20/9/11

Overview: Mercedes-Benz SLK 55 AMG

Topping the third-generation SLK series launched last March, the new SLK 55 AMG is not surprisingly the most powerful SLK yet built. Its new directly-fuelled 5.5-litre atmospheric V8 engine features cylinder shut-off to cut fuel consumption and offers 422PS and 540Nm. Its NEDC fuel consumption of 8.4l/100km (33.6mpg) overall, and CO2 emissions of 195g/km are around 30 per cent. lower than those of its predecessor.

AMG’s sports suspension complements the substantial engine output: the Copny’s Direct-Steer system, torque vectoring brakes and three-stage ESP are deployed, as well as the expected AMG braking system.

The new atmospheric V8 — known as M152 — is a close relative of the M157 biturbo used in the E 63 AMG, CLS 63 AMG, S 63 AMG and CL 63 AMG. Numerous design features are identical, such as the displacement, bore and stroke dimensions, cylinder offset, stop-start system and the direct injection technology. Distinguishing features over the M157 include new intake air ducting, new cylinder heads, modified valve drive, an adapted oil supply system and a different crankcase. The engine’s dry mass is a moderate 187kg.

The M152 power-unit packs in a fair few notable features. Direct injection by way of piezo injectors is at a pressure of 20MPa; combustion is spray-guided, and cylinder shut-off is used to cut fuel consumption. This last feature has a chequered history stretching back at least 30 years, but it’s fair to say that it has now reached maturity, as Volkswagen is introducing the same feature on some of its engines.

The V8’s crank-case is die-cast from aluminium and features ‘spectacle’ honing. Liners are of Silitec. Each cylinder has four valves, and both camshafts have continuous timing adjustment. The compression ratio is high, even for a directly-fuelled engine, at 12.6:1. A stop-start system is standard, as is ‘generator management’ — the alternator charges only on the overrun where possible.

AMG Cylinder Management

AMG’s Cylinder Management cylinder shut-off system cuts off cylinders two, three, five and eight under partial load. In similar form, this technology is also used in the (roughly) 750PS V8 engines used in Formula 1. Clearly, efficiency also plays a major role in motor racing; here, two or four of the eight cylinders are cut off when high outputs are not required — for example, when cornering at low speed, in the Safety Car phases, or for pit stops.

The cylinder shut-off function is available over a wide range of crankshaft speeds: from 800rpm to 3600rpm 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. Some 230Nm is available in four-cylinder mode. As soon as the driver leaves the partial load range, cylinders two, three, five and eight are activated. The switch from four- to eight-cylinder operation is immediate. At an engine speed of 3600rpm, the activation process takes 30ms.

Valves closed, fuel delivery and ignition deactivated

This is made possible by 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 7200 rpm.

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 reduce wear. The Silitec cylinder liners are cast-in.

Spectacle honing, which we mentioned previously, is another measure to reduce friction. 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 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.

Measures to improve engine efficiency include

  • lightweight 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-dependent between 10MPa and 20MPa.

The forged crankshaft of 38MnS6BY steel alloy rotates in five main bearings and has eight counterweights. It was designed to have low rotating masses. A two-mass viscous damper mounted at the front eliminates vibrations. Each connecting rod journal on the crankshaft carries two forged, cracked connecting rods.

The exhaust valves, which are subject to high thermal loads, are hollow and sodium-cooled. The infinitely variable camshaft adjustment operates within a range of 40 degrees on the intake and exhaust sides. Depending on the engine speed, valve overlap can be varied. The variable camshaft adjustment is carried out hydraulically by way of four pivoting actuators. These are electromagnetically actuated and controlled by the engine control unit. The camshafts are driven by three high-performance ‘silent’ chains — quieter than cylinder roller chains.

The stop-start system, like the ylinder cut-off function, is active in the economy-orientated trans­mission mode ‘C’. The engine restarts when the brake pedal is released or the accelerator is depressed. A crankshaft sensor 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 M152 always starts in eight-cylinder operation.

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 working, the driver is informed by the symbol ‘ECO4’, while ‘ECO8’ stands for eight-cylinder operation. (The ‘Eco’ here seems a little perverse.) 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.

We have mentioned that alternator management is used. Where possible, the battery is charged only on the overrun; when the engine is under load, the generator is kept at a low voltage. This apparently saves around 0.15 litres per 100 kilometres over the NEDC cycle, and up to 0.2l/100km in city traffic with its frequent overrun and braking phases.

For the first time AMG is using an exhaust system that uses 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.

Each of the two rear silencers has a flap which is controlled according to engine speed and load. At low loads and engine speeds below 2000rpm, 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. Reading between the lines of AMG’s press release, it is clear that four-cylinder operation was a particular concern so far as exhaust sound was concerned: it just isn’t impressive enough without some sonic massaging.

