Climate agreement, nitrogen oxide values and cheat software are making life hard for diesel engines. What are the alternatives? Thanks to the latest climate agreement, the global efforts to reduce CO2 emissions are now really picking up steam. And in German city centres, bans on diesel vehicles are becoming a realistic threat. Even if trucks are not affected: the cheat software affair has certainly not made diesel’s reputation any better. But is road transport at all possible without diesel? What kinds of engines does the future hold? In any case, the efficiencies that can still be achieved with diesel engines are limited, and the experts of the Shell commercial vehicle survey 2016 expect no more than a 14 percent improvement in energy utilisation. Further types of propulsion systems at a glance:
Natural gas engine
- Technology: Both petrol and diesel engines can be converted relatively easily for the use of natural gas. The former are available as pure natural gas engines as well as in mixed forms that use a gas-petrol mix. The use of natural gas in diesel engines requires so-called dual-fuel engines, as natural gas alone would not ignite. A further development, which so far has not been offered in Europe, is high pressure direct injection (HPDI), which injects gas and diesel under extreme pressure. This enables a large proportion of gas and efficient combustion.
Another technical difference concerns the storage of gas. The storage of cryogenic natural gas in pressurized tanks is common in passenger cars and light trucks. The better alternative in terms of energy efficiency is the storage as a liquefied natural gas (LNG) in cooled tanks. However, Germany for example, lacks a suitable filling station infrastructure.
- Emissions: Depending on the technology used, gas engines emit between six and 20 percent less CO2 than an equally powerful diesel engine.
- Marketability: Trucks with gas engines have already reached series production maturity and are available everywhere with various technologies.
- Economic efficiency: Commercial vehicles with gas engines are currently about 30 percent more expensive than diesel vehicles. They are also subject to higher maintenance costs. However, the fuel costs are much lower. Higher numbers of vehicles will lead to falling prices in the long term, meaning that cost effectiveness will rise in the future.
- Technology: Vehicles with internal combustion engine, additional electric motor and battery are currently available particularly for passenger cars and light commercial vehicles. And if they have a plug for external charging, they are called plug-in hybrid electric vehicles. This technology is becoming increasingly popular for passenger vehicles. Initial pilot projects have also already been carried out with trolley-hybrid busses, which can load their batteries via overhead contact line during certain parts of their route.
- Emissions: When it comes to the pure vehicle emissions, the CO2 emissions can be reduced by about one third when using a plug-in hybrid system. However, the exact numbers also depend on the type of technology that is used for the generation of the charging current.
- Marketability: The first small series of plug-in hybrids for light trucks and city buses are now available on the market. However, significant improvements are needed here, especially in terms of the batteries. Furthermore, the hitherto underdeveloped infrastructure for electric charging is still an issue.
- Economic efficiency: The vehicle costs compared to diesel engines are currently very high (+ 30% for buses). However, long term considerations of costs in the commercial vehicle sector are not yet available. Real economic efficiency can only be expected in the distant future with further advances in user numbers, expected battery technology and electric infrastructure.
- Technology: In fuel cells, hydrogen reacts with oxygen, thus releasing electrical and thermal energy as well as water. Vehicles with fuel cells require smaller batteries than hybrid or purely electric vehicles, and fuelling is much faster than charging the battery of an electric vehicle. A disadvantage is the high technical effort for the design and manufacture of tanks, the still short service life of fuel cells and the lacking hydrogen infrastructure.
- Emissions: Hydrogen drive trains are zero emission engines when it comes to operation. However, the overall picture also has to include the energy required for the production. Nevertheless, hydrogen fuel cells are the cleanest form of propulsion for the future.
- Marketability: So far only a few prototypes exist in the commercial vehicle sector.
- Economic efficiency: The cost of the vehicles currently significantly exceeds all other drive types. Cost-effective operation can be expected only in the distant future.
Electric power train
- Technology: Electric vehicles are technically easy to implement. The problem here lies in the energy storage. With the current technologies, the battery for a fully fledged long-haul truck would weigh about ten tons, assuming the same ranges as with a diesel engine. An interim solution might be the possibility of charging during the journey through an overhead line or induction loop. Initial tests have already been carried out in Europe and the USA.
- Emissions: No emissions are caused during operation. However, the energy costs that occur during the production of the drive train as well as during the generation of the electricity have to be included in the balance. In the end, the exact numbers depend on the used electricity mix.
- Marketability: The technology is available on the market, and even in larger series for passenger cars. Small commercial vehicles, in particular for urban transport, are also ready for series production. So far only prototypes exist for large commercial vehicles.
- Economic efficiency: The acquisition costs are at least one-third above those for diesel vehicles. Furthermore, pure electric vehicles can still not be considered for long-distance transport. Actual economic efficiency is to be expected only with further advances, particularly in battery technology.
To sum it up, internal combustion engines will remain the instrument of choice in long haul freight services for the foreseeable future. First and foremost, this means the most efficient diesel engines possible and, albeit with some limitations, natural gas engines. Propulsion methods with severely reduced or even no emissions at all will play a significant role in last mile delivery services only.