The engines for the new SLK excel with a considerably lower fuel consumption combined with a comparable or higher output than those in the preceding series, as well as sporty and emotional appeal. There will initially be three models of the SLK available at its market launch, all of them powered by new engines with direct injection.
The four-cylinder engines in the SLK 200 BlueEFFICIENCY and SLK 250 BlueEFFICIENCY develop 135 kW (184 hp) and 150 kW (204 hp) respectively from a displacement of 1796 cubic centimetres. The SLK 200 BlueEFFICIENCY has the lowest CO2 emissions in its segment. With the enhanced, optional seven-speed automatic transmission 7G-TRONIC PLUS it consumes just 6.1 litres of premium petrol (NEDC, combined) over 100 kilometres, corresponding to 142 g of CO2 per kilometre. This sporty model sprints from 0 to 100 km/h in 7.0 seconds, and achieves a top speed of 237 km/h (240 km/h with manual transmission). The SLK 250 BlueEFFICIENCY is equipped with an automatic transmission as standard, and consumes only 6.2 litres (NEDC, combined) per 100 kilometres (144 g of CO2 per kilometre). From standstill it reaches 100 km/h in 6.6 seconds, and has a top speed of 243 km/h.
The V6 engine in the SLK 350 BlueEFFICIENCY generates 225 kW (306 hp) from its 3498 cubic-centimetre displacement, using this power to accelerate from 0 to 100 km/h in 5.6 seconds (top speed 250 km/h). Its consumption is 7.1 litres (NEDC, combined) per 100 kilometres (167 g of CO2 per kilometre). The V6 engine has been completely newly developed. Its most important hallmarks are 3rd-generation direct injection, piezo injectors and multi-spark ignition.
Thanks to the great efficiency, it was possible to reduce the tank capacity of the new SLK-Class to 60 litres (a 70-litre tank is available as an optional extra). Nonetheless the operating range is more than 900 kilometres, or more than 800 kilometres for the SLK 350 BlueEFFICIENCY. From autumn 2011 the new SLK will also be available with a powerful diesel engine.
Four-cylinder with a bite
The four-cylinder engines for the new SLK follow the so-called downsizing principle, with the reduced cubic capacity compensated by homogeneous direct injection and turbocharging. This leads to advantages such as lower weight and reduced in-engine friction. A smaller engine also moves the operating range of the characteristic map towards more favourable values, with consequent fuel consumption benefits.
The direct injection system of the four-cylinder engines has an injection pressure of up to 140 bar. In the interests of improved efficiency, this allows the compression ratio to be increased versus a conventional duct injection system. The fuel evaporating directly in the combustion chamber also lowers the temperature in the combustion chamber, reducing the engine’s tendency to knock. The engine is homogeneously charged.
The same turbine rotors are used to generate charge pressure in both output variants of the four-cylinder engine. The single-flow turbocharging of the four-cylinder units operates more efficiently than a mechanical charger, as no engine output needs to be devoted to the charger.
Other characteristics of the four-cylinder engines include a four-valve concept with forged intake and exhaust camshafts, as well as camshaft adjustment on the vane-cell principle. This enables the valve timing to be rapidly and variably adjusted. The result is high torque even at low engine speeds, and a higher specific output.
The crankcase and cylinder head are cast from aluminium alloy. The crankshaft is likewise cast, and has eight counterweights. To compensate the secondary masses inherent to a four-cylinder in-line engine, two Lanchester balancer shafts rotate within the crankcase at twice the crankshaft speed.
An electronically controlled thermostat ensures logic-controlled engine warm-up (no coolant flows through the cylinder head when the engine is cold), and therefore intelligent thermal management. The warm-up phase is shortened as a result.
Sporty engine note
A sporty engine note produced by a new sound generator lends an acoustic backdrop to the 4-cylinder variants of the new SLK. This generator is located immediately in front of the throttle flap, and “enriches” the engine note with elements of the intake noise. It filters out certain frequencies from the intake noise and conducts them into the vehicle interior via a pipe connection – an ingeniously simple and very convincing solution for a sporty and emotionally appealing engine sound.
New V6 engine with innovative technology
The six-cylinder engine is a member of the new V6 family for which Mercedes-Benz has developed the currently most efficient form of direct injection. Known as BlueDIRECT, this innovation establishes the V-engines from Mercedes-Benz as worldwide benchmarks and makes a further reduction in fuel consumption by 20 percent possible by in-engine measures alone.
The V-angle of the new V6 engine is only 60 degrees. This enables the balancer shaft for primary vibrations to be omitted. As a result, the driver notices an excellent level of comfort.
