Alternative fuels: how much do they reduce the carbon footprint of transport?

In recent years, priority has been given to alternative fuels to the detriment of fossil fuels. In the automotive sector, the transition to these new energy sources represents a revolution that can contribute to the decarbonization of the sector. But what is the real benefit that these new energy sources can offer in terms of reducing the carbon footprint of road transport?

To answer this question, it is necessary to consider both the life cycle of the fuel or energy source and that of the vehicle. Regarding the life cycle of the fuel or energy source, the analysis considers all the stages of its life cycle, from production, transformation and distribution, to the phase of use of the fuel in the vehicle. It is what is known as “from the well to the wheel” (“well to wheel”, WtW).

This complete fuel cycle is divided into two phases:

  • A first stage “from the well to the tank” (“well to tank”, WtT), where the emissions generated in the process of obtaining, transforming and distributing each fuel are considered.

  • A second stage “from tank to wheel” (“tank to wheel”, TtW), in which the greenhouse gas (GHG) emissions generated in the vehicle itself are counted.

One of the lines of work of the Repsol Foundation Chair in Energy Transition – Sustainable Mobility from the Higher Technical School of Industrial Engineers of the Polytechnic University of Madrid.

Different bibliographic data allow comparisons in terms of mass of CO₂ equivalent emitted per kilometer traveled, considering methane (CH₄), nitrous oxide (N₂O) and carbon dioxide (CO₂) as the main gases emitted by the sector. The latter is the most relevant due to the magnitude of its issuance.

Thus, for example, according to the results obtained by the Latest report published by the JEC consortium (Joint Research Center, EUCAR and Concawe), you can establish the comparison in relative terms shown in figure 1 for the entire life cycle of the energy source. In it, the value of 100 is assigned to gasoline (consumed in internal combustion vehicles).

Figure 1. Relative comparison of GHG emissions associated with the energy source and for different vehicle technologies. Analysis for the whole cycle (WtW) and disaggregation for the WtT and TtW stages. Gasoline in internal combustion vehicle = 100.
Own elaboration based on data from Prussi et al., 2020

Gasoline and diesel

Regarding conventional fuels, diesel used by diesel engines has a lower carbon footprint than gasoline (around 13%). The GHG emissions of diesel (diesel) both in the procurement, transformation and distribution phase as well as in the use phase are lower than those of gasoline. In this second phase, the reduction in emissions is between 8-15% for the European area, according different sources.

On the other hand, gasoline and diesel hybrid technologies allow for reductions of 18% and 26%, respectively, compared to traditional internal combustion engines; 17% and 37% in the fuel use phase. Therefore, the hybridization option of conventional technologies reduces the life cycle impact of the energy source.

Natural gas and liquefied petroleum gases

The use of compressed natural gas (CNG) reduces the impact by 18% compared to gasoline for the entire life cycle. But with regard to diesel, the reduction is barely 5%.

Regarding liquefied petroleum gases (LPG), the reduction is 16% compared to gasoline, so results are similar to natural gas. While natural gas generates a greater impact in the phase of obtaining and distributing the fuel, liquefied petroleum gases obtain worse results in the emissions emitted through the exhaust pipe of the vehicle itself.


In the case of biofuels, it must be considered that the CO₂ emissions generated during combustion are analogous to those captured by the plant species in its growth, therefore, in this regard, its emission can be considered neutral, as shown schematized in figure 2.

Additionally, some processes for obtaining biofuels from different resources not only result in the desired product, but also in other flows or by-products, for which an additional environmental benefit is obtained, if the emissions avoided by not producing the material are discounted. which the co-product can replace in its use. These ways of obtaining are multiple, and can use different resources (including waste).

By choosing representative production routes for each biofuel, based on its availability in the medium term and on the production processes implemented in Europe, bioethanol reduces the carbon footprint of the entire life cycle by 28% compared to gasoline, presenting negative carbon footprint values ​​in the obtaining phase due to the absorption of CO₂.

On the other hand, biodiesel shows reductions of 54% compared to gasoline throughout the cycle, presenting negative emissions for the first stage due again to the accounting of CO₂ absorption.

Electric vehicles

For the electric cars and taking the European electricity generation system in 2016, the life cycle carbon footprint of the energy source is reduced by 68% compared to gasoline. This reduction is 100% in the use phase, that is, zero emissions are produced.

