Press Release
New SLK V6 Engine from Mercedes-Benz
Under higher engine loads, the camshaft adjustment feature is also used to optimize the valve overlap in line with the engine speed so that the combustion chambers are efficiently supplied with fresh mixture – which makes for a high output and torque.
The camshafts are controlled by electrohydraulically operated vane-type adjusters (see illustration). These are located at the front ends of the camshafts and are controlled by four integral hydraulic valves. The intake camshafts are driven by a duplex chain; the exhaust camshafts are moved directly by the intake camshafts via a braced pair of gear wheels.
In addition to this fourfold, variable camshaft adjustment a number of other measures contribute to the exemplary performance characteristics of the new V6 engine:
- flow-optimized intake ducts, which are designed for the best possible throughput and feature innovative tumble flaps;
- specially developed valves, which have a shaft diameter of only six millimetres and scarcely interrupt the flow in the intake duct;
- compact combustion chambers for a high compression ratio (10.7:1) and volumetric efficiency;
- a newly developed, two-stage intake manifold.
As in all the latest Mercedes engines, valve clearance adjustment is hydraulic and therefore maintenance-free.
Aerodynamics beneath the bonnet: improved airflow characteristics
The V6 developers devoted a great deal of attention to anything that would contribute to optimal aspiration. Sophisticated computer programmes were used to calculate airflows, for instance helping to optimize the flow of air from the twin-chamber air filter. This is where the ducts interface with a component which is highly important for the engine's functions, namely the hot-film mass airflow sensor or HFM. The housing of this unit accommodates an electrically heated sensor element which measures the mass of the intake air and provides the engine management system with important basic information for the composition of the combustion mixture.
In the interests of optimal engine aspiration the Mercedes engineers also improved the HFM unit further in terms of flow characteristics. The oval shape of the airflow sensor housing and a modified mesh with a low air resistance are two important results of this detailed improvement work.
Open flap from 3500 rpm: two-stage intake module for a controlled air supply
The air intake is variable in accordance with the engine load and engine speed by virtue of an intake module in well-proven magnesium alloy. The length of the intake pipes leading to the cylinders is varied by means of flaps. At high engine speeds - from approx. 3500 rpm - the flaps are open and the air takes the shortest distance to reach the combustion chambers, producing a high engine output.
At low engine speeds the flaps are closed, increasing the length of the intake duct. This creates pressure waves which assist the intake process and fundamentally improve the torque generated in the lower engine speed range. No less than 305 Newton meters are already available from 1500 rpm, which corresponds to approx. 87 percent of the maximum torque.
The fuel supply rail with the solenoid valves is located in a protected position inside the V-shaped cylinder banks. Injection takes place in the intake ducts.
Turbulent airflow: tumble flaps in the intake ducts
Electro-pneumatically driven flaps at the end of each intake duct are the special feature of the intake module in the Mercedes six-cylinder engine. These make a significant contribution to fuel economy.
Mercedes engineers refer to these as tumble flaps, which in some measure indicates their purpose: they literally cause the fuel/air mixture to tumble, increasing the turbulence of the airflow and causing it to enter the combustion chambers at higher speed with a more uniform distribution. The result is better, i.e. more complete combustion.
Under partial load the tumble flaps pivot to the open position, optimizing the airflow and increasing the speed of combustion - an advantage that makes itself particularly noticeable in terms of fuel economy where the mixture is lean as a result of exhaust gas recirculation.
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. Situation-related control of the tumble flaps is based on various characteristic maps which are stored in the engine management system.
Due to the tumble flaps in the intake ducts fuel consumption of the V6 engine can be reduced by up to 0.2 litres per 100 kilometres, depending on engine speed - while improving smoothness at the same time.
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.
Central location: high-performance control unit for all engine functions
The tumble flaps, fuel injection, ignition and numerous other engine functions are controlled by the Bosch ME 9.7 engine management system, which also communicates and exchanges information with the other onboard control units via a databus. In the interests of the shortest possible electrical paths the engine control unit is centrally located above the intake tract and is integrated into the engine design.
The microprocessor in the control unit carries out a continuous diagnosis of all engine functions. This includes monitoring of the catalytic converters, the ignition system, the electrical performance of the purge valve and the oxygen sensors. Should a fault develop in one of these important emission control components, the warning display "Check Engine" lights up in the instrument cluster. At the same time, the data are stored in memory so that service technicians can immediately identify and remedy the cause of the fault.
