It may be an old question on fuel economy.....?

While open windows save more below 35 MPH, above that A/C is cheaper.

 

The A/C in a Prius has ZERO direct impact on MPG. The compressor and HVAC blower are electric, variable speed motors.
Running on high with 4 or more bars showing on the fan meter, (max cool) it can drain the traction battery to the point that you might see a drop of 1-2 MPG. Once the fan meter shows 3 bars or less you can run the A/C all day with no impact. None of this has anything to do with the vehicle speed. If you run the car in ECO mode and keep the HVAC set to the max comfortable temp the A/C costs you nothing. Cool-down of a very hot car is what drains the system quickly, but once its cooled down you're getting free A/C.

 

You can trace every bit of energy in a non-plugin Prius back to gasoline. I can very clearly see that I lose a couple MPG for a tank when I'm using the AC quite a bit.

Don't forget that the AC is also there to cool the HV battery. A breeze feels cool to us because it helps sweat evaporate, the battery can't sweat. I used to just take the heat in the name of mileage, but I decided the battery's health was worth losing a couple MPG.

 

I must strongly disagree with much of the above post.

While the 2010+ Prius has a more efficient A/C system than any previously marketed passenger car, it still requires energy, which requires extra fuel, which reduces MPG. And this happens even when the battery gauge doesn't drop at all.

The only time it is effectively 'free' is in special corner cases, such as long downhill descents where the traction battery fills up and gravitational energy must be dissipated through various waste mechanisms, such as friction brakes or engine compression. It isn't truly free because it derives from gasoline that was paid for at the pump. It is 'effectively' free only because energy that must be discarded can be put to a second use as A/C. This doesn't apply to flatland driving, or even to short downhills where the battery doesn't fill up.

I live in a much cooler summer climate than most of NM, and even I see the MPG impact of A/C. It isn't large here, but isn't zero.

 

Fuzzy1 is correct again.

The old cliche about air conditioning using less fuel than open windows above X mph (where X can be 35, 45, or any other magic number a writer favors) is essentially true. However, the actual value of X depends on car model, temperature, which windows are open, how far they're open, and numerous other variables.

 

All regenerated energy is "free" to the extent that it does not have any impact on the amount of gasoline burned. Even the energy supplied by the engine to charge the battery tries to utilize excess engine capacity. If you are regenerating energy through braking and slowing, you can utilize it to run the A/C for very little cost, energy wise. Steady freeway driving will have a much larger impact on mileage since there is not a lot of opportunities to use regeneration.

 

Nope.

 

I must dispute most of this. Absent the filled traction battery situation (mostly on long downhill slopes), every bit of electricity sent to the AC unit is electricity diverted away from later propulsion use. It cuts overall MPG, thus isn't free.

Even that 'excess engine capacity' put into recharging the battery, is later used for propulsion while the engine is allowed to shut down or slack off and avoid burning some fuel. But if the AC has already consumed those electrons, then the engine needs to burn some more fuel.

 

The energy, as noted in my post, is pretty much free. It does not cost any energy to slow down...most cars convert kinetic energy to heat, but the Prius is a better design and puts some of it into the battery. The mpg increases come when you use the energy at some later time. There is an energy cost to running the A/C, but it is mitigated by the regen systems. If the battery became discharged (not charged) then the engine would run to charge it and run the A/C. That almost never happens in normal driving.

 

Lets try this from another angle. We claim ALL the energy in the car comes from gasoline. It gets moved around, rotating the engine, the motors, charging the battery, etc. but it all comes from gasoline.

Where does the A/C energy come from such that it is NOT from gasoline? (the Prius PHV has another source of energy, a plug so it is totally different)

 

In our 2008 I have many times proven to myself that the A/C drops the mpg average by close to 2mpg.

 

I find it so amazing it is believed by some that there is such a thing as no energy consumed while using the air conditioning, or that regeneration from braking is somehow free energy that comes from nowhere.

Energy is stored and changed. All energy that moves the Prius from point A to point B comes from gasoline. If any other systems are used, such as lights, heat, air conditioning, radio, and so on, that energy first came from the gasoline.

When the ICE runs it consumes gasoline. Part of that energy in gasoline becomes heat that is lost (radiator, etc); part of that energy charges the traction battery; part of that energy goes to the transaxle and moves the car forward.

When the ICE charges the TB, that SoC can be used to do many things, including making the car move, running the AC, or charging the accessory battery. If that stored TB energy is used to run the AC pump, it is not available to move the car.

When the car slows, some (not all) of the gasoline energy consumed making the car move forward is recaptured through regenerative braking.

I know for most of you I am preaching to the choir; we get it.

If you can truly make a hot room cold without using energy, I know someone at General Electric (GE) that wants your secret. You will make billions.

