S 400 HYBRID: the world’s most efficient petrol-engined luxury saloon
The S 400 HYBRID is the first passenger car model with hybrid drive to be introduced by Mercedes-Benz. Its combination of a further developed V6 petrol engine and a compact hybrid module makes the S 400 HYBRID the world’s most efficient luxury saloon with a petrol engine. In the New European Driving Cycle, the combined fuel consumption is only 7.9 l/100 km (29.77 mpg/US, 12.65 km/l). This produces the world’s lowest CO2 emissions in this vehicle and output class -just 186 grams per kilometre.
These exemplary figures go hand in hand with outstanding performance potential: the 3.5-litre petrol engine develops 205 kW/279 hp, while the electric motor adds another 15 kW/20 hp and a starting torque of 160 Nm (118 lb-ft). The combined output is therefore 220 kW/299 hp, with a combined maximum torque of 385 Nm (284 lb-ft). Moreover, the new S 400 HYBRID is the first series-production model to be equipped with a particularly powerful lithium-ion battery specifically designed for automotive use. This is another pioneering step by Mercedes-Benz towards the electrification of the automobile.
The Mercedes-Benz S 400 HYBRID is based on the S 350, and features an extensively modified drive train. This encompasses a further development of the 3.5-litre V6 petrol engine, an additional magneto-electric motor, the 7G-TRONIC seven-speed automatic transmission specially configured for the hybrid module, the necessary operating and control electronics, the transformer and a high-voltage lithium-ion battery.
The compact hybrid module is a disc-shaped electric motor that also acts as a starter and generator. The system offers a double benefit, as it both helps to save fuel and increases driving enjoyment. This is partly due to the booster effect of the electric motor, as it powerfully backs up the petrol engine with additional torque of up to 160 Nm (118 lb-ft) during the high-consumption acceleration phase. The driver benefits from the combined action of these two units in the form of even more impressive torque characteristics and smooth, effortlessly superior acceleration.
The hybrid module also has a convenient ECO start/stop function, which switches the engine off when the vehicle is at a standstill – for example at traffic lights. When it is time to move off again, the electric motor almost imperceptibly restarts the main power unit. This makes a significant contribution to fuel economy and environmental protection: because the engine restarts first time, and practically instantly, emissions are also minimised during the starting phase.
When the vehicle is braked the electric motor acts as a generator, and is able to recover braking energy by a process of recuperation. Working in finely tuned partnership, the electric motor supplements the braking effect of the combustion engine and the wheel brakes to deliver a smoothly progressive braking action. The recuperated energy is stored in a compact yet highly efficient lithium-ion battery in the engine compartment, and made available when required. This complex system is managed by a high-performance control unit, which is likewise located in the engine compartment.
The centrepiece of the modular, very compact and highly efficient hybrid drive system is the new high-voltage lithium-ion battery, which was specially developed for automotive use and is the first such unit worldwide to be introduced in a series-production vehicle. In this way Mercedes-Benz is making a trailblazing contribution to the electrification of the car, with the S-Class once again playing the role of the technological trendsetter.
Major advantages over conventional nickel/metal hydride batteries include a higher energy density and better electrical efficiency, together with more compact dimensions and a lower weight. Thanks to space-saving installation in the engine compartment, where it replaces the conventional starter battery, the generous interior space and boot capacity remain unchanged. The lithium-ion battery not only stores energy for the electric motor and the electrical refrigerant compressor, but is also connected to the 12-Volt onboard network via the DC transformer to supply power to other standard consumers such as the headlamps and comfort features. The completely newly designed battery system consists of the cell block with its lithium-ion cells and the electronic cell monitoring system, the battery management function, the high-strength housing, the cooling gel, the cooling plate, the coolant feed and the high-voltage connection.
The 3.5-litre V6 petrol engine with variable valve control has been thoroughly re-engineered and improved. In the process the development engineers made use of the advantages offered by the Atkinson principle, where the expansion phase is longer than the compression phase. The intake valve is kept open slightly longer between the intake and compression phases, which improves the engine’s thermal efficiency while reducing the specific fuel consumption and untreated emissions. A new cylinder head, different pistons and a modified camshaft with different camshaft control increase the output by 5 kW/7 hp to 205 kW/279 hp – while reducing fuel consumption at the same time.
Especially on rural journeys and on motorways, the S 400 HYBRID achieves a further efficiency improvement by moving the operating point of the petrol engine to produce a lower specific fuel consumption. The extremely high start-off torque made possible by the boost effect of the electric motor gives the driver a particularly exhilarating feeling of powerful acceleration, while fuel consumption and emissions are reduced.
The compact, disc-shaped electric motor, which is space-savingly installed between the engine and the 7G-TRONIC 7-speed automatic transmission, improves efficiency even further. This is a 3-phase AC external rotor magneto motor, which develops a peak output of 15 kW/20 hp and a starting torque of up to 160 Nm (118 lb-ft) with an operating voltage of 120 Volts. This compact motor also acts as a starter and generator, adopting the functions of both these conventional ancillary units.
Sophisticated interaction with the internal combustion engine makes numerous additional functions possible that positively influence the emissions and agility of the S 400 HYBRID in equal measure. Moreover, this disc-shaped motor effectively dampens torsional vibrations in the drive train, thereby further reducing noise and vibrations in the interior. The result is even more ride comfort for both driver and passengers.
The overall system offers extensive benefits: firstly by helping to save fuel, and secondly by increasing driving pleasure with the help of the “boost” effect, where the electric motor gives powerful assistance to the petrol engine with its maximum torque of 160 Nm (118 lb-ft) right from the beginning of the fuel-intensive acceleration phase. This means that the hybrid drive system of the S 400 HYBRID moves off powerfully even from very low engine speeds, as the torque curve impressively confirms. The additional torque of the hybrid module also has a consistently positive effect during subsequent acceleration phases. In all driving situations, the driver therefore benefits from the interaction between the two units in the form of powerful responsiveness and muscular torque – but without an increased fuel consumption.
The S 400 HYBRID accelerates from zero to 100 km/h in 7.3 seconds, and reaches an electronically governed top speed of 250 km/h. The S 400 HYBRID betters the already very favourable NEDC (New European Driving Cycle) fuel consumption of the conventionally powered S 350 by up to 2.2 litres per 100 kilometres. CO2 emissions are reduced by about 21 percent.
In addition the hybrid module features an extremely comfortable and efficient start/stop function, which already switches the engine off when the vehicle is rolling to a stop at less than 15 km/h, for example before stopping at traffic lights. When it’s time to move off again, the electric motor immediately and imperceptibly restarts the main engine as soon as the driver releases the brake pedal or operates the accelerator. This likewise contributes to fuel economy and environmental conservation: as the engine restarts practically instantly, emissions are also minimised during the starting phase. The vibrations and jolts that are unavoidable when starting with a conventional starter are also reduced to a minimum.
Steering and climatic comfort remain unchanged, as both the steering servo pump and the refrigerant compressor are electrically powered. Both systems therefore continue to operate even when the vehicle is at a standstill with the engine automatically switched off. Even the partial vacuum required for brake servo assistance is generated electrically. The intelligent control logic is able to detect whether the driver is executing a turning or parking manoeuvre. In this case the automatic start/stop function is temporarily deactivated, so that these manoeuvres can be carried out in comfort.