OFFICIAL PRESS RELEASEStuttgart, Germany, Jul 16, 2008
Hybrid, gas and hydrogen drive: New concepts for low to zero emissions
Since 2000 the Canadian bus manufacturer Orion has been a group member company. Since 1997, Orion, together with BAE Systems, has been advancing the development of the diesel-electric drive and is now the world�s leading maker of hybrid buses.
As in the case of the Cito � and unlike the hybrid Sprinter � the 260 hp six-liter diesel does not serve Orion as a drive engine, but, by means of a generator, produces electricity which is then available for the electric drive. By feeding the braking energy back into the batteries mounted on the roof, as much as 45 percent fuel can be saved compared with a normal diesel bus. At the same time it goes easy on the brake linings. The energy recuperation system is particularly well suited for stop-and-go traffic in downtown areas, where normally a great deal of energy is wasted due to frequent braking.
After testing ten vehicles, New York City Transit bought 125 hybrid buses in 2001, and two years later ordered another 200 and thoroughly compared them with an equal number of natural gas and diesel buses. The result of the study, conducted from September 2004 through May 2005, was convincing. With regard to emissions, but also energy consumption, acceleration and useful life, the hybrid bus wins by a mile against the competitors. The hybrid drive cuts the nitrogen oxide emissions by 40 percent and the particulate emissions (thanks to a particulate trap) by as much as 90 percent.
New York City Transit was impressed and promptly ordered 500 more vehicles, which were to be followed by another 389 units. Toronto, too, has ordered 150 hybrid buses, another 56 units are going to San Francisco, and in each case there is an option for follow-up orders. A special advantage of the Orion VII HybriDrive is its use of well-tried, robust components. However, the dual drive with lithium-ion batteries makes itself felt in the price.
Mitsubishi Fuso also goes into large-scale production with hybrid buses: The Aerostar Nonstep HEV
The Japanese subsidiary Mitsubishi Fuso has also been manufacturing hybrid buses since 2006: Aerostar Nonstep HEV is the name of its low-entry urban bus with the characteristic rooftop structure housing the batteries. In the rear of the bus is a transversely mounted 125 hp diesel engine featuring common-rail injection, exhaust gas recirculation, oxidation catalyst and particulate trap. Versus a conventional diesel drive the nitrogen oxide emissions are 68 percent lower, the particulate emissions 76 percent. Super-single tires on the rear axle create additional space inside the bus.
Experimenting with the flywheel: The gyro drive
A mechanical alternative to the battery which utilizes braking energy too is the so-called gyro drive. Originally the gyro bus was developed in the early 1950s by the Oerlikon engineering works in Switzerland and presented at the German Transportation Expo in Munich in 1953 in combination with an electric drive motor, as alternative to the trolleybus. A hydrogen-filled housing contained a flywheel which was wound up with the aid of electric energy. For this purpose the bus had to be docked onto a �charging� station for two minutes, enabling it then to travel up to six kilometers; the flywheel supplied the electric traction motor with energy via a generator.
In contrast, the gyro drive which Daimler-Benz studied in the late 1970s with grants from the German Federal Ministry for Research and Technology and in cooperation with Bosch, MAN and several university institutes involved a combination of flywheel and diesel engine. An O 305 standard regular service bus and two minibus variants with large, high windows served as test vehicles.
The flywheel stored the braking energy and the excess output of the diesel engine not required for drive purposes. Diesel engine and flywheel worked together, electronically controlled, to achieve optimum system efficiency. This enabled the vehicles to make do with a smaller diesel engine than usual. For short distances � for example in pedestrian malls or routes through tunnels � the flywheel, noiseless and emission-free, could take over to drive the vehicle alone.
The 17-seat city bus was fitted with a 65 hp diesel engine. The flywheel weighed 115 kilograms and took up 750 Wh of energy. By means of two electric motors, connected by power collecting gears, and a four-speed power shift transmission, the flywheel gave energy back to the drive unit. In the O 305, by contrast, four hydraulic motors transferred the energy from the flywheel to the drive unit. With 1500 Wh and a weight of 226 kilograms the gyro drive was designed with twice the power of the city bus. The diesel engine developed 130 hp.
