With the new S 250 CDI BlueEFFICIENCY, Mercedes-Benz is presenting the world's most fuel-efficient luxury saloon at the Paris Motor Show 2010. As the first four-cylinder engine in the more than 60-year history of success of the S-Class, the highly-efficient turbo diesel unit achieves fuel consumption levels of just 5.7 litres per 100 kilometres in the NEDC cycle, which corresponds to a CO2 output of 149 g/km. As a result, the new S 250 CDI BlueEFFICIENCY is the first 5-litre car in the luxury segment and at the same time the first vehicle in its class to undercut the 150-gram mark in terms of CO2 output. With a peak output of 150 kW (204 hp) and a maximum torque of 500 Nm, the four-cylinder CDI also guarantees a superior driving experience which is typical of the S-Class.
The luxury saloon accelerates from 0 to 100 km/h in 8.2 seconds, and reaches a top speed of 240 km/h. The combination of exemplary economy with a high level of driving dynamics is achieved thanks to the latest generation of common-rail technology, two-stage turbocharging with high and low-pressure turbochargers, and also the standard-fit ECO start/stop function, which Mercedes-Benz is offering in conjunction with the new, extensively revised 7-speed 7G-TRONIC PLUS automatic transmission.
The 150 kW (204 hp) engine in the latest S-Class variant already features in numerous Mercedes-Benz models, boasting outstanding efficiency. The maximum torque of 500 Nm is available from an engine speed of just 1600 rpm.
Fuel-saving: ECO start/stop function
The drop in the S-Class's fuel consumption down to well below 6 litres per 100 km is attributable in part to the new ECO start/stop function. This system shuts down the engine as soon as the vehicle comes to a stop – e.g. at traffic lights – and the driver continues to press the brake pedal while it is stationary. The engine starts again immediately as soon as the driver releases the brake pedal or presses the accelerator pedal.
The ECO start/stop function will not switch the engine off for example if the desired interior temperature has yet to be reached, if the driver has switched off the system or if drive programmes "S" or "M" are active. An intelligent control system ensures that all functions and comfort systems otherwise remain in operation during the stop phase.
An effective double act: two-stage turbocharging for exceptional pulling power
A key feature of the diesel engine is its two-stage turbocharging. First introduced by Mercedes-Benz for the diesel engines of production cars in the C and E-Class, this system provides for powerful torque right from low revs and performance on a par with that of a six-cylinder engine.
The compact module for the two-stage turbocharger consists of a small
high-pressure (HP) turbocharger and a large low-pressure (LP) turbocharger. These are connected in series, and each has a turbine and a compressor driven by this turbine. The HP turbine is located directly at the exhaust manifold and initially allows exhaust gas to flow through it; it then rotates at up to 215,000 revolutions per minute. The HP turbine housing features an integral bypass duct, which can be opened or closed by means of a charge-pressure control flap triggered by a vacuum cell. If the flap is closed, the whole exhaust stream flows through the HP turbine, such that the exhaust-gas energy is available solely for the HP turbine drive. This means that the optimum charge pressure can be built up at low engine revs.
As the engine speed increases, the charge-pressure control flap opens. A portion of the exhaust stream then flows through the bypass duct to relieve the HP section. Downstream of the HP turbine, the two exhaust gas streams join up again, and any remaining exhaust gas energy drives the HP turbine at a maximum speed of up to 185,000 revolutions per minute. To protect against overloading, the LP turbine is also equipped with a bypass, which is opened or closed by means of a wastegate. Once the engine reaches medium revs, the HP turbine's charge-pressure control flap is opened so wide that the HP turbine ceases to perform any appreciable work. This allows the full exhaust gas energy to be directed with low losses into the LP turbine, which then does all of the turbine work.
Raring to go: improved cylinder charging even at low revs
The two compressors are likewise connected in series and are in addition connected to a bypass duct. The combustion air from the air cleaner first flows through the low-pressure compressor, where it is compressed as a function of the LP turbine's operating energy input. This pre-compressed air then passes into the high-pressure compressor, which is coupled to the HP turbine, where it undergoes further compression. The result is a genuine two-stage turbocharging process.
