16/10/11

Overview: Mercedes-Benz B-class

One glance at the new B-class will confirm that, however far-reaching the technical changes under the surface, Mercedes does not want its newcomer to look too different from the original B-class. We have seen a similar caution recently in the styling of BMW’s new 1-series.

The language of the Stuttgart marketing folk centres around words like ‘agility’ and ‘efficiency’. The latter, of course, is a preoccupation throughout the industry. The new car is substantially lower than before — 1557mm against 1603mm — helping it to achieve a startlingly good drag coefficient of Cd 0.26 for the cheapest petrol version — the others manage Cd 0.27 — despite the marque’s current vogue for blunt and aggressive frontal design; details such as airflow around the front wheels, underbody design, a serrated wheel arch spoiler and management of the cooling airflow are implicated. An Eco Technology package will be offered in due course — we don’t have details at present — which will result in the drag coefficient dropping to a quite extraordinary Cd 0.24.

All power-units are new, directly-fuelled and turbocharged, paralleling BMW’s engineering decisions for the new Einser. The two gearboxes on offer are also new: a seven-speed dual-clutch unit and a closely-related six-speed manual. There are also a number of new driver-support systems.

Dramatic improvements in fuel consumption from one model to the next have become commonplace, but we don’t want to belittle the achievements of Mercedes in improving the B’s rolling-road results substantially: the best improvement, model-by-model, is 21 per cent. We would be a little more comfortable if we could be sure the improvements over the NEDC test régime reflected rel improvements in everyday driving. But that’s a moot point.

All the engines have been developed in-house, which is relatively unusual: most manufacturers farm out the final stages of engine development to specialist companies such as Ricardo. Both diesel models (B 180 CDI and B 200 CDI) now manage an overall 4.4l/100km, though they are just about dissimilar enough to produce different CO2 outputs — 114g/km and 115gkm respectively. Their predecessors used 5.2l/100km.

The new dual-clutch transmission has a big effect on the fuel and CO2 returns for the automatic versions. The new B 180 CDI emits 115g/km with the dual-clutch box: this is an improvement of 21 per cent. on its predecessor’s 146g/km.

The petrol-engine variants have also improved. The B 180 and B 200 now use 5.9l/100km for CO2 emissions of 137g/km and 138g/km respectively, undercutting their predecessors by 10-12 per cent.

We have mentioned that the new B-class is lower overall than its predecessor, by 45mm; the seat height in relation to the road has also been reduced, in this case by 86mm — in other words, the seat is lower in the car by 41mm, and headroom has improved slightly. The seating position is more upright, though, apparently in response to customer requests, and the rear floor has been lowered.

The B-class can be had with a system called Easy-Vario-Plus. This allows for the reorganisation of the interior to a certain extent, helping with the carrying of bulky items. Features of the system include fore-aft adjustment of the rear seats by up to 140mm — which increases the luggage compartment volume from 486l to 666l — and a front passenger seat whose backrest can be folded forward onto the seat cushion.

Engines and gearboxes are built at several plants: Untertürkheim, Hedelfingen, Rastatt, Gaggenau and Kölleda. The simultaneous development of power-units and transmissions allowed for a good degree of coordination — for example, in integrating the stop-start function which, as we would expect, is standard on all B-class cars. A four-point mounting system is used for the drive-train, comprising an engine mount, a transmission mount and two self-aligning supports. A hydraulic damper is integrated into the transmission mount.

The new four-cylinder petrol engines (known internallly as M270) mark the launch of a completely new engine series. The combustion process is based on the third-generation of Daimler’s direct petrol injection system which was introduced last year with the Blue Direct V6 and V8 engines.

The new four-cylinder engine has been designed for both transverse and longitudinal installation. In the new B-class, it is available at first with a single displacement of 1.6 litres and with either of two outputs: 122PS in the B180 and 156PS in the B200. Their torque maxima of 200Nm and 250Nm respectively are available from a commendably low engine speed of 1250rpm. A two-litre variant of this engine will follow later.

