Alternative fuels for trucks and buses: From diesel to natural gas and BTL to hydrogen

Nov 12, 2007
  • Strategy: from fossil-based diesel to BTL and then hydrogen
  • Biodiesel in pure form requires certain measures
  • NExBTL a useful addition to biodiesel
  • BTL as a fuel of the future for diesel engines
  • Long-term practical trials of BTL in the Actros and Atego
  • Quiet and low in emissions: Citaro and Econic NGT awarded
    the "Blue Angel"
  • Biogas can replace natural gas as a fuel
  • With hydrogen and the fuel cell to the Zero Emission Vehicle
In parallel with the development of low-consumption, environmentally friendly drive systems, Daimler Trucks and Daimler Buses are involved in the development of alter­­nativ­e fuels. These are an important means of avoiding emissions and be­coming less dependent on fossil fuels. Ambitious political objectives in the EU, the USA and Asia are providing an impetus for their introduction. The demand for ener­gy is set to increase by around 20% within the EU between 2000 and 2030, while energy production in these countries is forecast to decrease by around 25% in the same period. The EU Commission has therefore formulated clear goals: by as early as 2010, biofuels and biogas are to account for almost 6% and 2% respectively of the total fuel consumption within the EU. For the year 2020 the Commission has prescribed proportions of 8% biofuels, 10% biogas and 5% hydrogen. To achieve these goals the EU is subsidising alternative fuel projects, and the authorities in other regions of the world are taking a similar approach.
Strategy: from fossil-based diesel to BTL and then hydrogen
A clear, future-proof strategy is necessary to avoid confusing businesses and consumers with the variety of alternative fuels mentioned above. Daimler Trucks and Daimler Buses have formulated just such a strategy on the basis of three decades of research and intensive practical in-vehicle trials. The way to the fuel of the future leads from fossil-based diesel to BTL (Biomass To Liquid = fuel from biomass) and finally to hydrogen for fuel cell powered vehicles.
Diesel fuel is changing
For the foreseeable future diesel will remain the number one energy source for heavy commercial vehicles. Worldwide availability, a well-established infrastructure and highly developed engine technology with respect to performance and environ­mental protection make diesel the clear front-runner among fuels. Nonetheless, the diesel of the future will be different from the current product for reasons associated with environmental protection, energy costs and the security of energy supplies.
With respect to conventional fuel, Daimler Trucks and Daimler Buses are strongly in favour of sulphur-free diesel fuel with the lowest possible aromatics content, such as is used in the industrialised countries. Biodiesel is increasingly being added to diesel fuel even now, and the EU has announced a target content of 5.75% by the year 2010.
Biodiesel is not only gaining in importance as an alternative fuel within the EU, but also in other regions of the world such as NAFTA. The relevant requirements and standards vary greatly from region to region, however. There are very significant differences between North America, Brazil and the EU, for example, and a stan­dar­disation process would be very desirable. The same applies to all other biofuels and alternatives.
Biodiesel in pure form requires certain measures
All in all, biodiesel reduces CO2 emissions by around 50 percent. It is free from sulphur and aromatics, and biologically degradable. In high concentrations the reverse side of the coin is incompatibility with some plastics and rubber, poor low-temperature performance, an inadequate shelf life and higher nitrogen oxide emissions during combustion. Some fleets have even gone beyond biodiesel as an admixture, and are refuelling their vehicles with pure biodiesel to save costs, how­ever owing to the limitations of biodiesel this is not possible unreservedly.
All Mercedes-Benz commercial vehicles have been approved for biodiesel since 1988, though initially with the proviso of shorter oil-change intervals. New Mer­cedes-Benz trucks and buses are optionally available with extra equipment for bio­diesel, and retrofitting is possible for vehicles already in operation. With a pack­age consisting of modified unit pumps for the fuel injection system, a fuel pre-filter with a heated water separator and an auxiliary tank for conventional diesel fuel to ope­rate the auxiliary heater, oil-change intervals are now approaching those for en­gines with conventio­nal diesel fuel. And not least, the operating life of the auxiliary heater is maintained thanks to operation with conventional diesel fuel.
Using biodiesel is not possible in vehicles with an EEV emissions classification, as biodiesel generates up to 20% more nitrogen oxide emissions than diesel fuel based on mineral oil.
The use of non-estered, practically unprocessed vegetable oils in commercial ve­hicles is definitely not recommended. This raw material for biodiesel is subject to inadequate checks and causes damage to valves, injection nozzles, pistons and piston rings. There is also a risk of oil dilution and partial breakdown of the engine oil, with potentially serious consequential damage.
NExBTL a useful addition to biodiesel
Using biofuel admixtures in a higher dosage than the 7% currently known is now under discussion, and following a proposal by VDA, Daimler Trucks and Daimler Buses recommend the biofuel NExBTL as an admixture. This is based on hydrated vegetable oils or animal fats, and is already produced industrially. Whether as an admixture or in its pure form, NExBTL is able to supplement or partially replace diesel fuel without problems.
BTL as a fuel of the future for diesel engines
First-generation biofuels such as biodiesel made from rapeseed or sunflowers, or bio-ethanol made from sugar-beet or cereals as a substitute for diesel, only use part of the relevant plants to produce fuel. Accordingly they are sometimes in competition with food production. The same applies to NExBTL as a hydrated vege­table oil. All this will change with the advent of second-generation (BTL) bio-fuels, for which the entire plant is used for the production of fuel. This requires a smaller growing acreage and saves more CO2.
