Due for launch in January next year, the new Mercedes CLS represents a technical evolution of its seven-year-old predecessor. As we have come to expect, the engine line-up offers an increase in outputs and a reduction in emissions; lightweight construction techniques have been used, and the drag coefficient is — as usual for a Mercedes — impressively low, at Cd 0.26 for all models.
The CLS suspension system is a re-worked version of that used on the new E-class. At the front, wheel location is by three-links; at the rear, a lightweight multi-link arrangement is used, attached to a subframe.
For the first time in a Mercedes, electromechanical power steering is used, offering control over both the weighting of the system and also the power it consumes in use. Mercedes quotes a remarkable fuel saving of ‘up to’ 0.3l/100km (9.4mpg) just from switching from hydraulic assistance to an electromechanical system.
LED headlamps are standard, with each unit using 71 diodes. Bi-xenon lighting is available as an optional extra. The LED units have an estimated service life of 10,000 hours, which is around five times longer than you could expect from a xenon light. The LED headlamps give a light spectrum that approaches the colour of daylight, which is considered useful in minimising eye-strain.
Colour Temperature (Kelvin)
Mercedes’ Intelligent Light System is standard. This is familiar from models with bi-xenon headlamps, and features five light functions: country mode, motorway mode, enhanced fog lamps, active light function and cornering light function.
Mercedes CLS: LED lighting.
Adaptive High Beam Assist is also fitted. This is a system that aims to get the best vision for the driver without causing glare for other motorists. If an oncoming vehicle or a vehicle in front with its lights on is detected, the system dims the headlamps and continually adjusts the headlamp range in such a way that the cone-shaped beam of headlamp light does not reach these vehicles.
Technology suppliers like Bosch and Hella are increasingly offering modular driver-assistance systems off-the-shelf, and we have seen an explosion in their use over the last year or so. Even relatively mainstream models now incorporate systems to help the driver park — in some cases, even operating the steering by way of a motor on the column — to remain in lane, to stay awake, to avoid ploughing into the back of a traffic queue, or simply to be aware of a vehicle in the car’s blind-spot. As takeup of these systems increases, so costs will drop, and it won’t be long before relatively humble models feature simple ‘low-modular’ incarnations of many of them.
The CLS is not a humble model. It comes loaded with a dozen or so driving assistance systems intended to help to prevent accidents and to reduce the severity of an accident when it becomes inevitable.
The Active Blind Spot Assist and the Active Lane Keeping Assist are new to the CLS. Active Blind Spot Assist uses short-range radar to warn you, if you are about to change lanes, that there is a danger of collision. If you ignore the warnings and the car comes dangerously close to the next lane, the system will intervene by applying braking force to the wheels on the opposite side of the vehicle, using the car’s Electronic Stability Program. This obviously creates a yaw movement which swerves the car out of danger.
The Active Lane Keeping Assist system is now also linked to ESP. This system cuts in if the Mercedes inadvertently drifts over a solid line to the right or left of a lane. A camera attached to the windscreen monitors the lane markings. If the car does drift, the system gently brakes the wheels on the opposite side. At the same time, a display on the instrument cluster warns the driver of his folly. If broken lane markings are crossed, the system controls an electric pulse generator in the steering wheel which generates vibrations for a short time. Before the braking system intervenes, the steering wheel always vibrates to provide a warning.
Active Parking Assist including Parktronic
The new CLS can detect a parking space and operate the steering for you while you’re backing into it. The system uses electomechanical direct steering and a series of ultrasonic sensors. Its essential functions are:
Measurement of the length and depth of potential parking spaces at a vehicle speed of up to 36km/h;
Calculation of a suitable parking course based on the current position of the vehicle;
Control of the steering in order to maintain this parking course.
Parking spaces are measured using two ultrasonic sensors integrated into the right and left sides of the front bumper. Depending on the speed, these sensors are used to measure parking spaces and also to support the Parktronic functionality. If you are driving at less than 30km/h, a ‘P’ symbol is displayed in the instrument cluster to inform the driver that the active parking space search function is enabled. If a parking space is found, an arrow appears next to the ‘P’ symbol. By default, the arrow indicates parking spaces on the passenger side. If the turn indicator is set to the driver’s side, parking spaces on that side will be displayed. It is not necessary to switch on the parking space search function. If the ultrasonic sensors detect a pavement, the vehicle will be parked parallel to this.
