The new Mercedes-Benz B-Class: The drive - High tech under the bonnet
A model change that takes place under the bonnet, too: the new B-Class features new petrol and diesel engines as well as new manual and automatic transmissions. All transmissions and engines have been developed in-house, with production taking place at the plants in Stuttgart-Untertürkheim and -Hedelfingen, Gaggenau/Baden and Kölleda/Thuringia. The simultaneous development process enabled perfect coordination of engines and transmissions, e.g. with regard to implementation of the ECO start/stop function which features as standard on all B-Class models.
Common features of the new drive systems are the use of state-of-the-art technologies for maximum efficiency, very smooth running, high tractive power right from low revs, sustainability in terms of impending emissions standards and low weight. For the purposes of transverse installation the engines, all of which are turbocharged, and the two transmissions are very compactly designed, providing the B-Class with an exemplary small turning circle for a front-wheel drive vehicle of eleven metres. The engine/transmission suspension by means of two hydro-mounts – an unusual solution in this market segment – and two self-aligning supports provides for unusually
good decoupling of the drive units from the body.
The new four-cylinder petrol engines (internal designation M270) mark the launch of a completely new engine series. The combustion process is based on the third-generation Mercedes-Benz direct injection system which was introduced last year with the BlueDIRECT V6 and V8 engines. Its use in the B-Class marks this technology's debut in the compact segment. The new four-cylinder engines have been designed for both transverse and longitudinal installation. In the new B-Class they are initially available with a displacement of 1.6 litres, as the B 180 rated at 90 kW (122 hp) and the B 200 with an output of 115 kW (156 hp). Their maximum torque of 200 and 250 Nm respectively is available from an engine speed of 1250 rpm.
The new four-cylinder diesel engine is a further development of the OM651 deployed in the C- to the S-Class, a common rail direct-injection engine of the third generation. With its displacement scaled down to 1.8 litres and numerous optimised points of detail, this is the first time that the compression-ignition engine has been mounted transversely in a Mercedes-Benz car. The B 180 CDI generates 80 kW (109 hp) of power, while the B 200 CDI has an output of 100 kW (136 hp). The maximum torque of 250 / 300 Nm is available from 1400 / 1600 rpm.
Another Mercedes-Benz premiere is the new 7G-DCT dual clutch transmission in the B-Class. This transmission is extremely compact, extremely variable with regard to adaptation of the engine speed thanks to seven gears, features an electric oil pump for start/stop capability, shifts gear without any interruptions in tractive power and combines the comfort of an automatic with the efficiency of a manual transmission.
The new manual transmission designed along similarly compact lines as a three-shaft transmission is a close relative of the DCT. Extremely easy gear shifting, low shift forces, low internal friction and a low weight are among its most important characteristics.
The new petrol engines: high tech in the compact class
Direct injection with fast-acting piezo injectors for multiple injection, fully variable cam adjustment on the inlet and exhaust side, controlled oil pump, switchable water pump, high compression (10.3:1) despite turbocharging, scavenging for instantaneous turbocharger response, fast and convenient start/stop system – the technical details of the new generation of four-cylinder petrol engines read like a catalogue illustrating the fine art of high tech engine construction.
With regard to the combustion process, the new all-aluminium engines with two overhead, chain-driven camshafts are closely related to the BlueDIRECT V engines which were originally introduced in the S-Class in mid-2010 and have since been setting new benchmarks worldwide in terms of performance and fuel consumption under everyday conditions in the luxury and top-range models from Mercedes-Benz. Direct petrol injection with spray-guided combustion, which Mercedes-Benz was the first car manufacturer to introduce in series production, has been developed further by the engineers into third-generation direct petrol injection with spray-guided combustion. The system pressure is up to 200 bar and is variably optimised according to the engine's characteristic map. Completely newly developed piezo-electric injectors allow up to five injections per power stroke for the best possible mixture formation, while multi-spark ignition helps to ensure reliable combustion of the mixture. The upshot is outstanding fuel efficiency combined with minimal emissions.
In conjunction with continuous adjustment of the intake and exhaust camshaft, the direct injection system also provides the essential basis for so-called scavenging.