The new four-cylinder diesel engine is a development of the OM651 unit used in 2.1-litre form in a variety of other Mercedes-Benz cars. For the B-class, the swept volume has been reduced from 2143cc to 1796cc and assorted, unspecified detail changes have been made.

This smaller ‘D18’ variant of the OM651 is available with two outputs: 109PS in the B 180 CDI, or 136PS in the B 200 CDI. Torque maxima are respectively 250Nm from 1400rpm and 300Nm from 1600rpm. These figures compare with 120PS and 136PS for the 2.1-litre engine in the C 180 CDI and C 200 CDI respectively; corresponding torque outputs are 300Nm at 1400rpm and 360Nm at 1600rpm. Interestingly, the ‘200’ derivative of the larger unit offers no advantage in outright power, though it does deliver a convincing 20 per cent. more torque.

Dynamics

The new B’s centre of gravity should be lower than its predecessor’s, and this is in fact the case. Another factor mitigating for better handling is a new four-link rear suspension arrangement. So far as driver assistance is concerned, the car’s Electronic Stability Program (ESP) has been upgraded, so te newcomer should certainly be a measure more idiotproof than before. A sports suspension set-up is available as an optional extra: this drops the ride-height by another 20mm and includes sports dampers with amplitude-selective damping and a ‘direct-steer’ system in which the steering ratio becomes noticeably more direct as the steering angle increases. (Amplitude-selective damping means that the damping rate varies with wheel displacement.)

The electromechanical power-steering of the B-class has been redesigned: the electric motor of the servo assistance system is now located directly on the steering gear as a dual-pinion EPS system. We have seen elsewhere the advantages of EPS in terms of efficiency (that is, low power consumption) and the functions that it makes available. Mercedes cites a number of these:

  • Counter-impulse in case of oversteering;
  • Corrective steering when braking on road surfaces offering different levels of grip (split-fraction braking);
  • Mitigating the extent to which the front-wheel drive influences the steering;
  • Compensating crosswind and road gradients;
  • The electric power steering also makes Active Park Assist possible.

The new B features a radar-based collision warning system — increasingly common in larger cars, though something of a rarity at this level. This gives a visual and acoustic warning to alert the driver to obstacles which the system has identified as a potential poblem; it also prepares the brake assist system, which cut in as soon as the driver presses the brake pedal sharply. An adaptive brake assist system is standard.

The falling prices of off-the-shelf safety and assistance technology from companies like Bosch and Delphi is making these quite sophisticated systems available in smaller cars, and the new B is certainly a beneficiary of this trend. Systems available on the newcomer include

  • Adaptive high beam assist;
  • Blind spot assist and lane keeping assist;
  • Attention assist (standard);
  • Speed limit assist (speed limit sign recognition);
  • Active parking assist;
  • Brake hold function;
  • Hill hold function;
  • Linguatronic;
  • Reversing camera;
  • Distronic Plus.

The Pre-safe preventative occupant protection system features in the new B-class. This system appeared for the first time in the S-class in 2002. Its main features are reversible belt tensioning, the closing of side windows and the sunroof in case of critical lateral dynamics, and the adjustment (if it’s fitted) of the electric front passenger seat with memory function to an ideal position for maximum effectiveness of the restraint systems. Pre-safe is always activated in response to critical longitudinal and lateral dynamics, and it can also be triggered if some of the car’s assistance functions cut in.

In the rear, belt tensioners, belt-force limiters and belt height adjusters come as standard on the outer seats. Rear sidebags are available as an optional extra. Integrated child seats and automatic child seat recognition are available. The driver and front passenger are provided with new pelvis bags which are able to cover the pelvis and the entire upper part of the body. For head protection, a window-bag is fitted as standard.

So far as ‘infotainment’ is concerned, there are two levels of functionality available. The Comand [sic] Online multimedia system is the superior system, offering internet access when the car is stationary. On the move, the user can call up a Mercedes-Benz app. which Mercedes claims is simple enough to use while driving — we beg to differ. Integrated apps include Google Local Search and Weather and the facility for downloading a route which has previously been configured on a PC using Google Maps and transmitted to the car.

The bodyshell of the new B has been designed using what Mercedes calls the Energy Space concept. This means that the car is ready to accommodate versions with alternative drivetrains: appropriate interfaces in the body shell allow the main floor panel to be modified and a step to be produced for versions with ‘alternative’ drives. An underfloor compartment covering part of the area under the rear bench seat offers space for a battery-pack.

The seven-speed 7G-DCT dual-clutch transmission is making its début in the new B-class. It is perforce a compact unit, as it must be for transverse installation, and boasts an electric oil pump for use with a stop-start system. The new three-shaft six-speed manual transmission, known as F-SG 310, is a close relative of the DCT.

Increasingly, ‘virtual’ prototyping is being used to iron out the more fundamental problems with a new design of component or assembly, and the same applies to electronic systems. Daimler uses a driving simulator to carry out initial testing and calibration of safety systems before they are tested in real cars. The simulator is at the Company’s Technology Centre in Sindelfingen. It has a 360° screen and a 12m-long rail for transverse or longitudinal movements. Allegedly the most powerful in the industry, it went into operation at the end of 2010.

A dome measuring 7.5m in diameter is mounted on six individually extendable cylinders forming a ‘hexapod’. A complete vehicle is installed in the dome. The inner surface of the dome serves as a projection panel on which road traffic situations are displayed true-to-life, with moving pedestrians, oncoming traffic and housing. The handling of any current or future Mercedes model can be simulated mathematically, and the test driver experiences the calculated vehicle movements realistically and in real time.

We have mentioned the four-link wheel location at the rear of the B-class. In this design, forces are absorbed by three control arms and one trailing arm. The purpose of a multi-link system like this is to separate — as far as possible — lateral and longitudinal forces. Wheel carriers and spring links are made from aluminium, reducing unsprung masses.

Torque vectoring brakes are used to balance the car’s handling as part of the current ESP system. The electric power steering feeds torque into the steering column to help the driver decide how to deal with a tricky situation. The engineers’ aim apparently was to provide gentle intervention from the ESP system wherever necessary, to allow experienced drivers to get the best from the car and to remind less the skilled of the physical limits involved in driving.

All variants come with disc brakes on all wheels. The rear callipers and the brake boosters are made of aluminium. The hold function which is familiar from the larger model series has been adopted on the B-class: this is engaged by pressing the brake pedal firmly after bringing the car to a halt, and the brake will remain engaged until the driver presses the accelerator. On versions with manual transmission, the hill hold function automatically prevents the vehicle from rolling back unintentionally when starting on a slope. An electric parking brake is used: this operates by means of actuator motors acting on the callipers of the rear axle. When the parking brake button is pressed at speeds of over 4km/h, the parking brake acts as an emergency brake, activating all four wheel brakes by way of the ESP’s hydraulic unit. With automatic transmission, the parking brake is released automatically when the driver accelerates.

We have mentioned the B’s excellent aerodynamics, and the measures taken to smooth the air-flow around the front wheels. Additionally to this, the main floor panel features extensive cladding up to the rear wheel arch, followed by additional cladding of the rear axle. The rear exhaust silencer has also been designed with aerodynamics in mind.

A self-adjusting radiator shutter is used on the basic petrol-engine model in the B-Class. It provides infinite adjustment of the air-flow through the engine’s cooling module and the engine compartment by means of a circular louvred shutter installed in parallel with the fan. When there is no need for external cooling air, the shutter is closed completely.

The Eco Technology package, which will be optionally available for all versions of the B-class later on, incorporates additional measures designed to reduce drag. These include a lower ride-height, the sealing of joints at the rear window and the cladding of chassis components at the rear axle.