At the time of testing, the gyro drive had the advantage over the lead battery of a much smaller weight and a longer life. However, its capacity was not nearly as high that of efficient batteries. With the diesel engine switched off, the gyro bus only managed a distance of around 1000 meters, including starting off twice at bus stops. Larger flywheels would have required too much space. The gyro drive could not compete with new, lighter battery types like the zebra battery, the nickel-metal hydride battery or the lithium-ion battery.
The Olympic approach: First tests with natural gas in 1971
As the 1972 Olympics approached, commercial vehicle manufacturers were involved in a neck-and-neck race to develop new, environment-friendly drive systems whose effectiveness had not been proven in the least. �We refer to articles published recently about the development of natural gas engines by MAN,� we read in a letter of commercial vehicle and engine designers Rubi, Hartmann and M�ller-Berner to Daimler-Benz Development Chief Hans Scherenberg dated July 23, 1971. �They report on a new development proposed by MAN involving the use of liquefied natural gas for vehicle drive systems.�
The engineers learned from a telephone conversation with the MAN engineering department that the news about the reduction of nitrogen oxide emissions by up to 80 percent came from American publications. �Unlike MAN, which has not carried out any tests to date, for the past several weeks we have a gas engine running on a test stand on compressed natural gas. We have not managed as yet to beat the emission figures of a diesel engine, which are a great deal better than those of the comparable gasoline-powered four-stroke engine.�
Just six days later a meeting took place with two representatives of Munich�s municipal authorities after it became known, again through the press, that the local public transport operators there had begun to convert a few vehicles to a liquefied petroleum gas drive system (propane-butane mixture). An internal memo recorded the results of the meeting: �Operation on propane/butane is viewed in Munich only as a temporary solution and is to be superseded in the medium term by liquefied natural gas.�
Finally, on November 26, the press department suggested going public with the Mercedes-Benz natural gas bus after newspapers and television again had reported about forthcoming tests at MAN and about a liquid gas tank truck that was supposed to deliver the fuel for the propane-butane buses. The natural gas bus was to be presented to a �small group of ten to 15 commercial vehicle journalists� and then demonstrated before running television cameras. For after all, Daimler-Benz �currently is the only company that can immediately come out with a ready-to-operate natural gas bus.�
Soot- and irritant-free: The OG 305 natural gas bus
A press release dated May 31, 1972, described the first natural gas-powered bus: �This natural gas bus has the advantage of low-noise, low-odor combustion, and its exhaust emissions are almost entirely free of irritants and soot. However, this environment-friendly design requires making certain concessions. As natural gas only can be carried along in liquid form in the vehicle, thermally insulated tanks that permit a storage temperature of minus 162�C for natural gas had to be developed. In the OG 305 they are arranged underfloor and naturally result in a considerable increase in weight.�
The four insulated tanks made by Linde held 286 liters of natural gas, enough for a range of 400 kilometers. Depending on pressure state, the tanks emitted the natural gas in a liquid or gaseous state of aggregation. If the pressure were to exceed 4.2 bar at some point, two pressure relief valves blew the gas off into the open air. The system, tested by the German technical inspection authority (T�V), was licensed for road use without reservations. The engine relied on spark ignition. It was a modified six-cylinder, model M 407 hG, with a compression ratio of 1:11, an output of 172 hp and maximum torque of 677 Nm.
The press release remarked in detail on the emission figures: �Today the Mercedes-Benz natural gas engine undercuts the limits for hydrocarbons and nitrogen oxides by more than 20 percent and gives off only a twelfth of the permissible carbon monoxide gas. As natural gas contains no lead and sulfur compounds, this engine cannot produce any such pollutant emissions. However, the price for these advantages is higher operating costs.� Hans Scherenberg also came to a rather skeptical assessment in 1977: �Since natural gas is not available in adequate quantities in our country, realization of this drive system does not appear very feasible here. But it may be very interesting for other countries.�
Liquid gas: The LPKO 1113 waste disposal vehicle
Although, in the long term, natural gas seemed to be the more interesting alternative on a global scale, in the early 1970s Daimler-Benz looked into liquefied petroleum gas (LPG) as a fuel. This is a propane-butane mix occurring as a byproduct of oil refining. Serving as basis this time was an LPKO 1113 refuse vehicle with a gross weight of twelve tons. It was tested under realistic operating conditions. The spark-ignition six-cylinder M 352 G/2800 had a compression ratio of 1:10.4 and developed 100 hp. The tank was a 120-liter container, built by Siegel, with an operating pressure of six to eight bar; this sufficed for a day�s work.
