Toyota Prius Forum banner
1 - 19 of 19 Posts

·
Registered
Joined
·
27 Posts
Discussion Starter · #1 ·
I read somewhere (I think) that the Prius does not use normal engine parts when the AC is on so it does not need the engine to be on when the AC is on. Is the correct? If so, does it use more gas up when the AC is on when you are driving or does that not use gas either?

Does that make sense?
 

·
Registered
Joined
·
2,243 Posts
The AC is electric powered. So it doesn't run off the engine as with conventional cars. However, nothing is for free. So when you use the AC, you draw down the main battery quicker which means that the ICE has to run more to keep the battery charged.
 

·
Premium Member
Joined
·
888 Posts
The whole reason for electrifying the a/c was to improve fuel efficiency and find another area of overall vehicle efficiency. Specifically, mechancial a/c runs at the speed of the engine when engaged. However electric a/c can be run at potentially any speed.

Electric multi-speed or vairable speed a/c calls upon intelligence about a/c demand in the cabin and then adjusts its operating speed to meet the needs inside the cabin. So, if you are cooling off a hot cabin after having the car sit in the sun for a long period of time, the a/c compressor will be told to run at full power. But as the cabin reaches the temp setpoint on the auto a/c system the speed of the compressor will be reduced to only provide enough a/c capacity to maintain the cabin temperature.
 

·
Registered
Joined
·
610 Posts
:) My view: The '04-'05 Prius AC system is super clever. Runs the compressor only as fast as needed by weather conditions... and can be adjusted a degree at a time with the steering wheel button. Very much more efficient that the "belt-driven off the ICE" system on other cars. Even with the "translation loss" in the ICE re-charging the traction battery, it is an ingenious approach. Just one of the engineering features of the Prius.

And... it works very well in the Dallas TX summer temperatures often more than 100º F.
 

·
Premium Member
Joined
·
2,815 Posts
Depends - which Prius?

The AC compressor on the Classic Prius runs conventionally off of a
belt on the gasoline engine. If you need more cooling/dehumidifying,
the gasoline engine will run just to run the AC pump (will not show
up on the energy monitor). Unless you have the MAX button on, the
AC will cycle on and off as needed, so your gasoline engine will cycle
on and off as needed (even if you're just idling).

The AC compressor on the 2004-? Prius runs off of the NiMH traction
battery. As needed, it'll cycle on and off as you need more
cooling/dehumidifying.
When the hybrid battery charge gets below a certain threshold, the
gasoline engine will come on to recharge it (just like in regular
driving), even if you're idling, and this will be on the energy monitor.

So, either way, you'll end up with lowered MPG, as the engine will have to come on to provide power at some point...

Do note that the AC is not used on the EPA fuel economy tests...
 

·
Registered
Joined
·
27 Posts
Discussion Starter · #6 ·
So when I am driving down the road at 70 mph with the AC on, the AC is running on electricity, thus not causing any extra gas? Is that right?
 

·
Registered
Joined
·
370 Posts
yarzy said:
So when I am driving down the road at 70 mph with the AC on, the AC is running on electricity, thus not causing any extra gas? Is that right?
No. The AC is running off the battery, but since the AC is causing a drain on the battery, the gas engine has to run that much more to keep the battery charged. So, while the AC is indeed electric, it is indirectly using gas.
 

·
Registered
Joined
·
27 Posts
Discussion Starter · #8 ·
Got it...

One more question about that last reply...

If I was driving at 70 mph for 1 min and my battery was full, and I clicked on the AC, would I notice a drain on the battery or would the battery just recharge itself just as fast as it drains?

Thanks guys!
 

·
Registered
Joined
·
1,233 Posts
The battery charges and recharges continuously. The computer is designed to keep the battery at 6 bars (60% true SOC). If you have a "full" battery (8 bars which is only 80% SOC), it'll use the battery more to get it back to 6 bars. Likewise if you're below 6, it'll want to charge more to get it back up to 6. This is of course given the circumstance that fuel mileage and emissions come first so sitting in traffic, it will let it drain to 2 bars (pink) before turning on the engine to charge the battery.
 

·
Registered
Joined
·
7,161 Posts
Think of the battery as a buffer. It is not efficient to run the engine at idle or even at charge-idle. By charge-idle, I mean the minimum engine speed to provide battery charge. So, if the car can delay running the engine until it can operate at the most effficient RPMs and make use of all the power when it is running at those RPMs, it will.
The system can do 3 things with the power it creates with the ICE:
1. move the car mechanically (yes, I know, it can't do that alone)
2. Provide electrical power to charge the battery, or power MG2.
3. waste it in heat when 1+2 is less than engine output.

We of course want to avoid 3 as much as possible.

By the way, the AC compressor does have a minimum speed, at which is starts cycling on and off like any other single speed system.
 

·
Premium Member
Joined
·
888 Posts
yarzy,

You have gotten a lot of very good responses from everyone. However, the Prius power management schema is very complicated. Those of us who have delved into the technical items regarding this vehicle have come to an understanding about how engine RPM is translated into a combination of ground movement and electrical generation while the vehicle is moving.

