Mercedes-Benz performance division AMG has developed a new, naturally aspirated, 5.5-litre V8 engine for which it is making some impressive fuel-efficiency claims, thanks to what it calls cylinder management.
The system, a first for AMG, shuts down cylinders two, three, five and eight automatically under partial-load conditions, reducing overall fuel consumption by 30 percent, says AMG, while still providing big-inch V8 power and torque when needed, just a press of the loud pedal away.
The new engine has been specially developed for the new SLK 55 AMG roadster, which will make its world debut at the Frankfurt motor show in September - making it the only roadster in its class that still has a V8 engine.
Even with power up 45kW to 310kW and peak torque boosted from 510 to 540Nm compared to the previous SLK 55, average fuel consumption is quoted at 8.5 litres/100km, 3.5 litres/100km less than before, with CO2 emissions down to a provisional 199g/km.
The new engine is based on the 5.5-litre V8 twin-turbo engine, introduced in 2010 in the E63, CLS63, S63 and CL63 AMG models. It shares the same bore and stroke, cylinder spacing, stop-start system and direct fuel-injection, but with a new, much lighter crankcase, revised intake air ducting, new cylinder heads, a revised oil-supply system and of course, the new cylinder management valvegear.
And this is how it works:
When the engine is running under partial load with the transmission in the "C" economy mode, anywhere between 800 and 3600rpm, a complex system of 16 hydraulic elements keeps the intake valves of cylinders two, three, five and eight shut, while the electronic engine management programme shuts down their ignition and fuel-injection.
That leaves the engine running as a V4 on cylinders one, four, six and seven. AMG says it still produces 230Nm in four-cylinder mode, enough for most round-town driving situations, with fult tilt boogie available almost seamlessly in about 30 milliseconds.
The crankcase of the new engine is diecast aluminium, with cast-in Silitec liners and grey cast-iron bearing caps that are bolted to the crankcase, as well as on the sides, for extra rigidity.
The piston rings are carbon-coated, as are the piston skirts, to minimise internal friction, and there are special ventilation holes in the crankcase to minimise pumping losses. The main oil pump has an electrically controlled pressure stage and there's a separate, secondary oil pump for cylinder shut-off.
The forged, alloy-steel crankshaft runs in five main bearings and has eight counterweights as well as a two-mass viscous damper at the front to reduce vibration. Each journal carries two forged, cracked connecting rods.
Four overhead camshafts operate the 32 valves via low-friction cam followers. Valve timing on both inlet and exhaust cams is infinitely variable through 40 degrees.
An engine stop/start function is standard on the new SLK 55 AMG and is always engaged when the transmission is in "C" mode. As soon as the car stops - at a red light or in traffic, for instance - the engine is automatically switched off, and a crankshaft sensor reads the positions of all eight pistons.
Then, when the driver touches either the brake or accelerator pedal, it sprays some extra fuel into whichever cylinder is closest to TDC (after TDC, of course, not before!) and fires that cylinder, which is enough to turn the engine over twice and get it running in full eight-cylinder mode without using the starter motor.
The stop-start function will only work when the when the battery has enough charge to start the engine (just in case!), and the engine is up to optimum temperature. It also won't stop the engine if the inside of the car hasn't yet reached the selected temperature and the aircon compressor is still running under full load.
Whenever the engine is on the overrun or under braking, kinetic energy is used to charge the battery rather than being wasted as heat in the usual way, to keep the load on the alternator down when the engine is running hard; AMG claims an average fuel saving of about 0.15 litres/100km - more in city traffic - thanks to energy regeneration alone.
This is also the first AMG engine with a sports exhaust system that has an exhaust flap on each side. At low loads and engine speeds below 2000rpm the flaps stay closed, sending the exhaust gases through an additional damping element, so that the engine sound is pleasantly muted.
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 go straight through for a deeper, more authoritative engine note - and at full taps they open all the way to give the engine a serious V8 bark that would not earn you any brownie points coming home late at night.
Like all AMG engines, each new 5.5 V8 is assembled by hand in the AMG engine shop by a single technician, and bears his signature on the engine plate.
SPECIFICATIONS
Displacement: 5461cc.
Bore x stroke: 98.0 x 90.5mm.
Compression ratio: 12.6:1.
Power: 310kW at 6800rpm.
Torque: 540Nm at 4500rpm.
Engine weight (dry): 187kg.
Power/weight ratio: 1.66kW/kg.