In summers, I usually drive with the AC on at all times. In the past, I've noted many times that my usage of the in-car AC is not to reduce the temperature of the whole passenger cabin, but primarily to keep throwing cool air at me. Based on this observation, today I conducted a small experiment - I would run the AC for 5 minutes, then turn it off for 1 minute (while keeping the fan running at all times), then again turn it on for 5 minutes, then turn it off for a minute, and so on, to see the effect on my user-experience.
I observed that my experience was sufficiently comfortable during the 1 minute period when the AC would be off. My overall experience was good, minus the manual on and off I had to do (which is automatable anyway). Crucially, however, nearly 16% of the fuel normally spent on air-conditioning was saved, whilst causing me negligible discomfort.
I concluded that if my car had a feature which allowed me to automatically run the AC in this on-for-five-minutes and off-for-one-minute mode, I would be able to save a delicious 16% fuel spent on the AC.
Some things deserve mention here:
- This fuel-saving is true only in cases where the usage of AC is similar to my usage
- It should be possible to change the durations for auto-on and auto-off (for example, someone may want to set this as on-for-nine-minutes and off-for-one-minute, to save 10% of the fuel spent on AC)
- The intermittent on-off nature of this mode brings this hypothetical AC somewhat closer to a variable cooling AC. For example, in the 5+1 minutes mode, a binary representation of the power-status of the AC is: 111110111110111110... For each 6-minute period, the AC effectively runs at 0.83 cooling level. The smaller the durations of on and off are made, the closer this AC will get to a truly variable cooling AC (for example, for 50+10 seconds mode, the effective cooling per minute - 0.83 times - is closer to a truly variable cooling AC, than an AC with a 5+1 minutes mode)
- It's important to remember that an intermittently-running AC is not a true variable cooling AC. While an intermittently-running AC achieves reduced cooling mathematically - on a time-averaged basis, a truly variable cooling AC will actually cause reduced cooling on a continuous basis. Its values for a 6-minute period will look like this: 0.83, 0.83, 0.83, 0.83, 0.83, 0.83
- Some might say that the use of a thermostat also leads to fuel-saving as well as variable cooling (for example, an AC that was on for 6 hours in an 8 hour period effectively ran at 0.75 level, saving 25% fuel). While thermostat does save fuel, it shouldn't be considered as variable cooling because use of a thermostat usually leads to longer periods of on and off, leading to situations when it gets too cold, or slightly hot, thus violating the objective of a variable AC. So while the use of a thermostat can be thought of as a sinusoidal wave with higher amplitude and higher time period (as well as higher average value), intermittently-running leads to a wave with lower amplitude and lower time period
- It is assumed that the AC is built such that it can happily bear these multiple power-ups and power-downs, without breaking its jaws
- When I would turn off the AC, it would keep throwing sufficiently cool air at me for ~30 seconds (this is to be expected, since the heat-exchangers should be expected to remain cool for a while), thus really utilizing the previously-done cooling
- It's easy to replicate this feature in room ACs. Current ACs allow power-saving and cooling-control on the basis of temperature (using a thermostat), it'll be good if they incorporate power-saving and cooling-control features that work on the basis of time