The new M152 engine is based on the AMG M157 5.5-liter V8 biturbo engine which first debuted on the 2011MY CL63 AMG and S63 AMG. Numerous components and systems are basically identical, e.g. the displacement, bore/stroke, distance between cylinders, ECO stop/start system and direct injection technology. Distinguishing features versus the M157 include new intake air ducting, new cylinder heads, the modified valve drive, an adapted oil supply system and an optimized crankcase.

The M152 features direct injection at a pressure of 2,900 psi, spray-guided combustion and piezo-injectors in conjunction with map-controlled cylinder shut-off, an all-aluminum crankcase with spectacle honing, four-valve technology with continuous camshaft adjustment, a high compression ratio of 12.6 : 1, an ECO stop/start system and generator management, and revs to a maximum of more than 7,000 rpm.

AMG Cylinder Management. Under the AMG Cylinder Management cylinder shut-off system cylinders two, three, five and eight are cut off under partial load. This technology is similar to what’s used in the 750 hp V8 engines used in Formula 1. Since efficiency also plays a major role in this form of motor racing, two or four of the eight cylinders are cut off when cornering at low speed, during Safety Car laps or for pit stops.

The cylinder shut-off function is available over a wide engine speed range from 800 to 3,600 rpm if the driver has selected transmission mode “C”— Controlled Efficiency. The AMG main menu in the instrument cluster informs the driver whether cylinder shut-off is active, and whether the engine is currently running in four or eight-cylinder mode. 170 lb-ft of torque is still available in four-cylinder mode—enough power to ensure plenty of acceleration in most driving situations. As soon as the driver has a need for more power and leaves the partial load range, cylinders two, three, five and eight are activated. The switch from four to eight-cylinder operation is immediate and imperceptible, leading to no loss of occupant comfort, according to Mercedes-AMG. At an engine speed of 3,600 rpm the activation process takes no longer than 30 milliseconds.

The cylinder shut-off system is enabled by an engine management system with 16 hydraulic actuators and a complex oil supply system in the cylinder head. The selectable actuators are integrated into the cylinder heads, and keep the intake and exhaust valves of cylinders two, three, five and eight closed. At the same time their fuel supply and ignition are deactivated. This reduces the load-change losses of the four deactivated cylinders. It also increases the efficiency of the four remaining cylinders because the operating point is transferred to the higher load range. The actuators are compact and lightweight, allowing tight valve train operation and engine speeds up to 7,200 rpm.

All-aluminum crankcase with Silitec cylinder liners. The crankcase of the new AMG 5.5-liter V8 engine is made of diecast all-aluminum construction. The low (dry) engine weight of 412.4 lbs (187 kg) is the result of lightweight construction methods. The bearing covers for the main crankshaft bearings are grey cast iron, and are bolted to the crankcase as well as on the sides for high rigidity.

The piston rings are carbon-coated to minimize internal friction and optimize the wearing characteristics. Cast-in Silitec cylinder liners ensure low friction for the eight pistons. Spectacle honing is another measure to reduce friction and therefore fuel consumption: in this process, the cylinder liners receive their mechanical surface treatment when already bolted in place. As honing would no longer be possible with the cylinder head installed, a jig resembling a pair of spectacles is bolted to the crankcase. The cylinder liners distort as if the cylinder head were in place, and are only then given their mechanical surface treatment. As a result, any static distortion of the cylinder liners caused by tightening the cylinder head bolts can be completely eliminated.

Other measures to optimize efficiency include:

• weight-optimized cast aluminum pistons with special piston skirt coating;
• special ventilation holes in the crankcase;
• oil pump with electrically controlled pressure stage;
• separate oil pump for cylinder shut-off as a controllable, low-friction vane cell pump;
• engine cooling on the particularly efficient cross-flow principle; and
• electronically controlled fuel pressure, fully variable and demand-related between 1,450 psi and 2,900 psi.

The forged crankshaft of high-grade 38MnS6BY steel alloy rotates in five main bearings, has eight counterweights and has been optimized with respect to torsional rigidity, inertia, low rotating masses and a long operating life. A two-mass viscous damper mounted at the front reliably eliminates vibrations. Each connecting rod journal on the crankshaft carries two forged, cracked connecting rods.

