The high-tech naturally aspirated V8 engine, enthrals with a combination of advanced technologies which is unequalled anywhere in the world. No other petrol engine features direct injection at a pressure of 200 bar, spray-guided combustion and piezo injectors, all in conjunction with map-controlled cylinder shut-off, an all-aluminium crankcase with “spectacle” honing, four-valve technology with continuous camshaft adjustment, a high compression ratio of 12.6 : 1, a start/stop system and generator management – while revving to a maximum speed of over 7000 r/min.
All of these measures lead to high thermodynamic efficiency, which is a precondition for high fuel efficiency and low exhaust emissions. Moreover, deciding in favour of a large-displacement, naturally-aspirated eight-cylinder unit with cylinder shut-off resolves the conflict of aims between high output/torque delivery and low fuel consumption figures.
AMG Cylinder Management
It is the AMG Cylinder Management cylinder shut-off system in particular – completely newly developed by Mercedes-AMG – that significantly improves efficiency: cylinders two, three, five and eight are cut off under partial load, which lowers fuel consumption considerably. In similar form this technology is also used in the approx. 750 hp V8 engines used in Formula 1. As efficiency also plays a major role at the pinnacle of motor racing, two or four of the eight cylinders are cut off , for example, when cornering at slow speed, in the Safety Car phases or for pit stops.
With a fuel consumption of 8.4 litres per 100 km and CO2 emissions of 195 grams per kilometre (NEDC combined), the naturally-aspirated AMG V8 engine comes in at or below the level of its competitors, however these are unable to match either the output or torque of the SLK 55 AMG – and above all, they do not use large-displacement eight-cylinder engines.
The cylinder shut-off function is available over a wide engine speed range from 800 to 3 600 r/min 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. No less than 230 Nm 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 whatsoever. At an engine speed of 3 600 r/min the activation process takes no more than 30 milliseconds.
Valves closed, fuel delivery and ignition deactivated
This is made possible by intelligent interaction within the highly effective engine management system, with 16 hydraulically compensating elements and a complex oil supply system in the cylinder head. The compensating elements are integrated into the cylinder head, and keep the intake and exhaust valves of cylinders two, three, five and eight closed when cylinder shut-off is active.
At the same time their fuel supply and ignition are deactivated. This not only enables the load-change losses of the four deactivated cylinders to be reduced, but also increases the efficiency of the four remaining cylinders. This is because the operating point is transferred to the higher load range. The compensating elements are compact and lightweight, allowing taut valve train operation and engine speeds up to 7 200 r/min.
All-aluminium crankcase with Silitec cylinder liners
The crankcase of the new AMG 5.5-litre V8 engine features a diecast all-aluminium construction. The low (dry) engine weight of just 187 kilograms is the result of uncompromising lightweight construction methods. The bearing covers for the main crankshaft bearings are of 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 minimise internal friction and optimise the wearing characteristics. Cast-in Silitec cylinder liners ensure low friction for the eight pistons. So-called “spectacle” honing is another measure to reduce friction and therefore fuel consumption: in this sophisticated and complex 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 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 optimise efficiency:
- weight-optimised cast aluminium 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
- electronically controlled fuel pressure, fully variable and demand-related between 100 and 200 bar
The forged crankshaft of high-grade steel alloy rotates in five main bearings, has eight counterweights and has been optimised 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
Perfect charging of the combustion chambers is ensured by large intake and exhaust valves, of which there are four per cylinder. 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 high 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, which have considerable advantages in terms of noise comfort compared with cylinder roller chains.
Controlled Efficiency start/stop function as standard
The Controlled Efficiency start/stop function further contributes to the low fuel consumption. This system is standard equipment on the SLK 55 AMG, and active in the fuel economy transmission mode “C”. Once the driver comes to a halt e.g. at a red traffic light, 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 move off quickly. Intelligent technology guarantees a comfortable and immediate starting procedure: a crankshaft sensor which recognises the direction of rotation registers the resting position of all eight pistons.
For an automatic engine start, the cylinder with the most favourable 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.
A framed “ECO” symbol in the AMG main menu shows the driver that the Controlled Efficiency start/stop function is active. When cylinder shut-off is active, the driver is informed by the symbol “ECO4”, while “ECO8” stands for eight-cylinder operation. In the two more performance-oriented driving modes “S” (Sport) and “M” (Manual), the start/stop function is always deactivated. If required, the driver can also switch it off whilst in “C” mode. In this case neither a “start/stop” 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 on the overrun or when braking, kinetic energy is used to charge the battery rather than being wasted as heat in the usual way. 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 and makes for fuel savings of around 0.15 litres per 100 kilometres according to the NEDC standard, and up to 0.2 litres per 100 km in city traffic with its frequent overrun and braking phases.
New AMG sports exhaust system with integrated exhaust flaps
For the first time AMG is using a sports exhaust system featuring an exhaust flap on each side. This technology resolves the conflict of aims between an emotional sound when driving in a sporty manner and a more discreet engine note in the partial load range. The main focus of the sound design was on achieving a perfect synthesis of perceptible dynamism and the long-distance comfort that is a hallmark of Mercedes-Benz.
Each of the two rear silencers has a flap which is variably activated on a logic-controlled basis depending on the driver’s power requirement and the engine speed. At low loads and engine speeds below 2 000 r/min the flaps remain closed. This causes the exhaust gases to cover a longer distance and flow through an additional damping element, so that the engine sound is pleasantly subdued and irritating frequencies are effectively suppressed – resulting in a harmonious sound impression especially in four-cylinder operation when cylinder shut-off is active.
When the driver accelerates, the flaps open at an angle of 15, then 30 and up to 50 degrees so that some of the exhaust gases cover the longer, acoustically dampened distance and some the shorter distance. This produces a sonorous engine note. Under full load at higher engine speeds in eight-cylinder mode, both flaps are fully opened so that the occupants are able to enjoy the decidedly muscular sound typical of an AMG V8. In short, they are left in no doubt about the performance potential of the 310 kW naturally aspirated AMG eight-cylinder engine.
The two-pipe AMG sports exhaust system with two chromed twin tailpipes has a pipe diameter of 65 millimetres from the manifolds to the pre-silencer. The diameter is reduced to 60 millimetres from the pre-silencer to the rear silencers.
Effective emissions technology with new catalytic converter boxes
Low exhaust emissions, compliance with country-specific standards and a characteristic engine sound – the requirements for the exhaust system of the new, naturally aspirated AMG 5.5-litre V8 engine were manifold and complex. The SLK 55 AMG complies with the current Euro-5 emission standards.
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. 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.