The new CL-Class is equipped with the most powerful Mercedes engines: the eight-cylinder unit in the CL 500 is a new development which celebrated its debut in the S-Class in autumn 2005, while the Stuttgart engineers have developed the biturbo V12 unit of the CL 600 further to generate 12 kW/17 hp more output and 30 Newton metres more torque than before. In specific terms this means 380 kW/517 hp and 830 Newton metres in an engine speed range of 1900 to 3500 rpm. Despite this higher output, the V12 engine consumes 0.4 litres less fuel per 100 kilometres than the previous CL 600.
The new eight-cylinder unit in the CL 500 develops 285 kW/388 hp from a displacement of 5.5 litres, an increase of more than 26 percent versus the previous V8. At 530 Newton metres the torque also comfortably exceeds the maximum for the preceding engine by around 15 percent. This high level of torque is available from 2800 rpm and remains constant throughout a wide engine speed range up to 4800 rpm, providing ideal conditions for powerful acceleration and fast intermediate sprints.
With a specific fuel consumption of 233 grams per kilowatt hour (g/kWh) at peak efficiency and 345 g/kWh at part-throttle (2000 rpm, 2 bar), this engine is easily the most fuel-efficient unit in this displacement and output class.
Lightweight construction for smooth running and dynamic agility
A crankcase of diecast aluminium with low-friction aluminium/silicon cylinder liners is the solid and durable core of the new Mercedes-Benz eight-cylinder engine. It houses a five-bearing, forged steel crankshaft with induction-hardened main and big-end bearings. The connecting rods are also of forged steel, with specific areas milled away to make them around one fifth lighter than those of the previous V8 engine in the CL-Class.
Saving weight was not the only important aspect during the design work on the new engine, however. The engineers were just as determined to achieve the best possible results where smooth running and flexibility are concerned. Lightweight construction principles were helpful in this respect, for the lower the moving masses in the crankcase, the lower the vibrations and the more responsive the engine is to movements of the accelerator pedal.
Shifting camshafts for a perfect charge cycle and higher torque
Mercedes engineers used an intelligent valve control system, achieving remarkable progress and a major contribution to the good performance characteristics and low fuel consumption of the unit. In addition to four-valve technology, continuously adjustable intake and exhaust camshafts ensure that the cylinders are optimally supplied with fresh mixture. The valves are opened at precisely the right moment in any driving situation, significantly improving the gas cycle in the combustion chambers and reducing energy losses.
The camshafts are controlled by electro-hydraulically operated vane-type adjusters, which are located at the forward ends of the camshafts and are controlled by four integral hydraulic valves. The intake camshafts are driven by a duplex chain, while the exhaust camshafts are directly driven by the intake camshafts via a braced pair of gears.
The engineers at Mercedes-Benz have optimised the effect of this four-fold, continuous camshaft adjustment by using what are known as shifting camshafts. These control the opening of the exhaust valves and improve the engine’s charge cycle even further. Depending on the ignition sequence, the exhaust cams are designed in such a way that the valves open at different times during the exhaust cycle. This reduces the pressure fluctuations in the exhaust tract which are inherent to a V8 engine, and which lead to fluctuations in the residual gas content in the cylinders. The valve lift interval on the exhaust sides of the Mercedes eight-cylinder engine is as follows:
Cylinder 3, 4, 5, 7: 180 degrees crankshaft with 2 mm valve lift
Cylinder 1, 2, 6, 8: 190 degrees crankshaft with 2 mm valve lift
The newly developed shifting camshafts improve the torque and smoothness of the V8 engine by ensuring a more uniform residual gas content, a higher knock limit and better cylinder charging in the lower to medium engine speed range. At 2000 rpm, for example, the effective mean pressure is around six percent (10.3 : 11.0 bar) higher than in a comparable engine without shifting camshafts.
