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does keeping lights switched on consumes fuel?

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this question is bugging me a lot..

i know lights should be kept on even in daylight for saftey reasons,,

but for the sake of question does keeping it off saves fuel?

i will be happy to get the reason behind the facts..

:busted:

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In theory, yes the lights will use gas. You can't make power from nothing. The electricity that the lights use comes from the charging system of the bike. The more electrical load on the charging system, the more resistance to rotation that system will have, and more power from the engine is used to generate that electricity.

However, the difference in fuel consumption is so small that I don't think you would ever notice a difference if you rode without the light on. If you strapped 100 super powered halogen headlights on (with a charging system that could handle that) then you might notice a difference :busted:

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In a car it does, in theory at least. A lot less than air conditioning or anything else though. Those led drl's consume barely nothing and no one will never be able to notice the difference and I doubt no one would notice a 2x35W halogens either. IMO led drl's are better because they are more visible in bright daylight, not because they take less power.

In a bike it makes no difference since charging system is built differently. A bike stator always outputs maximum power, excessive is then dumped into frame (ground).

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Here's a back of the envelope estimate of fuel consumption. Let's say all your lights consume 75 Watts total. That's about 1/10 of a HP. The stator is quite efficient, but let's be conservative and guess it's 85%, so the lights will require about 0.12 HP. Using typical brake specific fuel consumption (BSFC) of 0.55 lb/hr/HP, that's about 0.064 lb/hr. If gasoline density is about 6 lb/gal, fuel consumption is about 0.01 gal/hr. If your engine burns 1 gal/hr (your mileage may vary), the lights require about 1% of total fuel consumption. It's not a lot, but still measurable.

Someone check my math, but I think it's in the ballpark.

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Here's a back of the envelope estimate of fuel consumption. Let's say all your lights consume 75 Watts total. That's about 1/10 of a HP. The stator is quite efficient, but let's be conservative and guess it's 85%, so the lights will require about 0.12 HP. Using typical brake specific fuel consumption (BSFC) of 0.55 lb/hr/HP, that's about 0.064 lb/hr. If gasoline density is about 6 lb/gal, fuel consumption is about 0.01 gal/hr. If your engine burns 1 gal/hr (your mileage may vary), the lights require about 1% of total fuel consumption. It's not a lot, but still measurable.

Someone check my math, but I think it's in the ballpark.

mathematically measureable but I doubtt highly any difference in fuel consumption would be measurable "in real world". no control over atmosheric,track/trail conditions and human error would be able to fall in that 1 %

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on a hot day more gas will probably evaporate then that. and your back of the envelope math seems to be right if the assumptions you made are accurate, which also seem to be fair from general intuition.

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Gravelfreak is correct, nearly all bike regulators work by shunting to ground any power that is not being used.

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Gravelfreak is correct, nearly all bike regulators work by shunting to ground any power that is not being used.

Good point. Efficiency is not much of a concern with bike electrical systems.

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Gravelfreak is correct, nearly all bike regulators work by shunting to ground any power that is not being used.

If the bike has permanent magnets in the rotor then the output from the stator depends upon the engine speed. In simple terms if the bike doesn't use the electricity then the regulator has to turn it into heat, thats why the req has cooling fins. No lights or other load on the charging system at high RPM means the regulator has to eat everything the stator can produce. Some times it fails.... Leave the lights on, for safety and reliability of the charging system.

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Leave the lights on, for safety and reliability of the charging system.
That is a very good advice. I bet it would prevent almost all regulator failures that happen.

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If 700 watts = 1hp ..... You would need a charging system that could power 7 100 watt lights to loose a single horse...

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If 700 watts = 1hp ..... You would need a charging system that could power 7 100 watt lights to loose a single horse...
That assumes 100% efficiency, which is impossible, but in the context of a 100 watt or smaller system, your point still stands.

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That assumes 100% efficiency, which is impossible, but in the context of a 100 watt or smaller system, your point still stands.

