The Prius Experiment

[Drifter]

Originally Posted by Drifter

I would hope so - or that the heat is the last resort. If you have multi-zone A/C I don't see any other way than using the heat for the sections with higher temperatures.

I look forward to testing it this summer...

I tried to do some testing on a long drive yesterday, but there is a lot of variation just keeping the A/C on the same setting. According to the scangauge, my AC watts were 2,000 when first starting the car after it was parked in the sun for an hour. After a mile or two A/C watts dropped to 1600, then 1100, and eventually 600-800.

ECO mode seemed to immediately lower A/C watts by 50-100 when driving steady state. The inverse was also true - turning off ECO mode caused A/C watts jump 50-100... I also never saw more than 1600 watts in ECO mode when first starting after parking, so that hot car start may be where the ECO setting makes the most difference (~400 watts). The downside is ECO mode does not cool a sweltering car down nearly as well.

Steady state driving 55mph in the bright sun with 91 degree outside air temperatures took 600-800 watts to keep me comfortable. Turning the A/C off and putting all 4 windows down 1/2 way seemed to add 0.5-1.5 horsepower (~400-1100 watts). I wasn't as comfortable, both temperature and noise, and without a clear efficiency advantage I think I'd just run A/C instead.

[ps2fixer]

The way AC works, it has to pump the refrigerant and do phase change and such. So initial turn on it has to work harder to cool, besides the extra heat load of the car being warmer as well. Kind of like cold starting an engine. Once it's ran for a while, it should take less for it to catch up. I didn't think the numbers would be so wildly different though. The variable speed pump isn't something I'm used to though, normally it's just cycle time of the pump. At first it's 100% duty cycle, then it will cycle on and off as needed (pressure and temp based).

I haven't done much AC tested with my car, but the little I've messed with it, it really hurt my mpg. But the hottest day I've seen so far was around 88F and windows cracked 3-4in was fine for me.

[Drifter]

Originally Posted by ps2fixer

The way AC works, it has to pump the refrigerant and do phase change and such. So initial turn on it has to work harder to cool, besides the extra heat load of the car being warmer as well. Kind of like cold starting an engine. Once it's ran for a while, it should take less for it to catch up. I didn't think the numbers would be so wildly different though.

Part of the wild variance is that as the interior cooled I reduced the fan speed which then put less load on the A/C system. With the fan on max, A/C wattage dropped from 2000 to maybe 1400 or so before I started getting cold. It may have continued to decline more if I'd wanted to stay in an icebox...

But the hottest day I've seen so far was around 88F and windows cracked 3-4in was fine for me.

I'm windows down till about 45 mph. If outside temperatures are below 75, I just roll the windows up and use fresh air vent with the fan on.

Windows down 3-4 inches keeps me cool enough below 90F, but I have to crank up the volume to listen to an audiobook or podcast and that noise fatigues me by the end of the day. Headphones would solve it, but they're illegal to wear while driving here.

I haven't done much AC tested with my car, but the little I've messed with it, it really hurt my mpg.

Yeah, it really hurts the slower go. Looking at the power required to maintain speed according to the ecomodder calculator:

30mph: 2.9kW (750W A/C would require 26% more power)
35mph: 3.6kW (750W A/C would require 21% more power)
40mph: 4.5kW (A/C adds 17%)
45mph: 5.6kW (A/C adds 13%)
50mph: 6.8kW (A/C adds 11%)
55mph: 8.3kW (A/C adds 9%)
60mph: 10.1kW (A/C adds 7.5%)
65mph: 12.1kW (+6%)
70mph: 14.4kW (+5%)
75mph: 17.0kW (+4.5%)
80mph: 20.0kW (+3.8%)

[Blacktree]

I filled up the tank in the Prius yesterday, and:

1) Holy crap, gas is expensive!
2) Running the A/C every day really hurts MPG.

Average fuel economy seems to have dropped by 8-10 mpg. The trailer hitch install was near the end of that tank of fuel. So it had little if any effect. But I didn't expect the A/C to have such a large effect on fuel economy.

I also noticed the Auto A/C basically does the same thing I was doing manually. It starts out at full blast to knock out the heat, then settles down to maintain a steady temperature. So now, I just let the auto A/C do its thing. Less fiddling with controls means less distractions while driving.

Also, RockAuto finally got some control arms in stock. Hopefully they're the correct ones this time...

[Phase]

Windows down in Cali versus Florida is completely different for driving. In Florida it’s a lot harder to drive with windows down because then you let all the humidity into the car. In west coast areas, it’s so much nicer to go with windows cracked or down since it’s still dry air outside

The ac in my Ioniq is extremely efficient. The temperature increase outside offsets any mpg loss my ac gives. So say I have windows cracked at 75 degrees outside, then it goes to 90 degrees outside and I put my ac on, my mpg stays the exact same, if not slightly improves actually. Mpg boost for hotter temps in my car is greater than the ac loss with my car and tinted windows. I’m sure I could get amazing mpg driving around Phoenix when it’s 115 outside and leaving the ac off, but I don’t want a heat stroke in the name of hypermiling

[Blacktree]

OK, we have another fillup under the summer conditions. The trailer hitch doesn't seem to have affected fuel economy. You can see the data in my fuel log. But now the car is averaging mid-50s mpg. With the A/C off, it was averaging low-mid 60s. That's a significant difference. You would expect Toyota to install more efficient A/C on a Prius, but whatever.

