Mercedes-Benz has announced details of the third hybrid model in its new S-Class line-up - the S500 Plug-In, which will premiere at the Frankfurt motor show in September ahead of its market launch in 2014.
Combining a new turbocharged three-litre petrol V6 with a heavy-duty 80kW electric motor and an externally rechargeable battery delivers about 30km of zero-emission pure electric driving, and a claimed overall average fuel-consumption of three litres per 100km with CO2 emissions of 69g/km.
A new high-voltage lithium-ion battery provides 10 times the energy storage of the batteries in the more conventional S300 and S400 hybrid models, with the option of being charged from an external source through a socket on the right side of the rear bumper.
Four operating modes can be selected at the push of a button:
HYBRID: The S500 operates as a conventional hybrid vehicle.
E-MODE: Electric power only.
E-SAVE: The S500 drives on petrol power only, keeping the fully charged battery in reserve for later pure-electric operation.
CHARGE: The S500 drives on petrol power only while charging the battery.
NUTS AND BOLTS
The three-litre turbo V6 is rated at 245kW and 480Nm, the electric motor at 80kW and 340Nm; the combined figures will be a little lower than the sum of those numbers since the petrol engine produces peak power at high revs and the electric motor does its best at very low revs.
Together, however, they launch the big E-class limo from 0-100km/h in 5.5 seconds and on to 250km/h flat out.
It's when you slow down, however, that the S500 Plug-In Hybrid gets really clever. What Stuttgart calls an anticipatory energy management system uses satnav data from the next eight kilometres of the planned route, as well as real-time conditions and driver input, to adjust for inclines, downhill stretches, bends and speed limits, managing energy flow to improve efficiency.
For instance, it will use battery power to maintain speed up a hill, rather than more petrol - as long as there is a down-slope on the other side of the crest that will replenish that power!
RECUPERATIVE BRAKING
This S500 uses brake-by-wire technology to convert as much momentum as possible into amps rather than waste it as heat.
When you press the brake pedal, a pedal-travel sensor measures how much braking you're asking for, and uses the electric motor as an alternator, switching in more coils as required to generate more electricity, at the same time applying increasing drag to the drivetrain and slowing the car down.
When that's no longer enough, the system will seamlessly bring in the disc brakes as well - although it will always use the electric motor to recover as much energy as possible.
When you're coasting on a down-slope, however, without putting your foot on the brake pedal, the system switch off the V6 and use the drag torque of the motor to charge the battery while preventing the car from 'running away' downhill - but, on the other hand, it won't slow you down until you press the brake pedal.
The aim is, as far as possible, to use 'recovered energy' rather than petrol to keep the battery charged.
MBSA says it is considering releasing the S500 Plug-In Hybrid in South Africa, depending on when the infrastructure to support it is in place.