In the development of the new V6 engine, the Mercedes engineers trained their sights on output, torque, fuel economy, comfort and exhaust emissions in particular, and set new standards in each area. Their successful approach involved incorporating innovations into the engine construction which have a noticeably positive effect in various fields, rather than representing individual solutions as such.
Fourfold variable camshaft adjustment
By incorporating the four-valve technology and four overhead camshafts which are now staple ingredients of sports car engines, the Stuttgart-based engineers had already laid the foundations for exceptional power development. However, they refused to stop there, broadening their horizons to include a system which allows the interaction of the 24 valves to be controlled as required according to the engine load, thereby ensuring an ultra-fast charge cycle in the cylinders: continuous camshaft adjustment. The angles of the intake and exhaust camshafts can each be continuously adjusted by 40 degrees, allowing the valves to open or close at the optimum moment in any driving situation.
At low throttle this technology allows exhaust gases to flow directly from the combustion chamber back into the intake port. The camshafts are controlled in such a way as to keep the exhaust valves open for a short time whilst the intake valves are opening. During this split second, some of the exhaust gases are able to flow from the exhaust port into the intake port. The vacuum in the intake manifold assists this process. This valve overlap when expelling the used gases and drawing in the fresh combustion mixture ensures efficient internal exhaust gas recirculation. This reduces the amount of energy lost during load changes in the cylinders, leading to substantially lower fuel consumption.
With the throttle open wider, meanwhile, camshaft adjustment also serves to optimize valve overlap in line with the engine speed so that the combustion chambers are supplied as efficiently as possible with fresh mixture. The result is an increase in power output and torque. The camshafts are controlled by electrohydraulic vane-type adjusters positioned at the front end of the camshafts and controlled by means of four integral hydraulic valves. The intake camshafts are driven by a duplex chain, whilst the exhaust camshafts are moved directly by the intake camshafts via a meshed pair of gear wheels.
In addition to this fourfold continuous camshaft adjustment, a series of other measures also contribute to the exceptional power development of the V6 engine:
* Flow-optimized intake ports, designed to allow optimum throughput and featuring innovative tumble flaps;
* The specially developed valves, which have a shaft diameter of only six millimetres and thus only represent a minor hindrance to flow in the intake port;
* The compact combustion chambers, which promote a high compression ratio (10.7:1) and impressive efficiency;
* The newly developed, two-stage variable intake manifold.
As with all the latest Mercedes engines, valve clearance adjustment is hydraulic and, as such, maintenance-free.
The development team for the V6 devoted a great deal of their attention to measures contributing to the best possible engine aspiration. The engineers used sophisticated computer programs to make flow calculations, the software helping them to optimize the flow of air from the twin-chamber air filter. Here the ducts interface with a component which is of key importance for the engine functions: the hot-film air-mass sensor. Airflow-related further development work has been carried out on this component in order to optimize the air supply. The oval-shaped housing of the air mass sensor and a modified mesh with low air resistance are two of the most significant products of this detailed improvement process.
The intake module: two-stage design ensures a controlled air supply
Air supply can be adjusted according to the engine load and speed with the help of a proven magnesium intake module. The length of the intake pipes which lead to the cylinders is altered by the use of flaps: the flaps are opened at high engine speeds – from approx. 3500 rpm - and the air flows the short distance to the combustion chambers, helping to generate impressive output.
At low engine speeds the flaps are closed and the length of the intake duct increases. This creates pressure waves, which aid the intake process and ensure a fundamental improvement in torque levels at low engine speeds. Indeed, 305 Newton metres of pulling power – some 87 percent of maximum torque – is available at 1500 rpm.
Turbulent airflow: tumble flaps in the intake ducts
The key features of the intake module in the Mercedes six-cylinder engine are electro-pneumatically driven flaps at the end of each intake port which make a significant contribution to enhancing fuel economy. Mercedes engineers refer to these as “tumble flaps”, a term which describes their role in causing the fuel/air mixture literally to tumble. This increases the turbulence of the airflow, which consequently enters the combustion chambers at greater speed and spreads out evenly. The end result is improved – i.e. more complete – combustion.
