Set for launch at the end of 2011 in saloon and estate bodies, Mercedes’ E 300 hybrid could be the first diesel hybrid in production — if PSA and Volkswagen don’t get there first. The E-class is a series-parallel hybrid, able to run on diesel, electric power or both, and it features a lithium-ion battery pack — the first committed firmly to production.
The technical basis for the E 300 Bluetec Hybrid is the E 250 CDI with a four-cylinder diesel engine developing 204PS. The diesel engine works in partnership with a compact electric motor developing 20PS. The lithium-ion battery pack is familiar from the petrol-electric S 400 Hybrid announced last year. Total system outputs are 224PS and 580Nm.
Mercedes-Benz E 300 Bluetec Hybrid.
Overall fuel consumption in the European test cycle is 4.1l/100 km (NEDC) or 68.7mpg; CO2 output is 109g/km. That’s the same CO2 yield as a Skoda Fabia 1.4 TDI 80PS GreenLine, which runs with particularly long gearing in the interest of economy.
At low revs, the electric motor assists the diesel engine, giving a boost effect. This allows the diesel to run at lower engine speeds than would normally be needed to deliver the required torque. This benefits low-speed acceleration, because electric motors produce their maximum torque at low speeds, and also fuel economy.
Mercedes rather amusingly stresses that its hybrid drive system ‘outperforms current six-cylinder diesel engines in terms of output and driving enjoyment’, a comment we assume is intended as a dig at BMW.
On longer journeys or when running at higher speeds, the electric motor ceases to be a useful aid to the diesel engine, and the car’s control system adjusts the load point of the diesel engine to improve efficiency.
The diesel power unit can be decoupled from the drivetrain by means of a clutch between it and the electric motor. This allows the Mercedes to cover short distances at speeds of up to 25mph using electric drive alone, so the diesel engine doesn’t often need to operate in stop-start urban traffic or when manoeuvring.
The combustion engine is switched off as soon as the vehicle starts to coast down from 50mph. This driving state is referred to by Mercedes as ‘sailing’. As you would expect, a stop-start system is fitted,
re-starting the engine as soon as the driver releases the brake pedal, presses the accelerator or reaches the limit of the electric motor’s usefulness.
Energy recuperation starts as soon as the driver releases the accelerator pedal. The electric motor then functions as an alternator and converts some of the car’s ‘spare’ kinetic energy into electrical energy, which is then stored in the lithium-ion battery pack. The motor acts also as a starter motor, providing ‘silent’ starting of the diesel engine.
As has become the norm with hybrids, the air conditioning compressor — like the power steering — operates electrically, so it’s independent of the internal combustion engine. What is less common is that boot space is unaffected by the hybrid powertrain, as Mercedes has managed to squeeze all of the componentry into the front end of the car.