Mercedes-Benz is set to introduce a new range of diesel plug-in hybrid models, which are being presented in pre-production form at the Geneva Motor Show. Based on the C- and E-Class, these PHEVs will combine the brand’s OM654 four-cylinder turbodiesel engine and third-generation hybrid drive system.

Looking at the individual aspects of the powertrain, the OM654 was first introduced in February 2016 as a replacement for the aging OM651 that was used in non-plug-in hybrid models like the W212 E 300 BlueTEC Hybrid and W222 S 300 BlueTEC Hybrid. Developed with hybridisation in mind, the OM654 offers more output, better efficiency, and thanks to its aluminium construction, is lighter than the engine it replaces.

As for the hybrid drive system, the third-generation unit (P3) is based on the 9G-Tronic automatic transmission, where the torque converter, clutch and electric motor are housed in a single unit. This differs from the first-gen (P1) and second-gen (P2) systems, where in the former, the electric motor was installed directly on the combustion engine like on the W221 S 400 Hybrid.

Meanwhile, the P2 system saw the electric motor being fitted at the transmission input with a clutch in between. This was used in the petrol-powered W222 S 400 h as well as the diesel-powered W222 S 300 BlueTEC Hybrid. In its latest P3 form, the separating clutch, torsional vibration damper and torque converter lockup clutch are integrated and connected within the rotor of the electric motor.

The electric motor itself was developed together with Bosch specifically for the 9G-Tronic transmission, and is accompanied by new power electronics. Mercedes-Benz quotes an output of 90 kW (120 hp) and 440 Nm, which is significantly more than the unit paired with the OM651 in the W212 E 300 BlueTEC Hybrid.

In electric-only mode, the top speed has been raised from 130 km/h to 140 km/h – 10 km/h more than what the W213 E 350 e can muster. There’s also a mention of a high-voltage electrical system, which is linked to the car’s powertrain as well as the climate control systems.

The company won’t disclose the exact outputs of both the C- and E-Class diesel PHEVs, but did state in its release, “in addition to a superlative driving experience with a total torque of 700 Nm, available from as low as 1,400 rpm, the new plug-in diesel hybrids will offer added comfort.” It is likely the petrol plug-in hybrid W205 C 350 e and W213 E 350 e will benefit from the new tech mentioned here, although it is yet unknown when such revisions will be implemented.

However, we are told an electric range of around 50 km (NEDC) is being targeted – the same as the W222 S 560 e. This is thanks to a higher-capacity 13.5 kWh lithium-ion battery, where a shift in cell chemistry from lithium-iron-phosphate (LiFePo) to lithium-nickel-manganese-cobalt (Li-NMC) allows cell capacity to be increased from 22 to 37 Ah. The increased capacity is more than what was found in the W222 S 500 Plug-in Hybrid (8.7 kWh), but comes at no increase to overall battery size.

Moreover, the previous 3.6 kW on-board charger has also been replaced with a 7.2 kW unit, which can fully recharge a battery in two hours when plugged into the automaker’s next-gen wallbox (up to 22 kW). On a conventional domestic power socket, the charging time is higher at around seven hours.

On the software front, the new PHEV models will come equipped with ECO Assist, which provides the driver with assistance in terms of predictive driving and fuel-saving. The feature is already present in the latest W222 S 560 e, and takes in account the route profile, speed limits and distance from vehicles ahead to concoct an efficient driving strategy. There’s also a number of operating modes for the plug-in hybrid powertrain, including Hybrid, E-Mode, E-Save and Charge.