The Active Parking Assist parking process begins when the vehicle is in front of the parking space indicated by the arrow, facing the direction of travel, and the driver has engaged reverse gear and confirmed a warning dialogue in the instrument cluster. The driver continues to control the accelerator and brake pedals whilst manoeuvring into the parking space, while the Parking Assist takes over the steering. The maximum speed of this parking process is limited to 10km/h. If the speed reaches 7km/h, a warning appears in the instrument cluster, announcing that ‘Parking Assist only functions up to 10km/h’, accompanied by a warning tone. The Parktronic function can be used without any restrictions being imposed. The system switches off if the driver intervenes in the automatic steering control process, switches off the system via the PTS (Parktronic Sensor) button, if both the driver’s seat belt buckle is released and the driver’s door opened, or is a system error occurs.
The Parktronic and Active Parking Assist system consists of the following components:
Ten ultrasonic sensors, two of which have an extended range;
PTS warning elements on the dashboard and in the roof lining at the rear;
A maximum of five manoeuvres are allowed (backwards-forwards-backwards, and so on). The wheels are then straightened out again. The final parking manoeuvre is notified in the instrument cluster with the message ‘Parking Assist completed’, accompanied by a confirmation tone.
Around a quarter of all serious motorway accidents are caused by drowsy drivers, making this factor an even bigger cause of accidents than drink-driving. Attention Assist is a possible answer to this, and is fitted as standard in the new CLS.
The system employs high-resolution sensors to observe driver behaviour, and can recognise whether the driver is tired or not paying attention. This is based primarily on measuring and analysing steering wheel movements, measuring around 70 parameters. Based on a variety of collected data, Attention Assist creates an individual driver profile during the first few minutes of each journey and compares this with sensor data and the driving situation as recognised by the vehicle’s electronic control unit. Alongside values such as the steering behaviour of the driver, the Mercedes-Benz system also measures driving conditions such as speed, longitudinal and lateral acceleration and indicator and pedal usage, as well as external factors such as the unevenness of the road.
The radar-based Distronic Plus proximity control supports the driver at speeds between zero and 200km/h by automatically adjusting the distance to the vehicle in front. It is able to apply the brakes to bring the vehicle to a complete standstill, and it can also accelerate the car again when clear space is available. In stop-and-go traffic, the system can detect if the distance in front is being reduced too quickly; it then warns the driver with both visual and audible signals.
With the introduction of the new generation of the CLS, the functionality of this system has been refined to react earlier to a vehicles that cuts in. Information from digital maps has now been incorporated into the system.
Assorted assistance systems
Adaptive brake: This offers assistance functions for drivers who have problems operating the parking brake correctly: a Hold function for waiting at traffic lights, and Hill-Start Assist which can help prevent rolling backwards when moving off on a slope.
Adaptive High Beam Assist: When vehicles are detected ahead, either oncoming or travelling in the same direction, this system automatically dips the beams and adjusts the range of the headlamps appropriate to the distance.
Brake Assist Plus: This system, is able to recognise an impending rear-end collision using radar sensors. It calculates the necessary degree of braking assistance and makes it available immediately when the driver presses the brake pedal.
Headlamp Assist: A sensor on the windscreen detects the lighting conditions and can be set to switch on the headlights automatically. Standard fare.
Speed Limit Assist: A camera fitted behind the windscreen detects speed limit signs at the roadside and compares this data to information contained in the GPS system. The relevant speed limit is then displayed in the instrument cluster.
Night View Assist Plus: The display in the dashboard shows a realistic grey-scale image from an infrared camera that monitors the road ahead. Any pedestrians detected are highlighted in the display with ‘photo corners’.
Pre-safe Brake: Autonomous braking if there is immediate danger of an accident. Initially the driver is given both an acoustic and an optical warning; if the driver does not react to this, the system brakes the vehicle autonomously. This occurs in two stages. Around 1.6 seconds before the calculated impact, the system decelerates the car with around 40 per cent. of the maximum braking power, or roughly 4m/s². This gives the driver an additional, haptic warning of the impending impact, and as a precaution activates the reversible Pre-Safe occupant protection system. If the driver still fails to react, the Pre-Safe brake activates the maximum braking power around 0.6s before the now-unavoidable collision. The Pre-Safe brake is active at speeds of between 30 and 200km/h when moving vehicles are detected in front of the car. The system also reacts if the car approaches a stationary queue of traffic, providing its speed is below 70km/h.
The blind spot and lane-changing
Active Blind Spot Assist warns the driver if it detects a risk of collision should the driver want to change lanes.
Short-range radar sensors housed on both sides of the rear bumper monitor the areas directly alongside and behind the car. If there is a vehicle in the blind spot, the system informs the driver by illuminating a red warning signal in the glass of the exterior mirror. If the driver fails to see this warning and indicates to change lanes, a warning signal sounds as well.
