Lights on = lower mpg?

[hockeydad]

Originally Posted by a64pilot

A Muller
Currently, 17 states mandate the daytime use of motorcycle headlights. The purpose of these laws is to make the motorcyclists more conspicuous and reduce the number of daytime multi-vehicle collisions. The effectiveness of the laws is examined by comparing the proportion of daytime fatal, front, and side-angle collisions between states with and without such laws. The comparison is based on all motorcycle fatalities recorded by NHTSA for the years 1975-80. The analysis shows no statistically significant difference between states with and without such laws, suggesting that daytime headlight laws are ineffective. Several explanations of this negative finding are explored.

Any motorcyclists out there? None that I have bought in the last 20 years or so have had the capability of turning the headlight off.
I believe that might just invalidate the above assertion.

Really? How is that? Just because the Government mandates something, does not mean it's right, or based on the findings of numerous studies? Most "safety related mandates" we have today are put in place because a fear of litigation. It only takes one case to bring on yet a new mandate. I guess the argument can always be made that if it save just one life, it's worth having the law.

That said, if it's a proven fact that it helps, why aren't DTL mandatory on cars? Why don't we have them on the 2010 Prius? More people are killed by drunk drivers...why don't all cars have a breath-alyzer lock-out so you can't drive over a certain limit? I guess just a matter of time until we've incorporate every possible "safety feature" to protect all, from the irresponsible few.

But things have gotten out of control, IMO. Ever been to a public pool lately? Back when I was growing up, every pool had diving boards - low and high. Not so much anymore. Is this a real safety issue for most people that go to the pool?? Or yet another over-reaction to a few that have hurt themselves on a diving board, and pools not wanting to face litigation?

[pakitt]

If the lights were consuming so much to affect by 2 or more mpgs, then Toyota would have adopted a more fuel efficient way to feed the lights (i.e. LEDs for all versions, or high efficiency lights for daylight - position lights, etc).
Since the lamps are what they are (standard halogen ones) - this means that the fuel consumption is likely minimally impacted by turning on the lights. I think A/C, heater and driving style play a much bigger role in FE.
The difference with a normal car though, is that in the Prius each thing that is turned off can leave more energy to the HV voltage battery that can provide electricity for more distance travelled or support the engine more often. Any portion of that going into a utility (e.g. radio) will reduce the HV charge and hence affect somehow the FE, likely visibly - depending on the length of a trip, averaging 0L/100km for 500m more, might mean a quite different consumption average. This is my experience on the 17km commute I do every day.
In normal cars, no matter what you do, the engine runs all the time. More utilities will increase the load on the alternator that needs to draw more current to keep the 12V battery charged while providing current to the utilities. You turn everything off, the engine still has to drag the alternator, the AC (even when not running and disconnected via the magnetic clutch), etc. We know, in comparison, how the ICE design of the Prius is optmised for avoiding continuous loads when not used (electric pumps and so on). So, if a normal engine is not burdened by several electric loads, there is in any case not much it can do with the portion of energy it could not waste (no HV battery to charge...).

[orange4boy]

It would be great if someone would do a good controlled test to see the mileage difference in the Prius with all accessories off and then with a bunch on.

Then you could calculate the losses as they relate to mpg.

The graham scanner reports the HV battey is supplying 1.5 to 2.5 amps of ~270 Volts to the DC-DC converter to run the basic requirements. that's .54 to .9 horsepower before calculating in the generator and battery losses, which means that the engine has to come up with a bit more than that accumulated power somewhere in the driving cycle. Since that draw is constant during ready mode, that's quite a few of watts all told.

I'd be thrilled to take a horsepower or two of load off the engine.

[qbee42]

Originally Posted by orange4boy

It would be great if someone would do a good controlled test to see the mileage difference in the Prius with all accessories off and then with a bunch on.

Then you could calculate the losses as they relate to mpg.
<snip>

No need to test. It's very easy to calculate based on a given load. There are a few posts explaining this calculation.

Tom

[Mike Dimmick]

The extra load of the headlights depends on how fast you're going, but it's typically not that much. A quick comparison of power usage:

Headlights: each headlamp is rated at 55W, each tail lamp at 10W (Gen 2, halogens). Total 130W.

Power required to balance aerodynamic drag at 70mph: 12,000W, plus any other sources of friction and tyre rolling resistance.

Drag at 30mph is about 990W. At 20mph, about 290W. At these speeds rolling losses are dominating. I don't have figures from the Prius but I can point you to Tesla's range information. If you multiply watt·hours per mile by miles per hour, you get watts. If you do this for the tyres you get a figure of 1,622W at 30mph and 1,058W at 20mph. The Prius has a base electronics load of about 300-400W.

Headlamp load is therefore fairly significant at lower speeds, but is not likely to have a massive impact on fuel economy.

[orange4boy]

Good post Mike.

2 things to add.

1) Your headlights are presumably on at all times, even when stopped so that has a cumulative effect.

2) The total watts required is higher because of efficiency losses. 13.8V/DC-DC/HV battery in-out/generator/engine. I have roughly figured the system to be about 65 to 70% efficient. So the engine needs to supply about 190 watts to get 130 at the bulbs.

[qbee42]

Originally Posted by Mike Dimmick

The extra load of the headlights depends on how fast you're going, but it's typically not that much. A quick comparison of power usage:
<snip>

I think what Mike really means is that the headlight load stays constant while the power required to move the car varies with speed. Because of that, the mileage reduction due to headlights varies with speed.

Tom

[orange4boy]

Did the numbers as per mikes post and headlights eat about ~1% at 70mph ~5% at 30mph Very rough figures.