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7 Facts About CO2 And Why It Comes Out Of The Exhaust.

Burning 1 liter of diesel produces 2.65 kg of CO2.

Source: helmholtz.de

It’s not negotiable, no chance! It is a law of nature.

Does it mean you cannot influence the CO2 emission?

No, it doesn’t mean that!

The CO2 emission of a vehicle can be reduced: 
> by lowering the fuel consumption 
> by using different types of fuel 
> by using different drive train technologies.

In this article, you will learn a lot about carbon dioxide and how much of it is created by burning different types of fuel.

Please take a look. The article will help you to understand the topic and its connection to climate change.

What is CO2?

Carbon dioxide (the chemical formula is CO2) is a molecule of one carbon atom and two oxygen atoms.

CO2 is a non-toxic, odorless, colorless and non-flammable gas.

This chemical element is an integral part of our everyday life.

It flows out of the exhausts of (almost) all trucks and cars, and we humans breathe it out ourselves.

  • We breathe in oxygen-containing air,
  • we eat carbonated food,
  • we produce strength and CO2 in our muscles and
  • then exhale the CO2 with the air exiting our body.
  • Our exhaust gas, the air we breathe, contains approx. 4% CO2. (source)

The atmosphere on earth consists of 0.038% CO2. Because of this very small proportion, it is referred to as a “trace gas”. The same applies to some other chemical elements in the air.

For what purposes do we humans happily use CO2?

CO2 can be found in many beverages.

CO2 pressure bottle
CO2 pressure bottle.

When CO2 forms a chemical bond with water, it is called carbonic acid.

Most of the carbon dioxide in fizzy drinks is not carbonic acid, as many people might believe, but dissolved carbon dioxide gas.

It escapes from the liquid in the form of small bubbles and creates the desired tingling effect.

At -78 °C (-108.4 °F) CO2 is frozen. It is then called dry ice because it changes directly from solid to gaseous.

If very low temperatures are required in industrial applications, CO2 is used as a coolant. After it has done its job, it simply evaporates.

At entertainment events, CO2 is used to produce fog. You may have experienced that yourself.

What’s the role of CO2 in our existence?

Carbon dioxide plays a very important role for the life on earth.

Carbon dioxide is a greenhouse gas. It ensures that solar energy is retained in the earth's atmosphere. For this reason, the comfortable temperatures set in, which enable us and the other living beings to live here on our planet.

In this context, the importance of CO2 is so great that scientists declare the effects of other greenhouse gases, such as methane, nitrous oxide, F-gases, etc., in carbon dioxide equivalents.

The carbon dioxide equivalent says how much stronger the greenhouse effect of these other gases is compared to the effect of CO2.

Over millions of years, a balance has developed on earth.

  • Plants divide CO2 into oxygen and carbon using photosynthesis
  • Animals and humans generate CO2 with their metabolism from oxygen and carbon.
  • A perfect cycle of nature that has been stable for millions of years

But it was not always like that.

How did the carbon get stored underground?

For many, many billions of years, the earth’s atmosphere consisted only of carbon dioxide, and it was incredibly hot on earth. There was no life.

Around 3.5 billion years ago, bacteria appeared in this hostile environment and split the CO2 into oxygen and carbon through photosynthesis.

Photosynthesis uses the energy of sunlight to break down the CO2.

Oxygen became part of the air and formed ozone, which has protected us from cosmic rays ever since.

The carbon from this process was deposited in the ground.

Carbon containing fuels from geological prehistoric times are called fossil energy, they embody stored solar energy!

The conditions on earth which make life possible exist now for 350 million years. (Source: Wissenschaft-im-Dialog.de)

But we humans have been intervening in this cycle for a little more than a hundred years now. It started with the onset of the industrial age.

We take carbon (crude oil, natural gas, coal) out of the ground and convert it back into CO2. The work which results from this process is used to drive machines. The overall goal is to improve our prosperity and wealth.

By using fossil fuels, we are restoring, piece by piece, the state the earth was in before we existed.

– Not good at all! –

We are in the process of destroying the delicate balance that is the basis of our existence.

How is CO2 created?

In order for a CO2 molecule to be created, a carbon atom and two oxygen atoms have to exchange electrons and form a chemical bond. During this process, heat is released.

This is called an exothermic reaction or combustion.

A Flame at a candle
CO2 is a product of combustion. It always emerges when heat is generated by burning carbon. It is inevitable!
The only possible way to produce less CO2 is by burning less carbon.

I will go into this in detail in many blog posts.

There is still one alternative: Combustion with a deficit of oxygen.

This creates carbon monoxide (CO).

