What impact does a vehicle with a specific powertrain have on the environment? To date, this has mostly involved looking at the emissions produced while driving. Yet this approach does not go far enough: The life cycle assessment (LCA) shows that while the vehicle is used, attention also needs to be focused on the provision of fuel and also on the process of manufacturing and recycling vehicles. Increasingly, results of this type are also of interest to the legislator.
To assess the ecological impact of a diesel or gasoline engine vehicle, focus in the past has largely been placed on fuel consumption or CO2 and pollutant emissions on the road. One weakness of this tank-to-wheel approach (TtW) is that it makes no allowance for fuel production. “As far as the ecological consequences of traffic are concerned, it is indeed very important as to where crude oil comes from as the basis for gasoline and diesel or how LPG and CNG are produced for gaseous-fuel vehicles”, explains Torsten Semper, project manager for drive system concepts and sustainability at IAV. “In addition, there is the impact of adding biogenic fuel components to biodiesel and bioethanol.”
Broadening system limits for an objective comparison
The well-to-tank approach (WtT) focuses on the ecological effects of providing fuel and energy for driving mode, and combining WtT and TtW produces the well-to-wheel approach (WtW). It covers all ecological impacts from primary energy well to the wheel. This also takes account of electricity generation for externally chargeable hybrid and electric vehicles – nonetheless, this approach is not complete. “It doesn’t allow for the effects of producing and recycling on vehicle life cycle assessment either”, Semper says. To get the most objective comparison of all drive concepts, system limits need to be broadened. This all-encompassing approach is called life cycle assessment (LCA). It evaluates the entire product life cycle.
To compare the ecological impact of different drive concepts, IAV has modeled a compactcategory vehicle with various drive system versions in LCA software. This assesses all materials, energies and emissions. Among other criteria, it was based on parts lists containing information on the materials and weights used as well as details on the methods employed for manufacturing all components. “We linked these data with database information on the specific properties of the materials and production processes in the LCA software”, Semper reports. “We followed a similar approach to recycling at the end of the vehicle’s life. Here, a key input variable was information on the recycling processes and recycle rate.”
Simulating energy consumption in WLTC as basis for life cycle
To model the life cycle, IAV’s experts have simulated vehicle longitudinal dynamics as the basis for determining energy consumption in the WLTC (Worldwide Harmonized Light-Duty Vehicles Test Cycle) for the particular drive concept. Together with database information on the processes involved in producing fuels or electric energy and the emissions identified in production and recycling, they could then establish the emissions resulting from whichever drive concept.
The results were evaluated using the CML method that is capable of categorizing the consequences for the environment and quantifying them in equivalents. These categories, for example, include the greenhouse gas potential, the potential for acidification, the potential for summer smog and eutrophication potential. There is also the possibility of including other environmentally relevant criteria, like the need for primary energy or the consumption of land and water, for this assessment.
Key impact of mileage and the use of carbon-neutral energy sources
Identifying the potential of greenhouse gases was based on a specified mileage of 200,000 kilometers. The gasoline engine vehicle was shown to cause the highest on-road emissions, followed by the diesel and hybrid versions. Of course, the battery-electric and the fuel-cell vehicle came out best – with zero emissions. The picture changes when we also include the emissions from providing electricity or fuel, like gasoline, diesel and hydrogen: The conventional drive systems gain significant ground – all the more so after including the emissions from production and recycling. This reveals that the production of partially and fully electric drive systems or fuel cell drives emits significantly more greenhouse gases. Nonetheless, comparing the vehicles using conventional energy sources puts the battery-electric vehicle first. In the case of the fuel cell vehicle, hydrogen in most cases being generated by reforming natural gas steam prevents a better result.
Schematic diagram of the product life cycle
Comparison of drive concepts (200,000 km mileage) using fossil-based and carbon-neutral energy sources
The picture would change in an interesting way if mileage were differentiated in relation to the drive type over vehicle life – about 220,000 kilometers for the diesel engine model as well as for the fuel cell vehicle and 80,000 kilometers for the e-vehicle which is mainly driven in urban traffic and does not reach the high mileages. In this case, the emissions for the electric car during production dominate – giving this model the worst life cycle assessment in this scenario.
When using carbon-neutral energy sources, the greenhouse gas emissions are significantly reduced for all models in the usage phase. Here too, it becomes clear that the percentage of total emissions resulting from production and recycling will increase in future as more and more carbon-neutral energy sources are used.
Differentiated picture when looking at all environmental impacts
Only looking at greenhouse gas emissions, however, does not reveal the full picture. If other environmental categories or environmentally relevant criteria are also included, the different drive concepts show different strengths and weaknesses. The fuel cell vehicle and the battery-electric vehicle do best when it comes to the formation of summer smog. Plug-in hybrid and e-vehicle show particular weaknesses in terms of their acidification potential.
“The study reveals that analyzing the environmental impact from different drive concepts must not be based on driving alone”, Semper says, summarizing. “Only a complete life cycle assessment provides a fair comparison of vehicle types.” Studies of this type are still not prescribed in law – but experts expect such analyses to become compulsory after 2020. “We are already getting inquiries about them from OEMs”, Semper says. “This is why IAV is analyzing the influence a vehicle’s different life phases have on the environment.”