Tuesday, August 11, 2009

Car AC that intermittently turns on and off can save a lot of fuel

A few days back, I wrote about my years-old desire for a refrigeration compressor that has variable cooling capability. Today, while I was driving back from my company, I got an idea which can save a potentially significant amount of fuel spent on the in-car AC. At its core, this idea is related to the variable-cooling "HyperCompressor" idea I wrote previously.

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:
  1. This fuel-saving is true only in cases where the usage of AC is similar to my usage
  2. 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)
  3. 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)
  4. 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
  5. 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
  6. 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
  7. 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
  8. 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
Click to read my thoughts on a weakness in Google's Chrome


  1. Nice idea but when you turn the ac back on it will take more energy than normal running to cool the heat exchanger down again. Unless you let the temperature in the car increase significantly above the temp with the ac on then you are using a similar amount of energy. You are just using it in bursts of higher energy rather than a constant baseline. In terms of energy you never get something for nothing. The still cool heat exchanger you are using for 30 seconds was paid for when you turned the ac on. Turning it off a minute before you reach your destination would be a pure gain. What I would be more interested in would be ac that turned itself off when the outside temperature was low enough not to need it ie every morning when I drive to work. Perhaps modern ac does this anyway and I am pointlessly hitting the ac off button every morning?

  2. In 1908, Henry Ford, presented the Model T for a simple $950, making it reasonable to the everyday person; notwithstanding, during its 19 years of creation its value brought down to as meager as $250.