Fuel Ecomomy with Air Conditioning question

Hello.

I don't know how exactly AC on my new Accord works and I am curious to know
what happens when I change temperature level with A/C on.

There are 2 possible scenarios:
1) A/C cooling system works at some constant level and temperature is
controlled by a amount of outside air in the mix which goes to a cabin
2) A/C cooling system works more or less depending of temperature selection.

With 1 scenario it does not matter how much you cool down, energy
consumption is the same. In other words, with regards to fuel consumption,
it does not matter exactly what temperature you select, as long as you
already have A/C on.

With 2 scenario, the less temperature the less consumption and vice versa.
In other words if you have every degree of temperature drop costs you X
pennies.

Which of the two is correct?

 

I think the most common implementation is to switch the a/c compressor
on/off with an electronic clutch. You can hear it go "CHUNK!" when it
engages and a lighter "chunk!" when it disengages.

So that'd be scenario 2.

 

There are a couple of different systems for controlling A/C. The most
common is to have a thermostat on the evaporator coil that turns the A/C
compressor clutch on and off. With moderate outside temperatures the A/C
will cycle on and off to maintain constant temperature at the
evaporator. The hotter it is, the longer the "on-time" and the shorter
the "off-time". If it gets hot enough the A/C will stay on contiguously
and eventually not be able to keep the cabin cool. If you turn on the
heater in addition to the A/C it will have a similar effect as warmer
outside temperatures. It will increase the "on-time" and decrease the
"off-time", in turn increasing your average fuel consumption.

The other type of A/C system does not have a clutch but uses a
compressor with variable compression. It is still controlled by a
thermostat at the evaporator, and turning on the heater will cause the
compressor to increase compression, consuming more fuel.

 

I don't know much about AC's, but I have a very hard time believing that if
you turn on the heater, it would be be similar to having warmer outside
temperature. According to your explanation, the air flow is

outside air > heater > AC > cabin

I always thought it was:

outside air > AC > heater > cabin

Or maybe it makes a difference if you have fresh or recirc air. Maybe you
meant with recirc?

cabin > AC > heater > cabin

 

According to your explanation, the air flow is

Agree, that is the typical configuration.

Yes. When I have the A/C on I usually has the system on recirc as well,
and when writing my post it did not occur to me to list the case of
heater on, A/C on and recirc off. In that case you would not see any
difference in fuel consumption as you turn on the heater.