Four-valve technology with variable camshaft adjustment. Optimal charging of the combustion chambers is ensured by large intake and exhaust valves. The exhaust valves, which are subject to high thermal loads, are hollow and sodium-cooled. Four overhead camshafts operate the 32 valves via low-maintenance, low-friction cam followers. The infinitely variable camshaft adjustment within a range of 40 degrees on the intake and exhaust sides depends on the engine load and engine speed, leading to outstanding output and torque values.

This also results in consistent idling at a low speed. Depending on the engine speed, valve overlap can be varied for the best possible fuel/air supply to the combustion chambers and efficient removal of the exhaust gases. The variable camshaft adjustment is carried out hydraulically via four pivoting actuators. These are electromagnetically actuated and controlled by the engine control unit. The camshafts are driven by three high-performance silent chains.

Controlled Efficiency stop/start function as standard. The Controlled Efficiency stop/start function further contributes to the low fuel consumption. This system is standard equipment in the new SLK55 AMG, and permanently active in the fuel economy transmission mode C. Once the driver brakes to a stop the V8 engine is automatically switched off. Once the brake pedal is released or the accelerator is depressed, the engine is immediately restarted and the car is able to accelerate quickly.

A crankshaft sensor recognizes the direction of rotation and registers the resting position of all eight pistons. For an automatic engine start, the cylinder with the most favorable piston position receives an injection of fuel into its combustion chamber. The precise piezo-electric injectors greatly assist this process, as they make particularly fast starts possible. The M152 always starts in eight-cylinder operation.

The engine management system ensures that the engine is only switched off if certain conditions are met. The starter battery must have sufficient charge, for example, and the engine must be at the necessary operating temperature for efficient emissions control. The same applies to the interior temperature selected by the driver: if this has not yet been reached, the engine is not switched off when the car comes to a stop. The onboard network management system makes sure that active audio, telephone or video functions are not interrupted by the stop/start function, and that the air conditioning continues to operate.

A framed ECO symbol in the AMG main menu shows the driver that the Controlled Efficiency stop/start function is active. When cylinder shut-off is active, the driver is notified by the symbol ECO4, while ECO8 stands for eight-cylinder operation. Should one of the above criteria prevent activation of the system, this is shown in the central display by the message “Stop/start inactive” and an ECO symbol with no frame. In the two more performance-oriented driving modes “S” (Sport) and “M” (Manual), the stop/start function is always deactivated. If required, the driver can also switch it off while in C mode as well. In this case neither a stop/start nor an ECO symbol appears.

Generator management helps to save fuel. A generator management system makes a further contribution to fuel economy: whenever the V8 engine is in overrun or when braking, kinetic energy is used to charge the battery. In all other operating modes a combination of onboard network and generator management enables the generator to be kept at a low voltage. This reduces the load on the engine as well as fuel consumption when in city traffic with its frequent overrun and braking phases.

Engine electronics. All the engine functions are executed and controlled by a Bosch MED 17.7.3. control unit. This unit not only controls the direct fuel injection, cylinder shut-off and variable oil supply, but also communicates with all the other onboard control units. The microprocessor has more than 30,000 different parameters and functions stored in its memory, and is able to perform up to 260 million individual operations per second. To reduce the load on the engine control unit, the eight individual ignition coils have an integral electronic module (ignition amplifier) at each cylinder. These ensure a strong ignition spark at all engine speeds and under all load conditions. Eight high-voltage powerstages are responsible for highly precise actuation of the piezo-electric injectors.

Aftertreatment. Air gap-insulated manifolds ensure a rapid catalytic converter response. For efficiency and to save space, this concept has a tandem catalytic converter housing on each side of the vehicle: adjacent to the firewall, two thin-walled ceramic substrates are grouped into each housing. This solution makes the previous, additional underbody catalytic converters unnecessary. The two ceramic substrates differ to ensure rapid and efficient emissions control: the front one is coated with palladium, while the rear one has a bimetal coating of palladium and rhodium. One lambda sensor per row of cylinders is located in front of each catalytic converter housing, and there is a lambda diagnostic sensor between each of the two thin-walled substrates.

The lambda sensors are necessary for demand-related lambda control. In all operating conditions, the constituents of the intake mixture can be precisely controlled to avoid damaging the catalytic converters. This also benefits the fuel consumption under full load, as the mixture can be leaner than in engines without this control system.