The first four-cylinder engine in the more than 60-year success story of the S‑Class attains a combined fuel consumption of just 5.7 litres per 100 kilometres, corresponding to CO2 emissions of 149 grams per kilometre. This makes the S 250 CDI BlueEFFICIENCY the first car in the luxury segment to consume under 6 litres of fuel per 100 km, and the first vehicle in its class to attain CO2 emissions below the 150-gram mark. With a peak power output of 150 kW (204 hp) and maximum torque of 500 newton metres, the four-cylinder CDI guarantees the superior performance which is a hallmark of the S‑Class. It accelerates the large saloon from 0 to 100 km/h in 8.2 seconds, with a top speed of 240 km/h.
The 2.2-litre CDI engine already delivers its maximum torque at 1600 rpm. The high torque at low engine speeds is a result of two-stage turbocharging. This technology, which is employed in a series-production passenger car diesel engine for the first time in the C and E-Class, is predominantly responsible for the high output at the level of a six-cylinder power unit.
The compact module for the two-stage turbocharging 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 to take the initial flow of exhaust gas, and rotates at up to 215,000 rpm. 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, so that the exhaust gas energy is available solely for the HP turbine drive. This means that the optimum charge pressure can already be built up at low engine speeds.
Improved cylinder charging even at low rpm
The two compressors are likewise connected in series, and are in addition connected to a bypass duct. The combustion air from the air filter first flows through the low-pressure compressor, where it is compressed as a function of the LP turbine's output. 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 major advantage of this sophisticated, demand-related control of the combustion air supply using two turbochargers is improved cylinder charging, and therefore high torque even at low engine speeds. Fuel consumption is also reduced. When the car is driven, this concept makes itself felt by harmonious driving characteristics with no turbo-lag, a favourable torque curve across the entire engine speed range, great responsiveness and excellent performance.
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.
Credits: Daimler AG
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