Petrol engine

The new M270 unit is directly fuelled with piezo injectors. A multiple injection régime is used, with up to five injections per power stroke and spray-guided combustion. Maximum fuel delivery pressure is 200 bar (20MPa). The mixture is ignited by multi-spark ignition.

The oil pump has variable capacity, and a switchable coolant pump is used, both reducing parasitic losses when the management unit determines that demand is low. Considering the engine uses forced induction, the mechanical compression ratio is high, at 10.3:1. Head and block are cast from aluminium alloy, and a hollow crankshaft contributes to the dry mass of 137kg. The twin camshafts are chain-driven.

A two-piece water jacket is used, with cross-flow cooling and fine cooling ducts measuring only three millimetres between spark plugs and injectors. Thermal management is used to help the engine get to operating temperature quickly: when the unit is cold, a switchable coolant pump with a ball-valve prevents coolant flowing through the engine, so the combustion chambers heat up quickly after starting up the engine. The thermostat is electronically controlled, and coolant temperatures are adjusted according to driving style and ambient temperatures. The thermostat itself is also a ball valve.

The M270 features cylinder spacing of 90mm and is designed for compactness, largely through the arrangement of the power take-off points. The 1.6-litre variant features a short stroke of 73.7mm with a bore of 83.0mm.

The control unit of the new 7G-DCT transmission communicates with that of the engine. High levels of fuel efficiency at high engine loads are claimed, as the mixture does not need to be enriched to cool the cylinder head at road speeds below 120mph.

Diesel engine

The OM651 engine first appeared in 2008, with a swept volume of 2143cc, and it is used right across the Mercedes-Benz range. The 1.8-litre ‘D18’ variant is new and was developed for the B-class. Compactness for transverse mounting was perforce a development priority. As before, the block is cast iron and the head is of aluminium alloy; the cast iron liners have been reworked in the interest of reducing friction. A two-piece water jacket in the cylinder head provides cooling in the area of the combustion chamber plate, allowing high ignition pressures and a high specific power output.

To compensate for the second-order vibration which is inherent in four-cylinder in-line engines, there are two Lanchester-type balancer shafts at the bottom of the engine block running on low-friction roller bearings. A dual-mass flywheel is used.

The belt drive for the power take-off points, the turbocharger mounting position and the air ducting have been modified for the purposes of transverse mounting. Cylinder spacing is 94mm and the camshafts are driven by spur gears.

The reduced swept volume was achieved by shortening the stroke from 99.0mm to 83.0mm. The markedly longer connecting rods deliver lower lateral forces, reducing frictional losses. The bearings of the two Lanchester balancer shafts have also been redesigned to reduce friction.

A new, single-stage turbocharger is deployed, with adjustable guide blades.

Similarly to its big brother in its latest versions, the engine incorporates vibration isolation between belt drive and crankshaft to allow the stop-start system to work unobtrusively.

The rail pressure of the injection system has been increased to 1800 bar (180MPa). The maximum ignition pressure is 180 bar.

The oil injection nozzles and the coolant pump are activated only when they are needed. The oil pump is a variable-capacity unit.

Dual-clutch 7G-DCT transmission

The new box is an automated three-shaft ‘manual’ transmission consisting of two sub-transmissions, each with its own wet-plate multi-disc clutch. Both actuation of the clutches and gear shifting take place automatically and without any significant interruption in torque transmission. The hydraulic fluid is actively cooled to help maintain the unit’s durability despite the comparatively low filling volume of six litres.

The use of seven forward gears allows a wide ratio spread of 7.99 without particularly wide gaps between gears. Mercedes claims that the dual-clutch unit is nine per cent. more efficient than the CVT that was used in the previous B-class and that its mechanical efficiency compares with that of a manual transmission. At 367mm long and weighing 86kg, the 7G-DCT is quite compact and light.

