Mercedes installs a racing-derived V8 engine with a flat-plane crankshaft in the Mercedes S-Class W223 facelift. Why did Mercedes choose this solution?
The S 580 4Matic version of the Mercedes S-Class W223 facelift features a substantially modified V8 engine under the hood. The M177 Evo (Evo from Evolution) engine abandons the cross-plane crankshaft with a 90-degree angle between crank pins (crank pins arranged in two planes) in favor of a flat-plane crankshaft with a 180-degree angle between crank pins (crank pins arranged in the same plane).
The flat-plane crankshaft engine originates from motorsport, and Mercedes previously used this solution in the radical Mercedes-AMG GT Black Series based on the first generation AMG GT.
That is why introducing it in a luxury limousine such as the Mercedes S-Class is surprising. However, Mercedes had serious reasons for adopting this solution, and Oliver Vollrath, head of engine development at Mercedes, explained to the German magazine auto motor und sport that the flat-plane crankshaft solution is better suited to meet the new Euro 7 standards with their stricter emission limits.
The new Euro 7 regulations introduce mandatory emissions testing under load, which means the engine can no longer operate with a rich fuel mixture at maximum load. It is essential that the engine runs with an air-fuel mixture at lambda = 1 across most speed and load ranges.
To achieve this, fundamental modifications were required, and these were easier to implement on an engine with a flat-plane crankshaft. Here is why.
In the previous generation Mercedes S-Class S 580, the M177 V8 engine had a firing order of 1-5-4-2-6-3-7-8, which meant a short 90-degree interval in the firing sequence between cylinders 4 and 2 and between cylinders 7 and 8, which are on the same cylinder bank. This resulted in an uneven firing order.
In the new M177 Evo engine, the firing order is 1-5-4-8-3-7-2-6, which means a uniform firing order and also that ignition occurs alternately in one cylinder from each bank. A uniform firing order—meaning symmetrically distributed exhaust pulses—proves advantageous for emissions because it leads to optimal flow between the two exhaust paths.
In addition to the technical aspect, there is also an emotional one. The uneven firing order generates a slightly irregular exhaust sound—a characteristic rumble that, in the previous S 580 generation, was quite well masked.
In the new model with a flat-plane crankshaft, the engine has a completely different sound, similar to Ferrari V8 engines. Furthermore, another advantage is that flat-plane crankshaft engines rev higher, and as engine speed increases, the sound becomes more aggressive and penetrating.
At low revs, however, the M177 Evo V8 sounds rather ordinary. Essentially, a flat-plane V8 behaves like two four-cylinder engines sharing a common crankshaft, with two connecting rods mounted on the same crank pin.
Returning to emissions, the new M177 Evo V8 engine operates according to the Miller cycle. What does this mean? It means that the intake valves close earlier, before the piston reaches bottom dead point. As a result, a smaller mass of air enters the combustion chamber, theoretically leading to lower power. However, Mercedes increased the boost pressure to 2.2 bar to compensate and used a high-tumble intake method that gives the fuel high flow velocity, creating turbulence in the combustion chamber for more efficient combustion.
In addition, the V8 engine is supported by a second-generation 48V mild hybrid system, featuring a new electric motor that weighs only 23 kg and produces 23 PS and 205 Nm. It is integrated into the 9-speed automatic transmission rather than being belt-driven.
With the flat-plane crankshaft solution, the rev range has increased significantly. While the old engine delivered 503 PS at 5,000 rpm, the new engine develops 537 PS between 5,500 and 6,100 rpm. Maximum torque has also increased from 700 Nm at 2,000–4,500 rpm to 750 Nm at 2,500–4,500 rpm.
Although in theory flat-plane crankshaft engines have reduced torque and power at low revs, Mercedes promises better response even from idle speed of 660 rpm thanks to the mild hybrid system and the two twin-scroll turbochargers, which are still mounted inside the V (the “hot V” solution).
We will return with driving impressions and information about how the new flat-plane crankshaft engine sounds after the first test drive. It should also be noted that the new M177 Evo V8 engine is produced in Affalterbach and will also be used in AMG models.
