The number one among urban regular-service buses is now even more attractive: in the new Citaro NGT, Mercedes-Benz offers an attractive alternative to the diesel-powered Citaro. The Citaro NGT (Natural Gas Technology) impresses with its quiet running and reduced CO2 emissions – both are crucial advantages in the heavy traffic of city centres. Practical people will be convinced furthermore by the Citaro NGT’s low weight and a correspondingly large number of passenger seats, the engine’s strong performance, low fuel consumption and extended maintenance intervals. Mercedes-Benz offers the new powertrain variant as a Citaro NGT solo bus and as a Citaro G NGT articulated bus. The Citaro NGT can be operated on natural gas or on renewable natural gas – in the latter case, the Citaro NGT is actually virtually CO2 neutral.

Mercedes-Benz steps on the gas: clean and quiet through the city
The Mercedes-Benz Bus and Coach unit moves into high gear: the new Citaro NGT with natural gas engine sets standards in terms of environmental friendliness. The Citaro NGT is also approved for the use of renewable natural gas pursuant to DIN 51624 without restrictions. This makes the carbon footprint even more attractive because in this case the natural gas bus operates virtually CO2 neutral.

The natural gas engine has even lower noise levels than the already smooth-running and quiet OM 936 diesel engine, noticeable over the entire engine rev range. Depending on the driving status, its noise emissions are up to 4 dB(A) lower, which corresponds to the subjectively perceived noise level almost being cut by half.

Both factors – low CO2 emissions and noise levels – predestine the new Citaro NGT especially for use in packed city centres and for driving through residential areas.

Specialised market segment of around 1000 natural gas urban buses
The segment of urban buses in Western Europe amounts to about 10,500 new registrations a year on average and remains on a steady level. The highly specialised European market for urban buses with a natural gas powertrain presently comprises just under 1000 units a year. Mercedes-Benz expects this number to increase further in future in light of the discussions about the environmental burden in population centres or about the preservation of resources. The very fact that the Mercedes-Benz Citaro NGT is available with a state-of-the-art natural gas engine is also expected to stimulate demand.

Distinguishing feature is the striking cover
The Citaro with natural gas drive is available as a Citaro NGT solo bus and as a Citaro G NGT articulated bus. Both can be identified by the striking cover for the gas bottles on the roof. It increases the overall height of the natural gas buses to exactly 3389 mm. All other dimensions remain unchanged from the diesel-powered Citaro. For reason of weight distribution, the gas bottles are positioned above the front axle on the solo bus and above the centre axle on the articulated bus.

The cover is made of polyurethane plastic and is fitted with an aluminium frame mounted on the inside. It is attached to the roof via a rail system, has a side service door for easy access to the valves of the bottles and can also be popped up completely.

Lightweight and robust gas bottles made of composite material
Under the cover are the newly developed gas bottles of the Citaro NGT. They are made of a composite material with a plastic core, a casing made of carbon fibres and – a new feature – additional glass fibres. This combination of materials is very lightweight on one hand and highly robust on the other. The gas bottles are filled from the engine compartment of the bus as standard. Optionally, the filler neck can be repositioned above the front right wheel arch, which corresponds to the position of the filler neck of the diesel-powered Citaro.

At the same time, the capacity of the new-generation gas bottles was increased considerably from 190 to 227 l. The advantage: the number of containers can be reduced while the overall volume remains the same. Compared with the predecessor model, the Citaro NGT therefore requires one gas bottle less while offering an unchanged operating range. This results in noticeable weight savings of 15 to 45 kg, depending on the number of gas bottles.

That number can in turn be specified as the application demands. Two different covers are available, which hold either four to six gas bottles or seven to eight as needed. In practice, the Citaro NGT has the same operating range as a diesel-powered Citaro.

Low extra weight means more passengers
The additional weight of the Citaro NGT resulting primarily from the gas bottles has been reduced considerably compared with the predecessor. On a solo bus, this additional weight will be just 485 kg. Apart from the new gas bottle system, this also comes courtesy of the considerably more compact engine.

The passenger compartment of the Citaro is not affected at all by the natural gas drive. On the contrary: due to the weight savings, the passenger capacity of the Citaro NGT is significantly higher than that of the predecessor model. The Citaro NGT solo bus in standard specification now transports up to 96 instead of previously 93 passengers. The Citaro G NGT articulated bus has even a capacity of 153 passengers, up from 149 passengers previously.

New M 936 G gas engine: textbook example of downsizing
The new Mercedes-Benz M 936 G natural gas engine with a displacement of 7.7 l is presently the most compact natural gas engine in its class. It delivers the performance of a diesel engine while setting standards for noise and exhaust emissions. The engine is especially compact and lightweight. Weighing just 747 kg including the primary catalytic converter, it is about 230 kg or almost 25 percent lighter than the predecessor engine with a displacement of 12.0 l, making this engine a textbook example of successful downsizing.