The key benefit of this sophisticated, on-demand control of the combustion air supply by means of two turbochargers is the improved cylinder charging and, consequently, high torque even at low revs. What's more, fuel consumption is reduced. During normal operation, the advantages of this concept can be seen in the harmonious driving characteristics without turbo lag, a good torque curve across the entire engine speed range, spontaneous throttle response and excellent performance.
Plenty of push: fourth-generation common rail
The two-stage charging is complemented by a large intercooler which lowers the temperature of the compressed and heated-up air by around 140 degrees Celsius, thus allowing a large volume of air to enter the combustion chambers.
A further feature of the innovative and highly efficient diesel engine is fourth-generation common rail direct injection. The rail pressure of 2000 bar is crucial in order to deliver 150 kW (204 hp) of engine power and maximum torque of 500 Nm in combination with low untreated emissions.
Fine-tuned for fuel economy: oil pump with two pressure stages cuts
A newly developed controlled oil pump also contributes to the low fuel consumption of the S 250 CDI BlueEFFICIENCY. This pump operates at two pressure levels. At low engine speeds and loads the pump runs at a low pressure of two bar. The high-pressure stage is activated at the upper load and engine speed levels. Thanks to this innovative control concept, the lubrication and cooling points of the engine can be supplied with significantly lower drive energy than would be possible with an uncontrolled pump. In this way, the Mercedes-Benz development engineers have managed to lower fuel consumption in particular during modes of driving which require lower levels of power output, such as in town.
Another special feature is the thermal management, which enables faster warming-up of the engine and therefore also contributes to the reduction in fuel consumption. The engine experts at Mercedes-Benz have also reduced the friction in the belt system to drive the ancillary components by lowering the tension forces. The diesel engine has additionally been equipped with a new, friction-optimised vacuum pump.
Dynamically controlled: engine mounts damp vibrations
To ensure hallmark S-Class comfort, dynamically controlled engine mounts feature as an innovation in the new S 250 CDI BlueEFFICIENCY. The rigidity and damping of these mounts - which are connected to the engine control system via a databus - alter according to engine speed, load and vehicle speed, thus preventing the transmission of unwanted vibrations and noise from the engine to the body.
Designed for maximum efficiency: the new 7G-TRONIC PLUS
The highly efficient diesel engine is combined with the enhanced 7G-TRONIC PLUS automatic transmission offering a further reduction in converter slip and optimised efficiency. The distinguishing feature of this transmission is its new torsional damper, which provides for comfortable driving in the low rev range and a marked reduction in slip of the torque converter lockup clutch even at low loads. This technology has enabled the Mercedes-Benz development engineers to lower the engine speeds – which also contributes to reduced fuel consumption – without compromising on signature S-Class comfort. In addition, the optimised damping of rotational irregularities and vibrations in the transmission also enables an even faster response to driver commands via
the accelerator pedal.
A new hydraulic circuit and an optimised converter characteristic also contribute to improved efficiency and starting performance. Thanks to a separate transmission oil pump, the 7G-TRONIC PLUS is also start/stop-capable. This function ensures spontaneous starting when the internal transmission oil pump is not operative because the engine is switched off.
CO2 champion: world-leading fuel economy thanks to trailblazing technology package
The new S 250 CDI BlueEFFICIENCY demonstrates in impressive fashion that state-of-the-art, highly efficient four-cylinder engines enable fuel consumption levels of well below 6 litres per 100 km in the luxury segment too, without compromising on ride comfort and driving dynamics. In addition to the four-cylinder CDI engine, a comprehensive technology package covering the entire drivetrain is also deployed to this end. The equipment involved here includes the ECO start/stop function and the revised 7G-TRONIC PLUS automatic transmission. The overall result is a combined NEDC fuel consumption level of 5.7 litres of diesel per 100 kilometres and the lowest CO2 emissions of any vehicle in this class worldwide, at just 149 g/km.