If the driver ignores the audible warning as well, and the Mercedes comes dangerously close to the next lane, the system will apply braking force to the wheels on the opposite side of the vehicle using the Electronic Stability Program. This creates a yaw movement which steers the car away from the impending collision.
The system deactivates as soon as the driver steers against the effects of the braking intervention or if the vehicle accelerates.
If an accident can no longer be avoided in spite of correcting the car’s direction, the Blind Spot Assist system can mitigate the consequences of a collision through another course correction. If the system detects vehicles or obstacles just a short distance away on the opposite side, it will adapt its braking intervention, using data from the front sensors for Distronic proximity control. Braking interventions to correct the car’s course can operate between 30 and 200km/h. The effect is limited to longitudinal and lateral deceleration of 2m/s². Visible warning in the exterior mirror is active up to a speed of 250km/h. When ESP is switched off, Active Blind Spot Assist is deactivated.
Active Lane Keeping Assist with ESP support
Active Lane Keeping Assist is now linked to the car’s ESP system. This system cuts in if the Mercedes inadvertently drifts over a solid line to the right or left of a lane. In this case, Active Lane Keeping Assist uses the ESP system to brake the wheels on the opposite side to the direction of drift; a display on the instrument cluster warns the driver at the same time.
If broken lane markings are crossed, the system triggers an electric pulse generator in the steering wheel causing vibration.
Before the braking system intervenes, the steering wheel always vibrates to provide a haptic warning.
Active Lane Keeping Assist relies on a camera fitted to the inside of the windscreen. The images from the camera are fed into software that can recognise lane markings using a contrast algorithm. The image processing software determines the position of the vehicle.
In addition to the camera-based optical system, the new generation of Lane Keeping Assist also evaluates radar signals: this is effectively a safety backup. Only when both systems concur is braking force applied to correct the car’s course.
The Mercedes assistance system also assesses the behaviour of the driver to determine whether the vehicle is leaving its lane intentionally or unintentionally. Braking intervention to correct the car’s course is available between 60 and 200km/h, but it is not deployed if:
ESP is deactivated;
the caris on a bend of radius less than 150m;
the vehicle’s tyres are in emergency run-flat mode;
when braking or accelerating by more than 2m/s², or driving on a bends with a lateral acceleration of over 2 m/s².
The steering wheel will not vibrate if the driver:
is accelerating before overtaking or joining the motorway;
steers into a bend;
cuts a corner intentionally;
uses the indicators;
is moving back into the original lane after overtaking;
Also, Lane Keeping Assist is deactivated immediately if ABS, ESP, Brake Assist or another active safety system intervenes.
Body and structure
Happily, mass-reduction is back after a decade or more in the wilderness. The new Mercedes CLS is a good paradigm for the trend: lightweight materials combined with new production processes allow better strength-to-mass for the bodyshell, while the fitment of increasingly elaborate on-board systems pulls in the opposite direction.
The main talking point so far as the CLS’s body is concerned is the frameless aluminium doors. These are made from deep-drawn aluminium panels with extruded sections; in comparison with conventional steel doors they’re around 24kg lighter. Individual elements of the doors are joined using a combination of gluing and riveting. This door design meets the same seal requirements as Mercedes’ conventional designs, including the need to withstand 80 bar (8MPa) of water pressure.
Elsewhere in the structure, there is the predictable increase in the use of aluminium and high-strength steels.
The bonnet, front wings, boot lid, parcel shelf, various support profiles and substantial parts of the suspension and engines are all made of aluminium. The front end is a hybrid construction made of aluminium panels and plastic strengthened with fibre-glass. The one-piece aluminium crash boxes in the front area are fitted in the side members.
The large front bumper with its integral radiator grille is made of polypropylene. The CLS uses adjustable fixtings in the nose to allow panel fits to be tweaked.
About 72 per cent. of all panels used for the bodyshell of the new CLS are made from rigid and ultra-rigid steel alloys. The ultra-rigid high-tech alloys, which have three to four times more tensile strength than conventional rigid types of steel, account for around eight per cent. of total mass. They are deployed in areas in which there could be extreme material stresses in the event of an accident: for example, the B-pillars and side frame of the roof are required to channel the energy of a side impact which avoiding deformation, so that the passenger doors can be opened after the crash.
Compared to the previous CLS, the front and rear crumple zones have been enlarged, and an improvement in energy flows is claimed. The front crumple zone has four independently-acting impact levels (load-paths), meaning that impact forces can be distributed over a wide area while bypassing the passenger cell. The four load-paths are:
Sectional panels above the wheel arches form the upper side-member level. From here, impact forces are channelled into the A-pillars and, subsequently, into the roof frame.