The transmission is supplied with lubricant by two pumps: one mechanical, one electric. The electric pump maintains oil pressure when the engine is shut off by the stop-start system, so the unit doesn’t suffer a period of ‘dry’ operation each time the car restarts in traffic. Additionally, the electric pump is used to increase the maximum pumping pressure (and flow volume) under high loads; this arrangement allows a smaller mechanical pump to be specified.

The hydraulics for the transmission’s parking lock, along with drive and reverse functions, are electrically activated from a small lever mounted on the steering column.

Three gearshift programs are available to the driver.

  • Economy: The transmission performs gear shifting automatically, keeping engine speeds down as much as possible.
  • Sport: Also fully automatic; the shift and response times are shorter.
  • Manual: The driver operates the transmission manually using shift paddles behind the steering wheel — these are standard with the 7G-DCT gearbox. Shift and response times are shorter still.

In Economy or Sport mode, the driver is still able to intervene manually in the gear-shifting process using the shift paddles. The transmission reverts to the selected automatic mode after the paddles have remained inactive for ten seconds, or after a longer delay when driving downhill or on winding roads.

The 7G-DCT is manufactured at Daimler’s Stuttgart-Hedelfingen plant.

Six-speed manual transmission

The new six-speed manual transmission has been developed alongside the 7G-DCT. The two transmissions share a large number of components and are both produced in Stuttgart-Hedelfingen. Like the dual-clutch unit, the three-shaft manual box is compact — 345mm long — and light — 46kg dry.

The selection of neutral (for the stop-start system) and reverse (for the reversing lights) is detected using a magnet integrated onto the gearshift shaft whose position is detected by a Hall sensor. A cable linkage is used, and the blutch actuation is hydraulic.

The ratio spread of 6.7 compares with, for example, 6.2 for BMW’s 120d manual box. Clearly, a wider ratio spread has advantages and disadvantages: narrower gaps between ratios allow the engine speed to be kept closer to the ideal for the driving conditions in town and at middling speeds, while a wider spread can offer a higher top for cruising. You pays your money and takes your choice.

The overhead camshaft with third and fourth gears and the reverse gear do not run in the oil bath, apparently reducing drag torque. Three-cone synchronisation is used for the first two forward gears; the other gears are provided with two-cone synchronisation. Shift forks are of cast aluminium.

Radar-based collision warning

Mercedes believes that a half of all rear-end collisions could be avoided, or their severity reduced, using the radar-based safety systems that are available off the shelf from companies such as Delphi. The cost of such systems is dropping as production volumes increase, and we have seen them filtring down steadily from the top of the market into humbler cars. Mercedes calls the systems Collision Prevention Assist and Adaptive Brake Assist.

Having said all this, the new B will not be available with these systems initially: they are due to appear within the first year of production.

The new assistance system gives a visual and acoustic warning to alert the driver of a potential problem, and primes the brake assist system; this is triggered if the driver brakes sharply.

The system works like this. First, it recognises an inadequate distance from a vehicle ahead in the speed range between 30km/h and 250km/h, and recognises when the gap is decreasing. If there is a danger of imminent collision, the driver is provided with visual and acoustic warnings.

Additionally, the colision warning system can recognise stationary objects ahead of the vehicle and issue warnings. It is programmed to recognise specific driving situations, such as bumper-to-bumper traffic, and it adapts the activation threshold for the warning and adaptive brake assist accordingly, calculating the braking force ideally needed to avoid an accident when the danger of a collision is detected, and making the best possible use of any distance remaining. This means that the driver behind also has better chances of preventing a rear-end collision.

The brake pressure is adjusted if the situation changes: if the vehicle ahead accelerates, for example, braking force is decreased, while if the gap ahead decreases, the brake pressure is increased further.

Preventative safety systems such as seat-belt pretentioners can be activated as necessary.

The new system meets the key Forward Collision Warning requirements of the NHTSA, the American road and vehicle safety authority.

Driving simulator

Mercedes describes its new driving simulator at Sindelfingen, which went into operation at the end of 2010, as a ‘photorealistic environment’. It’s also a ‘mechanical-realistic environment’, with the test car mounted in a pod that is thrown around by electric motors to simulate the loads a car would experience on the road.

