MPG vs. Temperature

i think you're on to something.

 

If elevation gain / loss isn't too much, the biggest factor that impacts a trip is wind direction and speed.

There are too many uncontrolled variables at play in your single trip to say what accounted for the difference in fuel economy.

 

The AC would have to work harder in the afternoon, and there's no free lunch: it's using energy that would otherwise be propelling the car. If you're indeed more downhill on the return trip, the impact is even more than indicated.

Up here at the 49th parallel we never get straight-down sun and oven-like temps (like Nevada), and manage to avoid AC use pretty much straight through summer: just control the cabin temp by rolling the windows down, as much as needed.

 

I suspect AC use could account for your drop in MPG on the return trip, but there are too many other variables that prevent me from declaring it is the main cause. All else being equal, and with no AC use, hotter temperatures improve fuel economy due to lower density air, and reduced pumping loss. This should help to offset AC use, but perhaps not entirely. Certainly running the AC more heavily will reduce fuel economy, all else being equal.

We don't know the average barometric pressure (air density) of your trip out, or the trip back. We don't know wind direction and speed. We don't know if you generally drove the same speed in both directions. What was the actual elevation gain and loss?

A 7% difference in fuel economy seems a little high to attribute to AC use alone, especially if you did run it some during the morning drive up.

If you had an aftermarket gauge like ScanGauge, UltraGauge, or Torque, you could see AC power use and compare how hard it's working between the trips.

 

In a hybrid (HEV), all energy comes from gas. Besides more a/c usage, winds typically pick up in the hot afternoons too. Hot air intake is also less efficient in a gas engine.

 

On I-5 between Salem and Portland, the wind typically blows from South to North in the morning, and then reverses direction in the afternoon. Different places will have different wind patterns, and any are subject to change at any given moment.

You've got to define what you mean by less efficient more precisely. In general, warmer intake air will improve fuel economy by reducing pumping losses. In this case, warm air in the intake increases fuel efficiency. The drawback is that peak power at wide open throttle will be reduced, because the warmer air is less dense, and less fuel can be injected. In this case, the power efficiency is reduced.

The reason that less dense air (warm) is more fuel efficient is that a larger throttle opening is required to produce the same amount of power as compared to more dense air (cold). An more open throttle reduces pumping losses because the pistons aren't fighting as much of a vacuum.

Some people modify their vehicle to have a warm air intake to slightly improve fuel economy. This comes at a cost of peak power at wide open throttle, so naturally more people choose to go the opposite way and install a cold air intake. A CAI will give a slight boost in top end power, but then again, what percentage of the time are we flooring our vehicles?

 

I'm speaking in terms of the OP's drives in the summer desert. The first was done in hot conditions and the return was even hotter.