Is tailpipe enough? How are vehicle emissions measured?

Tailpipe emissions? Let’s step back a bit first. Decarbonization is a major concern for the transport sector which despite the continuous progress in vehicle efficiency, continues to grow in terms of energy demand in mobility and freight transport. As a consequence, CO2 emissions from the sector are still increasing.

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

Here’s what’s important to understand – standards have been set according to the CO2 tailpipe measurement approach. In this article, we will try to explain what this means.

Let’s see if this is the real indicator and what we can expect in terms of decarbonization from the sector.

About CO2 emissions tailpipe measurement

The tailpipe CO2 emissions measurement has been set to define the vehicle fuel consumption. It’s calculated through the so-called “carbon balance”, summing up the carbon content from CO2, CO and THC (also measured) and converted to a fuel quantity.

You can use tailpipe CO2 emissions to compare vehicle efficiency when no other sources of energy are used. In the case of electrically rechargeable vehicles (for example PHEV or BEV), you would use and convert energy that cannot be measured at the tailpipe, making it invisible.

Again, when the dimension of renewable fuels (such as renewable gas) is introduced, the CO2  tailpipe measurement is completely irrelevant. Only in case the carbon content of the renewable fuel results different from the fossil one, a difference in tailpipe CO2 emissions can be measured. Such a difference is not what’s important here.

The consequence of the CO2 tailpipe approach is that BEV and PHEV result as the most effective technologies to comply with the emissions standard.

So what is the final scope from the regulation?

Vehicle efficiency

If the answer is to improve vehicle efficiency, it is clear that we should move in the energy dimension, so setting a target in terms of MJ/km or kWh/km.

Important to notice, in the current regulation, there is no target/request of efficiency on BEVs. This means that, as it is assumed to be zero emissions, it can also be…unefficient! Of course, this is not true as OEMs have interest in realizing efficient BEVs, with the aim to provide as much as vehicle range for the same energy.


If the aim of the regulation is decarbonization, meaning the reduction of GHG emissions from the sector, we are looking to the subject with the wrong glasses, getting only a partial sight on it. 


Because we measure only one component – the tailpipe emissions – ignoring the rest. In the next article, we will discuss how to improve this approach by making it more comprehensive. At the end of the day, if we trying to provide the right solution, we need to use an adequate measurement, right?

Stay tuned for the next article – talking about Well-to-Wheel Emissions!

In the meantime, feel free to share this article with a friend who is also interested in clean mobility.

We are also active on our social channels - let's connect!

Leave a Comment

Leave a Comment

© NGVA Europe. Transparency register: 1119946481-54. All rights reserved