EPA mileage testing, and hot air, well, carbon dioxide.
 

When the EPA says
What do they mean by carbon compounds, and precisely? Can they measure hydrocarbons of different lengths as well as carbon dioxide? Could subpar emissions systems be why certain cars test higher in terms of mpg?

If hydrocarbon emissions aren't burned in the cat as well as they could, then carbon dioxide emissions will read lower than they would compared to a vehicle with a cat that's operating well, and the mileage rating would be higher, even though in practice, the car is just as efficient with, or without, a decent cat. Could large manufacturers take advantage of this to inflate their mileage numbers?

If that is the way its worked out , its cracked.

Very simply put, they can measure the CO2 output, and then do calculations based on the known formulation of the test reference fuel. This is how they do it in Canada as well.

Ahh , no wonder the EPA figures are inacurate then.

If this is the case, then depending on the composition of the unburnt hydrocarbon that gets through, a car w/o a cat will get better mileage than a car with a cat when going by only carbon dioxide emissions. Take the comparison in section 5.5 of this. Going to emissions controls dropped hydrocarbon emissions by more than an order of magnitude. Not that the difference in emissions systems would be that bad, but considering that
There might be a significant financial incentive to reduce emissions system effectiveness. If the cafe standard is 30mpg and if a car gets 25mile/gallon combined, and a gallon of gas weighs 6.216lbs, then it would use roughly 100,000mg/mile of gasoline. Assuming the worst case senario of having a catalyst that's not functional for the ccombined test, then that's a 1% difference, which translates into .25mpg less, or a $14 tax per car. Now if this is multipied across an entire manufacturer's fleet of say, 5 million cars per year, then that's a significant chunk of tax, something like $70 million.

Now I wouldn't go so far as to speculate a manufacturer would use a vehicle in the EPA test that didn't have the cat functioning at all, but if the cat is functioning at reduced efficiency during the EPA test interval, then that is still a big chunk of cash, maybe a few million per year, for a manufacturer who's over the cafe standard. And it would be pretty hard to detect, since from what I've gathered, ECUs can be reprogrammed.

Not that this is what happens, or has happened.... But if I were a thrifty auto CEO, a million bucks isn't something to sneeze at. Not that this is even possible. Maybe they do the emissions and fuel economy testing with the same car and strict security, and no one slips anyone any cash.... Because bribery isn't likely? ;)

HC and CO2 aren't the same thing...

I'm not following the logic here for some reason.

Is there doubt about the validity of measuring a tailpipes output of CO2 to determine precise mileage?

On a side note, I have no idea where they get these "City" MPG figures. In my testing, that number has almost always been way high for mostly city driving, and with some Interstate to bump it up! The new guidelines that are phasing-in should help paint a better picture of real driving now, not something based on driving habits of the 1950's.

RH77

CxHy is converted to x CO2 when the cat oxidizes it. Now, from a practical standpoint, a car without a cat compared to a car with a cat may read 1% better mileage if the EPA only measures the amount of carbon dioxide, so it's a miniscule amount from th driver's standpoint. Otoh, from the standpoint of the cafe tax, 50mg/mi of HCs that aren't converted to CO2 can reduce the tax by a few million with a sizable fleet, provided the EPA only measures carbon dioxide out the pipe.

The actual mileage equations are in 40 CFR 600.113. They include hydrocarbons, CO2 and CO. I don't think the same tests are used for emissions and mileage, but CAFE and the window stickers come from the same tests.

They publish emissions data from the tests they do at https://www.epa.gov/fueleconomy/data.htm, in case you're interested.

Oh neato, at least they use the HC/CO content.... But, it seems there's still a big loophole.
Which, if I'm doing this right, means that a large manufacturer, like GM, can save ~$20-30million a year by insuring their vehicle fleet's fractional mileage is above y.x5mpg. I wonder how much has been "worked around". Hmmm... :thumbdown: