AC cooling off the engine??
 

Today I was on the highway in hot weather (81f) and was looking at the SC2 to
see how many MPG I was losing with the AC running.
With all the hills, I couldn't really pin it down. But, on average, I was burning
more fuel than I did on the trip out this morning when it was cool..
Cool=30.5 MPG and Hot=29 MPG..

Anyways, I was displaying the radiator water temp. It was staying right at 198
just about all the time. That's the normal temp even if it's 65 out..

Then, I noticed something interesting.. Each time I would turn on the AC,
the engine temp would drop down to about 188.. A 10 degree drop!
Turn off the AC and within a few minutes, the engine was back to 198.
Just a few miles of AC on, and it was back down to 188.

My theory is, the AC coolant being pumped into the coil sitting in front of the
radiator is still cool. Cooler than outside air temp.
So, when the in-take air comes in, it cools down a little on the AC coil
before hitting the radiator and cooling it off a little..

It seems like the extra load of the compressor making the engine work harder
would make it hotter.. But, it seems the AC coolant is making it run cooler..

So, what's the real story??

I don't know about the radiator/fan layout on a CRV, but I think the difference in temp would be mostly due to a "safety" feature. For example, a 2nd fan that kicks in automatically when the AC is turned on, or if you got only one fan, it turns faster, etc. even if coolant temp doesn't require it. Just to prevent more overheating.

Just my take on this phenomena, I know *some* cars are designed this way. Pop open your hood with the engine running and ask someone inside the car to start the A/C while you check the fans. Maybe I'm wrong though. ;)

But the coolant gaz return is still an interesting theory... I don't know if it'd be cool enough to make a difference in coolant temperature in the radiator. Maybe.

The dual fan theory may be closer to right. Most of these vehicles equipped with a/c will have a dual fan setup with one dedicated to the condenser. So it is likely that the second fan would and may be cooling the car a little more ...

I 3rd the 2nd fan.

Its the 2nd fan kicking in and the primary one switching to high.....

Dang!
 

I'll bet you guys nailed it..
For some reason, I didn't think about the two fans!!

I know the radiator fan never comes on unless the engine gets up to the 200 deg range.
But, I'm unsure if the AC needs a fan running when the car is going 55 MPH..?.
I guess they would just turn it on anyways, in case you were sitting at a long traffic light.. :)

I guess that those fans really do work, even if you are at highway speeds.

My crazy theory may have some merit.. IIRC, there was a guy around here installing insulation on his AC pipes under the hood.
He figured the pipes that were frosting up, were just wasting 'Cold' that he wanted inside the car, not under the hood..


Now that I think about it, it's nice that my AC still works okay with
80% of my grill intake blocked off.

http://i46.photobucket.com/albums/f1.../CRV/M018s.jpg

the refrigerant going to your A/C condenser is very hot..it goes straight from the compressor outlet to the condenser, so your theory of cool gas couldn't work.

They automatically turn the fan on when you hit the A/C button because that is the most efficient way to run the A/C. The more cooling you get in the condenser the more cooling you get in the car.

That is interesting that the guy was insulating his pipes...in any car I've seen the expansion valve in the A/C system was right in front of the evaporator in the car. But he does have an idea that you may get some heating of liquid refrigerant right before the expansion valve.

cooling?
 

I can't imagine either of the condenser fans coming on at 55-mph.The ram air should take care of that even with the grille block.The compressor load might move the engine closer to the BSFC peak,and engine run cooler,however it seems like the T-stat should maintain design coolant temp regardless.I suspect paranormal activity!

I can't say for certain this applies to every vehicle on the road, but every one i have owned turns on the condenser fan no matter what when the button is pressed. Literally, the fan is in the circuit for the A/C so there is no choice in the matter. When the button is pressed, condenser fan turns on. Most A/C diagnosis charts tell you to look at the fan when pressing the A/C button as that shows whether the circuit has power or not.

aerohead you are right at 55 mph it should get adequate airflow but there is no logic to prevent it from coming on based on speed, temp, etc.

Likewise, all my cars' tie the fans to the A/C. A/C on = fan on. With large amount of blocking in place, ram air is probably diminished, so the fans improve air flow through radiator even at highway speed. Also, the fans are in parallel. They both turn on together.

The frosty A/C hose is probably going from the evaporator to the compressor, not the other way around.

ChampagneSW2 -

Welcome to EM!!!!!

Bingo. I tested this with blackjackel's 2005 Corolla and it does the same thing. I didn't intuit that it was more efficient. I thought the reason was to make sure that the additional A/C load didn't overheat the engine (i.e. if A/C on when it's hot outside, then turn on fan).

Turning on the A/C in Saturns is a(gain a non-intuitive) way to cool off the engine.

CarloSW2

Thanks Carlo...nice car :)

well you're right it does serve the purpose of not overheating your car as well...so it serves two purposes of efficiency and preventing engine overheating.

Maybe some logic circuits need to be added in this case.?.

How about the AC fan only comes on IF the radiator temp is 200 deg (or more). Where main radiator fan voltage is already on..

Maybe the AC fan would never come on if you were zipping down the highway at a good clip.?.

My guess is. Better MPG!

Edit:
I just looked up the diagram. The relays for both fans are hardwired in parallel. They will always come on together. I wonder why they just didn't use one relay? Or maybe just a larger fan? :)

Maybe I could try a test switch to open the lines to those two motors
when I'm out on the highway.. And see how the AC would work without those two motors running.?.

At highway speeds your fans never turn on. Your thermostat opens up and coolant flows. Airflow is sufficient to cool the motor. Only if it continued to get hotter would the fans kick in - IE, stop and go where airflow is low.

The fans on your car run with the A/C, regardless of temperature. The extra airflow is cooling your motor to a lower point, until the thermostat cuts off cooling flow enough to maintain this new, lower temperature. You've observed a 5 degree swing. Seems about right.

You're seeing the swing because thermostats open over a RANGE. At 188 it only flows a little bit of water. However, it's enough to keep you cool with the fans running because the radiator is more efficient at rejecting heat with the fans going. Fans off, you're running at 193. The thermostat flows more water at 193, which helps your radiator to reject the heat with less airflow. Both are equillibrium cases.

Some cars have this "speed" logic. Later GMs do. The logic goes like this. Fan(s) run anytime the temperature gets above the set point(s). They also run if the A/C is activated. However with continuous speeds over 45 or 55 MPH, this A/C activation is disabled. Likely, your ECM is unaware of vehicle speed, and thus, this logic was not implemented on your generation of ECM/PCM/Body Control communications.

I'm installing electric fans on my truck and will have a manual switch allowing me to have fans with A/C around town, and to turn off that requirement on the highway. However, even with the fans decoupled from the A/C operation, they'd still kick on if the temperature got too hot. Worst case scenario, I notice the A/C isn't cold. Reach down, hit the switch, fans go. If you do put in a disable switch, be sure to do it in a manner such that the computer can still cut them on if things get too hot.

Two fans are better than one. In the case of failure, you've got another. Plus, radiators are usually wide and short rather than square. Two fans give better coverage in most cases. Two relays? Again, sometimes relays fail. Plus, they'll last way longer if they only switch the load of one fan rather than two.

Ive only got one fan and it comes on automatically with the AC. Except at over 70 mph. So says my service manual.

70 mph!! Maybe I should have driven faster!! ;)

Thanks for the comments guys. I learned a little more about my 'V'..