You’d think 415bhp would require a big-capacity engine. So how will the A45 S achieve it with just 2 litres?
MERCEDES-AMG A45S ENGINEA AMG’s new M139 four-cylinder engine has been designed from scratch, and shares nothing with the previous M133 installed in the original A45 and its derivatives, or the modified M260 found in the A35. As before, the unit is hand-built in the same Affalterbach facility that produces AMG’s V8 engines, but it now incorporates a suite of new technology designed not only to generate the mad 208bhp-per-litre specific output in peak ‘S’ form, but do it reliably.
‘AMG HAS GONE TO GREAT LENGTH STO KEEP BOTH THE ENGINE AND TURBO COOL’
The clean-sheet design starts with a more rigid aluminium block, with comparable toughness to that of a diesel engine. This is needed as the M139 has an immensely high load rating due to the physicality of producing so much power within so little capacity. The headers are of a non-symmetrical design that reduces back-pressure in the valvetrain, while down in the bowels the crank has been trimmed of 1.2kg, reducing inertia while not compromising on strength.
The turbocharger is a new twin-scroll unit with a peak boost pressure of 30 psi. Managing that boost is a new electronically controlled wastegate that streamlines the turbocharger’s breathing with more precision than a traditional mechanical set-up. The fuel-injection system is by direct and indirect injection, the latter only applying under heavy loads to essentially ‘fuel dump’ into the cylinders more quickly than the direct injection system would otherwise be able to do on its own.
All of this performance generates a considerable amount of heat, so AMG has gone to great lengths to keep both the engine and turbo – and the air they breathe – cool. The engine has been rotated 180 degrees, for instance, placing the exhaust and turbocharger on the bulkhead side of the engine bay. With the intake at the front of the car, cold air can now enter more effectively without the need to navigate a hot exhaust and turbocharger.
In addition to the usual intercooler and radiator is a front-mounted air-to-air chargecooler, while the block gets its own electric water pump and the headers a more traditional mechanical one. In the high-output S models, even the air conditioning system is used to cool the engine in extreme circumstances, such as when refuelling after a high-speed stint on the autobahn. Clever stuff.
What will the engine feel like when used in anger? AMG says that it has designed the M139 to ‘feel sporty’, and one look at the unit’s power and torque curves supports this assertion. In contrast to the flat, cliff-like torque curve of its predecessor, the M139, like the A35’s M260, instead has a more tailored curve that rises more progressively with the revs, augmented by a slight bump at around 4000rpm.
As four-cylinder engines go, AMG’s new arrival may deliver on power and tech, but the real test will lie in whether it’s an enjoyable engine to use. The previous M133, although impressively powerful, lacked a sense of occasion worthy of its outputs. Let’s hope the M139 lives up to its maker’s promises.