An aluminium crossmember connects the forward-extended side members and ensures that the forces are transferred to the side facing away from the impact. The crossmember and the forward-extended side members form the central impact zone.
The subframe to which the engine, steering and front axle are attached also serves as a load-path in the event of a frontal collision. It is made of high-strength steel and, depending on the engine variant, can be connected to the floor side members by means of supporting tubes. As a consequence, the subframe can deform in a predetermined way, absorbing energy, while channelling ‘surplus’ impact forces straight into the vehicle floor.
The side skirts have been extended forwards to support each wheel and prevent it from entering the footwell in the event of an offset frontal collision. Special struts and additional energy-absorbing elements are used for the wheel-arches; the struts are arranged diagonally and prevent the passenger cell from sinking in the event of an impact.
The firewall is a four-part construction. This design enables the material thickness to be varied according to the level of vulnerability to crash energy. The loads acting on the firewall during a frontal crash is greatest in the lower section, so the sheet steel used here is almost 50 per cent. thicker than the surrounding area.
Connecting points between the suspension and the bodyshell, which are required to withstand very high forces, are specifically reinforced to ensure that road-induced vibrations are not transferred to the body. Mercedes quotes gains in bodyshell strength of 28 per cent. for flexural strength and six per cent. for orsional strength.
The main floor assembly consists of different sheet-metal plates that either undergo flexible rolling or are laser-welded together and subsequently shaped. Flexible means that the high-tensile steel can be processed in such a way that areas with different steel thicknesses can be produced within a single component. The middle blank forms the transmission tunnel. Here the thickness of the panels varies between 0.7mm and 1.1mm, and between 1.55mm and 2.0mm for the tunnel reinforcements, depending on the stresses and loads to which they are subjected.
The continuous floor side members, the insides of which are further reinforced with extra sections, are important both for occupant protection and the rigidity of the bodyshell. Their front faces connect to the side members, thereby lengthening the load-bearing paths along which forces can be distributed in the event of an impact. At the rear, the floor side members extend as far as the crossmember beneath the rear seat unit to stabilise the entire floor structure.
Sturdy transverse aluminium sections known as transmission tunnel braces have been incorporated into the floor assembly. One is located beneath the transmission, and is designed to direct forces into the side of the vehicle in the lee of the impact in the event of a side-on collision. The second forms a connection between the two side members; it likewise braces the floor assembly and is able to channel impact forces into the floor structure at an early stage during a side impact.
Multi-piece side members and a robust, flexible crossmember made from ultra-high-strength steel form the key components of the rear-end structure. The rear side members are continuous, closed box sections with graduated material thicknesses. These are able to absorb high impact forces. The bolt-on flexible crossmember is manufactured using a flexible rolling process which also allows the material thickness to be varied as required along the length of the component. The material thickness on the outside of the crossmember, where impact loads are highest, is greater than on the inside.
Pasenger safety systems
The CLS’s airbag count includes two adaptive bags for the driver and front passenger, a knee-bag for the driver, two side-bags in the front seat backrests and two large window-bags which extend from the A-pillar to the C-pillar during a side impact. Pelvis-bags are fitted as standard for front seat passengers; in the event of a side impact, these can help to reduce loads on the pelvis. Side-bags are available for the rear passenger compartment as an optional extra.
Mercedes is offering adaptive belt force limiters for the rear seats as an option. These adapt to the size of passengers automatically. The system decides whether a rear seat passenger is large or small based on the belt extension. The belt’s grabbers and force limiters operate different régimes according to the size of the seat’s occupant: if the passenger is large, the maximum belt force is activated immediately, while a smaller occupant will experience a lower force.
Neck-Pro is the name Mercedes-Benz has given to a crash-responsive anti-whiplash head restraint whose development is based on analyses of real accidents. If the car’s sensor system detects a rear-end collision with a defined impact severity, it releases pre-tensioned springs inside the head restraints, causing the head restraints to move forward by about 40mm and upwards by 30mm within a few milliseconds. This increases the degree of support for the occupants’ heads at an early stage of the collision.
In 2002, Mercedes-Benz introduced an anticipatory occupant protection system called Pre-Safe. The system can initiate measures to protect the occupants if there is an imminent risk of an accident. The measures taken are reversible: if the accident is averted, the advance tensioning of the seat belts is halted automatically and the occupants are able to reset the positions of the seats and the sliding sunroof. The anticipatory occupant protection system is then ready for action again immediately.
