A Class Technical Glossary

All A-Class engines are equipped as standard with the Active Service System ASSYST. This system records the conditions under which the vehicle is operated, and takes this into account when calculating servicing intervals. A microprocessor uses information provided by the sensors monitoring oil level, oil temperature, coolant temperature, engine speed, vehicle speed and engine load to compute the actual wear on the engine oil and to produce an engine servicing plan based on actual needs.
The central display in the instrument cluster shows the driver how much further he can drive until the next service. The Active Service System includes an oil level sensor which warns if the oil level is too high or too low, and displays the shortfall in litres. It also detects if the oil is topped up, in which case it extends the servicing interval accordingly.

Automatic child seat recognition
The front passenger seat of the Mercedes-Benz A-Class estate is equipped as standard with technology which automatically recognises special rear-facing child seats fitted with a transponder system. The electronic unit transmits a coded interrogation signal via two aerials in the seat upholstery, which receivers (transponders) in the base of the rear-facing child seat decode and then reply to with a response code. This code is relayed from the seat aerials to the airbag electronics which thus 'recognise' that such a child seat is installed. If this is the case, the microcomputer automatically prevents deployment of the front passenger airbag. The "Airbag Off" indicator lamp in the centre console lights up to confirm that the aerials and transponder have exchanged signals correctly.

Automatic clutch system
For drivers looking for easier gear-shifting, Mercedes-Benz has developed a low-cost alternative to the automatic transmission, the automatic clutch. A sensor system detects that the driver wishes to change gear and ascertains the gear currently engaged, while a microprocessor evaluates the sensor signals and sends a command to the electric motor to open the clutch by means of two hydraulic cylinders. When the gear change is complete, the clutch immediately closes again. The control system develops a precise feel for different types of driving style: it can distinguish between fast, sporty gear-changing and slow, gentler operation of the shift lever and it manages the clutch engagement accordingly.

Automatic transmission
The (optional) electronically controlled five-speed automatic transmission automatically adapts its shifting patterns to the particular driving situation and the individual requirements of the driver. Like all Mercedes-Benz automatic transmissions the A-Class transmission features a hydrodynamic torque converter and torque converter lock-up with computer-controlled constant slippage, for maximised fuel efficiency, dynamism and comfort. Torque converter lock-up engages even at low speeds, and continuous computer-controlled slip when lock-up is engaged minimises vibration and noise. To engage the different gear ranges, the electronic control actuates five electro-mechanical hydraulic valves which in turn actuate the multi-disc clutches and the torque-converter lock-up clutch. By means of a switch, one of two shift modes, "S" (standard) and "W" (winter) can be selected. Winter mode features second-gear start and lower-rpm upshifts.

The A-Class has a MacPherson front suspension system with coil springs, twin-tube shock absorbers and a stabiliser. One advantage of this system is its compact design. The Mercedes engineers have modified the patented design of US automotive engineer E.S. MacPherson on one important point: on the A-Class the stabiliser is not involved in the task of wheel location but is connected by linkage to the spring strut, while a triangular link is responsible for wheel location. This arrangement offers greater flexibility when tuning the suspension, resulting in improved elastokinematics and reduced road noise and tyre vibration. The axle components together with the rack-and-pinion steering, the engine and the transmission are mounted on an integral sub-frame which is bolted to the body at eight points. The rear suspension of the A-Class features trailing arms with coil springs, a stabiliser and single-tube gas pressure shock absorbers. It has been designed so that the axle components are accommodated under the load compartment floor and no interior space is sacrificed. The excellent dynamic characteristics of this rear axle are largely attributable to special additional links which control the elastic deformation of the trailing arms and thus reduce toe-in changes.

Belt force limiters
The A-Class features standard-fitted belt tensioners on the front seats which in conjunction with the belt force limiters and airbags significantly reduce the forces acting on the chest region of the occupants. The belt force limiter is located inside the inertia reel and consists of a torsion bar which turns slowly when a force exceeding a pre-determined level acts on the belt strap, thereby producing a controlled reduction in the locking effect of the inertia reel. From a pre-specified point, the inertia-reel seat belt slackens and the force exerted by the belt strap on the occupant is reduced.

Brake Assist
This system, developed by Mercedes-Benz to shorten emergency stopping distances, takes over if a driver is too hesitant or gentle in applying the brake pedal in a critical situation. The system automatically develops maximum brake boost with split-second speed, so reducing the stopping distance significantly. Tests provide impressive proof of the effectiveness of Brake Assist: on a dry road, most drivers need up to 73 metres for an emergency stop at 100 km/h, since they apply the brakes too gently. With Brake Assist, the car can be brought to a standstill after just 40 metres, a reduction of 45 per cent.

