Decarbonization is a major concern for the transport sector as, despite the continuous progress in improving vehicle efficiency, energy demand from the sector is growing. Consequently, CO₂ emissions from the sector are still increasing. Looking to the light duty sector, in 2018 the average CO₂ emissions from the EU car fleet grow up by 1,7% compared to 2017 according to the European Environmental Agency EEA.

CO₂ standard emission targets have been set for 2025 and 2030. These targets have been revised for the light-duty sector and, for the first time, also introduced for the heavy-duty one.

Standards have been set according to the CO₂ tailpipe measurement approach.

In the following, we will explain why tailpipe measurement is a limited instrument to assess the carbon footprint of a vehicle.

1 – Why measure CO₂ emissions at the tailpipe?

The tailpipe CO₂ emissions measurement has been introduced to define vehicle fuel consumption. This is calculated through the so called ‘carbon balance’, which is delivering the total carbon content from CO₂, CO and THC (Total Unburnt Hydrocarbons) and then converted into the corresponding fuel amount.

We can use tailpipe CO₂ emissions to compare vehicle efficiency only when no other sources of energy are used. But, when electrically rechargeable vehicles (PHEV, BEV) are used, the electricity does not contribute to tailpipe emissions, thus providing a virtual zero emissions dimension. In reality, the carbon footprint from the electricity production is very different across Europe, with an average emissions of 106 gCO₂/MJ (see https://www.electricitymap.org).

Looking to the fuel dimension, when we use a renewable fuel (such as renewable gas), the CO₂ tailpipe measurement continues to translate the fuel consumption, but it is completely ‘blind’ towards the Well to Tank (WTT) benefits which are normally associated with renewable fuels.

Tailpipe emissions measurement cannot appreciate any difference between a fossil fuel and its homologue, renewable based, one.

This way of measuring only detects the renewable fuel if its carbon content is different from the fossil fuel one.

In the case of renewable gas, meaning biomethane or synthetic methane, the final product is exactly the same as natural gas. Consequently, CO₂ emissions, measured at the tailpipe do not differ. Even if irrelevant to the determination of the vehicle fuel consumption, it is not towards decarbonisation. In fact, the CO₂ emitted by the combustion of a renewable fuel is balanced by the CO₂ credit realized from the carbon capturing and conversion in the biomass used as feedstock.

As a result, the net CO₂ emissions generated by a renewable fuel is the one associated with the biomass to fuel conversion process and its distribution, as represented in the following figure:

For this reason, if we only look to CO₂ tailpipe emissions, we are not able to understand and measure the overall impact from the vehicle+fuel system towards GHG emissions.

With this approach, Battery Electric Vehicles and Plug-in Hybrids result as the most effective technologies to comply with the new emissions standard, but it results in contradiction with the ‘technology neutral’ approach.

Considering the complexity of future vehicle architectures and the growing mix of renewable energies sources used to produce fuels and electricity, the tailpipe approach results limited under a ‘net-zero’ emissions standpoint.

Under this perspective, tailpipe emissions measurement is like wrong glasses, allowing only a partial view of the entire problem.

2 – Well-to-Wheel (WtW) evaluation

WtW methodology is a first step to compare the impact of different solutions towards Greenhouse Gas Emissions (GHG) emissions.

WtW results out of the contribution of tailpipe emissions (Tank-to-Wheel, TtW) plus Well-to-Tank (WtT) emissions. These include the CO₂ generated during the production of the fuel, its transportation to the fuel station and dispensing to the vehicle tank.

When we compare different powertrain architectures and fuels, the combination of what is measured at the tailpipe and what is generated by the use of such a fuel (or electricity, or both as in the case of PHEVs), is shown in the following figure, relating to a so called ‘medium passenger cars’ (C-segment). Fuel economy and energy consumption for electrified solutions have been agreed internally at NGVA Europe (BEV consumption at 14,5 kWh/100km), while WtT data has been elaborated starting from the last dataset presented by Concawe at EUSEW and anticipating the Version 5 of the WtW Study from the JEC Consortium.

What we can conclude from this graph comparing conventional fuels with electrified solutions:

• On a TtW basis, electrified solutions offer the best performance
• Considering the current EU energy mix (106 g CO₂/MJ), WtT CO₂ contribution from BEV is approximately the double compared to conventional fuels.
• On a WtW basis, BEVs offer better performance compared to conventional fuel thanks to the higher efficiency of the powertrain.

Looking to natural and renewable gas

Natural gas has a direct effect on tailpipe emissions as, for the same energy, the CO₂ generated is approximately 23% lower than of conventional fuels.

In this section, we only compare CNG solutions with battery electric vehicles looking to today and 2030 energy mix.

For electricity production, we have considered the current EU energy mix (106 g CO₂/MJ) and what the JEC (which is a collaboration between the European Commission's JRC, EUCAR, CONCAWE) is projecting in 2030, corresponding to 72 CO₂/MJ. With an energy consumption by 14,5 kWh/100 km, the overall WtW emissions in 2030 corresponds to 37 g CO₂/km.

CNG car, with 94 g CO2/km tailpipe emissions, generates 114 gCO2/km at WtW level when fed with natural gas (fossil).

When considering different options coming from the conversion of biomasses (e.g. municipal waste and liquid manure) or from the combination of H₂ and CO₂ to provide synthetic methane benefits are much higher.

On a WtW basis CO₂ emissions are as follows:

• With biomethane from municipal waste à 17 g CO₂/km
• With Power to Methane à 7 g CO₂/km
• With biomethane from liquid manure à -171 g CO₂/km

These results show that we have a powerful technology TODAY to tackle GHG emissions:

• With pure biomethane from municipal waste, the WtW carbon footprint is better than the from BEV’s.
• With Power to Methane, the carbon neutrality target is achievable.
• When we convert liquid manure into biomethane, we come to negative emissions! What does this mean? It means that we are capturing and converting CO₂ emissions from the atmosphere, so we are providing the same function as a forest: absorbing CO₂.

So, simply moving from a tailpipe to a Well-to-Wheel CO₂ measuring approach, we realize that there are more solutions other than electrification that can effectively contribute to the decarbonization process.

Due to the immediate urgency and necessity to start curbing CO₂ emissions from the transport system, we need to maintain an open legislative frame where we can multiply and set choices for feasible solutions.

The great opportunity offered by natural gas infrastructure and vehicles is that they are fully compatible with 100% renewable gas with zero extra costs!

It is nonsense to think that electrified solutions and natural gas technologies would be competing. The transport system will continue to use internal combustion engines also in the different families of hybrid architectures and these engines must be fed with the cleanest fuels. And natural and renewable gas is here to get the job done!
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Find the related article 'The European Green Deal: time to start!' here, and 'Going beyond Well-to-Wheel: Life Cycle emissions' here.