�The exhaust gases are soot-free and produce next to no residues. The harsh emission regulations applying in California in 1973 (as of 1974 throughout the USA) are easily met. Even the 1975 California standards are met today,� an information sheet dated January 1974 reported but also stated: �The operating costs of the vehicle using liquefied propane-butane are higher than those of a diesel-engined vehicle because energy costs and consumption are higher.�
LPG as bus drive: The OG 305
�We have meanwhile converted the OG 305 natural gas bus, which we introduced in 1971, to run on liquefied propane-butane gas, and from April of this year up until a few weeks ago, we used it in everyday local public transport operation in Teheran,� Hans Scherenberg announced in a press release on November 24, 1977: �The results of this test were very good. Negotiations are in progress on the delivery of such buses to Teheran.� However, because of the Islamic Revolution, which began in January of the following year with demonstrations against Shah Reza Pahlavi and one year later forced him to flee Iran, these negotiations never were brought to a close.
Compressed natural gas (CNG): New beginning in Australia
One problem faced by the first natural gas bus in the 1970s was that sufficiently light and strong containers which could carry the methane gas in compressed form were not available at the time, so that the only practical option was to use liquefied gas cooled to a temperature of minus 161�C and carried in heavy gas cylinders. Of course, this increased the vehicle�s unladen weight, and 40 percent of the primary energy was lost in the liquefaction process.
It was not until the mid-1980s that safe, practical containers for compressed natural gas at a pressure of 200 bar became available. Daimler-Benz then tested three liquefied petroleum gas (LPG) buses and two natural gas (CNG) buses based on the O 305 in the Australian city of Perth. In Brazil, the further development of ethanol-fired spark-ignition engines for use with compressed natural gas began a short time later.
Three-way catalytic converter and lambda control: The O 405 N 2 natural gas bus goes into production in 1994
In 1994 the first Mercedes-Benz buses with natural gas drive went into production. They were low-floor solo buses of type O 405 N 2, which were soon followed by articulated buses, interurban buses, and finally standard buses. The M 447 hG engines were based on the corresponding six-cylinder diesel units which were fitted with an ignition system, three-way catalytic converter and lambda control and undercut the EURO II emission limits by more than 50 percent. Major orders were placed by Hanover and Mannheim, Greiz and M�hlhausen in Thuringia. A total of 216 units were produced at the Mannheim plant; more than half of them saw service in Germany. And 351 low-entry chassis of model O 405 NH were built for Australia.
Natural gas Citaro: Major order for Expo in Hanover
A generation change fell due at the turn of the millennium. Since then the new Citaro model has been providing the basis for Mercedes-Benz natural gas buses. From here on advanced turbocharged and intercooled engines of type M 447 hLAG featuring improved efficiency were used. Fitted with an oxidation catalyst as standard, they met the then current EURO II standard.
In time for Expo 2000 the Mercedes-Benz plant received the biggest contract to date for natural gas buses in Germany: Hanover�s Uestra local public transport service ordered 56 vehicles, 40 solo buses and 16 articulated buses. Their exterior and interior was the work of designer James Irvine: a special silver and green body with a raised, rounded roof which concealed the gas cylinders.
EURO IV and EEV: A newly developed generation of natural gas engines
Since 2003 the newly developed M 447 hLAG natural gas engine of the Citaro in its standard form has been complying with the EURO IV standard. Optionally, the lean-burn engines, tuned for low consumption, even comply with the particularly low limits for tax-advantaged Enhanced Environment-friendly Vehicles (EEV). Two performance levels are available: either 252 hp and maximum torque of 1050 Nm at 1000�1400 rpm or 326 hp and 1250 Nm.
Mercedes-Benz also has appreciably improved the high-pressure tanks for the natural gas, which are no longer made of metal, but of carbon-fiber-reinforced plastic, and so make possible either a 50 percent increase in tank capacity or an increase in the number of passengers. Five to eight gas tanks, each with a capacity of 190 liters, fit on the roof, depending on whether it is a solo bus or an articulated bus. This quantity definitely suffices for a normal day�s operation in urban traffic.