CO is a deadly breath poison. That is not a good idea. CO2 is definitely the better choice.

Why burning carbon?

Carbon-based fuels are safe and have an enormous energy density.

In order to get the combustion process going, energy must first be supplied, the “ignition”.

After that, the combustion generates enough heat to keep the reaction going.

The combustion continues until the fuel runs out, or the temperature drops below the ignition temperature. You know that under the term “extinguishing” a fire.

Carbon is characterized by the fact that it is difficult to ignite, but after ignition delivers a lot of heat from a very small volume.

This property explains why carbon is so incredibly popular as a fuel.

Carbon can be found in different forms.

  • Pure form –> Coal
  • Bound to hydrogen –> Oil, natural gas
  • Bound to oxygen –> Carbon monoxide and carbon dioxide

It is widely available and relatively easy to handle.

What happens during combustion inside an engine?

The diesel engine is an internal combustion engine.

Carbon-containing diesel oil is burned inside the engine’s combustion chamber in order to generate power.

I will explain exactly how the engine works in a separate article, a precise description would lead too far at this point

Here is a short version:

  • The working medium is air. In the end, the air in the cylinder delivers the power.
  • The engine takes the air from the environment.
  • The air provides the oxygen for combustion. This is very practical. The oxygen does not have to be taken along in the vehicle.
  • The number of oxygen atoms in the combustion chamber determines the maximum possible power output of the engine.
  • The air is compressed and becomes thereby very hot.
  • The fuel “Diesel” is injected into the hot air and ignites.
  • The diesel burns, releasing a large amount of heat into the air.
  • The air expands fast due to the sudden increase of thermal energy. The air pressure in the cylinder increases explosively.
  • The pressure is converted into force via the movable piston

So far, so good.

How does diesel oil burn?

Diesel consists out of several hydrocarbon molecules.

These molecules differ in the number of carbon and hydrogen atoms.

For example, Hexadecane with the formula C16H34.

One molecule of Hexadecane has 16 carbon atoms and 34 hydrogen atoms that can react with oxygen. It looks like this:

2 C16H34 + 49 O2 –> 32 CO2 + 34 H20

You can identify the two long chains of Carbon-hydrate (Hexadecane) and the Oxygen molecules from the air accompanying it.

After the explosion, the long chains are broken up and only small water and carbon dioxide molecules are left over.

Now we see the scientific evidence: When carbon is burned, carbon dioxide is always an outcome!

No matter how “clean” the combustion is, every combustion of diesel will leave CO2 behind.

The second product of the combustion reaction is water!

Really, there is a lot of water vapor coming out of truck exhausts!

Nobody talks about it because water is harmless. I guess a lot of people don’t even know this fact.

Speaking of clean combustion

Diesel is not pure carbon hydrate, and the air is not pure oxygen.

Everything that is in the cylinder can react with the oxygen in the air as soon as the ignition has taken place. So let’s take a closer look at that.

The air consists of 21% oxygen and 78% nitrogen. In addition, there are many other substances and gases in very small quantities in the diesel and in the air.

In the cylinder, we have carbon atoms, hydrogen atoms, and nitrogen atoms, which react with oxygen during combustion and form chemical compounds.

Besides the already mentioned carbon dioxide (CO2) and water (H2O), various nitrogen oxides (NOx) are created.

Nitrogen oxides are particularly critical.

They are poisonous, acid and have a greenhouse effect as well.

Fortunately, there is a way to convert the nitrogen oxides back into nitrogen and water vapor with SCR catalytic converters and Adblue fluid while still in the vehicle. (Adblue is also known as “diesel emission fluid” or DEF)

Unfortunately, there is no comparably elegant way to break down CO2 back into carbon and oxygen.

Because the combustion process is extremely fast and explosive, atoms and molecules that are not completely burned also remain in the exhaust gas.

Among other things, this is soot, also known as fine dust.

Soot are particles made of pure carbon that have not found oxygen atoms to dock on.

They are so small that we breathe them up into our lungs, where they can cause health issues.

These pollutants are also defused in the truck’s exhaust system. They are being burned afterward or filtered out of the exhaust gas in a soot filter.

CO2 is not only produced by burning fuel but also by producing and supplying it.

How much CO2 is released can be answered quite precisely:

Diesel fuel is standardized, and the corresponding values are officially published and specified for the calculation of CO2 emissions in the transport industry.

In the table, you find the values for diesel made from crude oil and for diesel with a blend of biodiesel, such as that used at the fuel stations in Germany.