At partial load, the tumble flaps pivot up, optimizing air flow and thus accelerating the speed of combustion – an advantage that becomes particularly noticeable in the extremely lean mixture produced by exhaust gas recirculation and one that helps to cut fuel consumption. Under greater engine loads, the tumble flaps are not required and fold down into the intake manifold where they cannot impede the intake process. The tumble flaps are controlled according to the situation in hand on the basis of calculations stored in the engine management system.
The tumble flaps in the intake ports can reduce the fuel consumption of the V6 engine by up to 0.2 litres per 100 kilometres depending on the engine speed, as well as enhancing smoothness.
Systematic fuel economy: heat management in all situations
In the interests of further reducing fuel consumption Mercedes engineers have developed an intelligent heat management system. For example, coolant circulation is interrupted during the warm-up phase to allow the engine to reach its operating temperature more quickly. This leads to improved oil flow and therefore significantly reduced friction in the engine. A drop in exhaust emissions is another benefit of this system. With the engine warmed up and working under full load, the heat flows are still directed in such a way that the engine oil and coolant remain constantly at optimum temperature. Here, the credit goes to an innovative, logic-controlled thermostat which is active in all operating situations.
The engine: the benefits of lightweight construction
The cylinder head and crankcase of the new V6 engine are made of aluminium. Plus, the pistons, connecting rods and cylinder liners are manufactured according to state-of-the-art construction principles which not only help to reduce weight but also noticeably enhance the responsiveness and smoothness of the engine. This is because the lower the moving masses in the crankcase, the greater the reduction in vibration and the more responsive the engine becomes to the driver’s prompting via the accelerator pedal:
* The pistons are made of iron-coated aluminium. The piston crowns – developed to take into account the valve angle (28.5 degrees) – are designed to ensure a favourable combustion chamber shape.
* Mercedes engineers have reduced the weight of the forged steel connecting rods by around 20 percent compared with other V6 engines, making a significant contribution to the remarkably smooth running of the new six-cylinder unit.
* The cylinder liners feature low-friction aluminium/silicon surfaces, a technology which has already proved its mettle in other Mercedes-Benz passenger-car engines. Minimal distortion, exceptional heat flow and impressively low weight are among the other plus points. Indeed, these liners are some 500 grams per cylinder lighter than their conventional cast-iron equivalents.
* The forged crankshaft is fitted with four counterweights. Four wide crankshaft bearings attached to the crankcase by transverse reinforcing struts also help to keep a lid on vibration.
* A balancer shaft between the two cylinder banks smoothes out the characteristic free vibrations of a V6 engine to ensure exceptional smoothness. The balancer shaft counter-rotates at the same speed as the crankshaft.
The acoustics: reduced noise, but enhanced sound
In addition to outstanding output and torque characteristics, impressive fuel economy and low vibrations, the new V6 engine also boasts certain audible benefits. As part of an exhaustive programme of testing, Mercedes engineers took acoustic measurements from almost all of the engine’s 210 separate parts - from the crankcase to the engine mounts, the pistons and the injection valves – and assessed their noise output levels and frequency.
Their aim was to ensure that the engine generated a pleasing sound in any driving situation. In order to achieve this, they not only measured absolute noise levels, but also sources of annoying noise which may not be especially loud but which produce obtrusive frequencies under changes in load and engine speed - and therefore detract from the driver and passenger’s enjoyment of the engine sound.
Noise configuration is developing into one of the key areas of engine development. Here, the engineers are faced with the challenge of keeping drive-past noise suppressed whilst shaping a pleasant and sonorous engine sound. To this end, the experts charged with creating the six-cylinder unit for the SLK-Class adopted an approach focusing on reducing noise while improving sound. A series of measures were developed with the aim of restricting engine noise, from a two-cartridge air filter with integral resonators to a sound-absorbent mat under the bonnet. At the same time, the sound specialists channelled their efforts into creating sports car vocals for the SLK-Class. They achieved this by emphasizing certain frequencies, predominantly through careful configuration of the twin-pipe exhaust system.