My great great great uncle Edison explained it to me like that when I was in third grade ha ha ha ha !

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That is a very good advice. I bet it would prevent almost all regulator failures that happen.

Not really an issue on most small bikes with half or full-wave charging systems. In fact lights on all the time can steal needed charging power to a battery to replace the massive draw of starting (80+ amps).

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Not really an issue on most small bikes with half or full-wave charging systems. In fact lights on all the time can steal needed charging power to a battery to replace the massive draw of starting (80+ amps).
Most (if not all?) small bikes like that run lights on a separate AC circuit anyway.

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Yes it does use more fuel but here's the math behind it. 745 watts is equal to 1 horsepower. When you count generation losses and such you're probably using about 1 horsepower to generate 500 watts. The lights on a typical motorcycle headlight is only using around 100watts. So having the headlight on is only sucking 1/5 of a horsepower from your engine. Even a small 250cc engine is producing about 30+ hp at the shaft. So even on a 250, the light is only using about 1/2 of a percent of the engines power (1/150 th to be more exact). That gas used on that small of an amount of power is VERY negligible. None the less, it is using up some extra gas.

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Yes in theory it would use more fuel with lights on, however I installed a switch on my XR250L, to turn off the lights when riding trail, and have ridden many, many miles of trail, with lights on, and lights off, there is no measurable difference in mileage either way. The switch is also usefull so I can turn off the lights when starting the bike, it is a very tough bike to get started, more power to the electronic ignition helps. Trick is to remember to turn it back on when riding roads.

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Yes it does use more fuel but here's the math behind it. 745 watts is equal to 1 horsepower. When you count generation losses and such you're probably using about 1 horsepower to generate 500 watts. The lights on a typical motorcycle headlight is only using around 100watts. So having the headlight on is only sucking 1/5 of a horsepower from your engine. Even a small 250cc engine is producing about 30+ hp at the shaft. So even on a 250, the light is only using about 1/2 of a percent of the engines power (1/150 th to be more exact). That gas used on that small of an amount of power is VERY negligible. None the less, it is using up some extra gas.
Did you read the thread at all? In a bike it makes no difference whether the lights are on or off, stator is producing all the power it can anyway, excessice is then turned into heat. And no, in a dirt bike it doesn't even affect ignition since ignition has a separate circuit and is completely isolated from everything. I don't know about the most recent FI bikes though.

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Yes it does use more fuel but here's the math behind it. 745 watts is equal to 1 horsepower. When you count generation losses and such you're probably using about 1 horsepower to generate 500 watts. The lights on a typical motorcycle headlight is only using around 100watts. So having the headlight on is only sucking 1/5 of a horsepower from your engine. Even a small 250cc engine is producing about 30+ hp at the shaft. So even on a 250, the light is only using about 1/2 of a percent of the engines power (1/150 th to be more exact). That gas used on that small of an amount of power is VERY negligible. None the less, it is using up some extra gas.
Did you read the thread at all? In a bike it makes no difference whether the lights are on or off, stator is producing all the power it can anyway, excessice is then turned into heat. And no, in a dirt bike it doesn't even affect ignition since ignition has a separate circuit and is completely isolated from everything. I don't know about the most recent FI bikes though.

I have a VERY hard time believing that. Where is the resistor that supposedly absorbs this "excess" power that is being generated? Unless there is some complex control circuitry that monitors how much power is being generated by the stator at any given instant and shunting all excess through a resistor that simply is not true. Why would a manufacturer design the system that way? Unless you have some good explanations on how why this works you are full of it.

Edited by Rocker01

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The Regulator/Rectifer shunts the excess to ground, that is why it gets so hot. This type of electrical system is fairly inexpernsive and the heat generation is not that great. A car, generatinr 2400 watts, would have a difficult time shedding all that heat plus the very noticable effect on fuel milesage, hence they use an alternator where the volatege in the field is varied as a factor of draw.

You need to do a little research next time instead assuming.

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