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Imperatives and incentives for meeting the Euro VI exhaust emissions standard
Special importance is given to the environmental footprint of regular-service buses, because these vehicles are very often used in areas that are particularly sensitive to environmental damage. For city centres, suburbs and the fast-growing urban areas in Europe and far beyond, environmental protection is a top priority alongside economic considerations when it comes to the purchase and operation of regular-service buses.
The Mercedes-Benz Citaro opens up a whole new chapter in this area by successfully meeting both environmental and economic objectives, as the first regular-service bus delivered to customers with a Euro VI-compliant engine. This has been achieved with an entirely new engine generation from Mercedes-Benz.
This was the ideal time for Mercedes-Benz again to take the lead in the engine development stakes, with several calls for tenders for procurement contracts stipulating Euro VI as mandatory. Incentive schemes are also now in place for meeting the new exhaust emissions standard.
Development of two completely new engine families
“BlueEfficiency power” was the motto for the development of these completely new engine model series for the Mercedes-Benz Citaro. The family comprises the in-line six-cylinder OM 936 with a displacement of 7.7 litres and the OM 470 with a displacement of 10.7 litres. Other shared features include excellent fuel, AdBlue and engine oil economy and long maintenance intervals for the diesel particulate filter – 120,000 km or two years in the case of the Citaro.
Impressive acceleration and smooth-running characteristics
The new engines impress immediately with their acceleration and power, combined with impeccably smooth running. The acoustics of the weight-bearing structures of the Citaro have also been very carefully designed, including a rigid framework structure for maximum vibration isolation.
High torque already at very low engine speed generates high but economical output at revolutions scarcely above idling speed. This makes for excellent acceleration performance from the bus stop or traffic lights. It also enables the gear change points in the automatic transmission available in the vehicle to be set relatively low, in the interests of fuel economy.
Even lower engine speed
The powerful performance of these engines has also allowed the developers in almost all cases to provide longer axle ratios than in previous engines. To take just two examples, the standard axle ratio in the rigid regular-service bus is now i=6.21 rather than i=7.37, and in the rigid intercity coach i=5.77 instead of i=6.21. This significantly lowers engine speed and noise levels, as well as reducing the number of comfort-impairing gear changes.
As well as being able to feel the new performance features of these engines, the driver can also see them, with the green area of the revolution counter marking the high-economy engine speed range, starting as low as 900 rpm in the Citaro.
So along with even higher fuel economy, the new Euro VI Citaro also offers smoother running and provides a more agreeable acoustic experience for driver and passengers alike, thanks to the lower engine speeds, plus optimised noise insulation and the fuel injection process.
Technical features of the new engines
The clean exhaust and low fuel consumption of these powerplants is based on a high-efficiency combustion process. Ignition pressures of over 200 bar and injection pressures of up to 2400 bar (OM 936) and 2100 bar (OM 470) set the new standard in this area. The engines also now feature cooled exhaust recirculation.
For exhaust gas aftertreatment, both engines rely on the BlueTec-6 technology developed by Mercedes-Benz. A sealed particulate filter and the SCR system successfully used by Mercedes-Benz over many years form an ideal combination. BlueTec-6 technology reduces particulate emissions to the limits of detectability, while nitrogen oxide emissions are broken down into harmless air constituents by the injection of AdBlue in the downstream SCR catalytic converter. The end result is further reductions of around 80 and 66 percent in comparison with Euro V engines, for nitrogen oxide and particulate emissions, respectively.
The exhaust systems have either one pipe (OM 936) or two (OM 470), and the low exhaust back-pressure further reduces fuel consumption. The exhaust system is located at the bottom right of the engine compartment in the regular-service bus, without any noticeable impact on space in the passenger compartment. It is dimensioned to provide sufficient ash storage capacity, and extends maintenance intervals for the diesel particulate filter to two years or 120,000 km.
