What are the expected real emission differences between new and old double decker buses?
An internal combustion engine produces two kinds of emissions:
- Combustion byproducts
The oxides are materials that even perfect combustion will produce. They are what is on the right side of the combustion chemical reaction.
Internal combustion engines usually use hydrocarbon fuels. Hydro = hydrogen, carbon = carbon.
Thus, the produced oxides are the oxide of hydrogen (H2O) which is water, and the fully oxidized oxide of carbon (CO2) which is carbon dioxide.
Of these, water vapor is a greenhouse gas but in equilibrium with oceans, so adding a little won't hurt.
However, much of the produced carbon dioxide increases the carbon dioxide present in the atmosphere. Historically there was very little carbon dioxide, but recently its concentrations have increased, causing climate change.
The oxides have a direct relationship with produced energy. If you produce a certain amount of energy, a certain amount of oxides are produced. The energy efficiency of internal combustion engines is rising very, very slowly indeed. Most of the advances in energy efficiency have already happened long time ago. So, given this:
A new bus has very slightly less CO2 emissions.
In contrast, combustion byproducts are non-fully oxidized oxide of carbon (carbon monoxide), oxides of nitrogen, volatile organic compounds and particulate matter. There is no physical law that links these byproducts to the obtained amount of energy. Thus, radical decreases can be and have been obtained.
The 4.0 and 1.5 you found in Wikipedia are related, not to carbon dioxide, but to carbon monoxide. Its concentration can be reduced markedly in newer engines. However, the test procedures are evolving too, and thus, sometimes you see a new test cycle that creates more emissions, necessitating a newer limit that is actually larger than an older limit. Usually, even in these cases, if you put a new engine and an old engine to the same test cycle, you will find the new engine produces less combustion byproducts.
No emission law controls the amount of carbon dioxide produced. There are entirely separate targets for the fuel efficiency of vehicles, which directly have an effect on the CO2 emissions. The laws controlling combustion byproducts are for combustion byproducts only, not for CO2.