The first new Mercedes-Benz Citaro mild hybrid buses (earlier post) have left the factory and will gradually go into service with Stuttgarter Strassenbahnen AG (SSB). They will be used to carry passengers on inner-city routes in Stuttgart. The vehicles offer fuel savings of up to 8.5%. In late 2018, the Citaro hybrids will switch to the new X1 express bus service introduced by SSB especially for low-emission buses with innovative powertrain technology.
The objective with the Citaro hybrid is to make the diesel engine even more economical while optimizing the degree of electrification for maximum overall economic efficiency.—Rüdiger Kappel, Sales Manager Mercedes-Benz Buses Germany
A disc-shaped electric motor is positioned between the engine and the automatic transmission. On overrun or when the bus is braking, the energy recovery phase, the motor acts as a generator to produce electric power, which is briefly stored by the Citaro hybrid in capacitors. When the bus accelerates again, the electric motor uses the stored energy to assist the diesel or gas engine with extra torque. The electric motor has a maximum output of 14 kW with a torque of 220 N·m.
On acceleration and as the vehicle pulls away, the combustion engine needs to deliver significantly less power thanks to the electric support. This saves fuel. The electric motor also provides assistance at idle.
The electric motor also serves as a starter-alternator in the current S-Class, while the additionally required cooler for the motor and inverter is used by Mercedes-Benz Trucks and the water pump can be found in many Mercedes-Benz cars.
Besides the series-produced parts used for the electric motor, energy storage and cooling, the absence of high-voltage technology is another key factor in keeping costs down. The 48 V system is classed as low-voltage technology and can be handled without risk of injury. The ancillary components remain unchanged and are driven conventionally. The maintenance intervals for the Citaro hybrid are also identical, at 60,000 km or once a year; the same applies to all maintenance points on the conventional powertrain.