AC commits pre-meditated murder on MPG

Is the A/C set up the same between the Gen II and the Gen III?

 

I'm not certain but I would love for someone with a Geniii to perform the same tests I have been conducting.

I tested the idea that auto vs. manual settings creates a greater hit on mpg and in my car this isn't so. Even after 50 miles with the AC maxed out I still observed the 10mpg hit. Using autox mode or manual settings made no difference when the desired temperature is low. I set the thermostat to 70° and outside temperature was 90 degrees.

Furthermore, I monitored horsepower and load on a hunch. Everytime I engaged the AC, HP jumped up by 2-3 and load (LOD on the scangauge) raised by 1-1.5. So at 50mph and AC off the scangauge read approx 14 HP. With AC on the scangauge read 16-17 HP. I find the results interesting. I tried to video the test but with the sun's glare you could not read the gauges.

 

I would certaintly be interested in seeing the power the generator is producing at various RPM's, and how this effects the way the power is distributed to the cars various systems. You are right in that we agree that the AC has an actual negligable effect itself on mpg.. The problem is it drawing the HV battery down at lower speeds, and that is where the visual draw on mpg comes in. I would be curious to hear from someone with one of those Li-ion add-on packs installed to see if the AC is able to draw off of that when the car is not in motion. I would assume so since the whole system ties in to each other.. I think the key to people like us that have to use the AC more due to hotter climates is to somehow tie in more battery power.. Granted that adds excess weight, but I think for those who commute by themselves like me the effect would be negligible.

 

I'm not saying AC has a negligible affect on mpg. I'm saying when used conservatively it will have a much smaller impact. We have plenty of anecdotal evidence that heavy AC use can have a large impact on mpg. This is why Toyota reduced AC demand when in Eco Mode on the GenIII.

 

This would only help if you are talking about a plug-in hybrid. So long as you are talking about non-plug-in, having a larger battery capacity wouldn't help. The additional battery capacity would still eventually need to be charged from the gasoline burned in the engine.

 

F8L, I had a lot of time to randomly check AC effect on instant mileage during my roadtrip, and the second half of my trip I ended up using it a lot less, since I also found it made a big difference. So to kinda corroborate your findings...

I only really checked it out when the coast was clear and I could keep my pedal very steady. Even with AC on lowest setting at 78°F, it would still knock it down right away by about what you say, maybe a little less, like 7-8 mpg. If I were in SHM, getting 60-90 instant mpg, it would definitely go down 10 mpg.

 

Thanks, Mike. It helps knowing this information is not unique to my car. This is especially helpful coming from some who pays close attention to detail.

Now we need a GenIII owner to perform this test.

 

The math here seems really really simple. It's an electric AC unit not much different from a window unit you might use in your house. A house window unit might draw 10 amps times 120 volts which is 1.2 KW. That's about 1.5 horsepower. (745W=1hp)

So why all the anecdotes? When running the AC, you have a fixed draw. Add to that the variable draw when driving. It shouldn't surprise anyone that fuel mileage goes down when driving slow. Just play around with the numbers in a spreadsheet.

 

Once the heat load inside the car has been removed, steady state AC power drain should be about equal to the amount of radiation entering the car. Since the roof is insulated, I estimate based on glass area: 3 meters squared on on-face irradiation is 3 kw maximum, and less when the sun is not overhead, clouds are in the sky, etc.

Last week I hit 70 mpg on my 25 mile trip with AC set to 77F and outside was 90F. But radiation was weakened by clouds.

As I have said before (and before ) the important trick is to have the interior of the car (and its thermal mass!) as close to ambient as possible before turning on AC.

 

I'd love to help, but don't have a scan gauge (do they work with the Gen III's?). I did some very non scientific observations when I was driving home from Santa Cruz on Sunday. With the A/C on, it did appear that my steady state MPG's dropped 5 - 10 mpg (the Gen 3 has the curved pips where each pip = 5 mpg), but it is kind of hard to see how many disappear and drive at the same time.

Interestingly, when I turned the A/C off, after a second or two, the mpgs would shoot up and above the original MPG point and then drop down a second or two later.

For example. I'm cruising along at 60 mph and the instant is bouncing between 60 - 65 mpg. Turn on the A/C, the pips drop to 50 -55 mpg. Turn off the A/C, the pips pop up to 70 - 75 mpg for a second or so and then back to 60 - 65 mpg.

