Fitting an automatic stop-start device to a car like the Mercedes S 63 AMG could be seen as tokenism or even dark humour. An S-class with any engine weighs over two tonnes, and the next generation of S 63 AMG, waiting in the wings as we speak, is powered by a 5.5-litre V8 petrol engine with two turbochargers. Nevertheless, a few per cent. of a lot of carbon is still quite a bit of carbon, so perhaps we shouldn’t be too cynical.
The aim behind the AMG S-class cars is to synthesise luxury and comfort with the performance and responses of a sports car. This is very difficult. For example, a large, heavy car with a substantial polar moment and a high centre of gravity does not and cannot behave like a small sportscar in a bend. The tendency to roll under cornering forces will be greater in a large saloon car than in a sportscar because of the different weight distribution, and body roll compromises handling irrespective of what kind of wheel location the designer chooses. So making a big car handle well calls for a substantial anti-roll force, which is very hard to deploy without damaging the ride comfort which is presumably the luxury car’s raison d’être.
Mercedes S 63 AMG.
Also, a large polar moment means that a great deal of force is required — first — to change the car’s direction, and then once more to straighten its course again. So very wide tyres have to be fitted, and they must have exceptionally low sidewalls if the car’s handling is not to feel soft. Likewise the bushes used to mount the suspension
must be very hard if they are to withstand high cornering loads without unwanted deflection. Wide tyres with low sidewalls and hard bushes exacerbate road noise transmission to the cabin. If you pay £100,000 for a luxury car, you could be forgiven for not expecting it to give you a headache.
The same problems apply when trying to make a large car’s powertrain feel responsive. Any fool can build a fast large car, but its responses to quickly-changing demands for power will be markedly different from those of a smaller car with a similar power-to-weight ratio.
The heavier car requires a much bigger proportionate increase in torque than the lighter car to change from steady speed driving to a given rate of acceleration. That’s because the torque required to maintain a steady speed is much less dependent upon weight than the torque needed to achieve a given rate of acceleration.
Another way of approaching the same question is to look at how the mass of a car affects fuel economy in town — with frequent acceleration and deceleration — and at a cruising speed. A vehicle’s mass has much less influence on fuel economy at steady speeds than it does under stop-start conditions.
So whatever engine you fit to the big, heavy car must be able to deliver more than outright maximum power or torque. Its combustion régime must be capable of increasing torque output from a low level to a much higher level very quickly while maintaining acceptable emissions.
So there is a lot that’s of interest about the most rarefied of S-class models, not least in how (or whether) AMG has achieved a balance of performance and refinement that justifies their cars’ high prices.
These pages are dedicated to the new drivetrain of the S 63 AMG, due to go on sale in September.
Known internally as the M157, AMG’s latest power-unit is a 5461cc directly-fuelled V8 petrol engine with a turbocharger integrated in the exhaust manifold of each cylinder bank. Charge air cooling is by way of an air-to-water intercooler mounted between the cylinder heads. The crankcase is die-cast from aluminium and has cast-in liners of silitec alloy. The crankshaft has five main bearings and eight counterweights. A two-mass viscous damper is mounted at the front of the engine to reduce vibration.
The forged connecting rods are mounted on the crankshaft journals by cracking and reassembly. The undersides of the piston crowns are cooled by oil-sprays.
Internal drilling of the block casting reduces the losses caused by air displacement around the block cavity and sump when the pistons are moving. The drilled ‘pulsation holes’, above the bearing blocks, connect adjacent crankcase cavities and allow some degree of pressure compensation between the individual cylinder cavities.
Fuel injection is by means of a common-rail system with piezo-electric injectors, in which the electrically-induced deformation of a small crystal controls the injection of fuel into the cylinder. An electric pump in the fuel tank delivers fuel at six bar to a high-pressure pump, which maintains a fuel rail pressure of between 100 and 200 bar; the pressure is varied according to the demand for fuel. Lubricant pressure also responds to demand, with the electric pump operating between two and four bar.
This engine has two oil coolers, working in different ways. When the engine temperature is low, the lubricant passes through a small internal oil-to-water heat exchanger which helps to distribute heat around the engine more evenly during warm-up. Once the engine reaches its normal operating temperature, a thermostat directs the oil to an external oil-to-air radiator.
To feed exhaust gas heat to the catalytic converters as effectively as possible, the V8’s exhaust manifolds are double-walled with an air gap to act as insulation.
Tandem catalytic converters are mounted on each side of the engine bay, with two thin-walled substrates in each of the two cat housings. Of each pair, the first uses a palladium catalyst, while the one behind it uses a palladium-rhodium coating.
There are two lambda sensors, one for each cylinder bank, fitted in front of each catalytic converter housing. A lambda diagnostic sensor is mounted between the two substrates in each of the two catalytic converter housings.
Both inlet and exhaust camshafts are continually adjusted according to engine load and crankshaft speed within a range of 40 degrees. The adjustment is carried out electromagnetically by means of a pivoting actuators at the end of each camshaft. The exhaust valves are hollow and cooled by sodium.
Mercedes S 63 AMG valvegear operation: both inlet and exhaust camshafts are adjusted constantly within a range of 40 degrees.
Standard outputs, with 1.0 bar of boost from the blowers, are 544PS and 800Nm. Of the 800Nm, 670Nm is available at 1500rpm. If this isn’t enough, you can opt for the ‘performance package’, which increases the boost pressure to 1.3 bar and the engine outputs to 570PS and 900Nm. A mechanical compression ratio of 10.0:1 is common to both variants.
At full load, the turbochargers operate at 185,000rpm, with a charging rate of 1750kg of air per hour. Turbo-lag has been reduced by dispensing with a dump-valve on the wastegate, a feature which the Company apparently believes to be unique. In fact, diesel engines have been built without any form of wastegate for 15 years.
The maximum speeds of both versions are electronically limited, as you might expect, but while the standard car is prevented from exceeding 155mph, the ‘performance’ version is allowed to run on to a laughable 186mph. We have yet to find an Autobahn with sight-lines that will support this sort of motoring, but perhaps that’s not the point.
S 63 AMG
* With AMG performance package
(1) Electronically limited
Sadly, fuel consumption is unlikely to matter to most potential AMG owners, and if you look askance at any car that can’t deliver 50mpg then you’re unlikely to approve of the new S 63. But the new car’s figures are, on paper at least, little short of astonishing. A vast, and vastly powerful, petrol-engined car has no business turning in a result of nearly 27mpg on the European test cycle. Its predecessor certainly couldn’t. While we don’t have the specific fuel consumption (SFC) figures to test AMG’s claim that the biturbo unit is more efficient that some diesels, there is no doubt that the new engine’s use of fuel is quite an achievement.
AMG 5.5-litre biturbo engine. The cutaway pipes are for engine intake air: the larger, outer pipe takes freshly-filtered air at atmospheric pressure to the turbocharger, while the inner pipe carries hot, compressed air to the rear of the engine, around a 180° corner and into the centrally-mounted air-to-water intercooler. Image: Daimler-AMG.