Have they lowered the quality of gas since the price went down?

[Magiver]

I check my gas mileage every time I fill up and there is no relationship to gas price and mileage. My gas mileage peaks when air temperature is in the 50-70F because I don’t use the A/C and the engine doesn’t run rich due to colder weather.

[kanicbird]

Many people have noticed a trend towards the reduced effectiveness of gas over the decades. Things they remember from the past that no one seems to remember anymore.

All I’m saying is you may be on to something.

[Magiver]

@kanicbird

Many people have noticed a trend towards the reduced effectiveness of gas over the decades. Things they remember from the past that no one seems to remember anymore.

All I’m saying is you may be on to something.

As was addressed above, adding alcohol to gas reduces the energy per volume. So people filling up a restored car from the 70’s will get poorer mileage today with the blended gases. But that has nothing to do with current gas blends. Alcohol is more expensive than gas so it is not added to save on the cost of production.

From what I’ve read, gasoline is blended based on regional EPA requirements and changes with the season. So it may be possible for seasonal changes to make a slight difference but I haven’t noticed. As I noted above, I check my mileage every time I fill up.

[SmackFu]

@Mr_Buttons

The whole article was about how people were getting about 10% less fuel efficiency once their neighborhood gas stations switched to 10% ethanol. It was a local reporter, so I can’t cite a big fact, but man-on-the-street interviews were returning what you suggested, the MPG dropped about 10% with the ethanol additive.

Wouldn’t that imply that the ethanol is getting 0 MPG?

[MatthewStabile]

I may have answered my own question.

I haven’t done any calculations, but from what it looks like, it might have been the few gas stations that I originally went to.

I am at about a half a tank (although I don’t fully trust the gauge enough to say this is more than an estimate) and I went about 160 miles so far, so it is looking like a considerable increase in mpg for this tank.

[matt]

@Santo_Rugger

I remember being involved in a discussion on this board about this before, but I thought the Carnot cycle showed that you get better mileage when it’s cold, because there is a bigger temperature difference between the intake and exhaust. My car runs at about 212F when it’s about 90F out, and about 205F when it’s 30F out. But the gasoline is still burning at the same temperature in the cylinder, isn’t it?

The Carnot cycle is a theoretical cycle that defines the limits of what is thermodynamically possible. The Otto cycle operates well below those limits.

Whether a heat engine benefits from a drop in outside temperature depends upon its mode of operation. A steam engine with a condenser will benefit, because the colder condenser will pull a better vacuum on the cylinder and so extract a bit more work while it exhausts.

A steam engine without a condenser, or a gasoline engine, will not benefit because the hot steam/exhaust is vented to the atmosphere. There is a greater temperature differential as heat energy transfers from the hot steam/exhaust gases to the cooler air, but there is no mechanism present to convert any of it into work.