Exhaust emissions under the EU-4 limits
In common with all the latest Mercedes-Benz passenger cars, the new SLK-Class also meets the stringent EU-4 exhaust emissions standards which come into force in 2005. The two striking tailpipes on the left and right of the car’s rear end fit in perfectly with the overall design of the car.
The emission control system is based on a dual concept: sophisticated engine-specific measures whose aim is to reduce untreated emissions and highly effective emissions treatment using catalytic converters located close to the engine, having a volume of 2x 1.4 litres in the six-cylinder unit. The SLK 350 the catalytic converters are each fitted with two oxygen sensors – one control sensor and one diagnostic sensor – with linear control. This means that the oxygen sensors are active immediately after a cold start and supply information relating to the composition of the exhaust emissions. The electronic control unit then processes this data for use in the control of the warm-up phase, for instance. The air-gap-insulated stainless steel exhaust manifold allows the catalytic converters to reach their operating temperature more quickly.
Among the systems at work inside the engine is the continuous camshaft adjustment, which permits more efficient exhaust gas recirculation within the engine under partial loads. Plus, the adjustable tumble flaps in the intake ducts of the V6 engine, which improve the combustion process, make an important contribution to minimizing the levels of untreated emissions produced by the engine. Added to which, a secondary air injection system has an afterburning effect on the exhaust emissions, causing the temperature in the exhaust ports to rise and allowing the catalytic converter to begin processing the pollutants at an earlier stage. This afterburning process also helps to reduce the carbon monoxide and hydrocarbon content in untreated exhaust gases.
Transmissions: five, six and seven-speed variants available
Mercedes engineers have further improved the six-speed manual transmission fitted as standard in the SLK 350 and given it an even sportier edge. The key to these improvements is a single-rod gearshift system, which allows drivers to make fast, sporty gear changes, whilst at the same time offering even greater shifting comfort thanks to the use of sophisticated damping elements.
By contrast to the manual gearshift system in the outgoing SLK, in which the vertical and horizontal movements of the gearshift lever were transferred to the transmission separately, a single gearshift shift rod now communicates both movements at the same time. This means an end to the indirect routes of the previous system and noticeable benefits for the driver in the form of fast and crisp gear changes.
Greater shift quality and improved precision allow the new single-rod gearshift to raise the bar in terms of driving pleasure. The driver enjoys a more harmonious shift feel, with the shift process benefiting from improved damping – and thus coming across as more rounded – thanks to the damping elements in the lower area of the shift lever and on the shift housing. In addition, the dampers cut the shift lever off extremely efficiently from the vibrations produced by the transmission and engine while at the same time reducing shift noise to a minimum.
A further change to the previous Mercedes six-speed manual transmission sees the reverse gear moved forward from its usual left rear position to the front left alongside first gear.
7G-TRONIC: seven gears for lower fuel consumption and even greater driving pleasure
The SLK 350 can be ordered as an option with the world’s first seven-speed automatic transmission (fitted as standard in the SLK 55 AMG). The 7G-TRONIC automatic transmission is brimming with technical innovations which ensure more powerful acceleration, extra-fast mid-range sprints, lower fuel consumption and a higher level of shift comfort.
The driving force behind the seven-speed automatic’s outstanding qualities is provided by a series of design features, the most important of which is the increase in forwards gears from five to seven. This allows a wider spread of ratios, as well as further reducing the differences in engine speed between the individual gears achieved by the five-speed transmission. As a result, the driver can rely on having the optimum ratio at his or her disposal for virtually any driving situation. In addition, the electronic control unit has even more scope in which to optimize the shift processes to achieve lower fuel consumption and greater comfort. For example, at 100 km/h the engine speed will be on average – depending on the driving situation - around twelve percent lower than with a five-speed automatic. This impressive engine speed adjustment system opens the door to lower noise, as well as improved fuel economy.
Mercedes engineers have achieved further advances as far as shift control is concerned. If the driver needs to accelerate quickly and therefore change down rapidly through several gears – i.e. kick-down - 7G-TRONIC avoids having to move through the gears in strict order. Instead, the transmission uses its direct downshift capability, shifting down by as many as four gears – as the situation demands – rather than just one at a time. This paves the way for direct gear changes from seventh gear to fifth, from fifth to third or from third to first, and even four-gear jumps from sixth gear to second, with only one valve opening and one valve closing in each case. The change in engine speed from the current gear to the target gear is extremely fast and spontaneous and feels more or less the same as a conventional single-gear downshift.
Where multi-gear downshift is not carried out directly – e.g. the frequent shifts from seventh into fourth or from seventh into third gear – the second part of the shift process is managed in such a way that it overlaps with the first, with the result that the driver senses nothing more than a steady change in engine speed. Shift times have been reduced significantly below the levels of the five-speed automatic transmission fitted up to now.
Like the proven five-speed automatic, the new seven-speed automatic transmission also boasts a torque converter lock-up clutch. This system is located in the hydrodynamic torque converter and largely eliminates slip between the pumps and the turbine wheel in many operating conditions. It manages this by establishing – where possible – a virtually rigid connection between the engine and transmission shafts and thus preventing power losses.
By contrast to conventional automatic transmissions, where the converter can only be locked up in higher gears, the lock-up clutch in the Mercedes-Benz seven-speed automatic is already active in first gear. In addition, in the interests of comfort the torque converter lock-up clutch features slip control, which allows it to run extremely smoothly. This is just another way in which Mercedes engineers have achieved unbeatable levels of shift quality.
Transmissions: manual gear-change using buttons on the steering wheel
In keeping with the dynamic and agile character of the new SLK-Class, drivers can operate the seven-speed automatic transmissions using gear-shift buttons on the steering wheel (optional). Plus, they can limit the gear stages or select a new gear manually, allowing them to fully exploit the engines’ extensive power reserves in any driving situation. And they always have the option, of course, of switching to “M” (Manual) mode at the touch of a button on the tunnel trim. The gear currently selected is highlighted in the central display of the instrument cluster.
When manual mode is selected, the automatic transmission still shifts up to the next gear when the maximum engine speed is reached.
In addition to manual mode, drivers can also make use of the “C” (Comfort) and “S” (Sport) settings. In these programs, the gears are changed automatically but the steering-wheel gearshift buttons are also active, so that drivers can switch to manual operation at any time.
With AIRSCARF, COMAND, a vario-roof and head/thorax side airbags, the new SLK Roadster is a remarkable car whichever way you look at it, packed with innovations no other vehicle in its class can match. The range of engines available for the new SLK is also outstanding. Customers can select from a choice of three petrol-engined variants developing between 120 kW/163 hp and 265 kW/360 hp and including - for the first time in this vehicle class - an eight-cylinder unit from the Mercedes-AMG stable.
The V6 engine in the SLK 350 is a totally new development boasting all the qualities you look for in a powerful, free-revving sports roadster engine. No other power unit in this displacement class can match the new six-cylinder unit when it comes to output and torque characteristics aimed firmly at enhancing driving dynamics and the over-all motoring experience. Added to which, the V6 newcomer also sets the pace in terms of fuel economy, exhaust emissions and comfort.
The six-cylinder powerplant develops 200 kW/272 hp at 6000 rpm from a displacement of 3498 cc. That equates with an output of just 57 kW/78 hp per litre which raises many an eyebrow in this displacement class. Torque levels are equally as exceptional, with 350 Newton metres available as low down as 2400 rpm and remaining constant up to 5000 rpm. That guarantees exceptional pulling power and rapid mid-range acceleration, but also relaxed driving in high gears. With the vario-roof down and an open road ahead of them, roadster fans could not ask for more.
NEDC combined consumption of 10.6 litres per 100 kilometres is further evidence of the state-of-the-art technology at work inside the V6 engine. Teamed up with the 7G-TRONIC seven-speed automatic transmission, the new power unit burns just 10.1 litres of fuel per 100 kilometres (NEDC combined consumption). This makes the SLK 350 some three percent more economical than the outgoing SLK six-cylinder model with automatic transmission, even though it can point to 25 percent greater output.
Wolfgang Zanker, Telephone +49 711 17-75847
Cypselus v. Frankenberg, Telephone +49 711 17-75846