Twin turbos, stop/start boost AMG's new Mercedes V8

  • 10-Jan-2011 10:00 EST
M157 AMG 5.5L V8.jpg

The new M157 5.5-L twin-turbo V8, with AMG's "Speedshift" 7-speed semi-automatic transmission, uses a wet start-up clutch in place of a conventional torque converter. The V8's induction system is a lesson in packaging. Note the intricate webbing on the sophisticated, reinforced-plastic intake manifold. AMG-Mercedes models powered by the new engine carry the "63" designation used previously with the 6.2-L V8.

Fuel efficiency regulatory pressures on luxury and performance brands may eventually see a reduction in performance from previous models. For the near-term, however, Mercedes-Benz’s AMG performance subsidiary is still able to boost performance even while cutting engine displacement and improving fuel economy.

“The performance category will have great improvements in fuel economy,” pledged Ola Kaellenius, general manager of Mercedes-AMG GmbH.

While AMG’s previous 6.3-L normally aspirated engine was designed and built in-house by the tuning shop, the new 5.5-L direct-injected twin-turbo V8 is based on a mass-produced Mercedes engine, with AMG-specific upgrades.

One appealing aspect of forced-induction engines is that their output can be varied by changing boost pressure. In the case of the AMG V8, it means that there are two sets of performance figures for the engine: the base specifications and those of the optional AMG Performance Pack. (Clearly any previous impressions that the AMG name itself promised maximum performance without any supplemental upgrade packages were mistaken).

The new engine produces 563 or 536 hp (420 and 400 kW, respectively) and 664 or 590 lb·ft at boost levels of 15.6 or 14.2 psi. These figures compare with 382 hp and 391 lb·ft for the base Mercedes-Benz 5.5-L V8. They also compare with 518 hp and 465 lb·ft for the previous normally aspirated 6.3-L AMG V8.

The new engine debuted in the 2011 S-Class sedan and CL coupe models matched to a 7-speed automatic transmission. The V8 is equipped with stop/start technology for maximum fuel efficiency. It does this by selecting the cylinder whose piston is in best position (related to TDC) for a restart and activating that cylinder's fuel injection and multispark ignition system to fire the engine almost instantly.

The electric starter motor assists with the restart. Mercedes engineers expect the technology to mature to where it will be able to perform stop/start without the use of the conventional starter.

Fuel economy for both the standard version and the performance pack is estimated at 22.4 mpg in combined driving on the New European Driving Cycle, a 25% improvement compared to the 6.3-L engine. Acceleration with the performance pack improves only fractionally over the base engine—from 4.4 to 4.3 s on a 0-60-mph (0-96.5 km/h) test, though the electronic speed limit is raised from 155 to 186 mph (249 to 299 km/h).

Sophisticated fuel, lubrication systems

The new V8 exhibits miserly control over every drop of fuel. Piezo-electric injectors spray fuel into the cylinder beginning on the downstroke, with two more squirts as the piston rises to create a stratified fuel mixture. A rapid multispark ignition system can fire four times per millisecond; Mercedes engineers say this creates a gas plasma with greater expansion than conventional ignition.

Once the mixture is ignited, the direct injection can provide up to two more injections to stabilize combustion as needed. Mercedes engineers claim a 4% savings in fuel efficiency, with the ignition and injection systems contributing equally to the total savings. Both systems are managed by a Bosch MED 17.3.3 computer which stores more than 30,000 engine parameters and performs 260 million operations per second.

An electric low-pressure pump delivers fuel from the tank to a high-pressure pump (87 psi/6.0 bar). Pressure in the fuel rail is controlled between 1500 and 3000 psi (103 and 207 bar) on a fully variable and demand-related basis.

Likewise, the oil pumps vary pressure to minimize wasted energy. At low engine speed and load the pump produces about 29 psi (2 bar) of pressure and the piston cooling jets are deactivated. As revs rise, pressure climbs as high as 48 psi (3.3 bar) and the cooling-jet nozzles open. The pump's scavenge side draws excess oil from the turbochargers to keep oil out of the intake and exhaust passages, to minimize emissions.

Oil circulates through a heat exchanger that draws heat from the coolant when the engine is cold for faster warm-up. At operating temperature a thermostat sends oil to an external oil-to-air cooler.

The cooling system varies its performance similarly. After a cold start, no coolant circulates. As the engine warms up, coolant circulates just through the engine. When it reaches 221° F (or 189° F under heavy load), coolant begins circulating to the radiator. Coolant circulation through the cabin heater is handled separately.

The Honeywell-Garrett turbochargers are welded to the exhaust manifolds for maximum heat transfer and minimum package space. In a unique detail, the turbo system forgoes the usual pop-off valve. Instead, the wastegate is vacuum controlled, allowing the engine to be dethrottled under partial load.

At its foundation, the cylinder block is die-cast aluminum, with a cast-iron bedplate that bolts the main bearing caps to the block.

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