Pre-Safe is linked to Brake Assist and the Electronic Stability Program, whose sensors detect potentially critical driving situations. Because these are ‘real-time’ electronic systems, they work on millisecond timescales.
The new CLS offers these preventive measures:
Previous incarnations of Pre-Safe responded only to emergency braking. The system in the new CLS can also be activated if the optional Distronic Plus has detected an impending collision, with the aid of its short- and long-range sensors, and a predetermined delay in the driver’s braking response has been exceeded. The occupants are prepared for the collision by the tensioning of the seat belts and (if the seat memory function is installed) the repositioning of the front-passenger seat. The latter feature allows the seat belts and airbags to offer better protection.
The active multi-contour seats, which arean optional extra, set out to seat the driver and front passenger more securely, limiting whiplash movements by the upper body in the event of an accident. If the Pre-Safe control unit detects a potential accident, it immediately activates air chambers in the seat cushions and backrests; these then envelope the seat occupants and give them more support.
When installed in combination with Distronic Plus, Pre-Safe uses the information provided by the short-range radar sensors in the front bumper to tension the front seat belts around 200ms before an unavoidable collision, reducing the forces exerted on the driver and front passenger during the crash.
Four new engines are available:
Diesel engines: four-cylinder CDI available for the first time; V6 CDI is 21 per cent. more economical.
Petrol engines: Blue Direct reduces consumption by 25 per cent. in the V6.
Start-stop function for reduced consumption in urban traffic.
7G-Tronic Plus automatic transmission standard on all variants.
All of the engines are now dubbed Blue Efficiency, and all boast increased outputs and reduced consumption compared with previous models. The three-litre V6 diesel engine in the CLS 350 CDI now offers an increase in output of 40PS while consuming 21 per cent. less fuel than its predecessor.
Daimler calls its direct petrol injection system Blue Direct. The third-generation system provides a 25 per cent. reduction in fuel consumption on the 3.5-litre V6 CLS 350 despite an increase in maximum power of 13PS; in terms of CO2 emissions, this engine now matches its diesel counterpart. (The diesel consumes smaller volumes of a more dense fuel.)
The CLS will become available then with 4-Matic four-wheel drive for the first time in the near future.
Mercedes’ four-cylinder 2.2-litre diesel is also being made available in the CLS. It’s an impressive engine in the E-class, though the extra mass of the big coupé will give it more work to do. Still, 500Nm is no joke, and the official CO2 emissions return of 134g/km is not to be sniffed at.
The three-litre V6 diesel has been completely revised. It now has improved engine management in the form of new-generation control units and new software, new sensors and actuators, and more efficient after-treatment in the maintenance-free diesel particulate filter system with reduced back-pressure. In addition to more effective cooling of recirculated exhaust gas using a variable bypass valve, the in-engine measures include a reduction of the compression ratio from 17.7:1 to 15.5:1, an optimised VNT turbocharger with low-friction shaft bearings for greater agility and high output, new injection nozzles and revised ducting in the intake tract. A package of measures has resulted in substantial fuel savings: these include generally lower in-engine friction (through increased precision in the finishing of the cylinder walls, amongst other measures), more efficient thermal management, an improved compound oil pump and modified cylinder head cooling. The idle speed has also been dropped to 520rpm.
The second generation of direct petrol injection made its debut in the CLS 350 CGI introduced in 2006. The six cylinder unit was the world’s first petrol engine with piezo-electric direct injection and spray-guided combustion, and as such achieved a fuel saving of around ten percent compared with its predecessor with port injection.
The V6 Blue Direct petrol engine in the CLS 350 differs from its predecessor in the following areas:
60° cylinder angle and omission of balancer shaft.
Extended lean-burn operation, lean-burn system with load monitoring from pressure information, new combustion system operating modes.
Resonance intake manifold.
Latest generation of piezo injectors.
Enhanced cooling circuit control.
Enhanced oil circuit control.
Increased output and torque.
Daimler was the first car manufacturer to put a spray-guided direct injection petrol engine into mass production. A spray-guided combustion system uses the injectors to direct fuel jets in such a way as to produce stratified charge: stoichiometric at the spark plug, lean elsewhere. Injection takes place, rather like a diesel engine, at the moment at which combustion is required to begin. The injection pressure is now up to 200 bar (20MPa). Newly-developed piezo-electric injectors allow up to five injections per power stroke, and multi-spark ignition is used.