However, in the phase of obtaining the energy source, emissions are up to 71% higher than gasoline, associated with the electrical energy production process. The electricity generation matrix in Europe in 2016 was characterized by the presence of 43% fossil sources, 29% renewable and 26% nuclear energy.

As renewables increase their contribution to electricity generation, as in the case of Spain, where by 2020 renewables accounted for 44% of the national electricity production, emissions from the first phase of the life cycle will be significantly reduced.

Currently, the results of the emissions associated with the generation of the electricity consumed vary significantly from one geographic area to another. Thus, within Europe there are important differences between countries such as Poland (great dependence on coal) and Sweden (great presence of renewables and nuclear).

Figure 2. CO₂ absorption-emission cycle in the case of biofuels.
Author provided

Synthetic fuels and hydrogen

The synthetic fuels they are becoming a relevant alternative for the decarbonisation of the transport sector. The advantage of these fuels lies in their independence from non-renewable resources such as oil and in that they can be used in existing vehicles, without the need to develop new technologies in the engines. Additionally, it is worth highlighting its unlimited production potential.

There are various routes for its production, which generate a wide range of values. These range from a 99% reduction with respect to gasoline for the entire fuel cycle by choosing wood residues with CO₂ capture as a resource, to increases of 188% by choosing coal.

Regarding hydrogen, there is a 50% reduction in the impact compared to gasoline throughout the life cycle. It is 100% in the use phase. On the contrary, depending on the hydrogen production process, in the obtaining phase an increase of up to 167% with respect to gasoline could be verified, considering the so-called gray hydrogen, which is the predominant production route at present. In this case, it is obtained from natural gas without capturing the CO₂ emitted.

Thus, gray hydrogen gives rise to greenhouse gas emissions that can have a greater impact on climate change than electric vehicles and those that use biofuels or synthetic fuels.

Other ways of hydrogen production from renewable energies and electrolysis (green hydrogen) or with CO₂ capture processes (blue hydrogen) lead to lower GHG emissions (even zero) in the obtaining phase. In this way, if wind energy is chosen as the route for hydrogen production, the reduction potential is 93% with respect to gasoline throughout the life cycle of the energy source, a value that contrasts with that obtained for gray hydrogen.

Which energy source has a lower carbon footprint?

comparing the results previously commented and representative of the situation at the European level, with the results obtained with the American model GREET, it is concluded that, beyond the quantitative differences obtained when considering information for different geographical areas, the comparison between the different energy sources follows practically the same trend (figure 3).

Gasoline of fossil origin is the fuel with the highest greenhouse gas emissions, followed by gas oil (diesel) and liquefied petroleum gases. Biofuels and synthetic fuels compete with electric vehicles in terms of their carbon footprint, although the path of obtaining the energy source (for electricity, biofuel and synthetic fuel) is the key factor in the differences in their footprint. carbon.

Figure 3: Relative values ​​of GHG emissions throughout the cycle comparing the JEC model and the GREET model for different fuels. Synthetic diesel: conversion of wood waste into diesel by hydrothermal liquefaction in JEC, e-diesel in GREET (CO₂ from the atmosphere + hydrogen from water with electrolysis process). Hydrogen: from natural gas without CO₂ capture. Electricity: European generation parent at JEC and US parent at GREET.
Author provided

With this range of possibilities, what measures should be taken to reduce the impact on climate change? The proposals that can lead to the decarbonisation of the sector are varied, with parallel and even complementary routes that may depend on geographical and temporal conditions, given the constant technological evolution. Solutions based on electric vehicles, biofuels and synthetic fuels with low or zero carbon footprint and hydrogen are gaining strength as the main options to reduce impacts on the climate.

In addition, we must not forget the environmental impact associated with the life cycle of the vehicle, which can be remarkably different between different technologies. This last aspect is also being addressed in one of the lines of work of the Chair.

This article is part of the coverage of The Conversation on COP26, the Glasgow climate conference.

Follow full coverage on English, French, Canadian French, indonesian language and español, here.

Javier Pérez Rodríguez, Professor of the Department of Industrial Chemical Engineering and the Environment. Member of the Environmental Technologies and Industrial Resources Group, Polytechnic University of Madrid (UPM)

This article was originally published on The Conversation. read the original.

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