EEV standard for vans and trucks: Sprinter and Econic with natural gas engine
Since 1996 Mercedes-Benz has also been offering vans with natural gas engines. The gas-drive Sprinter operates using a new sequential injection technique which the developers christened Natural Gas Technology (NGT). The 2.3-liter engine, which delivers its maximum torque of 182 Nm at 4000 rpm, develops 125 hp. The gas cylinders, with capacity between 130 to 210 liters, are underfloor and enable a range of up to 350 kilometers. Alternatively, a bivalent drive suitable for liquefied gas and gasoline is available.
Like the natural gas Sprinter the versatile low-entry Econic truck with natural gas drive already meets the EEV standards. The 6.88-liter M 906 LAG engine develops 279 hp.
Clean combustion, but hard to handle: Hydrogen as a source of energy
From the very beginning of alternative drive system development in the late 1960s, Daimler-Benz had an eye on hydrogen as a possible source of energy for the future. Hydrogen has only about 25 percent of the energy density of conventional fuels. On the other hand, no harmful emissions whatsoever are produced during combustion: hydrogen reacts with the oxygen in the atmosphere to form pure water.
The first problem to solve in using hydrogen to power vehicles was that of storage. High-pressure tanks or storage in liquefied form could not be realized for use in vehicles back then. For one thing, the strictest safety precautions had to be observed since in the event of an accident the hydrogen could combine with atmospheric oxygen to create an explosive mixture. Adequately safe storage tanks to achieve even small ranges would have been extremely heavy. Cooling to minus 253�C, on the other hand, would have required an unreasonably large input of energy.
So in 1967 Daimler-Benz awarded a research contract to the Geneva-based Battelle Institute to develop a titanium hydride storage system suitable for vehicle drive purposes. Storage in the form of attaching the hydrogen to metal hydrides (adsorption) was the only safe variant for a vehicle drive system at that time. Initial results were recorded in the minutes of the technical meeting of November 13, 1967:
- By adding nickel to the titanium, hydrogen adsorption is achieved and activation energy retired.
- In the tests to date, several hundred mA/cm� could be achieved at a constant voltage of 0.83 V.
- To date, 1500 discharge and recharge cycles have been performed with an air electrode.
- The system still operates without restrictions at minus 20�C.
- The energy density of the titanium hydride storage system is about 150 Wh/kg, i.e. it is about five times higher than that of a normal lead battery.
- Titanium and nickel today cost around DM 10 per kg, so that should make the price of the base material quite interesting.
- The cost per installed kW is about DM 150�200 for the titanium hydride storage system, DM 600�800 for the normal lead battery.
In 1975 it was finally ready: at the Frankfurt International Motor Show, Daimler-Benz introduced the world�s first hydrogen-powered minibus with a hydride storage tank. From practical testing of the vehicle the developers gained further valuable insights which were incorporated in the design of a new hydrogen minibus two years later.
Hydrogen-propelled city bus: Modified gasoline engine and TN chassis
At the 1977 Frankfurt Motor Show, Daimler-Benz presented the new city bus with hydrogen drive system. The basis was a vehicle from the new �Bremen� model series on a TN chassis. The modified gasoline engine developed 60 hp. Its range was 200 kilometers with tanks weighing 200 kilograms. �Since hydrogen operation must begin in conurbations for reasons of supply, we started with the city bus project on the basis of Mercedes-Benz vans,� a press release stated.