 

You can monitor the MPG using the instantaneous mpg indicator in the GenIII like you have been doing. The Scangauge simply allows me to check other parameters like HP and load. Your results are similar to mine.

It explains why all these people from hot places like Texas are reporting low MPG numbers in the low 40s. In the same conditions without the AC on full blast they could likely attain high 40s to mid 50s.

 

What I have monitored with the ScanGauge is AC watts (ACw) along with MPG. As Sagebrush stated, the best thing you can do is to get the cabin temp as close to ambient before running the AC. Open the windows and let out as much heat as possible.

While the AC watts is high, 1800 to 2000 watts, the mpg and my average mpg on the HSI screen are taking a hit. When the cabin temp finally gets cooled down and the AC watts drops back to the 500 to 800 watt range the mpg starts recovering to normal and the average mpg starts increasing again. This is at 60 mph highway speeds.

So, the sooner you can get the cabin cooled down the sooner the mpg hit begins to decrease. Of course, the higher the outside temp the longer it takes to cool down the cabin and the more of a mpg hit you take.

 

Therefore, short trips (less than 30miles) in high temperatures with the AC on will put a significant dent in your mpg. Longer trips or when ambient temperatures are low (closer to your AC temp setting) the mpg hit is minimal. I've tested this in the early mornings when ambient temp is 70 degrees and I set my AC at 70 degrees.

 

Exactly.

I have a daily 26 mile one way commute. With temps up to about 90F the cabin will normally cool down in about 5 minutes. Above 90 and especially in the 100+F range this summer, it would take 10 to 15 minutes for the cabin to cool down.

Short trips where the cabin never cools anywhere close to temp setpoint would definitely put a significant dent in mpg. The AC would never have a chance to drop below the 2000 watt range.

I have done the same thing in the morning with temps in low 70's and set the AC due to the high humidity. The AC watts are around 200 to 300 watts and minimal impact to mpg.

 

Thank you for the wattage data, JD. That helps confirm my observations with MPG loss and the HP jump when AC is activated.

 

Yeah. The distance I drive to work is what kills me in the afternoon when using the AC (only 11 miles).. Is why I love driving to work in the morning, leave my windows cracked so the car is as close to ambient as possible, and then drive home using the AC.

Cody sometimes that dip in your MPG bar comes from the car recharging something (have not figured out what that is yet).. Mine will usually dip for a mile or so, and then shoot up back to normal.. I am talking like a 10-15mpg dip though in the consumption bar. This is without the AC ever being on also.

 

If you are talking about the first few minutes, it is not recharging something it is burning hot to control emissions. Remember the Prius is a low emissions vehicle first and good gas mileage is just a secondary bonus.

 

It happens occassionally after I have been driving the car for awhile. I am aware of the catalytic converter/engine coolant warm up cycle that the car goes through in the morning/sitting for awhile. Just one of those things that happens about once or twice in my 11 mile journey to work or home. Or even when driving out and about town. Will be cruising along, one minute getting about 50 or so mpg on the readout bar, then boom it drops to down between the 25 & 50 line.. No noticeable change in engine power to me, but you can see it on the gauge. Then after a few moments it zips right back up to where it was. Is why I think it is either shifting from motor to generator in some effort to give a quick charge to something. I will have to take a video when I am driving, and create a seperate thread. I never thought much of it, and have accepted it as normal. If other people have not seen it though then maybe I have a problem.

 

Methods for cooling of your interior so as to avoid heavy AC usage:

1. Park in a shaded area. Under a tree may help but a better tactic is to park on the eastern side of a large building. As the sun moves to the west in the later portion of the day, the building will block the sun from hitting your car. With your windows cracked, the heat from the earlier part of the day will dissipate faster than if you parked your car in the full sun the entire day.
2. Tint your windows
3. Use a high quality windshield sun shade. Priuschat shop has a very nice one!
4. If you cannot park on the east side of a building try parking your car facing the west and use a quality windshield sun shade. This will reduce the number of windows directly facing the sun and thus not allowing as much of the suns radiation to enter the car.
5. Drive the car for the first few minutes with all the windows down to circulate the hot air and direct it outside.

 

Another comment on short trips is that I have good mpg during the week when commuting with 45 minute trips. On the weekends with short trips around town, I take a real mpg hit with AC. For those that have short commutes to work will really take significant dents in mpg as you say.

Good pointers on cooling down the cabin before turning on AC to limit mpg hits.