam lobes.

The N20 cylinder head had coolant passages that surrounded the injectors. Unlike the N55 engine, the N20 engine had no grooves on the block deck between the cylinders; instead, the N20 engine had bore holes between the cylinders (two on each side) which met in the middle.

An innovation for the N20 engine was that all the components for crankcase ventilation and the blow-by ducts were integrated into the cylinder head cover. Furthermore, a pressure control valve prevents an excessive vacuum from being generated in the crankcase.

Twin-scroll turbocharger

The N20 engine had a twin-scroll turbocharger with integrated wastegate and blowoff valves. At the turbine inlet, twin-scroll turbochargers have two separate ports in which exhaust gas is routed to the turbine vanes. For four cylinder engines like the N20 engine, cylinders 1 and 4 and cylinders 2 and 3 are brought together to form two ports each feeding one scroll. This results in pulse charging whereby the kinetic energy of the exhaust gas is used to increase turbocharger response (particularly at low engine speeds).

Like the N55, the four-into-two exhaust manifold for the N20 engine was air-gap-insulated and welded to the turbocharger.

Injection and ignition

The N20 engine had high pressure, solenoid valve injectors that injected fuel into the combustion chamber (direct injection). The N20’s ‘high precision injection’ (HPI) system used Bosch HDEV5.2 injectors and each injector had an inward-opening, multi-hole valve. The injectors were capable of system pressure of up to 200 bar. Similar to the N63 and N74 engines, the N20 engine had a Bosch high-pressure fuel pump that was driven from the exhaust camshaft via a triple lobe on the cam.

The N20 engine had Bosch MEVD 17.2.4 engine management with a map-controlled thermostat and hot-film air mass meter.