While the 3.5-litre V6 unit is an atmospheric engine, the new and technically similar V8 engine, due to be introduced during 2011 in the CLS 500, features two turbochargers. It too has direct injection, but was designed for countries without sulphur-free fuel and is therefore run continuously in homogenous mode — that is, injecting during the intake stroke and not offering stratified charge operation. Despite a significantly smaller displacement than its predecessor — 4633 cc against 5461 cc — the V8 has made gains in terms of power (408PS, up from 388PS) and torque (600Nm against 530Nm). At the same time, consumption has been significantly reduced: it is expected to achieve a saving in the region of 25 per cent.
The Blue Direct engines have two new operating modes for extended lean-burn operation.
Piezo injection control is used, with multiple injection pulses. This has allowed an extension of lean-burn operation for the new generation of V-engines across a wider range of the characteristic map while also providing two additional operating modes:
‘Homogeneous stratified combustion’: On the face of it, a contradiction in terms. HOS is a combination of homogeneous lean-burn and classic stratified combustion. With the engine unthrottled (throttle valve open), the first injection is sprayed into the intake stroke, forming a homogeneous basic mixture. Stratified injection then takes place during the compression stroke before ignition, and is a single or double injection pulse depending on the engine speed and load.
‘Homogeneous split’: In this homogeneous combustion process, more than 95 per cent. of the fuel is singly or multiply injected, followed by a very small ‘ignition’ injection to stabilise combustion. This is used when combustion conditions are difficult.
In order to ensure that the lean mixture is ignited reliably, the third-generation direct injection system uses Rapid Multi-spark Ignition (MSI). Following the first spark discharge and a brief combustion period, the coil immediately above the corresponding spark plug is recharged rapidly 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. The extension of the lean-burn operating phases offers improvements in fuel consumption at low engine loads.
Both the new V-engines have aluminium crankcases, pistons and cylinder heads. The crankshaft, connecting rods and valves are of forged steel. Engine friction has been reduced by 28 per cent. compared with the previous engine. Also, reduced parasitic power consumption by the ancillary components is claimed: a new coolant pump is used, improved thermal management, a demand-controlled oil pump, a volume-controlled high-pressure fuel pump and an intelligent alternator management system contribute to the savings.
New variable, hydraulic vane-type camshaft adjusters have been introduced for both intake and exhaust camshafts. These adjusters have an adjustment range of 40° with reference to the crankshaft and are 35 per cent. faster in their responses than previously.
Considerable compactness has been achieved for the camshaft adjusters by the use of a new, two-stage chain drive. This drives two short 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 good acoustic properties, along with low system friction.
The coolant ducting in the cylinder head is completely new. Reduction of hydraulic pressure losses and detailed improvements to the cooling system have made it possible to reduce the power consumption of the coolant pump despite increased cooling requirements.
It was possible to cut component weights by the concerted replacement of aluminium and steel by plastics: for example, in the thermostat, belt pulley, impeller, heater valve and hydraulic lines. Depending on the load on the components, the flow of coolant is regulated to maintain target temperatures between 80° and 105°. Hot coolant is supplied first to the cabin heater matrix, so during the warm-up phase the temperature inside the car now reaches the selected setting quickly.
In the interest of refinement, Mercedes has introduced a series of new and modified components into the drivetrain, such as a Hall-effect sensor for the crankshaft and a high-output starter motor.
The CLS’s stop-start régime is fairly standard. The engine is shut off when the vehicle is stationary and re-started when the brake is released. Clearly a rapid re-start is important. To achieve it, a Hall sensor is fitted to the crankshaft; this identifies the rotational direction of the crankshaft and enables the engine control unit to identify which cylinder is on the compression stroke — that is, which cylinder is in the best position for re-starting. When a re-start is necessary, fuel is injected into this cylinder first, speeding up the starting process. This is a similar solution to that adopted by Mazda.
An additional electric transmission oil pump supplies the clutches of the automatic transmission with oil pressure prior to re-starting. The starter motor is now designed to cope with eight times as many starting cycles, with the intention that it will last the car’s lifetime in continuous urban driving involving frequent auto-starting. The on-board electrical system is supported by a second battery.
It is not always expedient for the engine to be shut down automatically when the vehicle comes to a standstill. The auto-stop function is initiated only if a number of conditions are met:
The engine must have warmed to a predetermined minimum operating temperature.
There must be sufficient voltage in the on-board electrical system, the interior temperature should have been stabilised following the key start, the accumulator for the air suspension or brake system must be sufficiently full.
The relevant conditions relating to the driver must be met: the transmission selector lever must be set to D or N; no movement of the accelerator or the steering wheel; the driver’s foot must be on the brake or the HOLD function must be active; the doors must be closed, the driver’s seat belt must be fastened and the bonnet must be closed.
The stop-start system must not have been switched off via the ECO button.