As a result of a partial overlap of the opening times of intake and exhaust valve, part of the drawn in cold fresh air conveys the hot exhaust gas contained in the cylinder into the exhaust manifold, thereby improving the filling level substantially in comparison to the conventional mode of operation. Due to the increased mass flow in the exhaust line, the turbocharger additionally responds considerably more quickly at low revs in particular, avoiding turbo lag. As a result of the direct injection system, the fresh air is not yet mixed with fuel on entering into the cylinder, as is the case in engines with intake manifold injection. This avoids scavenging losses, that is, the escape of uncombusted petrol into the exhaust manifold. The new four-cylinder engine in the B-Class supplies its maximum torque right from an engine speed of
1250 rpm, for example, maintaining this maximum level up to 4000 rpm. In conjunction with the new 7G-DCT transmission, whose control system communicates with the engine control unit, the new four-cylinder engine responds highly spontaneously to movements of the accelerator pedal, while during cruising higher gearing can be selected, thereby further reducing fuel consumption and noise.
The turbo engines also retain their exemplary fuel efficiency at high load levels, as the mixture does not require enriching to cool the cylinder head up to a speed of 200 km/h (full-load enrichment). A two-piece water jacket with optimum cross-flow cooling and fine cooling ducts measuring only three millimetres between spark plugs and injectors convey the cooling fluid to the critical points. A new thermal management system has also been developed: in cold state, a switchable water pump with flow-optimised ball valve ensures that no coolant flows through the engine, providing for swift heating-up of the combustion chambers after starting up the engine. The thermostat is electronically controlled and the coolant temperatures are adjusted according to driving style and ambient conditions. The thermostat itself is also a flow-optimised ball valve. In the interest of high efficiency, the volumetric flow of the oil pump is also controlled, as in the V engines.
The new four-cylinder generation features cylinder spacing of 90 millimetres and is highly compact in design, thanks to the expedient arrangement of the power take-off units. The variant to be unveiled at the premiere with a displacement of 1.6 litres features a short stroke of 73.7 mm with a bore of 83.0 mm. The hollow crankshaft is a further factor contributing to the low weight of 137 kg.
Overview of technical data for the B-Class with petrol engine:
The new diesel engines: direct-injection models with displacement of 1.8 litres
Since its world premiere in 2008 the fourth-generation direct-injection diesel engine known as the OM651 has been setting standards in terms of performance and torque characteristics, economy, emissions and smooth running. It is in more widespread use than any other Mercedes-Benz diesel engine and serves as a model of efficiency and power right up to the S-Class.
A new variant with a displacement of 1.8 litres is now being introduced into the compact car segment for the first time on board the B-Class. The belt drive for the power take-off units, the turbocharger mounting position and the air ducting have been modified for the purposes of transverse mounting. With a cylinder spacing of 94 millimetres and camshafts driven via spur gears, transverse mounting and the appurtenant short design formed part of the specification for the engine from the outset.
The displacement was reduced by shortening the stroke (83 mm instead of 99 mm). The markedly longer connecting rods provide for a reduction in friction induced by lateral forces. The two Lanchester balancer shafts have also been optimised for reduced friction. The efficiency of the new, single-stage turbocharger has been optimised. It possesses adjustable guide blades.
Combining the engine control unit with the control system of the 7G-DCT involved a great deal of coordination work. Similarly to its big brothers in their latest version, the engine incorporates isolation between belt drive and crankshaft to enable implementation of the start/stop function. Other common features are
- common rail technology with a rail pressure increased to 1800 bar. The maximum ignition pressure of 200 bar also contributes to the high power output.
- The oil injection nozzles and the water pump are activated only when required, in order to save energy and fuel. The controlled oil pump additionally reduces oil flow and thus fuel consumption.
- The engine block is made of cast iron, the cylinder head of aluminium.
- A two-piece water jacket in the cylinder head provides for optimum cooling in the area of the combustion chamber plate. This enables an ignition pressure of 200 bar and a high specific power output.
- The cast iron barrels have undergone considerably finer honing than on the predecessor, also contributing to the reduction in fuel consumption.