Engine and AdBlue consumption dramatically reduced
Along with their outstanding fuel economy, the oil consumption of the new engines is down to around one half of that for their predecessors, which along with other design changes has also lengthened the filter cleaning interval. AdBlue consumption has also been trimmed back to only 2 to 4 percent of fuel consumption. That is about one half of the figure for the Euro V engine, another positive factor in the running cost equation.
Ingenious design details
The new engines are crammed with ingenious technical design solutions. A crossflow cylinder head with four valves per cylinder ensures an ultra-fast gas exchange process. The cylinder head and crankcase are joined with six screws per cylinder, for a rigid and therefore very stable connection. The two overhead crankshafts operating the valves of each cylinder are formed from a tube, to which the cams are shrink-fitted. This results in a structure that is both light and strong.
The new OM 936 in detail: compact and powerful
The subtleties of the new OM 936 include the VCP (variable camshaft phaser), based on the first such variable recess camshaft of this kind ever to be used in a diesel engine. Adjustment supports the regeneration of the PM filter: when regeneration is needed, the valve timings are adjusted in order to raise the temperature levels in the exhaust duct.
The turbocharging function of the OM 936 is tailored according to output level. The variant with output of 220 kW (299 hp) has an asymmetrical exhaust turbocharger with a two-pipe turbine. In contrast, the higher-power 260-kW (354-hp) variant available as an option in the Citaro LowEntry has a two-level supercharging process with two turbochargers.
Thanks to its compact structure with water pump fitted at the side, the engine is about the same length as its predecessors, in spite of the larger displacement. The OM 936 is actually 25 mm shorter than the OM 926 LA used in current models. The engines have favourable power to weight ratios – for example, the dry weight of the OM 936 is 650 kg (DIN 700020-A). The outstanding power performance of this engine is such that in some cases it can even replace larger-displacement and therefore heavier engines.
The OM 936 six-cylinder engine is available in the new Citaro in two power and torque levels:
Power / Torque
220 kW (299 hp) at 2200 rpm / 1200 Nm at 1200–1600 rpm
260 kW (354 hp) at 2200 rpm / 1400 Nm at 1200–1600 rpm
The regular-service bus specialist: the horizontally mounted in-line six-cylinder OM 936h
Both power levels of the in-line six-cylinder engine are also available in the Citaro in a horizontally mounted variant (OM 936h), specifically designed for regular-service bus use. The basic engine remains the same, since the design implications of a horizontal variant were factored in already at the concept stage. A high level of structural design input was required, however, since the entire periphery of the powerplant had to be developed specifically for this purpose.
In contrast with the vertical version, both levels of the horizontal engine have two-stage supercharging. The horizontal engine in the Citaro is a very compact structure, and because of the extremely narrow tolerances the radiator ducts are castings rather than the brass pipes normally used.
New features of the OM 936h include the fuel module, the oil coolant module, the oil sump, fitting the turbocharger with air and exhaust ducts, valve cover with oil mist separator and the arrangement of the belt-driven PTO units. The specialist profile of the horizontal engine called for a completely distinct development and testing process.
The OM 470 in detail: light and powerful
The new OM 470 engine is based on the OM 471 heavy-duty model range, but with some distinctive touches reflecting its intended areas of use. In particular, its weight of just 990 kg (DIN 70020-A) makes the new OM 470 ideally suited for buses, where every kilogram counts.
A key feature of the new OM 470 (vertically mounted only) is X-PULSE, the unique flexible common rail system developed by Mercedes-Benz, with all-electronic control and hydraulic pressure booster. In comparison with the pressure of a maximum of 900 bar developed in the common rail, the injection pressure in individual injectors is boosted to up to 2100 bar. The injection process can be modelled as required according to the operating environment.