The crankcase, pistons and cylinder heads of the V6 are of aluminium. The crankshaft, connecting rods and valves are of special forged steel.
The technology package in the new engine generation includes a number of new developments that are unique in this combination:
- In combination with multi-spark ignition, a further developed,
third-generation spray-guided direct fuel injection system with piezo-electric injectors offers further possibilities for fuel savings by means of a new, stratified combustion process with a considerably extended characteristic map and fuel-efficient lean-burn technology.
- Power consumption by ancillary units has been reduced. These include an optimised water pump with 2nd-generation thermal management, a demand-controlled oil pump, a volume-controlled high-pressure fuel pump and an intelligent generator management system.
Lightweight construction techniques and detailed improvements have also reduced in-engine friction considerably compared to the previous engine.
Third-generation direct-injection system
Spray-guided direct petrol injection, which Mercedes-Benz was the first car manufacturer to introduce in series production, has been developed further as a third generation. The system pressure is up to 200 bar, the pressure being variably optimised according to the engine’s characteristic map. Completely newly developed piezo-electric injectors allow up to five injections per intake stroke for the best possible mixture formation.
The crystalline structure of the piezo-ceramic changes in microseconds under an electric voltage, and with a precision of just a few thousandths of a millimetre. The central component of a piezo-electric injector is the piezo-stack, which directly controls the metering needle. With a response time of just 0.1 milliseconds, the fuel injection can be very sensitively and precisely adjusted to the current load and engine speed, with a beneficial effect on emissions, fuel consumption and combustion noise.
In the V6 unit, the multiple injections even in tiny quantities made possible with piezo-electric injection technology were used by Mercedes-Benz engineers to control a wider, usable characteristic map with the efficient lean-burn process, and to provide the conditions for further functions.
As the first new operating mode, Mercedes-Benz engineers have developed “Homogeneous stratified combustion” (HOS). As the name implies, HOS is a combination of homogeneous lean-burn and classic stratified combustion. With the engine unthrottled, the first injection is sprayed into the intake stroke, forming a homogeneous basic mixture. Actual “stratified” injection takes place during the compression stroke before ignition, and is a single or double injection depending on the characteristic map.
Another new operating mode is known as “Homogeneous Split” (HSP). In this homogeneous combustion process, more than 95 percent of the fuel is singly or multiply injected, followed a very small “ignition” injection to stabilise combustion. This is used when combustion conditions are difficult.
The characteristic map of the new Mercedes-Benz V6 engine is therefore basically divided into up to four areas:
- idling range àhomogeneous
- low partial load up to 4 bar and 3800 rpm àstratified
- medium partial load 4 to 8 bar and up to 4000 rpm àHOS
- high load and entire engine speed range àhomogeneous or HSP
Multi-spark ignition for optimal efficiency
The third-generation direct injection system also features rapid multi-spark ignition (MSI). Following the first spark discharge and a brief combustion period, the coil is rapidly recharged and a further spark is discharged. The MSI system enables up to four sparks to be discharged in rapid succession within one millisecond, creating a plasma with a larger spatial expansion than conventional ignition. Controlling this rapid multi-spark ignition enables both the time lapse before the next spark and the combustion duration for the relevant operating point to be optimally adjusted. This provides scope for optimising the centre of combustion and improving residual gas compatibility, especially during stratified charge operation. Fuel consumption can be reduced by roughly two percent in this way.
Fuel savings of up to 4 percent are possible alone by the use of piezo-electric injection technology in combination with multi-spark ignition, depending on the driving cycle.
Cylinder head with new camshaft adjuster
Mercedes-Benz engineers modified the variable, hydraulic vane-type camshaft adjusters for the intake and exhaust sides using the basic architecture of the preceding engine. These now have a larger adjustment range of 40 degrees with reference to the crankshaft. They were also able to improve the functionality, achieving a 35 percent greater adjustment speed and adjustability at an oil pressure as low as 0.4 bar. Despite the better performance, this new development excels with significantly smaller dimensions and low weight. For this reason the installation space on the longitudinal and vertical axes of the engine was able to be reduced by around 15 millimetres.
Two-stage chain drive for low noise
The extreme compactness of the camshaft adjusters was achieved by the new, two-stage chain drive. This drives two secondary chains – one per cylinder bank – via a primary chain and an intermediate gear. All three chains can be individually adjusted via a chain tensioner. This results in low tensioning forces and low chain dynamics, ensuring consistent timing and outstanding acoustic properties, with friction reduced even further. In short, the new chain drive is compact and ensures low-noise operation.
Exhaust system for stringent emission standards
The exhaust systems for the SLK were developed to meet the exhaust emission standards Euro 5 and ULEV, as well as noise emission standards. The exhaust system for the 4-cylinder petrol engine consists of a firewall catalytic converter, a centre muffler and two rear mufflers.