- To compensate for the second-order forces which are inherent to four-cylinder in-line engines there are two Lanchester balancer shafts at the bottom of the engine block running in low-friction roller bearings rather than conventional plain bearings.
- The two-mass flywheel has been specifically designed for high engine torque at low engine speeds in order to isolate the crankshaft's vibration stimuli, thereby contributing to the engine's excellent smooth running.
Overview of technical data for the B-Class with diesel engine:
The 7G-DCT dual clutch transmission: convenient and efficient
Drawing on over 50 years of experience in the development and production of automatic transmissions, Mercedes-Benz is venturing into new technical territory with the 7G-DCT dual clutch transmission: the new system is an automated three-shaft manual transmission consisting of two subtransmissions, each with its own clutch. Both actuation of the clutches and gear shifting take place fully automatically and without any interruption in tractive power. This enables a particularly comfortable but nevertheless dynamic mode of driving.
Thanks to its seven gears it offers an exceptionally large spread of up to 7.99. This means that a very short transmission ratio is available when moving off on an uphill slope with a high payload, for example, while during cruising the engine speed can be lowered considerably. This transmission is nine percent more efficient than the CVT employed in the B-Class to date and attains the efficiency of a manual transmission for the first time.
At a length of 367 millimetres and a weight of 86 kilograms, the 7G-DCT is more compact and lighter than the transmissions which have been available on the market to date. The clutches take the form of "wet" multi-disc clutches running in an oil bath. The specially developed hydraulic fluid is actively cooled, thus ensuring correct functioning of the transmission even under extreme conditions, despite the comparatively low filling level of six litres.
For the first time on this type of unit, the transmission is supplied with oil by two oil pumps – one mechanical and one electric. The electric pump maintains the oil pressure when the engine is switched off via the start/stop function. This means that the transmission is immediately operational when the engine is restarted and the vehicle can move off again without any delay. In addition, the electric pump is able to support the mechanical pump when peak loads apply, enabling a more compact and efficient design for the mechanical pump.
Another new feature is electrical activation of the hydraulics for the parking lock, which is locked by mechanical means. This "park by wire" function enables the transmission selector lever to be positioned as desired: it is located in the form of a steering column lever on the right behind the steering wheel in the B-Class. In combination with the electric parking brake, additional space has thus been created in the centre console for additional stowage facilities.
Three gearshift programmes are available to the driver:
- ECONOMY: In this mode, the transmission performs gear shifting fully automatically and particularly conveniently. The gears are selected with due regard to a particularly economical style of driving at low revs.
- SPORT: The transmission performs gear shifting fully automatically in this mode, too. The shift and response times are shorter, however.
- MANUAL: In this mode, the driver operates the transmission manually via shift paddles behind the steering wheel. Shift and response times are shorter still.
In ECO or Sport mode, the driver is still able to intervene manually in the gear-shifting process via 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.
A key factor contributing to the overall efficiency of the B-Class is the closely coordinated operation of the transmissions and engines. A continuous exchange of data between the control units ensures that the engines run at the ideal operating point at all times. The 7G-DCT is manufactured at Daimler's Stuttgart-Hedelfingen plant.
The six-speed manual transmission: convenient manual gear-shifting
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 Hedelfingen. This three-shaft transmission is also very compact (length 345 mm) and light (dry weight 46 kg) and incorporates a number of special features for particularly pleasant gear shifting. An integrated magnet on the gearshift shaft is detected by a Hall sensor. The idle position is identified in this way, enabling the start/stop function. The signal for reverse gear activates the reversing lights.
The large spread of 6.7 allows a reduction in engine speed while at the same time ensuring that sufficient tractive power is available when moving off with a fully laden B-Class and trailer up to a gross weight of 3.4 tonnes. The clutch is operated hydraulically and the gears are actuated via cables.
The overhead camshaft with third and fourth gears and the reverse gear do not run in the oil bath. This reduces drag torque, thus facilitating gear shifting at low temperatures in particular. The three-cone synchronisation of the first two gears serves the same purpose, while the following gears are provided with two-cone synchronisation. The weight-optimised cast aluminium shift forks are installed on anti-friction bearings on the shift rods, in order to reduce the shift forces.
Source: Daimler AG
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