DieselDiesel with Biodiesel
CO2 Tank to Wheel2,629 kg/l2,621 kg/l
CO2e Tank to Wheel2,665 kg/l2,657 kg/l
Co2e Well to Wheel3,174 kg/l3,069 kg/l
CO2 Emission pro Liter Diesel

I have extracted the following information from:

If you want, you can read more details yourself under the two links.

The 3 values for the CO2 emissions in the table differ in the scope.

  • The first value relates to the emissions that are emitted from in the vehicle, resulting from the combustion of the diesel. The scope of consideration starts in the vehicle tank and ends at the exhaust.
  • The second value also includes the carbon dioxide equivalent of the greenhouse gases that result from the combustion of trace gases in the engine. Furthermore, with Euro 5 and Euro 6, CO2 is also formed in the SCR catalytic converter, which is included here. The scope is tank to wheel as well. As you can see, the difference is less than 1.5%.
  • The third value shows a clear difference. The scope of consideration is drawn from the source of the fuel to the exhaust of the vehicle. In this case, it really starts at the well of the crude oil. It also contains the CO2 that is created during the production and provision of the fuel itself.

The CO2 footprint for the transport of goods is based on the WTW observation scope. It is used to determine how much CO2 was caused by the transport of goods overall.

To regulate vehicle CO2 emissions, the regulators use the tank to wheel value. The reason is, it is the scope of influence for the vehicle manufacturers.

Why are CO2 emissions calculated in g / tkm?

The last step to determine the CO2 emissions is still missing.

We now know the emissions per liter of diesel, and now we have to multiply by the amount of diesel used.

In freight transport, the value of CO2 emissions is related to the transported payload of 1 tonne and a distance of 1 kilometer. This gives the value in g / tkm. (Grams per ton and kilometer)

In this way it is possible to compare different means of transport such as shipping, rail, road freight transport or aviation.

The absolute amount of diesel consumed is often not known.

In these cases you can take the distance-related fuel consumption in liters per 100 km, multiply it by the CO2 value in kg or g and divide by the payload in tons times 100. You then get the CO2 value in kg / tkm or in g / tkm .

For this approach, the fuel consumption must be determined correctly. We will take a close look at that in my blog.

To relate the CO2 emission to the payload transported makes sense. Otherwise, the comparison would be misleading.

If I were to compare the CO2 emissions of a 40-ton truck with a 1000-ton train without taking the much larger load of the train into account, it would be unfair for the train.

In the public discussion, we encounter the CO2 emission value in two main fields:

  • The CO2 footprint in logistics. It looks at the CO2 emissions over the entire transport chain of delivery of goods. Based on this calculation, logistics companies measure how they are progressing on the way to CO2-neutral transport.
  • The CO2 regulation of the commercial vehicle industry. Its aim is to drastically reduce tank-to-wheel emissions from trucks over the next few years. The method by which this is measured is called VECTO. Of course, we will also take a closer look at that

Example values for CO2 emissions.

I have investigated the bandwidth of CO2 emission that different carbon-based means of transport have at full load.

The data are taken out of the special edition Verkehrsrundschau (Quelle: Andre Kranke; So ermitteln sie den CO2 Fussabdruck, Verkehrsrundschau, 23.09.2010).

g/tkmCO2 TTWCO2e TTWCO2 WTWCO2e WTW
Van342346408413
Truck32 -8533 -8638 -10239 – 103
Train17 – 6517 – 6620 – 7720 – 78
Barge8 – 538 – 5410 – 6310 – 64
Seagoing ship2 – 612 – 623 – 733 – 74
Cargo plane421 – 1712427 – 1736502 – 2043508 – 2067
CO2 Emissions, according examples from the special edition of the Verkehrsrundschau

One can see that even with the normalization to tonne-kilometers, there are still very large fluctuations, which have different reasons depending on the means of transport.

One thing is always the case: The more cargo a means of transport can load up, the lower the relative CO2 emissions are.

The van with a loading capacity of 0.5 tons is representative of the cars. You can see how bad the balance is when fully loaded. Not even thinking about it if there is only one driver in it.

Do petrol and natural gas leave less CO2 behind than diesel?

Petrol and natural gas have a higher hydrogen/carbon ratio than diesel. As a consequence, more water and less carbon dioxide are produced when they are burned.

This reads well. More water and less carbon dioxide would be good for the environment.

When we look up the CO2 Values in the literature (source: 51-52/2010 Verkehrsrundschau), we find the values in the table below.

Fuel TypeCO2 Emission TTWCO2e Emission WTW
Diesel with 7% Biodiesel2,621 kg/l3,069 kg/l
Petrol2,362 kg/l2,781 kg/l
Compressed natural gas (CNG)2,540 kg/kg3,229 kg/kg

As I have introduced already above, we typically look into different scopes.

The CO2 Emission Tank to Wheel (TTW) which we assign to the vehicle manufacturers.

The CO2e Well to Wheel (WTW) tells us the complete CO2 emission into the atmosphere.

At a first glance, the numbers look quite similar, with diesel being the worst in the TTW column. These numbers are often used when people talk about this matter.

CNG has a remarkable big difference between TTW and WTW. The reason is the large volume of the gas, which requires a lot of vessels to be transported from the well to the fuel station, where it enters the TTW scope. This number varies greatly, depending on the place of origin of the gas. Many times, it is outside of Europe.

For the following discussion, I put the TTW value in brackets, so that we can always see both scopes.

But let’s look a little deeper into the matter.

At first, we realize that CNG is in different units. We can convert everything into mass.

Fuel typeCO2 Emission DensityCO2 Emission per fuel mass
Diesel3,069 (2,621) kg/l0,83 kg/l3,697 (3,165) kg/kg
Petrol2,781 (2,362) kg/l0,74 kg/l3,758 (3,191) kg/kg
Compressed
natural gas (CNG)
3,229 (2,540) kg/kg3,229 (2,540) kg/kg

Now Diesel and Petrol move closer together. Gas still seems to be better.

But this is still not the end. We need to look at it from an energy point of view. Let’s add the caloric value, which stands for the energy released during combustion.

Fuel typecaloric valueCO2 Emission per Energypercentage
Diesel11,97 kWh/kg0,309 (0,264) kg/kWh100%
Petrol12,19 kWh/kg 0,308 (0,262) kg/kWh99,7(99,2)%
Compressed natural gas (CNG)13,16 kWh/kg0,193 (0,245) kg/kWh79,3(73,1)%

We see that diesel and petrol are very close to each other. Gas shows an advantage of 20%.

Now, that is still the input side of the story. What we need is the output side, and there it becomes complicated.

The internal combustion engine needs to trade the heat from the combustion into torque and rpm.

Because of the very different properties of the fuel, the engine has to deal with it in different ways. I am going to explain the diesel combustion process in a separate article.

Diesel oil is favorable for the purpose of Internal Combustion Engines. That's why the Diesel combustion process typically has better thermodynamic efficiency than the Otto combustion process, which is used in the case of petrol or gas fuels.

Engineers work hard to improve all sorts of engines. It can therefore not be concluded how big the difference in efficiency and in CO2 emission is, without the information of the particular fuel consumption.

According to the literature, the average Otto process is between 5 to 10% less efficient than the diesel process.

This would put the petrol engine at a disadvantage and reduce the advantage of the gas engine significantly.

Even more, the engines respond differently to certain load conditions.

The petrol engine tends to perform worse than a diesel engine, when it comes to CO2 emission, while a gas engine, fired with CNG, performs somewhat better.

Methane is a greenhouse gas!

Chemically, natural gas and biogas consist mainly of methane, which is a greenhouse gas with a carbon dioxide equivalent of 25. Under no circumstances should it get into the atmosphere!

If natural gas is used, it must under no circumstances be released in the atmosphere unburned. Otherwise, the climate impact is devastating.

Unfortunately, this is not uncommon. I guess you might have read or heard about leaking gas pipelines or blow-offs during transportation.

If you are considering using gas engines, please be aware of this. In my opinion, this is underestimated, when people consider Natural Gas as a climate-friendly alternative.

Is biogas a sustainable fuel?

We know the generation of biogas from biomass, which takes place in the present and therefore does not use the fossil fuels of the early days.

Instead of natural gas, biogas can be used as fuel in vehicle engines.

When burning biogas, CO2 is generated just like when burning natural gas.

However, the production of the biogas is based on carbon, which is sourced from regrowing, sustainable materials. This means that the CO2 balance is neutral.

That would be a suitable way of reducing the greenhouse effect.

I see it rather pessimistically that we would be able to produce as much biogas as we need for the transport of all goods. (In Germany in 2020 28,6 Million tons of diesel have been used according to statista)

The plants that do photosynthesis for us need a lot of space and grow relatively slowly.


That was a lot of information. I have touched on many topics without going great in detail. If you have questions, don’t hesitate to write them in the comment. I will do my best to answer.

Conclusion

  • CO2 is an inconspicuous gas that surrounds us every day.
  • The climate effect of increased CO2 content in the atmosphere must be prevented.
  • The CO2 emissions from trucks can only be reduced by reducing fuel consumption
  • In order to be able to compare different means of transport with regard to CO2 emissions, measurements must be made in grams per tonne-kilometer.
  • Natural gas engines are not a long-term alternative to limit CO2 emissions.

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