The new SLK 55 AMG: Top marks in all disciplines
Stuttgart
Nov 18, 2011
Engine and power transmission: Fun with eight, economy with four
  • High-tech: cylinder shut-off system inspired by Formula 1
  • Efficient: fuel savings of 30 percent
  • Exclusive: a combination of advanced technologies
  • Powerful: 310 kW (422 hp) and 540 Nm
  • Highly versatile: the AMG SPEEDSHIFT PLUS 7G-TRONIC
Superior power delivery, passionate response, high pulling power and a sonorous eight-cylinder engine sound – all combined with groundbreaking low levels of consumption: the naturally aspirated AMG 5.5-litre V8 engine in the new SLK 55 AMG provides pure excitement. The high-tech M152 powerpack enthralls 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, in conjunction with map-controlled cylinder shut-off, an all-aluminium crankcase with "spectacle" honing, four-valve-per-cylinder design with continuous camshaft adjustment, a high compression ratio of 12.6:1, a ECO start/stop function and generator management – while revving to a maximum speed of over 7000 rpm. All these measures lead to high thermodynamic efficiency, which is a precondition for high 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 decidedly low fuel consumption levels. The reduction in fuel consumption of 30 percent compared with the previous model represents a quantum leap forward.
Key data at a glance:
 
AMG M152 5.5-litre V8 engine
Cylinder arrangement
V8
Cylinder angle
90°
Valves per cylinder
4
Displacement
5461 cc
Bore x stroke
98.0 x 90.5 mm
Compression ratio
12.6:1
Cylinder spacing
106 mm
Output
310 kW (422 hp) at 6800 rpm
Output per litre
56.8 kW (77.2 hp)
Max. torque
540 Nm at 4500 rpm
Torque per litre
98.9 Nm
Maximum engine speed
7200 rpm
Engine weight (dry)
187 kg
Power/weight ratio
0.44 kg/hp
 
AMG Cylinder Management cylinder shut-off system
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 shut off under partial load, which lowers fuel consumption considerably. In similar form this technology is also used in the approx. 750 hp V8 engines found in Formula 1. As efficiency also plays a major role at the pinnacle of motor racing, four or six cylinders are disabled by shutting off the fuel injection when the driver has lower load demands, e.g. when cornering at slow speed, in the Safety Car phases or for pit stops. In the case of the M152, the intake and outlet valves also remain closed. With a fuel consumption of 8.4 litres and CO2 emissions of 195 grams per kilometre (NEDC combined), the naturally-aspirated AMG V8 engine ranks at the same the level as 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 3600 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. 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 3600 rpm 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 variable compensating elements and a complex oil supply system in the cylinder head. The variable compensating elements 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, and at the same time also increases the efficiency of the four remaining cylinders. This is because the operating point is transferred to the higher load range.
All-aluminium crankcase with Silitec cylinder liners
The crankcase of the naturally aspirated 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 made of grey cast iron and are bolted to the crankcase, from beneath as well as on the sides, for high rigidity.
The piston rings are carbon-coated to minimise internal friction and optimise wear 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 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 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 distorsion of the cylinder liners caused by tightening the cylinder head bolts can be completely eliminated. The positive effect of this process is reduced frictional losses.
Other measures to optimise efficiency include:
  • weight-optimised cast aluminium pistons with special piston skirt coating
  • optimised 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 made of high-grade 38MnS6BY steel alloy rotates in five main bearings, has six 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 two per cylinder. The exhaust valves, which are subject to high thermal loads, are sodium-cooled and for weight reasons also hollow. 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, which have considerable advantages in noise comfort compared to cylinder roller chains.
ECO start/stop function as standard
The ECO start/stop function further contributes to the low fuel consumption. This system is standard equipment in the SLK 55 AMG, and permanently active in the fuel economy transmission mode Controlled Efficiency "C". Once the driver comes to a halt e.g. at a red traffic light, the naturally aspirated 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.
The engine management ensures that the engine is only switched off if certain preconditions 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 ECO start/stop function, and that the air conditioning continues to operate.
A framed "ECO" symbol in the AMG main menu shows the driver that the ECO 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. Should one of the above criteria be preventing activation of the system, this is shown in the central display by the message "Start/stop inactive" and an "ECO" symbol with no frame. In the two more performance-oriented driving modes "S" (Sport) and "M" (Manual), the ECO start/stop and cylinder shut-off system are deactivated. If required, the driver can also switch the systems off while 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 naturally aspirated 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 litre per 100 kilometres according to the NEDC standard, and up to 0.2 l/100 km in city traffic with its frequent overrun and braking phases.
Highly efficient engine electronics for every function
All the engine functions are executed and controlled by a particularly efficient Bosch MED 17.7.3. control unit. This state-of-the-art engine computer not only controls the direct petrol 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 known as an 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.
