The www.howstuffworks.com site has an interesting article in their "question of the day" section today. They calculate the amount of extra gasoline that would be used if the U.S. mandated daytime running lights (DRL) as Canada has. The bottom line is that the U.S. would consume about 406 million extra gallons of gas each year (a couple gallons per car).<br><br>So now you know the real reason why the "green" Prius doesn't have DRL in the U.S. (yeah, like I know the inner secret thoughts of Toyota).<br><br>I also noticed when my sister followed me from Tucson to Phoenix that it's somewhat annoying to have DRL glaring into your rearview mirror for 2 hours.<br><br>P.S. The question of the day is archived if you don't get to it right away.

I went to the site but found their logic flawed - at least with respect to conventional autos.<br><br>Their calculations seemed to be based on the concept that the alternator runs only when electricity needed to operate the various electrical appliances in the vehicle. To the best of my knowledge the alternator is always engaged when the engine is running and therefore it makes perfect sense to use the continually generated electrical power to run the DRL's.<br><br>With the Prius, however, it seems that powering DRL's is necessarily consuming electricity which would otherwise be going to power up the battery which almost certainly would be causing the gas engine to run more often than w/o lights. <br><br>I would like to see an analysis of the cost with respect to the Prius since it is my custom to have the lights on whenever possible.<br><br>Just my opinion - I could be wrong.<br><br>Waiting in Maryland

While it is true that the alternator is always spinning when a conventional ICE is running, the load that the alternator places on the engine varies according to the electrical demands being placed. If no electrical systems are on, the alternator is basically "free-spinning" and placing relatively little load on the engine. If you are running the headlights, brake lights, blower fan, etc, then the alternator will be applying significant load to the engine (and using additional gasoline in the process).<br><br>p

kratzmark suggested that the alternator is always engaged in a normal (non-hybrid) car and therefore the electricity to power DRLs would otherwise just be waste. <br><br>On any reasonably modern car, the alternator's rotor field is controlled by the regulator to not generate electricity that is not needed. The rotor field has the largest effect on the load the alternator presents to the engine (via the fanbelt), friction being most of the rest of the load. So without DRL, the regulator won't need to power the rotor field as much and the alternator won't resist the engine as much.<br><br>Due to friction, electrical resistance, switching effects, etc. the alternator does require more power from the engine than it gives back to the electrical system. The howstuffworks article included this by estimating 70% efficiency in conversion of rotation to electricity in the alternator.<br><br>The reality check, of course, is to note that 2 gallons of gas for a year is a very small increase in the gas you're already using. Let's see, I buy about 10 gallons/week, hoping to cut that to 6-7 when my Prius arrives. That's about 500 gallons per year, so adding DRL would increase my current consumption by about 0.4% and my Prius consumption by about 0.7% (of 300 or so gallons per year).

Exactly. It's especially evident on my Subaru when I turn my headlights on or off with the car idling. When I turn the headlights on I can hear the engine struggle for a few cycles as it adjusts to the greater load from the alternator.<br><br>MJS

At 2 gallons/year this comes to under $4/year. My Insurance company will give me a $50/year reduction in premiums if I have DRL.