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English   Mercedes-Benz Citaro with new Euro VI engines
01.10.2012 von admin


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.


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