�Hydrogen - Drive System of the Future� is the title of an information sheet distributed at the 1977 Frankfurt Motor Show to explain the project: �With the limited availability of petroleum reserves in mind, Daimler-Benz investigates possibilities for the use of alternative fuels. In this context, hydrogen, together with the use of suitable metal hydride storage tanks, which have supplanted the previously known both voluminous and dangerous storage options (high-pressure cylinders and liquid hydrogen), gains increased importance. These hydride storage units, whose development is furthered by the Federal Ministry for Research and Technology, contain special metal alloys which absorb hydrogen and give off heat as they do. Waste heat from the engine then releases the hydrogen again for use in the drive system.�
An ingenious feature of the new city bus was that it integrated the heat produced during hydrogen adsorption and the heat required to release the hydrogen into a complex heat management system which made use of the waste heat of the engine, and also incorporated the heating and air conditioning system of the vehicle. For test purposes, five different hydride tanks were installed in the vehicle. They were based on the materials titanium/iron and magnesium/nickel and operated either as high-temperature accumulators at temperatures of 250 to 300�C, or as low-temperature hydrides between 80 and 90�C: �Fundamentally, various methods of releasing hydrogen are possible: using the engine coolant, the engine exhaust gases, or the heat contained in the air of the passenger compartment. This opens up interesting opportunities for vehicle climate control, heat recovery and exhaust gas cooling.�
A 1979 brochure shows an again modified vehicle with a 78-hp engine and now three hydride storage tanks, a 55 kilogram magnesium2-nickel tank and two titanium-iron tanks weighing 93 and 98 kilograms, respectively. Insulation included, the total weight of all three storage units added up to 400 kilograms. The gasoline equivalent of the stored hydrogen quantity of 5.5 kilograms was 20.5 liters.
A diagram explains the layout: The waste-gas-heated high-temperature hydride tank also serves as a stationary heater that delivers an output of eight to nine kilowatts. A downstream low-temperature hydride tank utilizes the residual heat of the exhaust flow. Another low-temperature hydride tank is connected to a heat exchanger and serves also as a cooling unit of the air conditioner, delivering three to four kilowatts.
�In a hydrogen drive system (sponsored by the Federal Ministry for Research and Technology) with a hydride storage tank, a large part of the engine heat is accumulated in the hydride when the hydrogen is extracted,� the brochure explains. �At the hydrogen filling station, this stored energy is recovered when the storage unit is recharged. Measurements by Daimler-Benz show that as much as 80 percent of the energy in the exhaust gas of a hydrogen-burning engine can be reused.�
Hydrogen demonstration project in Berlin: World�s first fleet test
In a demonstration project in Berlin, between 1984 and 1988 Daimler-Benz used five hydrogen-powered vans from the 310 model series introduced in 1982 and five 280 TE station wagons with a bivalent gasoline-hydrogen drive developed in cooperation with the University of Kaiserslautern, as early as the 1970s. This was the world�s first fleet test of hydrogen-powered vehicles used as taxis, mobile intensive care units, ambulances and for municipal purposes. The metal hydride storage tank of the vans was designed to give the vans a range of 120 kilometers. All in all the ten vehicles covered 680,000 kilometers.
Green light for the fuel cell: NECAR 1
Building on the experience of subsidiaries Dornier and Messerschmitt-B�lkow-Blohm (MBB) in aerospace research, and in cooperation with the Canadian systems manufacturer Ballard, still the leader in this field today, in the early 1990s Daimler-Benz began to turn its attention to a new revolutionary propulsion technology. Hydrogen reacts with oxygen in the fuel cell, too, but this is not an internal combustion engine. Instead, in the PEM (Proton Exchange Membrane) fuel cell � the only one of the different types of fuel cell which lends itself to vehicular drive uses � a proton-conducting membrane separates the hydrogen into protons and electrons, which can then be utilized as electric energy. The principle of the fuel cell has been known since 1839, but it required the successful research work of Ballard during the 1980s to enable its use in motor vehicles.
Nevertheless, in NECAR 1 (New Electric Car) of 1994 the 800 kilograms of components for the fuel cell drive system occupied so much room that the entire cargo space of the van from the MB 100 model series was completely filled out. But within just two years the dimensions were reduced so much that the system fitted underneath the rear bench of a V-Class vehicle. And yet the output of NECAR 2 was the same as the predecessor�s: 50 kilowatts.
The first fuel cell bus: NEBUS
Three years after the New Electric Car, Daimler-Benz was already able to present a New Electric Bus (NEBUS). It was an O 405 N low-floor urban production bus furnished with ten stacks of 150 fuel cells each, located at the rear, which together developed 250 kilowatts. In terms of volume, a 25-kilowatt stack had more than five times the power of the system in NECAR 1. A large cooling unit on the roof protected the power plant from overheating. The drive system made use of wheel hub motors, the same as in the articulated and interurban buses operating at the time.