Common-rail injection
With the state-of-the-art common-rail direct fuel injection used in the diesel models, Mercedes-Benz has achieved an ideal compromise between economy, torque, ride comfort and long life. Whereas conventional direct-injection diesel engines must repeatedly generate fuel pressure for each injection, in the A-Class CDI engines the pressure is built up independently of the injection sequence and remains permanently available in the fuel line. The common rail upstream of the cylinders acts as an accumulator, distributing the fuel to the injectors at a constant pressure of up to 1350 bar. Here high-speed solenoid valves, regulated by the electronic engine management, separately control the injection timing and the amount of fuel injected for each cylinder as a function of the cylinder's actual need. In other words, pressure generation and fuel injection are independent of each other. This is an important advantage of common-rail injection over conventional fuel injection systems. The CDI diesel engines offer flexibility in controlling both the main injections and the pilot injections: a few milliseconds before the main fuel injection occurs, a small pilot charge of fuel flows into the cylinder. This ignites immediately, thereby preheating the combustion chamber. As a result, better conditions are created for the main injection process, which has a favourable effect on noise.

Dynamic route guidance
If dynamic route guidance is specified, the "MB Audio 30 APS" radio (optional) uses the car phone to contact the traffic telematics service provider Tegaron Telematics. Tegaron Telematics analyses the information from Gesellschaft für Verkehrsdaten, a traffic data provider that monitors the traffic situation on German autobahns by means of some 3,800 infrared or ultrasonic sensors. Using the Short Message Service (SMS) of the mobile phone network, Tegaron transmits congestion warnings at regular intervals to the dynamic navigation system of the A-Class, which is therefore able to take the latest traffic information into account and recommend an alternative route in the event of a hold-up ahead. Dynamic route guidance by mobile phone is already in operation in Germany and will soon be available in other European countries too. A second way in which the "MB Audio 30 APS" radio can receive digital traffic data for dynamic navigation is via the Radio Data System (RDS) and Traffic Message Channel (TMC). Broadcasting stations transmit inaudible signals which are decoded and evaluated by the MB Audio 30 APS radio. These signals contain information about hold-ups identified either by induction loops in the road surface of autobahns or by the police. The information contained in these messages corresponds to the regular (audible) traffic news provided by the radio stations and is free of charge. RDS-TMC is already in operation all over Europe. Dynamic route guidance with this technology is currently operational in Germany, and preparations are also under way to introduce it in other European countries.


Electronic Stability Program (ESP®)
The standard-fitted ESP® system selectively controls the braking forces acting on the front and rear wheels in such a way as to reduce the risk of skids and slides and help the driver maintain control in critical situations. The system extends the technology of the anti-lock braking and acceleration skid control systems with a range of additional sensors which are used principally to detect skidding tendencies. The ESP® computer continuously compares the actual behaviour of the vehicle with the pre-programmed ideal values. The moment the car deviates from its ideal line, specially developed control logic causes the system to intervene with split-second speed, bringing the car back on track. It does this in two ways: by precisely controlled braking at one or more wheels and/or by reducing engine power. ESP® in this way both corrects driver errors and stabilises the vehicle in the event of skids due to wet, icy, gravelly or other adverse road surfaces where normally drivers would have little chance of maintaining control by steering or braking manoeuvres.

Sandwich concept
Compact cars like the Mercedes-Benz A-Class by definition have a shorter front section and thus present only limited deformation possibilities in an accident. Development work on the A-Class therefore focused on ways of making full use of the short crumple zone in the event of a frontal collision and clearing any components likely to adversely affect the deformation process out of the way. The components in question are primarily the engine and transmission, which undergo virtually no deformation and in a conventional-design compact car form a rigid block which can intrude into the passenger compartment, causing injury. The sandwich concept reduces this risk: in the A-Class the engine and transmission are positioned at an angle partly in front of and partly underneath the passenger compartment and in the event of a severe frontal collision can slide downwards out of the way. Thus the A-Class front end offers maximum possible deformation length. Without this "disappearing act" by the engine, the front section of the A-Class would have to be approximately 25 centimetres longer in order to offer occupants the same high level of protection. In a side impact too, the sandwich concept brings significant safety benefits due to the higher seating position of the occupants.

The standard sidebags in the A-Class are located in the front doors above the armrests. Sidebags for the rear are available as an option. In the event of a side impact, the sidebag tears open a seam in the door panel trim and inflates between the occupant and the door, thus preventing direct contact between the occupant and the door. This has a dual effect: as well as significantly reducing the forces exerted on the chest of the occupant sitting on the impact side, the sidebag also limits whiplash movement of the upper body.

In Germany the A-Class comes as standard with the tyre sealant compound TIREFIT. Optionally, a conventional spare wheel is available at no extra cost. There are good reasons for omitting a spare wheel. For instance, statistics have shown that on average, motorists in central Europe are only unlucky enough to get a flat tyre once every 150,000 kilometres. Thus they can go for ten to twelve years without ever needing the spare wheel. The TIREFIT set takes up very little space in the boot and, including the electric pump, only adds about 2.2 kilograms to the weight. Furthermore, TIREFIT frees up the spare wheel recess to be used as additional stowage space. The tyre sealant compound is based on a special latex solution, which is particularly straightforward and clean to use, without the need for any tools or a jack. The sealant is pumped through the tyre valve, the tyre is inflated using the electric air pump and the car can then be driven at a speed of up to 80 km/h. The sealed tyre must be replaced with a new tyre at the next Mercedes workshop.