These synthetically based BTL fuels give rise to great hopes for the future. If correc­tly processed they achieve the same quality as diesel fuel, and have a higher energy density. BTL fuels can be used in unmodified diesel engines, which are easily the most widespread power units in trucks and buses. They are able to use the existing refuelling infrastructure, and can either be added to diesel fuel in any ratio without problems or used in pure form as a direct replacement. Not least, they also exhibit a very favourable CO2 balance and have the potential to meet future exhaust emission limits. In the view of experts, BTL fuels could cover up to 20 percent of the total European fuel requirement.
Long-term practical trials of NExBTL in the Actros and Atego
This fuel has already successfully undergone fleet trials in passenger cars, and the next step is now imminent in the form of long-term fleet trials with 10 Mer­cedes-Benz Actros and Atego trucks in customer hands. Based in depots in Nuremberg and Munich, these vehicles will be in operation on a day-to-day basis over a period of three years.
Quiet and low in emissions: Citaro CNG awarded the "Blue Angel"
The current alternative to the diesel engine in regular-service city buses is natural gas. Both solo and articulated variants of the Mercedes-Benz Citaro are available with a natural gas engine, and around 900 of these are already in operation. The Citaro CNG has been awarded the "Blue Angel", a sought-after seal of approval issued by the Federal German Environment Agency for particularly environmentally friendly products. The Citaro CNG (CNG = Compressed Natural Gas) is the first natural-gas-powered, regular-service city bus to have bettered the limits prescribed by the EEV exhaust emission standard.
Particulates and sulphur dioxide are at practically undetectable levels in the ex­haust gases of this engine, while nitrogen oxide emissions are far below those of diesel or petrol engines. In addition the natural gas engine runs particularly quietly. Natural gas engines operate on the spark-ignition principle, however thanks to turbo­­charging, the horizontally installed six-cylinder in-line unit in the Citaro CNG develops a high torque. The tanks mounted on the roof are designed for pressures up to 500 bar, which corresponds to a 2.5-fold burst safety margin. Large numbers of CNG-powered vehicles from Daimler Buses are in operation in Australia, Thailand and Brazil. Orion also offers natural-gas-powered buses in North America.
Environmentally friendly municipal vehicle: Econic with natural gas drive
A particularly high level of environmental compatibility is also required of municipal vehicles. The response by Mercedes-Benz is the Econic NGT with a turbocharged natural gas engine. Around 600 of these specialist vehicles for municipal, collection and distribution operations are in operation with natural gas engines in Europe. The Econic NGT likewise complies with the Euro 5 and EEV exhaust emission limits, has been recognised as particularly environmentally friendly with the "Blue Angel" award, and excels with low noise as well as low exhaust emissions.
The special features of the Econic NGT include multipoint gas injection with single-coil ignition and a lean-burn configuration for a minimised fuel consumption. The Econic NGT is available in numerous two or three-axle versions with permissible gross vehicle weights up to 26 t. The gas tanks are safely stowed in a steel cage on the side of the frame. The potential of the Econic NGT has been demonstrated by one example in Gothenburg, Sweden: a start-stop system and hybrid drive for the waste collection body have reduced fuel costs by a further 20%. There is no lack of power either: with an output of 205 kW (279 hp) and 1000 Nm of torque, the Eco­nic is a highly capable performer.
Natural gas engines have a promising future in commercial vehicles by reason of their minimal pollutant emissions, considerably lower CO2 emissions compared to diesel engines and – with respect to the future-proof aspect – the world’s large deposits of natural gas. A comparatively favourable price for natural gas also helps to reduce operating costs. There is also political support for natural gas as a vehicle fuel: by 2010 natural gas is to account for 2% of the total fuel used within the Euro­pean Union, for 5% by 2015 and no less than 10% by 2020.
There are limitations in the form of the heavy, pressurised gas tanks and a lack of infrastructure, however the development of natural gas drive systems has not come to an end. An Econic NGT with hybrid drive is conceivable in the future, and the Eco­nic can also be operated with biogas.
Biogas can complement natural gas as a fuel
Biogas (methane) as an alternative to natural gas is among the fuels of the future. The high level of investment is a good indicator: last year more than € 1 billion was invested in biogas generating plants in Germany alone. There are now around 3500 such facilities in existence, more than three times as many as in 2000. Biogas is generated from biomass such as energy-producing plants and orga­nic waste, and can be distributed using the existing gas network. Emissions are very low, and com­bustion produces no additional CO2 because biogas is obtained from plant material and therefore part of the earth’s natural CO2 cycle. Biogas can be used in natural-gas-powered vehicles with no modifications. The limitations with respect to storage and the infra­struc­ture are identical to those for natural gas.
With hydrogen and the fuel cell to the Zero Emission Vehicle
With biodiesel as an admixture in low doses, BTL fuels ("SunDiesel") as a fully-fledged fuel for diesel engines and later biogas, environmentally friendly and some­times even CO2-neutral alternative fuels are either available now or will become so in the near future.
The next stage will lead to hydrogen as the fuel of the future. Currently obtained primarily from natural gas and crude oil, hydrogen will be generated by means of regenerative energy sources, i.e. using hydroelectric, wind and solar energy in the future. This will require large investments in an appropriate infrastructure, however. Hydrogen obtained by environmentally friendly means as an energy source for fuel-cell-powered vehicles will then turn the vision of the Zero Emission Vehicle into reality. In large-scale trials with 36 Mercedes-Benz Citaro buses on three conti­nents, hydrogen obtained by electrolysis with hydroelectric power and the fuel cell have already impressively demonstrated their potential.