In all versions of the OM 470, supercharging is carried out with an exhaust turbocharger with an asymmetric turbine casing and rigid geometry. Specifically for fitting in buses, the engine also has a very flat oil pan to save space. The new OM 470 is designed for use in articulated buses operated in areas with challenging topographical features and in large-capacity buses. It is available for the Citaro in two power and torque levels:
Power / Torque
265 kW (360 hp) at 1800 rpm / 1700 Nm at 1100 rpm
290 kW (394 hp) at 1800 rpm / 1900 Nm at 1100 rpm
Choice of two state-of-the-art transmission systems
Both new engines have fully automatic torque converter transmission systems, as before. Voith and ZF transmissions sytems are available, both in the latest version for compliance with the Euro VI exhaust emissions standard. The torque converter characteristic curve is adjusted for the low engine speeds in the new engines.
Euro VI compliance with up to 5 percent lower fuel consumption
In spite of Euro VI requirements and the associated exhaust gas aftertreatment process, the fuel consumption of the new Citaro is even lower than its Euro V predecessor, which already had excellent fuel economy. Measurements in practical operating conditions indicate consumption savings of up to 5 percent, with a corresponding reduction in CO2 emissions. For an operator with average mileages and fuel costs, this means extra savings of around 800 to 1400 euros per year.
Recuperation module for fuel savings
When it comes to the outstanding economy of the new Citaro, the development engineers have excelled themselves, with a new fuel recuperation module (24 volts) for even more fuel savings. The free electricity generated during the “overrun” phase will be stored, and used for energy supply to the vehicle during the “traction” phase. This will ease the load on the alternators, lengthening their service life, and further reduce fuel consumption.
The energy will be stored in “supercaps”, double-layer capacitors with a power rating of 6 kW and a capacitance of 1 Ah. This process alone will achieve a reduction of more than two percent in the fuel consumption and CO2 emissions of the Citaro, depending on operating conditions.
Battery/alternator management for improved economy
But vehicle economy involves more than just fuel consumption. For example, in the Citaro the starter battery temperature is constantly monitored, and a battery-specific characteristic curve is used to determine the optimum charge voltage. As well as keeping the battery in optimal condition at all times, this also increases battery life, thereby reducing costs.
The same applies to the new alternators. The synchronised operation of the up to four alternators in parallel mode ensures an optimum load distribution. This means a lower load and a significantly longer service life for these components.
However, the new Citaro is able to dispense with an automatic oil refilling function. Because of its minimal oil consumption, this function is not essential, but it is still available as a special equipment option.
A new equipment option is a tyre pressure monitoring system. This displays the current tyre pressure in the individual tyres, and also warns the driver of any possible concerns, such as unusually high temperatures. This prevents tyre deterioration, increases tyre service life and reduces fuel consumption and emissions by keeping tyre pressures at the correct level at all times.
Fuel savings even from the lower boarding height
Both passengers and transport operators benefit from a boarding height at all doors of just 320 mm. The height reduction of 20 mm at doors two and three is more convenient for passengers and also reduces kneeling time and air consumption, which adds up to superior economy. Air consumption for the kneeling operation is reduced by around 10 to 20 percent, with fuel savings in the order of 0.4 to 0.5 percent. In the event of more ground clearance being required by the terrain on specific routes, because of steep gradients or overland routes, for example, a chassis elevation system can be provided on request.
The electronically controlled air compressor previously used in the vertical-engine Citaro articulated bus is now fitted in all models in the range. The close-loop controlled air compressor is a two-level device, and therefore operates at significantly lower temperatures. It also works whenever possible in fuel economy mode, during an “overrun” phase. To ensure this is the case as often as possible, the system pressure for the suspension and door operation has been increased from 10 bar to 12 bar. This creates higher air reserves, and saves fuel. Rigid buses are fitted with a two-cylinder air compressor, and their articulated counterparts with a three-cylinder device.
The air processing unit (APU) is now electronically controlled. The unit combines an air dryer, pressure governor, four-circuit protection valve and pressure sensors in a single component. The APU and a parking heater are now fitted at the same location in all versions of the Citaro, simplifying servicing procedures.
Weight still the same, with every kilogram at a premium
In spite of the major addition of exhaust gas aftertreatment to the specification for Euro VI engines, the weight of the new Citaro remains largely unchanged, with no reduction in passenger numbers. According to seating and equipment options, the new Euro VI Citaro rigid bus therefore has capacity for up to 100 passengers.
With an eye on the increased weight involved in future compliance with Euro VI, numerous weight-saving measures were already incorporated in the redesigned Citaro introduced one year ago. To meet the Euro VI requirements, the rear of the Citaro has now been completely redesigned and weight-optimised, from the rear axle up. The changes to the rear axle segment alone resulted in a weight reduction of as much as 60 kg. The rear axle is designed for with high input torque levels, for engines with ratings of up to 1900 Nm.
Other weight-trimming measures included reducing the panel thickness of the steel wheel arches to 1.5 mm, without affecting strength characteristics. The new single-section roof made of glass-fibre-reinforced plastic (GRP) saves another 20 kg. Another new weight-optimised feature is the roof frame in the area section, and further weight savings of around 40 kg have been achieved with engine mounts made from aluminium.
Every kilogram was at a premium, as a wide range of detailed design features reveal. For example, the significantly lower consumption of AdBlue meant that the in-vehicle reserve tank volume could be cut from 38 to 21 litres, bringing significant weight savings. Another case in point was the open bearing frame for the radiator fan. This is now a single aluminium casting, resulting in less weight and increased manufacturing precision.
Body adjustment even more sensitive than before
The chassis of the Citaro – already recognised as an outstanding design – has also been further enhanced with some fine touches on the part of the developers. Placing the height sensors outside rather than inside the vehicle makes the body height adjustment process even more sensitive and unobtrusive than before. The optional chassis for poor roads, a combination of vehicle elevation and modified suspension/shock absorber settings, provides a higher ground clearance and increased approach and departure angles, creating the optimum chassis setting for intercity journeys on rough terrain.
Exterior: the new-look rear of the Citaro
The most striking features of the rear of the new Euro VI Citaro are the increased height and the tower-style engine arrangement. The developers have designed a new rear frame for the new engines, including a new-look rear, lengthened by 30 mm. The rigid bus is now 12,135 mm long, and the three-axle articulated bus has a length of 18,125 mm. The turning radius is not affected by the new dimensions.
Another standout feature is the new rear dome of the Citaro, with a distinctive decorative element at the side. This gives the vehicle a slightly wedge-shaped, dynamic contour. The new rear section increases the height at this point to 3095 mm, but the maximum overall vehicle height remains unchanged.
The eye is also struck by the dark-coloured glass servicing flap on the rear of the bus, alongside the rear window. This provides free access on the left to the components in the engine tower.
The elegantly rounded fascia panel above the rear window underlines the understated, classy look of the Citaro. It too is made of glass.
Lightweight engine compartment flap, LED rear lights
The engine compartment flap and rear lights have also been redesigned. The engine flap is a sandwich structure of two adhesive-bonded die-formed panels. This makes the component lighter, but actually stronger than before.
In terms of the rear lights, the attentive observer will note not only their distinctive shape, but also the power-saving and long-lived LED lamps for the tail lamps, brake lamps and turning indicators. Only the rarely used reversing lamps and rear fog lamps still use incandescent bulbs as before. LED lamps are also used for the high-mounted tail lamps.
Tower-style engine and radiator configuration
Inside the vehicle, the most striking feature at the rear is the engine tower. The tower height varies according to the engine type: horizontally mounted (OM 936h) or vertically mounted (OM 936/OM 470). In all engine variants, the radiator is placed above the engine. Accordingly, the radiator and the engine air inlet are well above the level of upwardly projected dirt and splashes.
Horizontally mounted engines are fitted with a vertical radiator, and vertical engines with a horizontal radiator, to save space. The cooling water surge tank is located on the roof on both cases, again to save space. However, the level gauge for the refill reservoir and filling connections are placed in the engine compartment, for greater ease of maintenance.
