Wednesday, February 25, 2015

An exercise in measuring exercise

This morning on an errand I took advantage of a feature of the car we bought after my wife started her new job.

You can track your average miles per gallon from whatever point you re-set the meter. And like on many cars, there's a trip meter that you can use to track your miles travelled from whatever point you re-set that one.

So this morning before turning on the car, I reset both of those meters and learned that my trip covered 2.5 miles, at an average gas usage of 17.8 miles per gallon.

In other words, I used 0.14 gallons (2.5 miles / 17.8 miles per gallon).

That's a generous 2 cups of gasoline (0.14 gallons times 16 cups per gallon = 2.25 cups).

How much energy is in that much gasoline?

An EPA document found through Wikipedia says that gasoline has 114,000 Btu per gallon, which can be converted to 33.41 kWh per gallon, or 44.79 horsepower-hours (HP-hr) per gallon.

So the gasoline I used this morning contained 4.69 kWh. If that were electricity, it would cost about $0.60 in New York. It's also enough to run a 60-watt bulb for 78 hours. If you replace that bulb with a compact fluorescent using only 15 watts, you could get 313 hours of light from that much energy. My errands took about 15 minutes.

The horsepower-hour conversion allows a comparison to our own work. An average-size adult human, reasonably fit, can work at a rate of about 1 HP for short bursts (I actually verified this with some students). maintain a work rate of about 1/8 of a horsepower over the course of a day, doing physical work like shoveling, lifting, carrying (this site gives a more conservative 1/10 HP, biking at 12 mph).

If you work an 8-hour day, you can produce something like 1 HP-hr (an eigth of a horsepower, maintained for 8 hours, which is the same amount of work as one HP, maintained for one hour).

My little outing this morning consumed 6.3 HP-hr (0.14 gallons times 44.79 HP-hr per gallon).

In other words, to exert the same amount of energy as my car consumed in 15 minutes, I'd have to do reasonably hard physical labor for more than six days.

If the weather had been good for biking, I would have done the same errand in about 20 minutes, consuming something like 1/30 of a HP-hr, and gotten some exercise in the bargain.

I'm ready for spring.

No comments:

Post a Comment