The simulator has a 360° screen. To deliver transverse and longitudinal movements, the pod is moved on a 12m-long rail.

The simulator is not intended to replace real test drives, but it will enable systems and components to be tested more conveniently throughout the development process. The new Mercedes B-Class was one of the first models to receive its final fine-tuning in the new driving simulator before it was driven on the road.

As well as testing vehicles and components, the simulator is used to conduct tests with people. Normal car drivers are able to approach the car’s physical handling limits with no danger, providing the development engineers with findings concerning the acceptance and operation of new safety systems, such as the radar-based collision prevention assist and adaptive brake assist.

The simulator cell is a hexapod mounted on six moveable supports. Inside there is a complete car in which the test driver is seated, surrounded by the 360° projection screen showing a realistic image of a traffic situation, with moving pedestrians, oncoming traffic and houses.

The vehicle’s controls are linked by data lines to the computerised system that controls the driving simulator. When the test driver turns the steering wheel, accelerates or operates the brakes, these reactions are registered by the control system and have (so far as the driver is concerned) the same effects as they would in real life. The scenery on the screen changes constantly, and the moving pod simulates the vehicle’s attitude on the road — for example, front-end dive when braking, or body-roll during fast cornering. The computer calculates the driving behaviour of the car more than 1000 times per second, issuing the relevant commands to the electrics. It can move the cell sideways by up to 12m at a maximum speed of 10m/s (36km/h). Some of the energy required to operate the simulator is recycled by means of energy recuperation when the pod is being braked; it is then fed into the power network of the Sindelfingen plant.

Mercedes-Benz B 180 CDI B 200 CDI B 180 B 200
Cylinders 4I 4I 4I 4I
Block/head Fe/Al Fe/Al Al/Al Al/Al
Valves 4 4 4 4
Aspiration/
geometry
T/f T/f T/f T/f
Injection
pressure
180MPa 180MPa
Bore/stroke TBA TBA 83.0/73.7 83.0/73.7
Swept volume 1796cc 1796cc 1595cc 1595cc
Compression
ratio
16.2:1 16.2:1 10.3:1 10.3:1
PS/rpm 109/3200 136/3600 122/5000 156/5300
Nm/rpm 250/1400 300/1600 200/1250 250/1250
Maximum speed 117 129 117 135
0-100km/h 10.9 9.5 10.4 8.6
Urban MPG
(l/100km)
52.3
(5.4)
52.3
(5.4)
35.3
(8.0)
35.3
(8.0)
Combined MPG
(l/100km)
64.2
(4.4)
64.2
(4.4)
47.8
(5.9)
47.8
(5.9)
CO2 g/km 114 115 137 138
Emissions EU5 EU5 EU5 EU5
Transmission
I
II
III
IV
V
VI
Final drive
M6 (DC7)
3.94
2.19
1.22
0.86
0.72
0.59
3.348
M6 (DC7)
3.94
2.19
1.22
0.86
0.72
0.59
3.348
M6 (DC7)
4.31
2.44
1.35
0.94
0.82
0.70
3.348
M6 (DC7)
4.13
2.44
1.35
0.94
0.82
0.70
3.348
Driven wheels Front Front Front Front
Kerb mass † 1400 1400 1320 1320
PS/t 77 97 92 118
Nm/t 178 214 151 189
Length 4359
Width 1786
Height 1557 1558 1557 1558
Wheelbase 2699
Track
— front
— rear

1552
1549

1548
1545

1552
1549

1548
1545
Cd 0.27 0.27 0.26 0.27
Tyres 195/65R15 205/55R16 195/65R15 205/55R16
Brakes Dv/D
Fuel tank 50l
† Manual transmission models, DIN kerb mass. For E.U. kerb mass, add 75kg. For models with dual-clutch transmission, add 30kg.
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