Optimum access, effective noise insulation
All components are also easily accessed from the inside, through separately opened maintenance flaps. Noise insulation is boosted with quadruple seals over the maintenance openings, plus a hollow chamber seal arrangement.
The engine tower features a completely new insulation concept, based on a sandwich configuration with dual panels and an inside insulation layer. The elements are adhesively bonded to each other. On the passenger compartment side, this insulation covering has a further plastic layer, which is both pleasant to the touch and easy to clean.
Further increase in corrosion protection and quality
The Mercedes-Benz Citaro has gained an excellent reputation over the years for its outstanding quality, based on meticulous workmanship and also the cataphoretic electrodipping (CET) paint finish. This “e-coat” provides highly effective corrosion protection for the frame structure. Implementation of Euro VI requirements has enabled the developers to further boost the quality of anti-corrosion protection.
For example, the roof panelling now consists of a continuous glass fibre-reinforced plastic (GRP) strip. The strip is supplied in the form of a roll, painted white and given a protective gel coat layer.
The new engines have a combination of clearcoat paint and a wax layer. Further enhancements have also been made in the application of the underfloor protection and the cavity protective coating. Structural design changes mean that deposits of dirt are no longer able to accumulate in the wheel arches of the Citaro. Another anti-corrosion measure is the use of adhesive rather than welded joints in the entrance supports. Even the hinges and locks are given an extra spot waxing coating.
Additional improvements have also been made to components such as the optionally fitted cassette ramp. The ramp housing is now watertight, which reliably prevents corrosion. Servicing procedures have been simplified by reducing the number of structural components and ensuring they are easily accessible. Sensors now operate over the entire length of the ramp, by means of step mats. The front edge is marked with a blinking LED strip.
A comprehensive corrosion test carried out on the new Citaro has successfully confirmed the effectiveness of all these anti-corrosion measures. The test consisted of a salt spray test plus exposure to the effects of heat and moisture, and a poor road and winter trial. The compressed test programme replicated use in tough conditions over a period of 15 years. The results provide ample confirmation of the long-term stamina of the Citaro.
Even safer than before
The Citaro was already recognised as the safety benchmark for regular-service buses, and has now raised the bar even higher. Its new emergency valves are disabled at speeds of more than 5 km/h, and are automatically reset after the opening or closing of the passenger doors.
At the design and production stages, close attention has also been paid to all the measures applied for increased fire safety. These include optimised electrical cable routes with safety in mind, high-grade steel fuel lines, the sandwich construction of the engine tower with internal isolation, and dripping pans under all fluid containers in the engine compartment. A fire alarm in the engine compartment is standard equipment, and can be combined with an automatic fire extinguisher device, also mounted in the engine compartment.
Ever-wider range of vehicles
With the introduction of the new Euro VI engines, Mercedes-Benz will progressively extend the range of new Citaro vehicles with further model variants to create a comprehensive family of urban buses.
During this year, the two-axle 12.1-metre-long rigid bus with two or three doors and the 18.1-metre articulated bus and the corresponding overland versions will be joined by the 10.6-metre-long compact Citaro K. Next year will see the arrival of the Citaro LE, in both city and overland versions, 12 and 13 metres long, respectively, and the right-hand drive variants. By autumn 2013, the range will comprise a total of 20 model variants.
The vertically mounted Euro VI-compliant OM 936 and OM 470 engines will be available for the start of series production, and will be followed by the horizontally mounted in-line six-cylinder OM 936h engine in mid-2013.
The new Mercedes-Benz Citaro in detail
The “face” of the Citaro remains a standout feature, with the distinctive A-zero columns at the front. The severe lines normally characteristic of regular-service buses are softened here by the almond-shaped headlamps and especially the round contours of the front end, which seem to greet the passenger with a friendly smile.
The driver station in the Citaro is set 60 mm higher than in the predecessor vehicle, duly reflected in a higher windscreen and destination display. The overall body height remains the same, however, so the A-zero columns segue elegantly into a three-dimensional decorative feature on the side of the roof. At the bottom of the front end is the characteristic rounded bumper.