There are ICE rpm sweet spots where the ICE is running most efficiently. The computer system is able to determine this based upon programming and onboard sensors. The beauty of HSD in general is that the computer's goal is to make the vehicle as efficient as possible regardless of what you are doing with the vehicle: accelerating, braking, cruising, going uphill, going downhill, etc.

I have found that when you are cruising on the highway at a posted speed of 70 mph on flat road, the MFD indicates that you being powered by the ICE and providing electrical power to the battery. Without going into what the PSD is doing specifically (it helps to see one of the online demonstrations) the vehicle is running the ICE at an RPM which is most efficient for that ground speed, and the surplus RPM is then directed into MG1 for power generation.

If you have 5 or 6 bars in this situation, and there is only minimal draw on the battery from the 12v system or A/C you will cruise at 6 bars on the battery most of the time. Even with A/C on at these speeds you should continue to see 5 or 6 bars. I have never experienced the car at cruise acting as if the ICE were stressed because it was having to generate power for a/c and help move the vehicle forward.

By removing all the belt driven accessories off from the ICE (except for the water pump), there is no power loss from the engine through accessories. This actually frees up a lot of the engine's power generation ability and directs it into one place, the PSD and through that MG1 and the drive train. There is lots of talk about electrical conversion loss, however I'm convinced through logic only, and not numbers, that there is conversion loss, if not overall power loss if the a/c system was belt driven.
 

·
Registered
Joined
·
7,161 Posts
I believe the inefficiency of a belt driven AC is because the engine is forced to carry the load continuously and at the same mechanical advantage, regardless of the ICE RPM. By making it electric, the battery can buffer the load, and the ICE can run at the optimum RPM to provide the necessary power. Otherwise, it becomes similar to a car that doesn't have a PSD, and the engine is bogged down at low ground speed/low ICE RPM, which is grossly inefficient.
 

·
Registered
Joined
·
319 Posts
DanMan32 said:
Otherwise, it becomes similar to a car that doesn't have a PSD, and the engine is bogged down at low ground speed/low ICE RPM, which is grossly inefficient.
Like my 3-cylinder '88 Chevy Sprint, where if I was using a freeway on-ramp I had to turn off the A/C to be sure I could accelerate effectively...
 

·
Registered
Joined
·
7,161 Posts
Right, although you could appear to have a power loss if you normally attempt to put all power sources (ICE and battery) to full use when accelerating. If you are expecting the 200V/500V system to be fully allocated to acceleration, but you are running the AC and it is on full blast, some of the electrical power would not be available for acceleration, thus feeling underpowered. It's a bit different though than trying to get efficient power (torque) out of ICE at near 0 RPM. Motors can easily do that, ICE cannot.
 

·
Registered
Joined
·
393 Posts
ChasSuitt said:
:) My view: The '04-'05 Prius AC system is super clever. Runs the compressor only as fast as needed by weather conditions... and can be adjusted a degree at a time with the steering wheel button. Very much more efficient that the "belt-driven off the ICE" system on other cars. Even with the "translation loss" in the ICE re-charging the traction battery, it is an ingenious approach. Just one of the engineering features of the Prius.

And... it works very well in the Dallas TX summer temperatures often more than 100º F.
I agree. Except put SOUTH FLORIDA where you have Dallas.
 

·
Registered
Joined
·
35 Posts
hmmm.........i live in so fla.........& lived in tx( fort worth & san antone) for 18 mos......& i must say.........in the 10 yrs of so fla.......it has never reached the death ray heat of tx!!! think about it........so fla only hits to 98.........i have NEVER seen 100 degrees in so fla........but when tx hits 100 it seems to stay! that nasty dry heat! i thought i was in hell! (hey! i missed my sticky humidity! lol)
cocoa
 

·
Registered
Joined
·
393 Posts
Cocoatreat said:
hmmm.........i live in so fla.........& lived in tx( fort worth & san antone) for 18 mos......& i must say.........in the 10 yrs of so fla.......it has never reached the death ray heat of tx!!! think about it........so fla only hits to 98.........i have NEVER seen 100 degrees in so fla........but when tx hits 100 it seems to stay! that nasty dry heat! i thought i was in hell! (hey! i missed my sticky humidity! lol)
cocoa
Yup, never hits 100 here.
 

·
Registered
Joined
·
4,873 Posts
Of course the big differance is the "humidity" Playing golf in Florida in 70 degree temps and you can sweat like a "you know what!" Play golf in Dallas in 100 degree temps with that proverbial 25 mph plus wind and zero humidity and it can be downright comfortable. And then again there is Pebble Beach or the Olympic club in San Fran for your "perfect" sub par rounds without nary having to kill one snake!
 

·
Registered
Joined
·
393 Posts
hyperion said:
Of course the big differance is the "humidity" Playing golf in Florida in 70 degree temps and you can sweat like a "you know what!" Play golf in Dallas in 100 degree temps with that proverbial 25 mph plus wind and zero humidity and it can be downright comfortable. And then again there is Pebble Beach or the Olympic club in San Fran for your "perfect" sub par rounds without nary having to kill one snake!
I have a lot of ocean breeze where I live, so it stays cool. Well, cooler.
 
1 - 19 of 19 Posts
Top