## 2 Literature Review
### Direct and Indirect Emissions  
In transport, approaches to calculating pollutants and greenhouse gases fall into two basic groups in terms of the direct or indirect environmental impact. Direct emissions are represented by the tank-to-wheel (TtW) approach, and indirect emissions by the well-to-tank (WtT) approach. The well-to-wheel (WtW) approach is a more comprehensive approach considering both direct and indirect emissions, but it does not deal with emissions over the entire life cycle of the product. Research by [PERSON] and [PERSON] [8] focuses on estimating tank-to-wheel efficiency functions for different vehicle types, such as internal combustion engine vehicles (ICEVs) and battery electric vehicles (BEVs). The study addresses the limitations of using a constant value to represent TtW efficiency, which may not adequately capture the variability of driving conditions. [PERSON] et al. [35] found that TtW efficiency varies depending on the engine power used, and the estimated functions exhibited different patterns for diesel, petrol, and electric vehicles. The results showed that the TtW efficiency for electric vehicles was higher than that for fossil fuel vehicles, reaching approximately 85% efficiency. For fossil fuel vehicles, the efficiency varied with engine power, with diesel vehicles showing higher efficiency than gasoline vehicles at low power levels. [PERSON] et al. [36] conducted research on emissions from articulated wheel loaders, which are non-road mobile machines used in the construction industry. The research conducted by [PERSON] et al. [37] deals with the environmental assessment of road freight transport services and presents a methodological approach to defining boundaries and activities within the transport system. It highlights the importance of integrated analyses to document the environmental sustainability of services, pursue policy objectives, and enable consumers to choose the most sustainable services. The study reviews previous life cycle assessment (LCA) studies in the transport sector and identifies limitations in the assessment of individual transport components. [PERSON] and [PERSON] [38] looked at a comparison of low-carbon alternatives for road transport, such as electricity, hydrogen, and biofuels, in combination with battery electric vehicles (BEVs) and fuel cell electric vehicles (FCEVs). The comparison is based on the overall efficiency and greenhouse gas (GHG) emissions involved in converting primary energy sources into the energy needed on wheels through a well-to-wheel analysis. The research considers factors such as the electricity generation mix, renewable energy sources,and the impacts of direct ethanol fuel cells (DEFCs) on GHG emissions and the overall efficiency of the chain. In research conducted by [PERSON] et al. [39], an analysis of WtW biomethane derived from biomass gasification for use in heavy-duty engines in the European Union was carried out. The research compared biomethane with fossil fuel alternatives such as compressed natural gas (CNG), liquefied natural gas (LNG), and diesel. Three different modern gas engines for heavy-duty vehicles were also considered: the spark ignition (SI), the dual fuel (DF), and the high-pressure direct fuel injection (HPDI) engines. The WtT research was based on a case study of the GoBiGas plant in Gothenburg, Sweden, which is the largest biomethane plant in the world. In the TtW part, three different engine technologies were compared. The results showed that WtW emissions from biomethane were reduced by 60-67%, 43-47%, and 64% when used in SI, DF, and HPDI engines, respectively, compared to diesel. HPDI and DF engines were found to be the most efficient technologies for biomass utilization, achieving significant emission reductions. The research by [PERSON] et al. [40] analyses the potential costs and impacts of achieving CO\({}_{2}\) emission targets for new vehicles in the EU. The research focuses on TtW emissions and examines scenarios involving electric drive vehicles (EDVs) and shifts between vehicle segments.