13/2/11

Overview: Mercedes C-class coupé

The new C-class coupé sees Mercedes-Benz entering a new market segment, offering a compact coupé for the first time and — techically — presaging the forthcoming new C-class saloon. The new two-door C-class is set to be launched at the Geneva Motor Show in early March, with deliveries beginning in June.

The C-class coupé has four individual bucket seats. The rear pair can be folded down to extend the boot space.

As always with a new model, the coupé’s telematics take a step on from earlier models. The displays are larger, while features include phone book transfer, display of text messages — we have yet to verify whether this is enabled when the car is moving — wireless music reproduction via Bluetooth, and a USB interface accommodated in the centre armrest.

The multimedia system COMAND Online now provides internet access for the first time. When the car is stationary, the driver can surf at will; Mercedes claims that its own on-line services load particularly rapidly and ‘are also easy to use while on the move’ — an interesting boast. The integrated services include weather information and a special destination search through Google, as well as the option of downloading a route that has been previously configured on a PC using Google Maps and sent to the car. The navigation system of COMAND Online also has added functions. New features include a 3D display with ‘plastic’ city views. Also new: routes covered can be recorded for re-use, specific personal destinations can be saved or imported via an SD card, and four alternative routes can be displayed on the navigation map, one of them a particularly economical variation.

Drivetrains

The new C-class coupé is available with a choice of two diesel engines — both versions of the 2143cc four-pot — and three petrol units. All are directly-fuelled and fitted with an automatic stop-start system as standard.

Mercedes C-class coupé: rear accommodation is strictly for two; seats fold.

All automatic versions use Daimler’s 7G-Tronic Plus seven-speed automatic transmission; this is standard on the C 250 and C 350 petrol versions. The C 220 CDI and C 250 CDI diesel variants and the basic C 180 petrol engine come with a six-speed manual transmission as standard.

AMG model aside, the most powerful engine in the range — at least, in terms of top-end horsepower — is the 3.5-litre V6 petrol engine in the C 350. This uses Daimler’s Blue Direct spray-guided injection system (see below) and delivers 306PS and 370Nm. (The C 220 CDI beats that torque figure by 30Nm.) Overall fuel consumption in the NEDC rolling road test is 41.5mpg.

The four-cylinder petrol engines, like the diesels, have direct injection and are turbocharged

At present, Mercedes-Benz is quoting only overall MPG figures — we consider the urban cycle results more informative — and even there we are presented with maxima and minima for each model. The differences, presumably, are a result of tyre equipment and other factors. Nevertheless, vague though the figures are, it looks as though the only variant which is more economical with a manual transmission — of those available with either the six-speed manual or the seven-speed automatic — is the entry-level C220 CDI.

Chassis

The ‘agility control’ suspension, which is standard equipment, is based on an amplitude-dependent damping system. When driving in a straight line, with low suspension deflection, the damping forces are automatically reduced in the interest of road noise and ride quality. When suspension deflection is greater — for example, when cornering at speed or performing evasive manoeuvres — higher damping forces are set. Speed-sensitive power-steering with variable centring and a constant steering ratio is standard in the U.K.

So far as wheel location is concerned, the C-class does not skimp. The front wheels use a three-link arrangement with McPherson spring struts. The lower control arm consists of two separate elements, which serve as torque and cross struts. Both are made of aluminium. The third link in the front suspension is the tie rod which connects the transverse steering gear to the wheels. The anti-roll bar is connected to the spring strut.

At the rear, each wheel is attached to five flexibly mounted, independently acting control arms which control five of the available spatial movements; each rear wheel retains freedom of movement in one plane only, namely during controlled compression and rebound.

As part of the AMG Sports Package, which is standard in the U.K., the C-class coupé will feature sports suspension and speed-sensitive sports steering.

Driver assistance & safety

The C-class coupé has seven airbags as standard: to wit, front airbags and sidebags for the driver and front passenger, a kneebag on the driver’s side, and headbags. The side protection system comprises a head-bag and a thorax-bag. To protect pedestrians, the coupé has an ‘active’ bonnet made of aluminium.

A number of driving assistance systems are fitted, including Attention Assist — detecting drowsiness — and the Distronic Plus proximity control system. The assistance systems are based on radar, camera and sensor technology, and cover frequent accident causes such as driving too closely, fatigue and darkness.

Some of the assistance systems give a warning in hazardous situations, while some actively intervene.

Mercedes-Benz
C-class Coupé (not AMG)
ABS Standard
Adaptive high beam assist Standard
Active lane keeping assist Optional
Active blind spot assist Optional
Attention assist Standard
Distronic plus Optional
ESP Standard
Headlamp Assist Standard
Speed limit assist Optional
Intelligent light systems Standard
Parktronic with parking guidance Standard
Pre-safe Standard
Pre-safe brake Optional
Lane keeping assist Optional
Blind spot assist Optional

C 63 AMG

The C-class saloons and station-wagons have AMG variants, so it is only right that the coupé should too. Power comes from AMG’s familiar 6208cc atmospheric V8, driving through the Speedshift MCT seven-speed automatic transmission. Headline outputs are 457PS and 600Nm. AMG’s performance package, comprising lighter internal engine components and a variable-length intake manifold, boosts top-end power by 30PS, but with no increase in maximum torque and a very marginal improvement in performance.

AMG’s Speedshift MCT seven-speed automatic transmission consists of a conventional epicyclic gearbox attached to a much less conventional wet-plate start-off clutch. There is no torque-converter. As we have come to expect, there are a number of driving modes, including a ‘Controlled efficiency’ mode for day-to-day driving. Driving in ‘C’ mode gives up-changes at relatively low engine speeds and a ‘soft’ accelerator response. The other modes — ‘S’, ‘S+’ and ‘M’ — deliver progressively faster gear-changes at higher change points. Gear-changes in S+ and M take 100ms, which is very quick for an epicyclic box.

Needless to say, running-gear is substantially upgraded, though the basic suspension design remains the same. The track is wider at both ends, and negative camber is also increased, providing better turn-in and bite. Bushes are stiffer and anti-roll bars are thicker. Changes to the spring and damper rates are claimed to benefit road noise levels as well as handling. The electro-hydraulic power steering is quite direct at 13.5:1.

The car’s ESP system has two stages — standard and sport — and can also be switched off.

The various electronic driver assistance systems fitted to the C 63 AMG coupé are set out here.

Mercedes-Benz C 63 AMG
ABS Standard
Adaptive high beam assist Optional †
Active lane keeping assist Optional
Active blind spot assist Optional
Attention assist Standard
Distronic plus Optional
ESP Standard
Headlamp Assist Standard
Speed limit assist Optional
Intelligent light system Optional †
Parktronic with parking guidance Optional †
Pre-safe Optional †
Pre-safe brake Optional
Lane keeping assist Optional
Blind spot assist Optional
† Standard equipment on non-AMG models.
Mercedes C 63 AMG coupé.
Mercedes-Benz
C-class coupé
C 220 CDI C 250 CDI C 180 C 250 C 350 C 63 AMG
Cylinders 4I 4I 4I 4I 6V 8V
Swept volume 2143cc 2143cc 1796cc 1796cc 3498cc 6208cc
PS/rpm 170/3000 204/4200 156/5000 204/5500 306/6500 457/6800
Nm/rpm 400/1400 500/1600 250/1600 310/2000 370/3500 600/5000
Maximum speed 142 148 137 148 154 154
0-100km/h 8.1 7.1 8.8-9.0 7.2 6.0 4.5
Combined MPG
(l/100km)
53.2-57.6
(5.3-4.9)
53.2-57.6
(5.3-4.9)
40.3-43.4
(7.0-6.5)
40.3
(7.0)
40.3-41.5
(7.0-6.8)
23.5
(12.0)
CO2 g/km * 128-139
(117-133)
128-139
(128-143)
150-162
(157-169)
152-163 159-164 280
Transmission M6 (A7) M6 (A7) M6 (A7) A7 A7 A7
Driven wheels Rear Rear Rear Rear Rear Rear
* Figures for automatic transmission; manual transmission in brackets.

Blue Direct

The most powerful C-class coupé, the C 350, uses Daimler’s 3498cc petrol V6. Unlike all of the other petrol and diesel engines in the range, it’s an atmospheric unit, and it uses a spray-guided stratified-charge direct-injection system called Blue Direct.

‘Stratified charge’ means that the fuel:air mixture ratio is not homogeneous throughout the combustion chamber. With the intention of running with a lean overall fuel:air ratio to save fuel, a stoichiometric mixture is presented at the spark-plug — ensuring a healthy initial flame-front — while the mixture in other parts of the combustion-chamber is much weaker. In modern engines, this ‘stratification’ in the fuel:air mixture is achieved using very finely engineered multi-hole fuel-injectors; this is called ‘spray-guiding’. Other methods to create a stratified mixture involve control of intake air or the use of a physical feature cast into the piston-crown.

Like other stratified-charge engines, the Mercedes Blue Direct unit has different operating modes, switching between them as engine loads change. To broaden the range of loads over which the engine can run lean, the Blue Direct system borrows piezo injection technology and multiple injection techniques from current diesel engine practice.

The Blue Direct engine has two operating modes that are worthy of note.

Homogeneous stratified combustion (HOS): An apparent contradiction in terms, this is a combination of homogeneous lean-burn and classic stratified combustion. With the engine unthrottled (throttle valve open), the first injection is delivered during the intake stroke. This forms a homogeneous basic mixture. The stratified injection takes place during the compression stroke before ignition, and is a single or double injection depending on the operating conditions.

Homogeneous split (HSP): 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.

To make sure that the lean mixture is ignited reliably, the current Blue Direct V-engines feature rapid multi-spark ignition (MSI). Following the first spark discharge and a brief combustion period, the spark-plug’s unit-coil recharges 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 V-engines

Blue Direct is a feature of Daimler’s new V-engine family. Both the V6 and the V8 derivatives have aluminium crankcases, pistons and cylinder heads; the crankshaft, connecting rods and valves are of forged steel.

Engine friction has been reduced compared with the engines’ predecessors: Mercedes claims a 28 per cent. improvement. Additionally, less power is required to drive the engine accessories. Quite apart from progressive engineering refinements, contributors to this reduction in parasitic losses include a demand-controlled oil pump, a volume-controlled high-pressure fuel pump and an intelligent alternator management system.

Variable hydraulic vane-type camshaft adjusters are used for both the intake and exhaust sides. These have an adjustment range of 40 degrees with reference to the crankshaft; they are also 35 per cent. faster than before. The extreme compactness of the camshaft adjusters was achieved by 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 with a chain tensioner. This results in low tensioning forces and low chain dynamics, giving consistent timing adjustment responses and good acoustic properties. Friction performance is said to be good.

The coolant ducting in the cylinder heads is completely new. Reduction in hydraulic pressure losses and refinements to the cooling system have made it possible to reduce the operating energy input of the coolant-pump despite an increased engine output and greater cooling requirements.

It was possible to cut component masses 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 various target temperatures between 80°C and 105°C.

Part of the new engines’ thermal management régime is the extended temperature range in which heat is primarily supplied to the interior of the car. This means that during the warm-up phase of the engine, the temperature of the vehicle interior now reaches the selected setting more quickly.

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