I am trying to find what Euro 4 standard is with regards to CO2 g/km. All I can find is that it is:

CO: 1.0g/km
THC: 0.10g/km
NOx: 0.08g/km

This makes no sense to me when trying to compare this to e.g. a CO2 g/km on sites like Autotrader where filter values are:

100 g/km CO2
110 g/km CO2
120 g/km CO2
130 g/km CO2
140 g/km CO2
150 g/km CO2
165 g/km CO2
175 g/km CO2
185 g/km CO2
225 g/km CO2
255 g/km CO2

2 Answers 2


The Euro 4 (and similar standards) are not about CO2 emissions. They are about other pollutants in the exhaust, as in your table. They apply to all cars, whatever the engine size and fuel consumption.

Note that CO (carbon monoxide) is not the same as CO2 (carbon dioxide). For a "perfect" engine the CO emissions would be zero, but the CO2 emissions depend how much fuel is being used.

The CO2 emissions are related to fuel consumption (miles per gallon or liters per 100km), and the numbers apply to particular makes and models of car.

The CO2 filter levels on sites like Autotrader are because in some countries (e.g. the UK) annual car tax rates are linked to the declared CO2 emissions.


There is a fundamental misunderstanding in the question. Perfect combustion of hydrocarbons (HC, H = hydrogen, C = carbon) produces the oxide of hydrogen and oxides of carbon.

The oxide of hydrogen is water, H2O.

The main oxide of carbon is carbon dioxide, CO2, although there is carbon monoxide too, CO. Only imperfect combustion produces CO. Perfect combustion produces only H2O and CO2.

Imperfect combustion produces various pollutants as well as H2O and CO2:

  • One is the carbon monoxide, CO, which I already mentioned. It is fatal even in very small amounts in the air.
  • Another pollutant class is oxides of nitrogen, often called NOx because there are quite many of them with varying x. They are a constituent of smog.
  • A third pollutant class is unburned hydrocarbons, called HC (although it's a mystery why they are not called HCx akin to NOx because there usually won't be as much H atoms as there will be C atoms). They are too a constituent of smog. Sometimes, methane is separately mentioned and then the non-methane parts are called NMHC and the total amount of methane and non-methane parts may be called THC.
  • A fourth pollutant class is particulate matter, called PM. They are typically hydrocarbons, but in particulate form. If they get to your lungs, they can cause respiratory diseases. PM is the main reason unfiltered diesel exhaust is dangerous when inhaled for long periods of time.

Now, these pollutants are some seriously bad stuff, and they should be minimized at all costs. However, both of the non-pollutants, H2O and CO2 are greenhouse gases: they increase the temperature of our planet due to radiative forcing. Of these, H2O has a balance between liquid and gas states, and adding more H2O just adds more of the liquid while keeping the gas in balance, so fortunately more H2O does not equate to a warmer planet. Also, there is so much H2O in the oceans that adding a bit of more won't hurt.

However, CO2 is a gas with no liquids in the usual temperatures on planet Earth. Thus, adding more CO2 adds to the amount of gas in the atmosphere directly. The main harm is radiative forcing which creates a warmer planet.

A particular amount of gasoline produces a particular amount of CO2: 1 litre of gasoline equates to 2.35 kg of CO2.

There are plants to limit CO2 which would equivalently limit fuel consumption. However, Euro 4 is so old standard that it does not apply to CO2 at all. I believe the CO2 limit standards in the future will be completely separate from Euro N standards where N is the version number. So even Euro 10 if we see such a standard ever won't probably limit CO2.

Limiting CO2 is a fundamentally different thing than limiting the harmful pollutants. Traveling a kilometer requires a certain amount of energy, and if the energy source is gasoline, at a certain efficiency a certain amount of gasoline needs to be burned. Limiting CO2 can only be achieved by better energy efficiency or a more aerodynamic/lighterweight car.

Making the picture more complex is that if the gasoline is created from biological sources, they have captured the carbon from the atmosphere. Thus, even if 1 litre of biofuel creates 2.35 kg of CO2, it won't add any net CO2 to the atmosphere because the biological sources have captured the CO2 already. Thus, limiting CO2 will be hard: how can the limits take into account biofuel usage? I think they'll just have to assume some percentage of biofuel used (such as 10%). We don't have enough biological sources of gasoline to replace all gasoline usage on this planet.


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