A transport expert claimed that a Euro 6 car has fewer NOx emissions than a Euro VI bus, surely this cannot be correct?!


Euro 6b (diesel car) - 0.080 g/km of NOx

Euro VI (bus) - 0.46 g/kWh of NOx

Need to convert from g/kWh to g/km. Assuming 1 gallon is equal to 36kWh and the bus does 7 miles per gallon.

7 m/g = 11.2km/g NOx/g = 0.46g/kWh 0.46g/kWh * 36kWh/gallon = 16.56g/gallon 11.2km/gallon / 16.56g/gallon = 1.49g/km

I make it that the bus would be 1.49g/km in comparison to the car that would be 0.08g/km. I make the single bus 18.6 times more polluting than a single car.

Is this correct?

  • Is it possible that they meant per passenger?
    – LShaver
    Commented Mar 31, 2021 at 14:22
  • Yes, I realised that when I was writing this! However, I wrote this spreadsheet that shows, based on emission limits alone, that the difference per passenger is not clearcut. For example to have parity between in emissions between a bus passenger & a car passenger (1.4 per car), the bus would need to achieve 8mpg with an average of 23 passengers...for CO you would need an avg of 32 passengers...these are very high occupancy rates. docs.google.com/spreadsheets/d/…
    – atreeon
    Commented Mar 31, 2021 at 21:59

1 Answer 1


Your calculation is incorrect.

1 gallon contains 36 kWh of heat, but only around 12 kWh of that will be turned into useful motive power in an engine with 33% efficiency. The rest, 24 kWh, are emitted as heat. I suspect the Euro VI standard for buses is 0.46 g / motive-kWh and not 0.46 g / thermal-kWh.

So 0.46 g / kWh would be 5.52 g / gallon or 5.52 g / 7 miles or 0.78857 g / mile or 0.49 g / km. Not 1.47 g / km (which I assume is what your calculation without rounding errors would give.)

So you need only 6 passengers on the average to make the bus less polluting. I think most buses are typically above this level.

  • Very interesting @juhist I'll do some investigation on this and try to find out if it is motive-kWh or thermal-kWh
    – atreeon
    Commented Apr 12, 2021 at 10:26

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