Thanks to the many years of development work on natural gas buses, safe high-pressure tanks for the hydrogen were meanwhile available. Seven 150-liter cylinders were placed on the roof of the NEBUS. They were made of aluminum with a casing of carbon-fiber-reinforced plastic, and each transported three kilograms of hydrogen compressed to 300 bar. On one tank filling of 21 kilograms of hydrogen, the bus could travel 250 kilometers. The top speed was 80 km/h.
The NEBUS attracted much attention and spurred on competitors to acquaint themselves in more detail with the fuel cell technology. The NEBUS operated in regular scheduled service for a few weeks in Oslo, Hamburg, Lisbon, Melbourne, Perth, Sacramento and Mexico City.
First fuel-cell-powered Sprinter in practical test: Under the flag of the Otto mail-order company
At the suggestion of Hermes Versand-Service of Hamburg (�Otto Versand�), a Sprinter was fitted with a fuel cell drive system in 2001. This was the first fuel-cell-powered van in the world to be tested under everyday conditions by a customer. The Sprinter was provided with a 55 kilowatt electric motor. One tankful sufficed for 150 kilometers, the top speed was 120 km/h.
Two-year field test: 30 fuel-cell Citaro as part of the CUTE project
In October 2002 DaimlerChrysler presented the next generation of the fuel-cell bus. In 2003, 30 Citaro low-floor buses began a two-year practical test in ten European cities, sponsored by the European Union within the framework of the CUTE project (Clean Urban Transit for Europe). In some respects the fuel-cell Citaro even was a step back behind the NEBUS. There was no more talk of wheel hub motors; the power was transferred by a conventional six-speed transmission. But the point was to test the fuel cell technology, so it made little sense to simultaneously take likewise little tested and still unreliable components on board: in all its parts except for the drive system proper, the fuel-cell bus corresponded as far as possible to a conventional urban bus.
Two approximately 80 x 60 x 60 centimeter stacks with 980 fuel cells each were accommodated on the bus roof this time. They each developed 125 kilowatts and powered a central 225 kilowatt asynchronous motor. The consistently high torque made itself felt in a positive way for hill starts: 1027 Nm were already available at 500 rpm and rose to 1045 Nm at 1000 rpm. The use of carbon fiber in the casing permitted raising the pressure in the hydrogen storage tank cylinders to 350 bar. Nine 200-liter cylinders were arranged on the forward section of the roof and stored a total of 43 kilograms of hydrogen, adequate for a range of 200 to 300 kilometers.
On the occasion of the 2003 UITP Congress in Madrid, the first trio of buses took the start. Hamburg, Stuttgart, Amsterdam, Barcelona, London, Luxembourg, Porto, Stockholm and Reykjavik followed. One aim was to test the fuel cell drive system under practical conditions which differed as much as possible: from the flat-as-a-pancake downtown parts of Amsterdam and Hamburg to the hills of Stuttgart and Luxembourg, but also from the summer heat of Madrid to the Icelandic winter. Before the two-year test was over, Perth in Australia joined it with three vehicles; in 2005 Beijing followed suit. When the two-year testing period was complete, each of the ten participating cities had gained an average of about 7,000 operating hours or 100,000 kilometers of practical experience.
Special attention was paid in the test to the production of the hydrogen, by means of steam reformer or electrolysis, because the energy balance and lifecycle assessment for the drive system can be conclusively determined only on the basis of the complete system. If the bus operates free of emissions, but the production of the hydrogen consumes all the more energy and simultaneously gives rise to high emissions, then little has been gained in the end. In some cities the hydrogen was produced in distributed fashion, which led to unexpected technical difficulties in Stuttgart, for example. With central production, on the other hand, additional energy was lost during the subsequent transport to the hydrogen filling station.
At the end of the two-year test, one thing was certain: with an availability rate of more than 90 percent the fuel cell drive system had given a better account of itself than expected. The new technology was extremely well received by local public transport operators, drivers and passengers so that seven cities promptly extended the test by another year. Hamburg even bought the vehicles from Stockholm and Stuttgart and now operates nine fuel-cell buses.
