“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Thursday, February 21, 2008

Stoplights (stop me if you've heard this before)

Never one to leave well enough alone (as an aside, this is one of the many expressions I never really understood until well into adulthood - another is "you can't have your cake and eat it too"), I've started sporadically keeping track of my stoplight experiences. I've tracked how many greens, how many reds, and approximately how much time was spent waiting. I say approximately because it's not so easy to determine when to start the timing at a light - do you start the stopwatch at first brake application? Or at a complete stop? What about slowing down but not having to stop? I'm trying to tie the timing to time not using fuel as efficiently as cruising, but there's a lot of judgment involved.

But it's looking like the earlier estimates I made (see here and here)for stoplight durations are fairly close. In the time I've been recording this data (only sporadically because it's quite distracting), I've encountered 59% green lights. I've suffered an average delay of 32 seconds. I've passed through an average of 32 lights each day. So that means that I'm losing an average of about 10:06 per day while stopped at 19 stoplights.

I try to minimize driving on weekends (though I haven't succeeded in eliminating it entirely) so I'll figure 280 days per year of losing 10:06 per day, for a total of 47.13 hours per year lost at stoplights. Burning about 0.5 gallons of fuel per hour at idle, if I don't turn the engine off at any lights, I'll burn 23.6 gallons of fuel. In my Land Rover LR3 HSE, that's a little over a single tank full and at $3.39/gallon (today) it's worth just barely less than $80.00.

This underestimates the loss, however, because it only counts idling fuel and not the fuel wasted in regaining energy lost to braking that has to be added by burning fuel. I estimated that in the second of the two posts listed above, so I'll just refine it here. I estimated stopping at 12 lights for 45 seconds each day for a loss of 9:00 per day, apparently a slight underestimation.

To finally squeeze the last blood from this turnip, I'll estimate that I slow from 35 m.p.h. to 0 on average at each of the 19 stoplights. It's not perfect, but it's as good as I know how to do. In any case, this wastes 322,150 joules of energy which takes, at 25% efficiency, 1,288,600 joules of heat energy from burning premium grade fuel to regain.

Using the figures above, and estimating 125,000,000 joules of heat energy available in a gallon of gasoline, I burn 54.84 gallons of fuel per year adding kinetic energy to my vehicle that I've wasted to heat my brakes stopping for stoplights. The total then is 78.4 gallons of fuel, or about 3.6 tanks full wasted. This number is quite close to my previous estimate, but now there's data to back it up. To me, the interesting aspect of this is the fact that well over 2/3 of the fuel wasted is due to getting back up to speed rather than to burning fuel while sitting still. Since kinetic energy is proportional to the square of speed, this stands to reason but it's still interesting to see it documented.

I'm still anticipating an experiment to determine fuel lost in restarting, but this data shows the potential savings from coasting to a stop without brakes (thus using instead of wasting kinetic energy) and turning off the engine - ideally as soon as the coasting begins. As with most of the other measures, it won't eliminate our need to import oil but it could help delay the crash.

No comments: