It’s back to basics for the Mercedes-Benz Citaro as its fuel efficiency is improved thanks to a simple yet robust hybrid kit. Not only is it uncomplicated, but there are no training requirements, says Daimler
A simple, cost-effective and maintenance-free way to reduce fuel consumption by up to 8.5% is how Daimler describes the mild hybrid package that will be available across almost all of the Mercedes-Benz Citaro range from 2018.
Although referred to as a hybrid by Daimler, It’s actually a straightforward kinetic energy recovery system (KERS) that has a reasonable payback period.
At the heart of the Mercedes-Benz Citaro hybrid is a motor-generator mounted between the engine and the gearbox that is connected to a pair of 24v supercapacitors that act as electrical energy storage.
When the bus brakes, the supercapacitors are charged. Slowing from around 30mph to stationary is enough to fill them. The stored energy is subsequently returned to the driveline, but in an intelligent manner. There is a also further string to the hybrid system’s bow.
Daimler points out that an engine is at its most inefficient when idling. To mitigate that, the motor applies a small torque at tickover, removing from the engine some of the load created by internal resistance and electric auxiliaries. That contributes to a further fuel saving.
Despite the differences between the Citaro’s hybrid package and those already in the market with other manufacturers, Daimler has named it correctly. As it points out, the definition of a hybrid in a vehicle sense is one that utilises more than a single power source.
Integral to the overall fuel saving are a high-efficiency ZF rear axle and electro-hydraulic steering. Either can be specified individually on non-hybrid Citaros, but they cannot be deleted from examples with the kinetic energy recovery pack.
As would be expected from a Mercedes-Benz product, the KERS system is mature and well packaged. The supercapacitors are out of harm’s way on the roof, and there is no impact on space within the cabin. The front-mounted steering motor can be heard at lower road speeds, but that’s it.
EvoBus (UK) has confirmed that it expects around 50% of the Citaros that it supplies to be equipped with the hybrid package once production is in full swing. While the 8.5% fuel saving is its headline figure, the manufacturer says that it also results in a 2.5% cut in the total cost of Citaro ownership.
The motor’s purpose is to support, and not replace, the standard six-cylinder OM 936 engine, although it’s also available in conjunction with the M 936 G gas-fuelled unit.
“The support element is an important detail,” says Daimler Buses Technical Explainer Frank Mandel, who adds that there is neither ‘engine off’ capability nor stop-start functionality on the Citaro hybrid. “Support from the motor comes not just when the bus is accelerating. It is delivered to the crankshaft when the engine is idling, because there is still an electrical load at that time.”
Peak torque output of the 14kW motor is 220Nm. The OM 936 delivers a maximum of 1,200Nm, but a control unit ‘blends’ the two sources, meaning that no additional get-up-and-go is available on the hybrid compared to a standard diesel Citaro.
“This is not an AMG package for buses,” says Mr Mandel. “The torque delivered by the motor substitutes for what would otherwise have come from the engine, and that generates the fuel saving.”
To accommodate the motor-generator, the engine and gearbox are split slightly. The KERS package adds 156kg to the unladen weight, but Mr Mandel adds that as all of its components are within the rear overhang, it actually reduces the imposed mass on the front axle.
The only alteration within the saloon is a slight extension to a raised area at the rear to accommodate the gearbox access hatch. When production of hybrid examples begins next year, all Citaros – regardless of whether they have the fuel saver fitted – will come with the extended pedestal. That will allow a retrofit of the KERS package to those buses that lack it.
Doing so would come at a higher cost than specifying it from new, and no detailed sums have been done in that regard. But if external money became available later in a bus’ life, it may be viable.
Another of the Citaro hybrid’s plus points concerns the low voltage of its KERS. At 48v it means that no high-voltage training is required for technicians, although the unit is free from a routine maintenance requirement apart from annual visual inspection.
The supercapacitor used is not new to the Citaro. Daimler quietly introduced it at Euro 6 as part of its smart alternator package, and a supercapacitor is below the cab floor, where it powers auxiliary items within the bus.
The smart alternator set-up contributed to an initial fuel saving at Euro 6, but Daimler has gathered consumption and emissions data from test Citaro hybrids for comparison with both non-hybrid Euro 6 buses and those from the previous emission level.
While the hybrid’s principle boast is of an 8.5% fuel efficiency gain over a conventional Euro 6, when combined with the earlier developments there is an even more significant improvement over Euro 5 EEV. In that case, fuel use has been reduced by 18%, while NOx emissions are cut by 83% and particulate matter by 98%.
As a result, Mr Mandel describes the hybrid as “nearly emission free.” But significantly, Daimler is clear that it does not represent a transitional step to zero emissions. A battery-powered Citaro is coming next year and a hydrogen fuel cell model will follow it, but diesel forms part of the long-term strategy.
Last week Daimler invited the press to Mannheim to drive two pre-production examples of the Citaro hybrid. Apart from the KERS package, they are standard left-hand drive specification buses. Power in both is from the OM 936 engine rated at 295bhp coupled to ZF EcoLife gearboxes, although the hybrid pack is also compatible with a Voith transmission.
In the cab, there are no differences to a conventional diesel: No additional controls and no lights on the dash that illuminate when the hybrid package engages. That’s for a reason, and it means that drivers already familiar with the Citaro require no additional type training on the hybrid. Although the motor’s output replaces a proportion of the engine torque when accelerating from a stop, there is no perception of it doing so.
The test buses’ gearboxes were set to an economy mode and so progress is made at a more sedate pace than some Citaros in the UK with a more aggressive shifting strategy, but there is no aural evidence of the motor’s support.
What is evident is a change in the steering feel. The electro-hydraulic pump works only when required, and at higher road speeds the wheel is very firm.
The only point at which the KERS equipment makes itself known is when slowing. Mr Mandel explains that when the accelerator is released, the motor-generator applies a small negative torque of around 20Nm before more resistance is felt as the brakes are applied.
A lot of work went into validating the optimum level of energy recuperation when coasting, he adds. “The braking torque is low because we want the bus to drive like a diesel. It is key that the driver does not notice the hybrid package’s presence.”
In left-hand drive markets, Daimler expects that most Citaros built will be equipped with the hybrid package once production is established. The economics of bus operation there are generally much different to those in the UK, but other manufacturers have shown that with the right payback period, buyers can justify the additional cost of a fuel-saving system, and early indications seen by EvoBus (UK) are that take-up will be respectable here.
The product is well packaged, and while its fuel saving potential is not on a par with some full hybrid systems, Daimler promises that the maintenance requirement will be minimal. That will be welcomed by those who have found the opposite to be true with some conventional hybrids.
Daimler has no plans to expand the mild hybrid package to other members of its PCV range. It relies on a modern electrical infrastructure such as that introduced on the Citaro at Euro 6 – and that no doubt leaves the door open to further developments on the best-selling citybus.
