Car versus Bus carbon emissions

I came across something online that claimed that diesel buses emit 200x the pollution of a normal passenger car. I looked into this, read some studies, and it’s not true.

The average vehicle on the road emits ~1# of CO2/mile (it’s higher for light trucks, lower for cars, but it works out quite well). I looked up some testing data, and a late-90s diesel bus on low-sulfur fuel (no trap, none of the new cool technology) puts out ~6.4# CO2/mile.

So an older diesel bus = about six cars, CO2-wise.

– There’s a lot of variance in how much pollution an individual bus generates
– This doesn’t account for the relative harm of the pollution generated by the different vehicles, and there’s a case to be made that the buses should account for non-CO2 products

But as a range… there’s just no way it’s 1-to-200.

5 thoughts on “Car versus Bus carbon emissions

  1. Evan

    Since carbon dioxide doesn’t usually count as a pollutant (only since global warming became a big thing has that misconception become popular), comparing the carbon emissions isn’t really relevant.

    I’m sure if you measured suspended particulate emissions you’d see a difference much closer to the 200:1 quoted.

  2. DMZ Post author

    Ah, but there’s the rub with diesel – diesel’s smoggy because of the particulate issue, but as a global warming issue, you’re cool — as an energy efficiency thing, you get more for your CO2/CO emission dollar with diesel.

    Now, the smog and particulate – they’re horrible, sure.

    But there’s no way I can find to figure how a diesel bus is worth 200 cars.

  3. Evan

    Sure. Global warming folks should really like diesel engines for that reason (they should also really dislike burning ethanol for the same reason – a lower heating value means more CO2/mile travelled), but you specifically mentioned pollution. Measuring CO2 is sort of beside the point.

    Pollutants would be things like suspended particulate or sulfur dioxide or lead or mercury (or platinum, palladium, and rhodium, the biggest pollutants produced by catalytic converters). You haven’t measured pollutants.

  4. DMZ Post author

    Ahhhh. Yes. I should have clarified – this came up in a global warming argument, and I should have put this up as “bus != 200 cars in terms of warming impact”. I really couldn’t claim to know the particulate count.

  5. Evan

    You might be able to work out particulate count, though. The American Lung Association claims that 66% of all particulate from on-road sources is produed by diesel engines (published April 2000). So if you knew the proportion of diesel engines on the road, and then assigned some approximate values for engine sizes for large trucks and cars and the like, you could probably get a number within about 20%.

    But on the global warming thing, diesel engines are more efficient than gas engines in terms of CO2/mile travelled. The higher compression ratios produce greater efficiency, and the diesel fuel has a higher heating value than octane or ethanol, so you burn less of it to produce the same amount of energy.

    The differences in heating values between various combustible fuels are often ignored when comparing their CO2 production, and that’s a major oversight. Any engineering or chemistry student could do the calculations to show how much heat you get from burning each of the fuels (and since the internal combustion engine runs on heat, that’s what matters).

Comments are closed.