AMG V8 uses cylinder deactivation for high efficiency
M152 V8 cuts fuel consumption and CO2 emissions by deactivating cylinders
AMG, the in-house tuning arm of Mercedes-Benz, has developed a 5.5-litre V8 engine with F1-inspired cylinder deactivation. Like an F1 engine, the new AMG M152 V8 can run on four cylinders to cut fuel consumption and CO2 emissions when power demand is low.
The M152 engine, which makes its debut in the SLK 55 AMG at the Frankfurt motor show in September, has 62PS more power than its predecessor, and 30Nm more torque - giving peak figures of 422PS and 540Nm. In the SLK, fuel consumption on the NEDC test is expected to be around 33mpg (8.5l/100km), up from 23mpg - though at higher speed and load conditions the new engine is unlikely to have such an advantage over the outgoing biturbo engine. CO2 emissions on the same test are expected to be 199g/km.
Development of the new engine began in 2008, a focus of early development being to optimise the cylinder head packaging, intake system and exhaust manifolds to accommodate the cylinder deactivation componentry. A cylinder head with clear plastic windows was used to develop the hydraulic deactivation system.
"Thanks to the innovative AMG Cylinder Management system and other high-tech components, our newly developed AMG M152 naturally aspirated eight-cylinder engine achieves the fuel economy of a four-cylinder with the muscular power delivery and emotional sound experience of a V8," said Friedrich Eichler, head of powertrain development at Mercedes-AMG GmbH.
M152: normally-aspirated 5.5-litre V8 based on M157 biturbo
M152 has a strong family resemblance to the 5.5-litre M157 V8 used in the E 63 AMG, CLS 63 AMG, S 63 AMG and CL 63 AMG. The basic dimensions - bore, stroke and cylinder spacing - are identical, and the two engines share stop/start systems and direct petrol injection technology.
Features exclusive to the M152 are:
- new intake air ducting
- new cylinder heads
- modified valve gear using continuously variable camshaft timing
- modified oil supply system
- redesigned aluminium crankcase with cylinder liners honed in situ
M152 uses a high compression ratio, 12.6:1 for good thermal efficiency. According to Mercedes natural aspiration and cylinder deactivation was adopted as it "resolves the conflict of aims between high output/torque delivery and decidedly low fuel consumption figures," though it's interesting to note this is exactly the opposite strategy to BMW - which is adopting turbocharged engines to large-capacity, normally-aspirated engines in its performance models.
AMG Cylinder Management: V8 runs on four cylinders at low speed and low load
The cylinder deactivation system fitted to M152 is called AMG Cylinder Management. When the driver selects a transmission mode called Controlled Efficiency, the engine automatically shuts down cylinders 2, 3, 5 and 8 between 800rpm and 3600rpm, which still leaves up to 230Nm available. A dashboard display shows when the system is operational.
Deactivation is carried out using 16 hydraulic actuators - Mercedes calls them "compensating elements" - which act as the pivot point for the finger followers which operate the valves. Normally these actuators (marked red in the picture above) are 'pumped up' with oil pressure, and the finger follower pivots on the end of the actuator to operate the valve. When oil pressure to the actuator is reduced the actuator provides no resistance to the motion of the follower, so the follower pivots about the end of the valve rather than opening it. The effect is similar to Fiat's MultiAir hydraulic variable lift and timing system used in the TwinAir engine.
The system automatically brings all eight cylinders back online by pressurising the hydraulic actuators for the valves, and restoring fuel and ignition (which are both cut off when the cylinder is deactivated) when revs rise or high power is required. At 3600rpm the switchover is said to take just 30ms (two engine revolutions), even though the changeover must occur when the hydraulic element for a given valve is not under load. Mercedes says the change is "imperceptible".
Lightweight construction and low-friction internals
The M152 has a dry weight of 187kg, impressive for a large-capacity V8. The low weight is largely due to the redesigned crankcase, an aluminium alloy diecasting, which has cast-in Silitec aluminium/silicon low-friction cylinder liners which are honed in-situ after the cylinder heads have been fitted, eliminating any distortion due to the head clamping loads.
The five grey cast iron main bearing caps are cross-bolted for rigidity. The forged steel (38MnS6BY) crankshaft has eight counterweights and a two-mass viscous vibration damper and carries forged, "cracked" connecting rods. The cast aluminium pistons have carbon-coated low-friction rings and a special skirt coating.
To improve efficiency M152 has an electrically controlled oil pump, a separate pump for the cylinder shut-off system, and electronically controlled fuel pressure which varies between 100bar and 200bar.
Bosch MED engine computer controls injection, oil supply and cylinder deactivation
All the engine functions are executed and controlled by a Bosch MED 17.7.3 control unit with more than 30,000 different stored parameters and functions, capable of performing up to 260 million individual operations per second.
To reduce the load on the engine control unit, each of the eight individual ignition coils has an integral electronic ignition amplifier at each cylinder which ensures a strong ignition spark at all engine speeds and under all load conditions. Eight high-voltage power stages are responsible for highly precise actuation of the piezo-electric injectors.