Relevant speeds must have been exceeded after starting with the ignition key or during manoeuvring, for example.
The observant will have noticed that if the driver follows best practice and applies the parking brake when the car is stationary, the stop-start system will not work.
Auto-starting takes place when the engine is in auto-stop mode: the engine has been shut down by the engine-stop function, and the ignition remains switched on. One of the following conditions must also be met:
The accelerator pedal is pressed.
The transmission selector is set to Reverse.
The transmission selector is moved out of Park.
The brake pedal is released and Hold is not activated; or the parking brake is released and the transmission selector is not set to Park.
The ECO stop-start function is switched off via the ECO switch.
The vehicle begins to roll.
A function that requires the engine to be running — raising the ride height, for example — is activated by the driver.
The stop-start function can also start the engine automatically as a comfort or safety function. The engine control unit starts the engine automatically, without any intervention on the part of the driver, when one of the following conditions is met:
One of the preconditions for shutting off the engine ceases to be met.
The driver releases his seatbelt or opens the driver’s door. In this case, automatic starting takes place in order to prompt the driver to switch off the engine properly using the ignition key. This ensures that the stop-start system is safely deactivated when the car is parked.
To reassure the driver that the stop-start system is working (or to warn him otherwise), an ‘ECO’ symbol is displayed on the multifunction display in the instrument cluster:
Green: All conditions are met: engine will be shut down when the vehicle stops.
Yellow: ECO is active, but conditions are not met.
No ECO symbol displayed: ECO is switched off or has been deactivated because of an error.
One rather curious decision has been taken as a result of the improvements in the CLS’s fuel economy returns: the fuel tank capacity of all models bar the five-litre V8 has been reduced to rather puny-looking 59l. An 80l tank is an optional extra, and understandably standard on the V8.
7G-Tronic Plus automatic transmission
All CLS models come with the 7G-Tronic Plus automatic transmission as standard; improvements in efficiency and refinement are claimed, with changes to the torque-converter and gear-trains. For the latest torque converter, Mercedes claims an provides an improved dynamic response, better durability and reduced noise and vibration levels thanks to a new hydraulic circuit, as well as improved dampers and torque converter housing. The extreme wheel-slip reduction of the torque converter housing, combined with larger mechanical damper de-coupling, helps to reduce consumption significantly. In addition, the gearshift program in ECO mode has been changed in favour of lower engine rpm at steady speeds.
The transmission uses a new automatic transmission oil (FE-ATF) with reduced viscosity. When the cooling limits are observed, the increased service life of the new oil results in oil change intervals of 125,000km instead of 50,000km. The improved electrohydraulic assembly of the transmission and the new friction-reducing materials used on various transmission parts, combined with optimised software, also lead to better shift quality.
The CLS will also be available with Mercedes’ 4-Matic four-wheel drive for the first time in the autumn of 2011.
Essentially an E-class set-up with extensive revisions, the CLS chassis sees electromechanical power steering for the first time. This feature is likely to find its way onto other Mercedes models in due course. Mercedes rather optimistically describes the CLS’s new power steering as a ‘pioneering innovation’, rather ignoring the fact that such systems have been used by other manufacturers for some considerable time. The Company is also keen to stress that its new system is more expensive to make than conventional hydraulic systems.
In the CLS, the steering gear and the supporting servo-motor form a single compact unit which is mounted as previously on an extremely rigid and low-weight integral support frame made of high-strength steel; this is positioned in front of the wheel’s centre line. As previously, the steering ratio becomes more direct as more lock is applied.
A remarkable fuel saving of 0.3l/100km (9mpg) is claimed just from the adoption of electromechanical power steering.
The sensors and control unit allow a greater degree of freedom in configuring the feel and weighting of the steering system under different driving conditions. Active damping enhances the feeling of safety when driving straight ahead at high speed.
Power assistance remains available when the engine is shut off, although the load on the car’s battery will clearly be substantial — the engine is not prompted to start in stop-start mode if the driver plays with the steering wheel while the vehicle is at a standstill. Perhaps more significantly, the new steering system has also allowed Mercedes to fit its Active Park Assist automatic parking system.
The new steering system also assists with braking on a surface whose level of grip varies from side-to-side. (This is known as a split-µ surface.) Brake on this type of surface, and the car will yaw, turning towards the side with the greater grip. Sensors recognise the putative yaw, and the steering is automatically turned slightly to encourage the driver to counteract the yaw. If he acts on this signal, the steering intervention is shut off.
