With the new S 250 CDI BlueEFFICIENCY, Mercedes-Benz presented 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 l/100 km (41.26 mpg US, 17.54 km/l) in the NEDC cycle (New European Driving 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 (368 lb-ft), 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 (62 mph) in 8.2 seconds, and reaches a top speed of 240 km/h (149 mph). 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 (368 lb-ft) is available from an engine speed of just 1600 rpm.
The drop in the S-Class’s fuel consumption down to well below 6 l/100 km (47.08 mpg US, 16.66 km/l) 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.
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.