Wheelbase and front overhang adjustments for Euro VI
The wheelbase and overhangs were already adapted for the transition to Euro VI engines in last year’s model change. Among the design priorities were a generously dimensioned driver’s station with enhanced collision protection and the requirement to retain all the existing seating configurations in the vehicles with the new Euro VI engines.
Accordingly, the new Citaro as a rigid bus is slightly longer than its predecessor, because of the longer front overhang (by 100 mm) and the extended wheelbase (by 55 mm). And because of the modular construction, these larger dimensions are also carried over into the Citaro G articulated bus.
The bumper is now in three sections for greater ease of repair. Two easily replaced scuffing protectors, placed on the right and left sides in front of the hem section, are fitted to prevent damage in the event of the vehicle coming into contact with the ground because of the longer overhang.
Daytime running lamps and low-beam headlamps, bi-xenon technology
The H7 headlamps of the new Citaro provide strong and efficient illumination of the road ahead. The LED daytime running lamps and LED turning indicators are integrated in the headlamp housing, further contributing to the safety and totally distinctive look of the new Citaro. Bi-xenon lights and fog lamps with integrated cornering light function are available as optional equipment.
Deep side windows
Another striking feature of the new Citaro’s appearance is the generously dimensioned side windows. They extend well down the side walls of the vehicle, giving the bus a transparent look. The three-dimensional moulded GRP wheel arches add variety and contrast to the body. The elastic material of the wheel arches prevents damage from minor impacts.
To make repairs easier, the side wall is divided into segments. The aluminium panelling reaches around the hem section at the base of the body structure, creating a smooth, continuous profile and also improving protection against corrosion. If desired, the side panelling can be segmented horizontally to facilitate repairs of minor damage.
Safe, weight-optimised structure
The bodyshell is still based on the proven annular frame technology, but with increased rigidity for better protection of the passengers in the event of a side collision. An outstanding safety feature in the front end is the crash element for optimum safety in a collision situation. Along with the strengthened A-zero columns and a clearly defined frame structure, this means the Citaro is already prepared for the future pendulum impact test according to European regulation ECE R 29.
The bodyshell of the Citaro has been developed to comply with the future ECE R 66/01 regulation, requiring a large survival space inside the vehicle if it overturns. The new regulation does not take effect until 2017, but the new Citaro already fulfils its requirements.
Every detail focused on weight and functionality
Already in the design of the Euro V Citaro, the engineers were looking to create an even stronger and yet lighter bodyshell. The structure is based on a wide range of detailed design measures. A good example is the multidimensional joint between the horizontal and vertical bodyshell elements on the window columns, reminiscent of a sword hilt, which is lighter, but even stronger than before. The same goes for the new castings in the doorframes, which are again stronger than before, in spite of being lighter and with thinner walls.
Bus operators have also noted the electrical system in the new Citaro, in the central electrics compartment behind the driver. The electrical system, based on PCBs, is characterised by its flexibility and extreme reliability. It can easily accommodate additional devices and is very clearly laid out and easily accessible for maintenance purposes. The design successfully eliminates a lot of complex and expensive wiring for the vehicle’s electrical equipment.
Practical and attractive interior
The inward opening doors, manufactured in-house, are light, low-maintenance structures, held firmly closed with an almost airtight seal by an optimised rubber seal on the door base. The doors impress with their light weight, strong frame and precise fit. As an equipment option, the front entrance of the Citaro can also be fitted with an exclusive pivot-and-slide door. This structure offers some highly practical features not usually associated with such doors. For example, the two-leaf variant can be fitted at the front, and the two leaves can be operated separately.
Lower, well-lit entrances
The passenger compartment in the new Citaro is one of the vehicle’s most inviting and attractive features. The positive impression begins before the passengers board the vehicle, with the entrance and exit areas brightly lit at all doors with a broad band of LED lights.
