New Mercedes-AMG A45 and siblings to get world-beating 500Nm 2.0 turbo with ‘normally aspirated engine’ revving characteristics
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Mercedes-AMG has commenced production of the world’s highest-output turbocharged four-cylinder engine, produced on a brand-new factory line at AMG’s headquarters in Affalterbach.

The new hand-made ‘M139’ 2.0-litre turbo replaces the ‘M133’ engine that was built at a different site, and while it shares the same displacement, it’s almost entirely new. It’ll power Mercedes-AMG’s fleet of compact pocket rockets, starting with the new-generation A45 AMG hot hatch.

It’s also designed to do at least 250,000km, with AMG’s head of engine development Ralph Illenberger saying “it’s not an engine built for short usage” – something you may not suspect of an engine so turned up.

The headline figures are peak power of up to 310kW and peak torque of up to 500Nm, up 30kW and 25Nm. That’s 155kW per litre of displacement. For context, its outputs better the 2.5-litre five-cylinder turbo used in the Audi RS3 from Ingolstadt, which makes 294kW/480Nm.

But Mercedes-AMG argues there’s a lot more to this new engine than output ceilings. At its genesis, Illenberger went to AMG boss Tobias Moers with the idea of giving the turbo engine “response compared to a normally aspirated engine”.

This idea is called “torque shaping”, and at its core it means moving away from a flat table-like torque curve and instead delivering this maximum torque higher up the rev band, between 5000 and 5250rpm, well on the way to its 7200rpm redline. Max power hits at 6750rpm.

This theoretically gives the engine a “more emotive driving experience” because the engine is more free-revving. We should caveat that a quick look at a torque graph at a factory presentation we just attended showed that there was still north of 400Nm before 2000rpm.

Like AMG’s BiTurbo V8 engines, the M139 is assembled by hand from a delivered block, under its ‘one man, one engine’ principal. This means one person builds the engine over a period of 2.5 hours start-to-finish before it goes to bench testing, and affixes a signed build plate.

The completely overhauled build room upstairs from the 4.0-litre V8 shop is a largely cable- and cord-free zone, with each tool connected to the engine assembly shop’s WLAN meaning torque settings on the auto screwdriver, for instance, can be automated. Every step is digitally logged meaning errors later in the engine’s life can be traced back forensically.

Each assembly technician is followed by a driverless transport system (an autonomous cart). This ‘shopping cart’ has precisely the components needed to complete the engine being assembled. Like IKEA but far more complex.

Like the M133, the M139 is fitted transversely, but the whole unit has been rotated by 180 degrees, meaning the turbocharger and exhaust manifold are placed at the rear closer to the firewall, with the corollary being an intake system positioned nearer the front of the car.

This improves the weight distribution and makes the engine flatter at the front, meaning the car design can be sleeker and have a lower drag coefficient. AMG boffins say it also enables better air ducting because there are shorter distances for air on the intake and exhaust sides.

Fresh air is used in addition to oil and water to cool the turbocharger. This is directed to the charger from the radiator grille, via the engine cover designed as an air deflector. Other engine components are cooled by an electric water pump and helped by the AC’s chiller system.

The new twin-scroll turbo (2.1 bar boost) is augmented by parallel flow passages accessing the turbine housing. As in the AMG GT’s 4.0-litre V8, the compressor and turbine shafts have roller bearings to reduce mechanical friction, meaning it reaches its V-max of 169,000rpm faster.

There’s a new electronically controlled wastegate actuator that’s designed to be more precise, and offers more flexible control of the boost pressure.

The engine gets a new chill-cast aluminium crankcase designed to be strong enough to handle the massive loads running through it (peak combustion pressures of 160 bar). The closed-deck construction means areas around the cylinder holes themselves are solid.

The cylinder linings themselves are coated in a patented Nanoslide mirror-like coating trickled down from F1 to reduce friction on the variably controlled valves. There are familiar Piezo direct injectors spitting fuel into the air chamber but also additional intake manifold injectors using solenoid valves, adding extra fuel to the mixture at higher engine speeds.

The sump features baffle plates so that despite the larger sump, and “even under high lateral acceleration forces”, there is always sufficient engine oil to lubricate all the relevant components.

Mercedes-AMG 2.0-litre M139 turbo engine specs:

  • Displacement: 1991cc
  • Power: 310kW at 6750rpm (AMG S) or 285kW at 4750rpm (basic version)
  • Torque: 500Nm at 5000rpm (AMG S) or 480Nm at 4750rpm (basic)
  • Redline: 7200rpm
  • Compression ratio: 9.0:1
  • Max pressure: 2.1 bar
  • Engine weight (wet): 160.5kg