Originally Posted By: actionstan
Hello All,
I was wondering.. when you turn on the AC in a vehicle and the compressor starts putting load on the engine.. does it put the same load on the engine all of the time? Or does it depend on how fast the blower motor is running.
For example if you use the AC but keep the blower on low, will you save fuel/load on the engine, V.S. running the blower on high with the AC on?
Just wondering because it may make more sense to run it high.. then turn it off when I am comfortable enough, then run it high again. I have a 99 SL1 and the compressor basically completely kills its power, and in the 2013 it idles rougher with the AC on so I like to minimize use as much as possible.
Thanks,
there are two types of compressors, clutch cycling and variable displacement.
for either one, the load that the compressor puts on the engine is directly proportional to the heat the refrigerant is moving from the evaporator to the condenser and how well the condenser is shedding that heat. so on a hot day with your blower on high- the most hot air is flowing over the evaporator coil to be cooled, putting the most heat in the system. now if your parked and you notice your radiator fans kick on high speed, that's because of all the heat in the refrigerant and it's unable to be shed at the condenser easily- high side pressure of the AC system is very high and that pressure difference from what the compressor is pumping is the load placed on your engine.
on a variable displacement compressor which is what most cars have now, i don't know what model years the clutch cycling got mostly phased out, the compressor varies its output (pressure) based on internal valving to keep low side pressure above 30 psi for r134a refrigerant. so when heat load is low the compressor swash plate is more flat and it's pistons pump less, when heat load is high the swash plate is angled most and the pistons pump at max capacity, as heat builds in the system and high side pressure rises, that pressure between low side and high side of what the compressor is causing is the load placed on the engine. you will never hear a normally operating variable displacement compressor clutch cycle off unless it's turned off or you're low on refrigerant and the clutch is tripped by the low pressure sensor.
in a clutch cycling orifice tube system, when it does cycle off under normal conditions that's because the heat load on the system is very low causing that so just prior to it cycling off the compressor is placing the least amount of load on the engine it could be.
and in either case a blower setting on low vs high puts less mass heat flow over the evaporator coil = less heat put into refrigerant = lower system pressure = less load on engine regardless of outside temperature.