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.
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 2000 rpm 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 – 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 performance-oriented eight-cylinder operation, 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 (422 hp) naturally aspirated AMG eight-cylinder engine.
The two-pipe AMG sports exhaust system with two chromed twin tailpipes on each side 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 standard, as well as meeting all the requirements of the US market (LEV-III standard, On-Board Diagnosis II).
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.
Long tradition of powerful AMG V8 engines
As is typical of AMG, engine production is carried out by hand in the AMG engine shop. An engineer assembles the AMG 5.5-litre V8 engine according to the traditional "one man, one engine" philosophy, from installing the crankshaft through to adjusting the camshaft as well as fitting all of the wiring. An AMG engine badge bears the signature of the responsible engineer.
Powerful eight-cylinder engines are an inseparable part of AMG's corporate history. Established in 1967, the company immediately caused a stir with the 300 SEL 6.8 AMG which succeeded in taking second place at the 24-hour race in Spa-Francorchamps (Belgium) in 1971. The AMG racing saloon was technically based on the Mercedes-Benz 300 SEL 6.3 with the M100 V8 engine. With an engine output of 184 kW (250 hp) at 4000 rpm and a top speed of 220 km/h, this luxury V8 saloon with the M100 V8 engine was Germany's fastest regular production car at the time. Classic tuning as well as an enlarged displacement from 6330 to 6835 cc resulted in an increase in output to 315 kW (428 hp) at 5500 rpm and in torque from 500 to 608 Nm.
Another important engine in the AMG story was the M117, its first eight-cylinder unit with four-valve technology. With a displacement of 5.6 litres, 265 kW (360 hp) and 510 newton metres of torque, this V8 accelerated the 300 CE 5.6 AMG to a top speed of 303 km/h in 1987. This made the coupé the fastest German car in series production, and American fans reverently christened it "The Hammer".
The supercharged AMG 5.5-litre V8 of 2001 is another milestone in the history of AMG engine development: the M113 K delivered up to 428 kW (582 hp) and a torque of 800 newton metres. The supercharged AMG 5.5-litre V8 of the SLR McLaren dating from 2003 was even more powerful – the M155 generated up to 478 kW (650 hp) and 820 newton metres.
2005 saw the debut of the naturally aspirated, high-revving AMG 6.3-litre V8 engine known as the M156, which developed up to 386 kW (525 hp) and 630 newton metres. In the new C 63 AMG Coupé Black Series, this eight-cylinder engine delivered 380 kW (517 hp). From 2005 to the present day, more than 71,000 M156 engines have been produced – a record in the history of AMG. Exclusively reserved for the SLS AMG, the M159 likewise has a displacement of 6.3 litres and delivers a peak output of 420 kW (571 hp), with a maximum torque of 650 newton metres.
Closely related to the new M152, the AMG 5.5-litre V8 biturbo engine delivers between 386 kW (525 hp) and 420 kW (571 hp) depending on the model, and a maximum torque of between 700 and 900 newton metres. The M157 is regarded as the most efficient engine in its output and displacement class. The combined NEDC fuel consumption of the E 63 AMG is 9.8 litres per 100 kilometres.
The supercharged AMG 5.5-litre V8, the AMG 6.3-litre V8 and the AMG 6.0-litre V12 biturbo were all able to win the Best Performance Engine category several times in the International Engine of the Year Awards.
Highly versatile: the AMG SPEEDSHIFT PLUS 7G-TRONIC
The perfect partner to the naturally aspirated AMG 5.5-litre V8 engine used in the SLK 55 AMG comes in the form of the AMG SPEEDSHIFT PLUS 7G‑TRONIC. This seven-speed automatic transmission has three transmission modes: Controlled Efficiency (C), Sport (S) and Manual (M), which can be selected by means of a button on the centre console.
In "C" mode, the Roadster generally moves off in second gear and makes use of the high torque already available at low engine speeds to shift up to the highest viable gear as quickly as possible. The resulting dethrottling effect on the intake side lowers fuel consumption. In modes "S" and "M", the engine and transmission take on a much more agile character, with higher engine speeds, more spontaneous gear changes and faster shift times. The automatic double-declutching function which is now active when downshifting as well as the brief, precisely defined interruption of ignition and injection when shifting up under full load shorten shifting times and increase emotional appeal when adopting a sporty driving style. The seven gears can be changed via the AMG shift paddles on the steering wheel. The upshift indicator integrated into the AMG instrument cluster informs the driver when a manual gearchange is advisable.
A double turbine torsional damper integrated into the transmission and fitted with centrifugal pendulum which adapts to engine speed helps to minimise vibrations in four-cylinder mode and enhances the impression of comfort experienced by the vehicle occupants.
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