However, the fuel-cell bus is still far from being able to compete with the conventional diesel bus. The cost price of a single vehicle was 1.25 million euros. Despite its surprisingly good performance, the service life of the fuel cell is still far less than that of a diesel engine. The consumption of the test buses ranged between 15 and 30 kilograms of hydrogen per 100 traveled kilometers, the equivalent of the consumption of 50 to 100 liters of diesel fuel (diesel has a higher gross calorific value).
Unless service life and efficiency are enhanced significantly and the price is reduced at the same time, the new technology consequently will not be able to keep up with conventional drive systems. Moreover, considering the additional weight of the fuel-cell Citaro � some three tons � there is still major potential for reducing weight. Further development will therefore necessarily focus on the reduction of cost, weight and consumption, as well as on efficient energy management.
Finally, the definitive energy balance and lifecycle assessment for the drive system have not even been computed yet. It may make sense to operate emission-free locally, and the efficiency of the fuel cell itself is distinctly higher than that of an internal combustion engine. But if, for instance, more energy is lost during the production of hydrogen from natural gas than in the combustion of that gas in a natural gas engine, then little will be gained. And hydrogen production through electrolysis will in the end only help to protect the environment if the electricity required for electrolysis is generated from renewables like wind, water or solar energy.
Of course, questions like this cannot be answered in theory, but only through practical testing. Bearing this in mind, the large-scale test with the Citaro fuel-cell buses has furnished the most comprehensive data material so far.
Alcohol in the tank: Methanol drive with energy recovery
In addition to gasoline, diesel fuel, natural gas and liquefied petroleum gas, there are other fuels basically suitable for use in vehicular drive systems. Owing to the different properties of the fuels, modifications to the engines are necessary in each specific case. As a matter of principle, alcohols like methanol and ethanol can basically also be used in the diesel engine, however their lower gross calorific value, the higher heat of vaporization and the required higher cooling performance constitute definite system drawbacks.
In a standard bus with methanol drive system introduced in 1981, Daimler-Benz took advantage of the heat of the coolant and methanol�s low boiling point of 65�C and, instead of the diesel engine, used a modified M 407 hG natural gas engine, many of whose parts were identical with a normal 11.5-liter production diesel. After a short preheating period of about two minutes before a cold start, the heat of the coolant sufficed to vaporize the methanol for use in the gas engine. Considering the short stops in urban regular service operation, any further warm-up usually was unnecessary.
With this configuration the circle was squared, as it were, and the methanol engine came close to a diesel direct injection unit in the utilization of energy. A diesel bus converted to methanol operation in June 1981 took up operation one month later in Auckland, New Zealand, as the world�s first methanol-powered public service bus. In early 1982 another methanol bus followed, going into practical operation with the Berlin local public transport service BVG. In May of the same year, a third methanol bus, a double-decker, went into service in Pretoria, South Africa. After two years of regular service and mileage of 100,000 kilometers, the Berlin methanol bus had given an extraordinarily good account of itself. It operated quietly, with low emissions, in particular with practically no particulate emissions, and impressed the drivers with its acceleration characteristics and its flexibility. As a result, BVG put seven more methanol buses into service in 1985. Methanol could be produced from natural gas or coal, but also from organic wastes, and was therefore considered a possible alternative fuel. The problem is not just the engine conversion, but even more so the provision of adequate fuel quantities and the establishment of a dedicated infrastructure for the fuel.
Ethanol drive: Widespread in Brazil, tested in the Netherlands
As sole country in the world, Brazil has had a network of ethanol filling stations since the 1980s. The vehicles are filled up with high-proof liquor, so to speak, manufactured from sugarcane. The Daimler-Benz subsidiary Messerschmitt-B�lkow-Blohm (MBB) offered conversion kits for diesel engines as standard and new vehicles with ethanol-powered drive systems from 1984. Ethanol-fired spark-ignition engines derived from diesel engines were included in the range in 1985.
Another trial was undertaken in 1992 by the Dutch city of Groningen which tested three O 408 regular service buses from Mercedes-Benz with ethanol obtained from sugar beets. First results were not long in coming: there were practically no particulate emissions, the nitrogen oxide emissions were 50 percent lower than a diesel engine�s, and thanks to a downstream oxidation catalyst the figures for hydrocarbons and carbon monoxide were also extremely low.
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