 

We do a very regular run, and I monitor fuel consumption for the trip with ScanGauge. My seat-of-pants impression is AC reduces the efficiency, by upwards of 10%.

 

I know myth busters did a whole episode devoted to trying to determine the reality in regards to this question.
The general consensus seems to be lower speeds, city driving, stop and go, roll down your window. Higher freeway speeds, more wind resistance, use your AC.

BUT....my opinion is that automobile cabin temperature is both a comfort and simultaneously a safety issue. I'm going to concentrate better if I'm not sweating off pounds like I'm sitting in a sauna.
So I say, Do what you want.

I say choose and adjust based on comfort, and damned be the efficiency.
If it's a beautiful evening, I'll roll down the windows just because I want the summer or fall air.
If it's a hellish scorcher? I'm blasting the AC until I'm released from that 7th circle of hell.

I own a Prius, so certainly fuel economy is a concern. But life is too short IMO to not enjoy the luxury of driving in relative comfort.

There were too many spartan years in my past where I couldn't even afford a vehicle with AC. Yes, they use to build automobiles without AC. Now that those years are behind me? Comfort leads my decision in this regard.

 

So well said. I don't track my fuel use or mileage. I drive a Prius because of the overall great gas mileage. If it's 53 on one trip, great. If it's 35 on another because I sat for awhile with the AC on, that's great also.

I've been around 6 decades and I like my creature comforts as you do.

 

Not really. Battery energy from regen is no different than battery energy from the ICE-MG1. And AC electric load is no different than other electric loads, including propulsion. For basic MPG purposes, EPA and otherwise, the regenerated energy is already counted as going back into additional propulsion. Any of that energy diverted to AC is thus robs MPG.

Even when the engine is already running for propulsion, not just to boost a discharge battery, it must run harder to maintain the charge level whenever any accessory load, AC or otherwise, is taking energy from the battery.

 

Fortunately, thermodynamics works even if you do not understand it. Burning gasoline is a non-reversible process. Whatever happens afterward is unrelated to the burning process. Regenerating the energy increases mileage by powering the MG. Not regenerating it does not lower the mileage. If, as is stated above, the energy comes from burning gasoline, then using it would lower your mileage and not using it would increase the mileage. The gasoline is burned to create kinetic energy. What happens to that energy has no relation to it's creation. It is usually converted to heat and wasted. Some of the waste energy can be converted into useful battery energy, but whether you save it or not, the amount of gasoline burned does not change. The engine efficiency is completely determined by the input and output temperatures and is not affected by anything that occurs afterward. That all being said, I usually see at least a 1 mpg drop when using the A/C. It would probably be higher if I drove more on the freeway.

 

Use of regen is built into the MPG expectations and ratings of certain car models. If that regen is diverted away from propulsion, then MPG is lowered.

... or into other forms of potential energy.

Much of that battery energy does not come from 'waste' engine energy. It comes directly from burning extra fuel to put more power onto the crankshaft, which in turn is sent to MG1 to power accessory electric loads.

The Prius engine is intentionally operated to seek an efficient operating point, and adjusting the battery charging current adds another degree of freedom to help reach the best efficiency. Thus, the incremental fuel burned to power the high voltage bus may be exceptionally efficient compared to normal operation (i.e. the margin efficiency may well be larger than the overall ICE peak thermal efficiency).

But that bus power is still not 'free' (outside the previously mentioned corner cases where power must be intentionally dumped), because if the accessory electric load is shut off, the engine could throttle down a bit and burn less fuel.

 

I agree with you Fuzzy1. Fortunately E=MC2 works even though it is not understood. The equation also works as MC2=E.

Potential energy in gasoline can be converted to potential energy in the Traction Battery. Mass in motion can be converted into energy and energy can be converted into mass in motion.

With every conversion there is lost heat. No conversion is 100% efficient.

I am not an expert in this area. I just know that all energy used to power the ICE, the Transaxle, the inverter (charging the TB), the electric motors, the A/C, and all other electrical systems comes from the gasoline in the tank. That gasoline energy needs to be converted, and some of that energy is waisted as heat.

An exception would be if a Prius was hauled up a mountain on a truck, so the Prius could get free mileage rolling back down the mountain while charging the TB for free. The truck that hauled it up the mountain used the energy required to get the Prius up the mountain. If the Prius drove up the mountain, it would use a lot of energy, creating potential energy from altitude. That potential energy of being on top of the mountain is used to put miles on the car and regenerating it to charge the battery without using additional gasoline.

The reason we use MPG as the bench mark is because that's where all the energy used was originally stored.

I know this friendly discussion will probably not change any minds. We all enjoy our Prii.

Cheers!