Using cars equipped with programmable electromechanical power steering, Mercedes carried out ’clinic’ tests in Europe and the U.S. to discover what steering feel characteristics the sample drivers liked. It seems that drivers on both sides of the Atlantic showed almost exactly the same preferences. The clinic trials tested tastes in the following areas:
Feedback: Results showed that all drivers would like to feel immediate resistance when they turn the wheel, as well as a defined increase in steering moments at increased speed and lateral acceleration.
Centre alignment and damping: This provides the feeling of secure straight-line stability at higher speeds.
Response rate: How dynamically does the steering wheel respond in returning to the straight-line position after cornering?
Physical effort: What is perceived as comfortable and appropriate when manoeuvring, driving in urban traffic, along winding country roads or on the motorway?
Turning circle diameter and the gearing of the steering rack.
The result was that Mercedes customers want precise steering which offers relaxed driving at speed, and therefore high levels of stress-free comfort on the motorway; on the other hand, they want good manoeuvrability with high gearing winding roads.
A total of around 250 parameters were recorded. Their assessment has been used to define the Mercedes-Benz steering characteristics, which in future will form the basis for tuning all vehicles with electromechanical steering. The underlying feature is that the steering is perceived as comfortable across all model series. As part of this, individual tuning according to vehicle type remains one of the core competences of the development engineers: the expectations of drivers at the wheel of an M-class are different from those at the wheel of an SLS AMG.
Front wheel location is by way of three-links with a McPherson strut: a reworked version of the E-class arrangement. There are two individual links — a pulling strut and a cross strut — in the lower link plane; these are positioned at a steeper angle than on the E-Class, raising the roll centre to 90mm. This results in improved anti-roll characteristics without any increase in the diameter of the anti-roll bar; it also adds negative camber on the outer wheel when cornering, providing a crisper turn-in and better grip. The anti-roll bar is connected to the suspension strut.
The forward-sloping pulling strut is a forged aluminium component, while the cross strut is of forged steel. The tie rod is defined as the third link, forming part of the rack-and-pinion steering system. According to Mercedes, the arrangement and design of the wheel control components, and in particular the way in which the lower A-arm is divided into two individual links, offer better axle kinematics than would a fixed A-arm. Also, the kingpin inclination is closer to the wheel centre; this minimises sensitivity to tyre imbalances and fluctuations in braking force.
The lightweight multi-link rear suspension is mounted on a subframe. Like the set-up for’ard, it has been adapted from the E-class. Almost all wheel control parts are produced in aluminium, to reduce the unsprung masses. An additional support has been added for the front bearing between the subframe and the body, giving an improvement in NVH isolation.
The standard coil-spring suspension of the CLS uses selective damping, whereby the dampers adapt to the conditions. The spring struts on the front axle consist of cylindrical, lateral force compensating coil springs, dual-tube shock absorbers and newly-introduced three-phase head bearings. The front anti-roll bar is connected to the spring strut, which assists in controlling the front wheels.
Air suspension is standard on the forthcoming CLS 500, and is an optional extra on other models. Known as Airmatic, it is combined with an electronically controlled continuously variable damping system which controls each wheel individually. A total of seven sensors monitor the driving conditions and body movements, relaying information to an electronic control unit; this also processes information on vehicle speed, steering wheel angle, braking torque and engine torque to determine the appropriate damping rate. Fast response speeds are claimed. In evasive manoeuvres, the damping forces are adapted immediately to the car’s driving dynamics. The driver can alter the vehicle’s damper settings manually.
Automatic ride-height control is a feature of the Airmatic system. The level of the vehicle does not alter as a result of loading. The ride height is lowered by 15mm at speeds of over 140km/h. Additionally, the driver can increase ground clearance on rough roads or on difficult upward slopes by pressing a button.
With the Adaptive Brake feature, the new CLS offers a variety of aids for the driver. One example of this is the Hold function: after braking to a standstill, briefly pressing the brake pedal a little further engages this function. The car is then held by the brakes, even if the driver’s foot comes off the brake pedal. The Hold function is deactivated automatically when the car moves off.
If the driver’s foot moves abruptly from the accelerator to the brake pedal before an emergency stop, the brake system increases the pressure in the brake lines and brings the pads into contact with the brake discs so that maximum braking power is available as soon as the driver hits the brake pedal.
Adaptive Brake operates a brake-drying function. When the windscreen wipers have been operating for a certain length of time, the film of water is periodically removed from the brake pads by gentle, automated applications of the brakes: not enough to slow the car, but enough to allow the brake pads to clear water from the discs. This automatic brake drying function is always active.
Finally, Adaptive Brake operates the ubiquitous hill-holding function.
Brake calliper housings are produced in aluminium or aluminium composite to reduce unsprung mass.