The emission control system for the V6 petrol engine has a firewall catalytic converter for the right and left cylinder banks. The exhaust gases then flow into storage-type NOx catalytic converters. These are followed by a twin-pipe centre muffler and rear mufflers designed as reflection/absorption mufflers on the left and right. The system is rounded off by two exhaust tailpipes on the right and left.
ECO start/stop function with intelligent direct-start
The exemplary efficiency, with fuel consumption reduced by up to a quarter compared to the preceding model, is also due to the ECO start/stop system which is fitted as part of the standard specification in all models – now also with the 7G-TRONIC PLUS automatic transmission.
Realising an auto-stop and auto-start function at the high comfort level of a Mercedes-Benz requires complex control logic, a number of new or modified components and a heavy-duty starter motor. Operating principle:
- As a general principle, the engine is switched off every time the vehicle comes to a standstill (auto-stop function)
- With an automatic transmission, there is always an automatic restart when one of the following conditions is met – the driver operates the accelerator, the transmission is shifted to R, the transmission is shifted from P, the driver releases the brake pedal and Hold is not activated.
- With a manual transmission, there is always an automatic restart when one of the following conditions is met – the driver operates the accelerator, the driver operates the clutch.
Restarting the engine (auto-start function) takes place almost imperceptibly; and moving off without any noticeable time lag compared to a stationary vehicle with its engine running. This is because the new start/stop system uses starter-supported direct-start. This means that when the engine stops, a new crankshaft sensor identifies the rotational position of the crankshaft and enables the engine control unit to identify the cylinder in which the piston is ideally positioned for restarting. The fuel is injected into this cylinder first, thereby speeding up the starting process. The engine is then briefly turned over by the starter, immediately making reliable injection, ignition and combustion possible. On restarting, a reduced fuel quantity can be injected directly into the first cylinder to ensure a fast starting procedure, e.g. at traffic lights.
An additional electric transmission oil pump supplies the clutches of the automatic transmission with oil pressure prior to starting, so as to enable a swift resumption of the journey after direct starting of the combustion engine via the ECO start/stop function. The starter motor (starter) has also undergone thorough modification: it is now designed to cope with eight times as many starting procedures, ensuring that it will last a car’s lifetime in continuous urban driving involving frequent auto-starting. In addition, the on-board electrical system is supported by a second battery.
Automatic transmission with fuel economy benefits
The SLK 200 BlueEFFICIENCY is equipped with the familiar 6-speed manual transmission, which excels with a short, sporty shift travel. The improved 7G-TRONIC PLUS seven-speed automatic transmission is available for this model as an option, and is standard equipment for the other two engine variants (a 6-speed manual transmission will be available as an alternative for the SLK 250 BlueEFFICIENCY at a later date). It has undergone extensive further development with the aim of achieving an engine connection with further decreased torque converter slip, a significant reduction in internal losses and therefore optimised efficiency. Mercedes-Benz developers subdued eccentricities and vibrations in the transmission by incorporating a new torsion damper, for example. This is designed as a twin-turbine damper, and also makes comfortable vehicle operation possible at low engine speeds.
In ECO mode this has enabled engine speeds to be reduced as a result, with a considerable contribution to fuel economy (up to seven percent). It also allows an even faster response to accelerator commands. A new hydraulic circuit and optimised torque converter mapping also contribute to better start-off performance. As a further fuel economy measure, a new transmission fluid with a lower viscosity and a long service life for the same cooling performance is used. This means that replacement of the transmission fluid is only necessary every 125,000 kilometres.
The optimised electrohydraulic control unit of the transmission and new reduced-friction materials in various parts of the transmission in combination with optimised software result in improved shifting comfort. Owing to an optional, additional demand-controlled ATF pump, 7G-TRONIC also can feature a start/stop function for the first time. This pump ensures readiness to move off when the internal oil pump is not operating.
Adjustable radiator shutter not only saves fuel
For the first time in a sports car, and depending on the engine variant and country version, Mercedes-Benz uses an adjustable radiator shutter in the SLK. This enables the airflow through the engine’s cooling module and engine compartment to be variably controlled. A circular, louvred shutter arranged parallel to the fan improves the aerodynamics, which in turn lowers fuel consumption. The engine also reaches its normal operating temperature more rapidly, heating comfort is improved and engine noise is suppressed even more.
Diesel variant and an AMG version to follow
In the fourth quarter of 2011 a powerful four-cylinder diesel engine will be available for the new SLK. In 2012 the SLK 55 AMG will follow with a new V8.
Source: Daimler AG