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 a 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 a decidedly muscular sound.

Warm-up of the catalytic converters (there is one on each side) is helped by twin-wall exhaust manifolds. 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.

Drive torque is tramsmitted to the rear wheels by means of an AMG Speedshift Plus 7G-Tronic automatic transmission, which features three operating modes: Controlled Efficiency (C), Sport (S) and Manual (M). The automatic double-declutching function when downshifting as well as the brief, precisely defined interruption of ignition and injection when upshifting under full load shorten shifting times. A double turbine torsional damper with centrifugal pendulum which adapts to engine speed helps to minimise vibrations in four-cylinder operation.

AMG’ suspension package for the SLK includes torque strut bearings on the front axle with higher rigidity, stiffer anti-roll bars all round as well as a more negative camber on the rear axle.

Another feature is torque vectoring brakes. This system produces a defined rotational movement of the vehicle about the vertical axis in fractions of seconds, through selective brake actuation at the rear wheel on the inside of the bend. The AMG brake system has internally ventilated and perforated brake discs on all wheels, measuring 360mm x 36mm at the front and 330mm x 22mm at the rear.

AMG’s three-stage ESP gives a choice of control strategies: the ESP button in the centre console allows the driver to choose between ‘ESP on’, ‘Sport Handling’ mode and ‘ESP off’.

The optional AMG Handling Package comprises AMG performance suspension with stiffer tuning, an AMG rear axle differential lock, composite brake discs at the front and a three-spoke AMG performance steering wheel finished in nappa leather with Alcantara inserts in the grip areas.

Panoramic vario-roof with Magic Sky Control

The standard vario-roof opens and closes in less than 20 seconds, transforming the open-top roadster into a closed-top coupé with rigid roof. Alternatively, you can opt for a panoramic vario-roof in tinted polycarbonate. The third option is the panoramic vario-roof with ‘magic sky control’. This glass roof switches to light or dark as required at the press of a button. When light it is virtually transparent, offering an open-air experience even in cold weather. In its dark state the roof provides welcome shade and prevents the interior from heating up when it’s sunny.

The frames of the three roof variants are made of magnesium. This makes each roof around six kilos lighter than in the previous model, giving the car a lower centre of gravity.

An alternative to the standard draught-stop is the new Airguide pivoting draught-stop. It consists of pivoting transparent plastic layers which are attached to the reverse of the roll-over bars. This allows different comfort (or perhaps discomfort) levels to be selected for two occupants, as the driver and passenger can individually pivot the units to the centre of the vehicle, controlling turbulent air flow from the rear. The advantage over a conventional set-up is that no installation or removal effort is required.

As you would imagine, the SLK 55 AMG has a good collection of driver-assistance and safety systems, though not the exhaustive list of equipment we would expect at these prices and at this performance level. Not so much a ‘system’ as a permanent feature is the fibre-reinforced roll-over bar; the seat belt tensioner have degressive belt force limiters, there are Neck-Pro active head restraints, automatic child seat recognition, the three-stage ESP we mentioned earlier, and Attention Assist drowsiness detection. Other assistance systems available as optional extras include

  • Distronic plus: this proximity control system automatically maintains a set distance from the vehicle ahead, braking the SLK to a standstill if necessary and also accelerating the car again up to its pre-set cruising speed when it is safe to do so.
  • The Intelligent Light System (ILS) provides five lighting functions: cornering light, country mode, motorway mode, active light function and enhanced fog lamps. The respective modes are activated depending on the driving conditions.
  • Speed Limit Assist is able to detect speed limit signs using a camera on the inside of the windscreen, and to indicate the detected speed limit in the instrument cluster or central display.
  • The anticipatory and reversible Pre-safe occupant protection system is able to activate protective measures for the car’s passengers as a precaution. The aim is to prepare the occupants and the vehicle for an imminent collision so that the seat belts and airbags are able to fulfil their protective function to best advantage during an impact. The Pre-safe Brake detects the acute danger of a rear-end collision and prepares for autonomous braking. The system is active at speeds of between 30km/h and 200km/h when moving vehicles are detected ahead.
Mercedes-Benz SLK 55 AMG
Cylinders 8V
Block/head Al/Al
Valves 4
VVT IE
Aspiration A
Injection pressure 20MPa
Bore/stroke 98.0/90.5
Swept volume 5461cc
Compression
ratio
12.6:1
PS/rpm 422/6800
Nm/rpm 540/4500
Maximum speed 250*
0-100km/h 4.6
Combined MPG
(l/100km)
33.6
(8.4)
CO2 g/km 195
Emissions EU5
U.S. LEV III
Transmission A7
Driven wheels Rear
* Electronically limited.
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