On a side note, I also replaced the front control arms. That was a nice improvement. The old control arm bushings were worn out. They made annoying squeaky noises. Plus the front end felt sloppy. I'm glad that's fixed. But working in 98 degree heat was NOT fun!

https://www.youtube.com/watch?v=V1NwbPUooIs

Also, I changed my mind about the pizza pans. I was going to drill holes in the wheels and mount the pans directly to the wheels. But I'll attach them to the trim rings instead. Actually, I think I'll do that before starting the BMW project.

[Phase]

Originally Posted by Blacktree

OK, we have another fillup under the summer conditions. The trailer hitch doesn't seem to have affected fuel economy. You can see the data in my fuel log. But now the car is averaging mid-50s mpg. With the A/C off, it was averaging low-mid 60s. That's a significant difference. You would expect Toyota to install more efficient A/C on a Prius, but whatever.

On a side note, I also replaced the front control arms. That was a nice improvement. The old control arm bushings were worn out. They made annoying squeaky noises. Plus the front end felt sloppy. I'm glad that's fixed. But working in 98 degree heat was NOT fun!

Also, I changed my mind about the pizza pans. I was going to drill holes in the wheels and mount the pans directly to the wheels. But I'll attach them to the trim rings instead. Actually, I think I'll do that before starting the BMW project.

well toyota did install a way more efficient AC in newer priuses...

[ps2fixer]

Funny, my Prius needs some front end work too, when I was changing the oil when I first got it, the driver's side front wheel had slop in both directions. I didn't confirm it but pretty sure wheel bearing in my case. It makes no noise at all, but the slop is more than it should be.

I have my hitch installed and I see an mpg drop, but I have the plastic cover in the back off still. In limited testing I'm seeing around 5% effect with panel off + hitch.


One thing that would be a good idea to check on is the inverter coolant pump. You can see it right behind the driver's headlight. With the car in ready mode the pump should be running and if you can get a finger on the hose you can feel it running, or open the reservoir and check for fluid flow. Mine was completely dead, worked fine for quite a while, but with the lower block and driving a bit faster than normal it tripped off on a trip to a near by city P0A93 was the code for it and it seems to only flag when the inverter is overheating (the metal below the top cover is a good place to feel how warm it is. If it feels hot, it's over heating, warm is ok, I think the normal temp was something like 120F, when mine set the code it felt just below scolding temp, maybe 160F ish).

On the AC pump, it might be mainly the logic used to control it that's killing the mpg. Electric motors and AC systems generally don't change too much for efficiency. Like "window shakers" run around a CEER (combined energy efficiency ratio) of around 10-12. The ratio is btu of cooling vs watts used. 12,000 btu that uses 1200w is a rating of 10. My old ac unit from 1990 was around 8 and the new one I just bought is 11.4.... 29.8% less energy used for the same cooling, but that's jumping over 30 years in tech in one leap, R22 vs R32 and such, not even the same refrigerant lol.

In the stores I saw a ranting range of around 10-11.4. I'm sure car AC systems will be all over the place since it's not a simple packaged setup, same pump design is probably used on several models of cars and such.

I actually went on a trip running AC at higher speeds and the mpg effect wasn't too bad, but the faster speeds was a huge hit lol. Around 45mpg going 72mph with AC on. Trip home at around 60mph and 55mph back roads I hit around 50mpg with AC on. It wasn't a super hot day, but I had passengers. I sure do like the 60-65mpg trips at 40mph better with ac off lol.

[Blacktree]

We have another fillup to add to the spreadsheet. It seems the suspension work improved fuel economy? Or maybe it was just the driving conditions. But after replacing the front control arms, I also had the alignment reset. The car actually feels like it rolls easier. The old worn-out suspension bushings must have been playing havoc with the suspension geometry. This latest fillup came in at 57.5 mpg.

Also, we now have 3 fillups under the summer conditions. That should be enough data to compare against. So now it's time for some pizza pans!

[redpoint5]

The higher the MPG of a vehicle, the more MPG are "lost" when running AC. A 100 MPG vehicle might lose 10 MPG with AC use, while a 10 MPG vehicle might lose 1 MPG. My point being that measuring AC energy consumption by effect on MPG might not be the best way to understand it.

There is a PID that can be monitored to see exactly how many watts the AC is consuming.

Off topic, but once when I was rotating tires on the Prius with the front end off the ground, I noticed it was extremely difficult to turn the wheel. On front wheel drive vehicles, usually it takes medium effort to turn a wheel and the other axle will rotate counter. In the Prius it seemed to take a lot of effort, and I don't know if that is normal or not?