Measuring transpo. emissions: Per-mile or per-gallon of gas burned. Which is more useful and why it may matter

It’s no secret transportation emits more greenhouse gas pollutants (carbon mostly) than any other single sector and that emissions from transportation worldwide are on the rise.

And, it should be no mystery either that for every gallon of gasoline burned in an internal combustion (IC) engine, 19.64 pounds of carbon dioxide from said burning is released into the atmosphere.

I always thought that seemed like a lot just from the standpoint of what a gallon of gasoline weighs.

As an educated guess, I would say a gallon of gasoline weighs about 5 pounds. If this is so, then what that means is 5 pounds of gasoline, when burned in an internal combustion engine, produces roughly 4 times its weight in released carbon. That’s quite remarkable considering.

What’s highly relatable here is that regardless of how far a person say may drive and no matter the type of internal combustion engine relied on, for each gallon of gasoline consumed, approximately 20 pounds of carbon emissions is released into the air as a result.

On the other hand, emissions released per distance driven is a whole other matter.

It’s a well-known fact that most, not all, but most internal-combustion-engine-equipped motor vehicles get better gas mileage on highways than on city streets. I remember when I had my previous car (an SUV) its highway mileage rating was 23 miles per gallon of fuel burned versus its in-town mileage rating of 17 mpg.

So, when it comes to measuring the amount of emissions per distance driven, it makes sense that what amount of emissions released is determined by the variation in IC-engine efficiency.

Which suggests that as engine efficiency improves, the amount of corresponding released emissions would be lower over a given distance traveled and the amount of corresponding released emissions would be higher over a given distance traveled as engine efficiency declines. There is variability.

In this sense it is more difficult to determine for, per-distance driven, what emissions are compared to measuring emissions released per gallon of fuel burned.

Therefore, when considering the measure of emissions from transportation, it may be more useful to apply the latter rather than the former.

Under-the-hood view of internal combustion engine

On the other hand, looking at the former might be more useful if one were to, say, want to know emissions characteristics of vehicles for highway versus urban driving. Interestingly, for a hybrid vehicle, I once checked the mileage ratings of one and I noticed that in this one particular case, the city driving rating was actually better, not by much but still better than what was associated with highway use – in this instance, 49 versus 48 mpg, respectively.

It is also interesting why on a per-distance basis some countries fare better than others in terms of their having lower amounts of pollutant emissions entering the atmosphere from vehicle exhaust pipes.

In this sense what is probably being reflected is the idea that one country’s vehicles are, overall, cleaner-burning than another’s which would account for or explain the difference but such figures related to this, unfortunately, do not tell the whole story.

That all said, it’s true both measures are useful, each in its own way. And, that said, what I believe is the most important takeaway here is that cars’ engines are getting cleaner, not the opposite.

Awesome that!

Image above: Christopher Ziemnowicz

– Alan Kandel

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