Friday, September 26, 2008

Lights on cars in the daylight

According to the Dail Mail, crazy European legislation forcing car drivers to keep headlights on ALL day could inflate fuel costs by up to £160 a year.

What are the numbers? Well, this is one that I worked out earlier. It's in chapter 9 of Sustainable Energy - without the hot air, on page 58. Let's say the four bulbs for the running lights on a car use about 100 W. Allowing for the engine's and generator's inefficiencies, this 100 W of bulb power requires a petrol power of 730 W. For comparison, the petrol consumption of an average car running along at 50 km/h and consuming one litre per 12km is 42000 W. So having the lights on while driving requires 2% extra power.

If you drive 50km per day and fuel costs £1.20 per litre then you spend £5 per day on fuel. Putting the lights on is going to increase your costs by 2%, which is 10p per day. That's £37 pounds per year, for a typical driver. Obviously the answers come out differently if we change the vehicle to a Hummer, or if we replace the incandescent bulbs by modern LEDs.

I hope this helps!


James Lee Vann said...

That is interesting, I never thought of the energy that Daytime Running Lamps would require. I think the modern LED idea is a good one.

We also have to calculate the energy savings from reduced accidents. The concept of running lights on all te time has repeatidly been shown to increase safety. If we see a reduction of accidents, that means less fuel consumption by emergency vehicles (police, ambulence, firetrucks...) less consumption by hospitals to fix people up, and less consumption to replace the damaged vehicles. We would also have less billing and clerical expenses for insurance companies- all activities that use energy.

Calulations are difficult, but if we break even on an energy usage basis, the benefits in reduced human suffering from accidents are by far worth it. If we use modern LED headlamps, we could even have a net energy savings!

Laura James said...

Do running lights really use extra fuel?

When the engine of a car is running, the alternator is working, and generating electricity. The electricity is used to charge the battery, or directly by electrical devices in the car (eg running lights, stereo etc).

Whilst you are driving, you are always making electricity through the alternator. What you use it for is up to you, but since you can't disable the alternator, you will be making that electricity come what may. You can then use it for running lights or to charge the battery or to power a stereo, or just burn it off as heat.

Now, if you are already using all the electricity your car generates, on lots of air con or whatever, you must either start to run off the battery (to have running lights as well) or must turn off some other system to power the running lights. However, on modern cars the battery is rarely used much (not more than 20% depleted, as a rule, the internet tells me) and I don't believe that most cars running all their electric systems would use up all the generational capacity. So running lights would just absorb "leftover" electricity.

So, unless I am mistaken, running lights don't use "extra" petrol...

Damon said...

Extra load on the alternator will ultimately cause extra load on (and fuel consumption by) the engine, else you have just invented a perpetual motion machine (or else you could make the car more efficient).



Laura James said...

Hmm, that's what I thought, but the alternator load (of moving magnetic fields around) is the same whilst the engine is running - it doesn't get more because the electricity generated will be used by other things. My understanding is it works continuously whilst the engine is running, and the difference is whether the generated electricity is used directly, for charging a battery, or "wasted", and that the alternator load is constant.

I don't disagree that you could make a car more efficient by removing or redesigning this part :)

IANAE in this area so may be wrong, but I have failed to find a good source explaining how this part of a car operates...

dave said...

Laura.. I'm afraid you're wrong. Power is always the product of 2 things, eg. electrical power = voltage x current, mechanical (shaft) power = torque x speed. So the engine may be spinning the alternator at a certain speed (and it therefore generates a certain EMF or voltage), but if we pull more current from it (by connecting other loads such as the lights), then (assuming the speed is constant for argument's sake) it will just 'draw' increased torque from the engine, which will burn more fuel as a result of having to provide this extra torque. Does this make sense? I think your assumption that the alternator sees a constant load is incorrect.

Brendon said...

"The concept of running lights on all te time has repeatidly been shown to increase safety."

I wonder if cars having their lights on all the time will increase the danger to motorcyclists. Currently we have a visibility advantage because when we have our headlight on we stand out. I wouldn't like to see that go - especially since energy wise, small motorcycles and scooters are an advantage over cars.

Joe said...

Hi, I really enjoyed reading the book, I must go back and read it all again and crunch the numbers in my own head!

Just a couple of things - I would appreciate your thoughts on the wind-assisted freighter ships, which are said to be more efficient than conventional shipping. Also, it is possible (though difficult) to travel as a passenger on a container ship around the world, which would presumably be significantly more energy efficient than flying, though very slow. Is it too much to imagine faster, mixed passenger/freight shipping?

Also you've not mentioned (IIRC) the possibilities of canal freight, which would potentially less fuel per tonne km than the equivalent road transport. The infrastructure is old, but largely there and unused. Even just using canals to ship freight from the edge of cities to the centre would avoid losses due to road congestion.

Regarding bikes, it is true that the Belgians and Dutch are streets ahead of us in terms of attitude and facilities. That said, their countries are largely flat. It is a bit hard to imagine people cycling to Ikea here as happens in other countries. I think we need to look at power-assisted electric bikes, which at present are fairly clumsy and the batteries don't last long.

Good luck


patohare said...

James Lee Van writes
"The concept of running lights on all the time has repeatedly been shown to increase safety."

Can you point me to all of this evidence?

Oleg Kuzin said...

Daylight Running Lights are standard on cars sold in Canada. Obliged by federal regulations, auto manufacturers have found an economical way to run dimmed lights by using LED's or other low consumption bulbs. I believe the expense of using these lights is negligeable. Now cars can be seen regardless of the time of day, reducing the number of accidents.