Then comes the bright, welcoming passenger compartment, with ceiling lamps providing both direct and indirect lighting, thanks to the reflective effect of the roof flaps. As an equipment option, the fluorescent lamps can be replaced with LED lamps.
The new-generation vehicles can also be equipped with light fittings on the side sections of the roof, with LED lights providing resting and reading zones. The clear lighting they provide in the rear section also helps to minimise the risk of damage due to vandalism.
Handrails with advertising, ticker ads and lighting
The upright handholds in the Citaro have the same outwardly curved shape as in the vehicle’s predecessor, with their even spacing contributing to the comfortable, uncluttered feel of the passenger space. Straight handholds are used at the front of the vehicle, with a wider interval to open the space up for passengers boarding the vehicle.
The horizontal handrails are in the form of oval extruded aluminium sections. This shape and the larger diameter give them excellent grip. The new shape also allows the integration of light fittings, replaceable ad inserts or even ticker ads. The stop request buttons around the circumference are easily located by all passengers.
Smaller cross duct, new stowage compartments
The cross duct behind the front entrance of the Citaro is flat, providing higher overhead clearance and a more spacious feel.
The luggage racks by the handrails are solid. The optional wheelchair place provides better protection for mobility-impaired passengers in the form of handrails with a wider radius of curvature and a glass parking wall for wheelchairs. Screen prints are placed on the glass partitions to make them easier to see.
Advanced air conditioning for passenger compartment
The continuous mid-section of the ceiling has an unobtrusive design with evenly spaced holes, used for uniform ventilation of the passenger compartment. Like its predecessor, the new Citaro can also be supplied with an optional, highly effective roof duct ventilation system with integrated heating. Heating in the lower area is with side wall radiators, or a convector heating system as a special equipment option.
As a further choice, filling the gap between ventilation and air-conditioning options, the rigid bus version can be fitted with a compact air-conditioning system. This takes the form of separate, independent units positioned over the roof hatches, requiring no ducting except for the power supply.
Passenger seating arrangements unchanged
One of the features retained from the previous model is the proven seating design with cantilever mountings. The seats are manufactured in-house. Standard equipment is the City Star Eco seat. In the Citaro Ü, because of the longer travel times, the Inter Star Eco intercity seat is used. Numerous variants of both these seats are available.
Ergonomically optimised driver’s workstation
One of the outstanding features of the Citaro is the driver workstation, which sets new standards for user-friendliness, ergonomics, instrumentation and comfort. The door into the driver’s cab is hinged at the rear and opens forwards. This ensures fast and direct access from the front door to the driver’s workstation and vice versa. Stowage compartments have been optimised by providing a large compartment for the driver’s bag and a drinks bottle compartment. The driver workstation partition screen is larger than in the predecessor vehicle, with a wider forward curvature.
Looking passengers in the eye
The driver’s pedestal and seat are set 60 mm higher than in the predecessor vehicle, putting the driver on eye level with passengers boarding the bus. The higher seat position also improves visibility, and the driver also benefits from the lower side windowsill.
Elegant instrument panel, practical steering-wheel control pad
The instrument panel meets the requirements of the Association of German Transport Operators (VDV) in terms of construction and adjustability, as does the steering wheel, and also already complies with the recommended specifications for the new European driver’s cockpit. The new elegant contours provide more knee room, and the bright colour of the upper part of the panel contributes to the friendly feel of this part of the vehicle. The soft surface is pleasant to the touch, and underpins the general impression of high quality conveyed by the Citaro.
The cockpit contains large, clearly defined round dials for road speed and engine speed. Between the instruments is a colour monitor for a wide range of additional displays. The monitor is operated from a control pad located in the steering wheel.
Logical layout of switches and buttons
The placement of the switches and buttons in the cockpit draws on the latest ergonomics research findings. For example, the bottom right row of controls consists solely of the buttons operating the doors, which are constantly in use. The less frequently used automatic transmission controls are placed in a horizontal row further to the front. The light switches are placed on the left, in a clearly separated position, well to the front.
