As the drive towards efficiency forces engines to become smaller and smaller, manufacturers are having to find ever more ingenious ways of ensuring their powerplants continue to develop the kind of prodigious outputs to which buyers have become accustomed. The latest example comes from Mercedes-AMG, which has today unveiled the world's most powerful 2.0-litre four-cylinder production engine, set to find its way into the manufacturer's new '45' model line.
The completely new unit, dubbed the M139, produces 387hp in its standard configuration and 421hp in S guise, besting the previously most powerful M133 by 40hp. Maximum torque has also increased by 20lb ft to 370lb ft between 5,000-5,250 rpm. Best of all, Mercedes reckons that the M139's power delivery is comparable to that of a naturally aspirated engine. That's thanks to a dynamically increasing torque curve at lower rpm for improved agility and increased torque at higher rpm, making the engine more free-revving right up to its 7,200rpm redline.
"We already set the benchmark in the segment with the preceding engine. This fundamentally new four-cylinder presented us with the challenge of doing even better. And we succeeded with a number of sometimes revolutionary solutions. With the M139, we have once again impressively demonstrated the engine expertise of Mercedes-AMG. Not only is the output per litre [211hp] unrivalled for a turbocharged engine, the high level of efficiency also demonstrates that the internal combustion engine still has further potential," said Mercedes-AMG Chairman, Tobias Moers.
So how exactly have they done it? For starters the unit has been rotated by 180-degrees, allowing the turbocharger and exhaust manifold to be positioned at the rear, with the intake at the front. This not only makes for the flattest possible profile, benefitting aero, but crucially accommodates improved air ducting "with shorter distances and fewer diversions". A new twin-scroll turbocharger contains roller bearings for the first time, minimising mechanical friction, and also benefits from a divided turbine housing and exhaust manifold, allowing the exhaust flow to be fed to the turbine separately and preventing the individual cylinders from negatively influencing each other.
An electronically-controlled wastegate allows for more precise and flexible control of the charge pressure, which can reach a maximum of 2.1 bar. That's nothing compared to the 160 bar combustion pressures within the new alloy cylinder block, though, it's as sturdy as a diesel's apparently, and sits alongside a new 16-valve cylinder head, all-aluminium crankcase, lightweight forged steel crankshaft and forged aluminium pistons.
The cylinders themselves are lined with Mercedes-AMG's patented Nanoslide coating. Bestowing them with a "mirror-like surface for minimal friction" which is twice as hard as a conventional grey cast-iron liner, it's the same technology found in Merc's Formula 1 engines. Elsewhere there are larger exhaust valves with variable Camtronic valve control and two-stage fuel injection, while an independently operated electric water pump and additional radiator aid cooling.
Finally, a newly designed production line at AMG's Affalterbach factory also sees a return to its "One man, one engine" policy, with each unit entirely hand assembled from start to finish. This provides all the benefits of a bespoke engine build, with employees still "optimally assisted in their work by digital tools". Jolly good.
Quite a lot to take in then, but the end result will hopefully be an A45 which delivers on the promise of a full-fat AMG hatch, after the A35 left us wanting something more. With the car currently in the advanced stages of testing, we shouldn't have long to wait to find out.