The Mercedes-Benz M 936 G natural gas engine is based on the state-of-the-art OM 936 turbodiesel.engine. This engine is already in successful use on board the Citaro and Citaro G Euro VI. The vertically installed six-cylinder mono-fuel engine runs on compressed natural gas (CNG). It has an output of 222 kW (302 hp) at 2000 rpm while delivering a peak torque of 1200 Nm consistently from 1200 to 1600 rpm. To some extent, it remains well below the Euro VI emission limits.

Natural gas engine as powerful as a diesel and even cleaner
Such figures, in combination with its impressive power delivery, place the single-stage turbocharged engine on a par with its diesel-powered counterpart. In the main operating range, the power and torque curves of the two engine types are congruent from idle speed to around 1500 rpm. Above this, the natural gas engine actually delivers a slight advantage in terms of power and torque. Only from revs exceeding 2000 rpm is the diesel engine superior to the natural gas engine – levels that an urban bus does not reach in real-world scenarios.

At the same time, the natural gas engine from Mercedes-Benz raises the bar for environmental friendliness particularly high, because the CO2 emissions of a natural gas engine are up to 20 percent below those of a diesel engine. Using renewable natural gas to power the bus makes the carbon footprint even more attractive because in that case a natural gas bus operates virtually CO2 neutral.

Specially matched to the needs of CNG operation
The cylinder block and four-valve cylinder head of the M 936 G were adopted from the diesel engine and merely adapted for operation on CNG. Newly developed items are the turbocharger, the charge-air ducting, the ignition system and the mixture formation with a water-cooled exhaust gas recirculation system. The ignition system in a natural gas engine is identical to that in a petrol engine. The M 936 G uses spark plugs with pencil ignition coils. They are housed in the same installation space that accommodate the fuel injectors in a diesel engine. The piston bowl now has a different geometry.

Clean combustion with high output and low emissions
Like a modern petrol engine, the natural gas engine operates with a stoichiometric air-to-fuel ratio of lambda = 1. This means that there is precisely the amount of air required for complete combustion of the fuel. This is also the range where the catalytic converter achieves its maximum purification effect. In the M 936 G, this results in especially clean combustion combined with a high power output and low pollutant emissions.The new combustion process significantly reduces NOx emissions compared with the predecessor, for example.

A turbocharger with asymmetrical turbine geometry and two wastegate valves ensures an excellent engine throttle response and the supply of the cooled exhaust gas recirculation system at the same time. Due to the practically soot-free combustion, the engine, unlike a diesel engine, does not require the customary SCR technology with AdBlue injection or a particulate filter. As in a petrol engine, emission control for a natural gas engine is by means of a three-way catalytic converter. It is comprised of a primary catalytic converter with two oxygen sensors and a main catalytic converter.

The new M 936 G natural gas engine has already been proving its merits in the distribution and waste disposal vehicle Mercedes-Benz Econic for a year. The base engine for the Citaro NGT is identical. Due to the basic dimensions being identical to the original Mercedes-Benz OM 936 engine, the packaging at the rear of the Citaro is largely the same. The modified installation position only required adapting components such as the oil pan and the belt drives, the fitting for the water, the air compressor, the hydraulic pump and the air deflectors – common adaptations for a bus engine.

Fuel consumption reduced by about 15 to 20 percent
The new natural gas engine is extremely efficient. In concert with more intelligent control of the ancillaries, this results in a savings potential of around 15 to 20 percent for the Citaro NGT over the predecessor model. Even more energy savings can be expected in conjunction with the optional energy recovery module.

The intervals for changing the engine oil and spark plugs have been extended substantially in comparison with the previous engine, from 45,000 km to 60,000 km. Based on this practical metric, this means that a Citaro NGT driven for a typical urban-bus distance of about 60,000 km a year only has to be taken in for servicing once every 12 months.

Automatic transmission with torque converter matched to the engine characteristics
The power is transferred as usual by automatic transmissions with torque converter from ZF and Voith. The shift points and the use of the torque converter have been specially tailored to suit the new gas engine’s characteristics.The same applies to the available final drive ratios. Due to the different engine speeds, the development engineers have chosen standard final drive ratios that are one step shorter than those of the diesel engine in both cases, ergo i = 6.19 for the solo bus and i = 6.98 for the articulated bus.

Tested extensively in cold, heat and at altitude
The new Citaro NGT meets the highest standards right from the start – including on reliability. To this end, the development engineers put it through arduous tests under the most demanding conditions for three years. This included winter testing in Scandinavia in biting cold and summer testing in Milan and Soelden, Tyrol. That’s where it simultaneously underwent altitude testing at elevations of up to 1600 m above sea level. Endurance testing was conducted in addition. The testing also represents a logistical achievement because the supply of natural gas is not assured in all of the classic test regions. All in all, the three test vehicles successfully covered a distance of some 130,000 km. Deliveries of the new Citaro NGT will start as early as autumn 2015. Full series production will start in spring of next year.

Source: Daimler Buses