The redesign process has included a new-look left windowsill, so that the controls located here are now within the driver’s immediate reach and view. Here, too, the stowage compartments have been reworked. The driver workstation is now lit with LED lamps.
Efficient cab ventilation
The electrically opened window panel is placed at the front, so the driver can summon up a refreshing flow of cool air when required. There are also two large air vents in the instrument panel.
Behind the large air inlet grille beside and behind the driver’s seat, there is space to fit a bulky pollen filter. The air discharge behind the driver’s seat delivers a high air flow rate through the cab. The front box for ventilating and heating the driver station provides more uniform demisting of the whole of the electrically heated windscreen.
High-comfort chassis
The Citaro chassis again impresses with its outstanding comfort and active safety, thanks to the front axle with individual wheel suspension. This aspect is even further enhanced in the Citaro. For example, the reworked steering system provides a more effective steering wheel return, which is invaluable during the frequent turning manoeuvres required during the operation of a regular-service bus.
For the first time in a city bus: maximum safety performance with ESP
The new Citaro can also be equipped with the ESP electronic stability program, as the first low-floor city bus to offer this feature. The Mercedes-Benz Citaro is therefore opening a new chapter in safety technology for city buses.
To the extent physically possible, ESP intervenes immediately to stop the vehicle from going into a spin by reducing the engine torque or optimum braking of individual wheels. This is a real boost to active safety, particularly on slippery road surfaces in autumn and winter and during higher-speed travel on intercity routes.
Simple and economical maintenance and repairs
The specifications for the development of the new Citaro also included a noticeable reduction in running costs. This was a tall order, given that its predecessor had already been an outstandingly economical city bus. The engineers focused mainly on the maintenance costs. Accordingly, with the introduction of the new model, the maintenance interval of the automatic transmission and rear axle has been increased from 120,000 km to 180,000 km (although for the articulated version, this depends on the transmission system; with the Voith gearbox, the maintenance interval remains at 120,000 km). Changes at the detailed level have also helped to trim costs: for example, the use of larger filters in the air inlet for ventilating the driver station means a longer maintenance interval for that component.
Significantly less risk of minor damage
Another source of costs is the minor damage typical of the operation of city buses in dense traffic. The designers have also addressed this issue, with bolted – and therefore replaceable – GRP scuffing protectors on the front overhang to protect the hem section from damage when pulling into bus stops, or when pulling out over slopes or embankments. Repair costs are also reduced by flexible corner protectors on the rear bumper and wheel arch, again made of GRP, and the option of horizontally segments side panelling also delivers repair cost savings.
Cost reductions from LED lamps
The numerous LED lamps both inside and outside the new Citaro significantly reduce electricity consumption. They also generally last as long as the bus does, so that tedious and time-consuming tasks such as changing bulbs are largely eliminated altogether during the life cycle of the Citaro.
The capacity of the windscreen washer fluid tank has been increased from 8 to 20 litres, which reduces the need for refills to a minimum.
Top quality, thanks to a graduated development process
The Citaro has benefited from a graduated approach to the implementation of new components. This is exemplified in the decision to fit Euro VI engines only in the second stage of the development operation, in a city bus that was fully ready for operation.
The basis for top quality in the series production of the new Citaro is the use of state-of-the-art development methods such as HiL (hardware in the loop, i.e. automated tests of electronic components) and CAE (computer-aided engineering). Examples of this kind of quality assurance include component strength analyses, the assessment of vehicle vibrations and crash test simulations.
Safety-related components underwent analyses by the FMEA method already in early development phases. Failure mode and effects analysis is used to test for problems before they occur. Engineers carry out innumerable computer-assisted design calculations or components, major assemblies and the vehicle as a whole to ensure design maturity at an early stage in the process. An extensive test programme involving a large team of function testers and a comprehensive stamina programme conducted at a range of European locations with extreme operating conditions were used to validate the development operation.
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