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27-07-2011 at 07:00: Fun With Eight, Economy With Four

Mercedes-AMG, AMG being Mercedes-Benz performance branch, has developed a new 5.5-litre V8 engine which combines powerful eight-cylinder performance with the decidedly economical fuel consumption of a four-cylinder.

The efficiency is achieved by the AMG Cylinder Management cylinder shut-off system newly developed by Mercedes-AMG. This new technology exclusively developed by AMG is inspired by Formula 1 racing. In the eight-cylinder racing engines that develop around 750 hp, the cylinder shut-off is likewise used primarily to save fuel.

Thanks to a fuel saving of around 30 percent compared to the existing AMG 5.5-litre V8 engine, the naturally aspirated, high-revving eight-cylinder demonstrates the great innovative strength of AMG. At the same time it is AMG’s demonstration of the potential still residing in the naturally aspirated eight-cylinder engine.

The distinctive AMG 5.5-litre V8 engine with AMG Cylinder Management and direct petrol injection will find exclusive use in the new SLK 55 AMG, which will celebrate its world premiere at the International Motor Show (IAA) in Frankfurt/Main in September. This means that the latest version of the SLK 55 AMG will remain the only roadster in its segment with an eight-cylinder engine.

The peak output has been increased by 45 kW to 310 kW and torque increased from 510 to 540 newton metres. Fuel consumption figures of 8.5 litres per 100 kilometres (NEDC combined, provisional figure) is 3.5 litres less than that of the preceding model. This figure is around 30 percent better which places the new SLK 55 AMG among the top performers in its segment. CO2 emissions have likewise been considerably reduced, as witness the figure of 199 grams per kilometre (NEDC combined, provisional figure).

“Top performer for fuel economy and maximum output: with the new 310 kW AMG 5.5-litre V8 engine we have once again created a real masterpiece,” said Ola Källenius, Chairman of Mercedes-AMG GmbH.

“The M152 sets an outstanding example for efficiency, while demonstrating the creativity and expertise of our engineers, who took their inspiration from Formula 1 when developing the cylinder shut-off system. This makes the new, naturally aspirated eight-cylinder the perfect power unit for the new SLK 55 AMG, which will celebrate its premiere at the International Motor Show in Frankfurt/Main in the autumn.”

*provisional figures
Key data* at a glance:

AMG M152 5.5-litre V8 engine Displacement 5461 cc Bore x stroke 98.0 x 90.5 mm Compression ratio 12.6 : 1 Output 310 kW at 6800 rpm Max. torque 540 Nm at 4500 rpm Engine weight (dry) 187 kg

The high-tech M152 powerpack not only enthrals with its extraordinary power delivery and torque, but also thanks to 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 technology with continuous camshaft adjustment, a high compression ratio of 12.6 : 1, a stop/start system and generator management – while revving to a maximum of over 7000 rpm.

All these design features lead to high thermodynamic efficiency, which is in turn the 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 decidedly low fuel consumption figures.

The AMG Cylinder Management cylinder shut-off system in particular, is a completely newly system developed by Mercedes-AMG, which significantly improves efficiency. Cylinders two, three, five and eight are cut off under partial load, which considerably lowers fuel consumption.

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 e.g. when cornering at slow speed, in the Safety Car phases or for pit stops.

With a fuel consumption of 8.5 litres per 100 km and CO2 emissions of 199 grams per kilometre (NEDC combined, provisional figures), 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 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 Newton metres 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. At an engine speed of 3600 rpm the activation process takes no more than 30 milliseconds.

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 selectable 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 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 crankcase of the new AMG 5.5-litre V8 engine is of 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 forged crankshaft of high-grade 38MnS6BY 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.

The Controlled Efficiency stop/start function further contributes to the low fuel consumption. This system is standard equipment in the new SLK 55 AMG, and permanently 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.

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 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 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 “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.

Development of the new, naturally aspirated AMG M152 5.5-litre V8 engine commenced in 2008. On the basis of the M157 eight-cylinder biturbo unit, the first drafts were calculated, designed and verified by simulation. Various model and component tests were followed by numerous test series before a concept engine was realised. The focus was on the greatest possible system availability of the cylinder shut-off function, with the aim of achieving the maximum fuel saving. The spray-guided direct petrol injection exclusively developed by Mercedes-Benz Cars and also used in this new AMG V8 engine made a valuable contribution.

During the conceptual phase for the M152 the AMG engine specialists concerned themselves with fundamental packaging trials, especially concerning the placement of the selectable compensating elements for the cylinder shut-off function. The design of the intake and exhaust ducts was decisively influenced by the compensating elements, the aim being to make best possible use of the installation space in the cylinder heads to achieve the maximum output and torque yield. Configuration of the camshaft sections was also a central consideration. The challenge was not only to achieve the desired output and torque figures, but also to realise a durable, dynamic valve drive with the selectable compensating elements despite the high engine speeds of up to 7200 rpm.

In addition to extensive simulations of the valve drive system, tests were conducted on the cylinder head test bench to assess the dynamic properties of the valve drive before it was used in a finished internal combustion engine.

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