Automotive Intelligence - the web for automotive professionals and car enthusiasts
August 26, 2011
The high-tech M152 powerpack 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 rpm. 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.
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 e.g. 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 199 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 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.
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 7200 rpm.
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 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.
The optionally available AMG Handling package enhances the vehicle's sportiness even further. It comprises an 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.
(August 23, 2011)