headlights on and gas mileage

The II has halogen projectors. If safety is more a concern than FE, use them. Really in the end your not going to see a significant hit I don't think. Try a tank with them on, and a tank with them off and see what happens.

 

Certainly there is some reduction in gas mileage, you are burning about 200 watts (1/3 HP) of lighting. Compared to being hit by a truck that seems a small price. Go for it.

 

Agreed that it's not likely to make a big dent in your mileage.

For some rough back-of-the-envelope calculations... Wikipedia tells me that gasoline has an energy density of 32 MegaJoules per Liter; this is about 33.6 kilowatt hours per gallon. Let's say about half of that energy gets wasted in friction, conversion loss (mechanical energy to electrical), more conversion loss (electrical energy to chemical in the battery), more conversion loss (chemical back to electrical), etc. So we're dealing with about 17 kilowatt hours per gallon. (Maybe more, maybe less, I'm just taking a guess here.)

I recall seeing sometime in the past on PC that the halogen headlamps are 60 Watts apiece (the LED headlamps are three 10-Watt lights, so 30 Watts per side). Somebody can correct me if I'm remembering incorrectly, but this is just a rough estimate anyway, so it doesn't really matter. So you're burning 0.12 kilowatt hours every hour, or 0.007 gallons per hour.

So if you're on the highway, getting 48 mpg at 65 mph with headlights off, you'll burn 1.354 gallons per hour. With headlights on, you'll burn 1.361 gallons per hour, a difference of half a percent. In the city, at 51 mpg and 22 mph, you're burning 0.407 gallons per hour; the 0.007 extra gallons per hour is a little bigger percentage here, but still under 2%. So worst case, you'll probably lose 1 mpg.

And, as others have said, if you're worried about people not seeing you, the $3 for a gallon of gas will buy you about 143 hours (almost 6 straight days) of running your headlights. Seems like a pretty small price to pay, all things considered.

 

This estimate is too optimistic. Prius engine efficiency alone is just in the mid- to high-30s percent range, and this fantastic for a gasoline engine. I think we are getting around 12 kWhrs/gallon at the crankshaft. Then start piling on the other losses you mention.

Don't forget the tail lights, license plate light, side marker lights, front running lights, and a slug of dashboard lights linked the headlight switch (more significant on my older cars than on this Prius).

All that said, I do keep the headlights on nearly full time.

 

In our country, it is mandatory to have headlights on all the time. Hence I never drive with headlights off (I would be fined if I do so :mod, and I still get well above 50MPG in my gen II (55-56MPG per tank usually). The effect of the headlights on the fuel economy is quite negligible, I would say. There are much more important factors in play, driving style being the determining factor.

 

I'm confused. You think that the safety factor of having headlights on during low visibility periods is worth sacrificing for a spoonful of oil? I vehemently disagree.

 

You drive with your hand out the window instead of using your headlights?

 

It probably uses just as much fossil fuel as your headlights.

The bulb is 55w and you have to recharge it with your car so you lose a lot of energy recharging and allowing the spot light to slowly discharge when not in use.

Why wouldn't you just flip your lights on when a car is approaching and turn them off after the car has passed?

Also, FYI you would be pulled over and ticketed in my state for using the spot light on the road. Then you would have to use more oil getting back up to speed, plus all the trees that would have to be cut down and more oil used to process all of the tickets you receive.

 

Spudd,
Using the car headlights and using your car-recharging lamp are roughly equivalent in one important respect--the energy used ultimately comes from burning gasoline.

 

Hey Spudd, try these!

 

I've heard that turning lights off to save fuel is common in certain lesser-developed regions. What they don't understand is that nearly all the fuel savings occurs after the crash. Once the vehicle and driver are wrecked, they don't need any more fuel.

 

In my calculations headlights burns fuel about 1 euro in 1000km(620miles). So it’s something but not much. And in here gasoline cost 1.45 euro per liter.

 

I really hope this is some kind of joke . You got us, bud ! Now, please tell us that it really was a joke, because if you don't, somebody here might actually believe you were serious !

 

The peak engine efficiency is 220 grams of fuel in, per kilowatt-hour of energy out, using whatever brand of fuel that Toyota used to make that measurement. The presumed energy content of that fuel is about 44.4 megajoules per kilogram so 44.4 kJ per gram. That gives us 220 x 44.4 = 9768 kJ of energy in to get 3600 kJ of energy out (one watt is one joule per second, 3600 seconds in an hour). Best engine efficiency is therefore 36.9%.

However, you're not always running at peak efficiency. The car delivers this level from about 10kW output to about 30kW output. The demand for power when cruising - and you're mostly cruising unless you live in New York or another big city where you just accelerate from one set of lights to the next - depends on the losses: to heat in conversion from mechanical to electric and back (if sending some power from MG1 to MG2 or vice versa); to friction in the drivetrain; to rolling resistance in the tyres; and (predominantly, at higher speeds) to drag. The car does stop the engine if demand is getting much less than 10kW and there's decent charge in the battery (and speed below 45mph).

The amount that lighting uses relative to the other losses is trivial and really doesn't affect the fuel consumption that much at higher speeds. At very low speeds it will be more significant, but generally I wouldn't worry about it.

 

I was so going to make a comment about how the headlights run on an insignificant amount of power in comparison to other things, but you did it with such eloquence that I'm afraid my comment just isn't necessary any more... To think you did all that math... Sigh...

... I think I'll avoid commenting on the other comments in this thread related to holding a spot light out the window... But I do shake my head in utter stupidity at the concept...

 

I believe there was a study done about consumption when daytime running lamps first came out in the 1990s. I can't seem to find it.

 

Be aware -most- DRLs are headlamps only or "bright" turn signal lamps only. No tail lamps, no side marker lamps. AND the headlamps are usually run in series so lower power consumption. Pearl's DRLs are quite a bit dimmer than when the headlamps are turned on.

Even though not quite accurate, go with the calculation already posted. Lights on -may- increase your fuel consumption by a few percent. Compared to wind which -may- increase your fuel consumption by 20 percent, or driving style, which -may- change your fuel consumption by as much as 40 percent. Or even fuel type used which -may- change your fuel consumption by as much as 10 percent.