Two-stage intake module and tumble flaps for better combustion
In addition, the V8 features further technical innovations for adaptive control of the combustion processes:
Two-stage intake module for a controlled air supply depending on the engine load and engine speed: the length of the intake ducts leading to the cylinders is varied by flaps. At high engine speeds – from approx. 3500 rpm - the flaps are open and the air flows directly to the combustion chambers. This allows high outputs to be achieved. At low engine speeds the flaps are closed to increase the length of the intake ducts. This creates pressure waves which support the intake process and improve the torque yield in the lower engine speed range. As a result 435 Newton metres of torque, and therefore more than 82 percent of the maximum torque, are already available from 1500 rpm.
Optimised airflow for the best possible engine aspiration. Mercedes engineers used highly sophisticated flow calculations to optimise the airflows in the eight-cylinder power unit. Particular attention was for instance paid to the airflow from the twin-chamber air filter, the housing of the hot film air mass sensor and the intake manifold.
Tumble flaps for more complete combustion: electro-pneumatically driven flaps at the end of each intake duct increase the turbulence of the air flow, which is thus more uniformly distributed in the combustion chambers. At part-throttle the tumble flaps pivot up, optimising the airflow and increasing the speed of combustion; under higher engine loads the tumble flaps are not required, and can be completely recessed into the intake manifold so as not to impede the intake process. The use of these tumble flaps has two positive effects in practical operation, namely smoother running and a reduction in fuel consumption by up to 0.2 litres per 100 kilometres, depending on engine speed.
Combustion of the fuel/air mixture is by means of a direct coil ignition system. The spark plugs project into the centre of each cylinder between the four valves, with the ignition coils located directly above them.
Electronically controlled thermostat for adaptive heat management
The developers of the new Mercedes eight-cylinder engine have also made a major contribution to fuel economy with a sophisticated heat management system. Coolant circulation is interrupted during the warm-up phase, for example, so that the engine reaches its normal operating temperature more rapidly. This in turn improves the oil flow and considerably reduces friction in the engine. When the engine is warm and under full load, the heat flows are directed in such a way that the engine oil and coolant are always at an optimal temperature. This is ensured by an innovative, logic-controlled thermostat which is active in all operating situations and adjusts the coolant temperature according to the style of driving and the prevailing conditions. This also enables the heat supply to the heater core to be controlled as needed.
Twin catalytic converters und linear oxygen sensors for low exhaust emis-sions
The emission control system of the new Mercedes eight-cylinder engine follows a two-stage concept and is therefore highly effective. In addition to in-engine measures such as continuous camshaft adjustment, adaptive combustion control by tumble flaps, internal exhaust gas recirculation and secondary air injection, which ensure low engine-out emissions, the new CL 500 is equipped with two close-coupled catalytic converters as standard. These consist of two individual monoliths or bricks, each of which is monitored by two oxygen sensors: a control sensor and a diagnostic sensor which analyses the exhaust flow in the gap between the two bricks.
This oxygen sensing is linear, which means that the oxygen sensors are already active immediately after a cold start, supplying information about the exhaust gas constituents for the electronic control unit of the V8 engine to process when controlling the warm-up phase. This enables the catalytic converters to reach their normal operating temperature more rapidly.
Thanks to its ultra-modern technology and efficient emission control system, the new eight-cylinder engine complies with the most stringent exhaust emission standards currently in force; it also has the potential to meet limits which are planned for the future.
Transmission: seven gears and three different modes for the CL 500
The eight-cylinder engine of the CL 500 is specified as standard with the 7G-TRONIC seven-speed automatic transmission, the only passenger car transmission of this type in the world. The V12 engine of the CL 600 is paired with a five-speed automatic transmission.
The conventional, centre-console-mounted automatic selector lever is replaced by a DIRECT SELECT shift lever on the steering column, which allows the driver to select the "P", "N", "R" and "D" positions by slight finger pressure. The commands are transmitted electronically by cable.
The S/C/M/ mode selector switch on the centre console allows the driver to choose between three different modes: Sport, Comfort and Manual. As well as the transmission characteristics, these modes also allow the characteristics of the accelerator pedal and the suspension (springing and damping) settings to be varied. In Manual mode, the driver changes gear using the steering-wheel gearshift buttons.
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