I turns out I burn away a kilometer of travel for every four minutes or so of running my van’s engine just to power the air-conditioner while stationary. Talk about a rude awakening.
I constantly keep track of fuel mileage. After all, any decrease in kilometers per liter I’d observe would signal a need for cleaning, adjusting, replacing or replenishing something on my vehicle. And, all would be good if I didn’t give in to running the engine for minutes on end just to run the AC and get some fleeting relief from the summer heat.
I lead an office-van life, you see, and at some day hours the lighter-jacked electric fan I use simply can’t cut it. I’ve taken to rationing my AC breaks, using my stopwatch to dole out 10 minute stretches of the engine idling and the air-conditioner running with my face just inches from the vents. But, that nod to conservation, that painful shut-off after 600 seconds of cool bliss, begs the question: how much am I giving up in mileage for every minute of guilt-ridden, ozone-burning running in place?
I finally dusted off the old algebra to come up with numbers I could believe and work with. With my 1.2 Liter gasoline-burning minivan, I move around at an average of 40km/h and at 2,500rpm in city traffic. From this I figured out that it takes just 4 minutes and 10 seconds to burn off a kilometer’s worth of fuel while idling the engine to run the AC. This roughly translates to 2 and a half kilometers of travel lost for every 10 minutes of idling, 5 kilometers for 20 minutes, 10 kilometers for 40 minutes, and a major commute’s 15 kilometers for 60 minutes or an hour of idle running.
See below for the math I used … needless to say, precise or not, I’ve taken to short walks outside my vehicle instead switching on the ignition.
A–average time to cover 1 kilometer in city traffic
= (1km / (40km/h/60minute))
= (1km / 0.667 km/minute)
= 1.5 minutes per kilometer
B–average rpm in city traffic at 40km/h
= 2500 rpm or revs/minute
C–average revolutions per kilometer in city traffic
= A x B
= 1.5 minutes/km x 2500 revs/minute
= 3750 revolutions per kilometer
D–idling rpm with AC on
= 900 rpm or revs/minute
E–minutes idling equivalent to a kilometer traveled
= C / D
= 3750 revs per km / 900 rpm
= 4.1667 minutes idling equivalent to 1km traveled
F–schedule of idling minutes to equivalent kilometers traveled
= Total revolutions / C …
10 minutes @ 900 rpm = 9,000 revs –> 2.4 km traveled
20 minutes @ 900 rpm = 18,000 revs –> 4.8 km traveled
40 minutes @ 900 rpm = 36,000 revs –> 9.6 km traveled
60 minutes @ 900 rpm = 54,000 revs –> 14.4 km traveled