OT: Petrol consumption

Running the headlights with the engine off uses even more petrol, since the alternator eventually supplies the energy, but at reduced overall efficiency. The headlights consume energy; where do you suppose the energy comes from?

Jerry

--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

It takes more torque to turn the generator when more electrical energy is being consumed. This torque is a bit like having the brakes lightly depressed while driving down the road. The larger the electrical load, the more the "brake" drags. This drag is more significant for a small engine car than it is for a much heavier vehicle. For one that gets 30 miles per gallon, I suspect headlights cost you about 1 mile per gallon.

You guess it takes about a 3 percent hit to convert chemicals into heat and photons, rather than forward thrust? I say 1 percent.

Let's propose that the headlight pair consumes 1000 watts. Let me assume that the power plant you propose produces 100 kW. Do you have better numbers?

How many km/l do you suffer from loading the alternator with

1000 W?

I'm from Texas, and I can use SI and/or metric. It's not hard! Although thinking of power plants in terms of kW is really weird. And forget about km/l. That's just too foreign.

[...]

Let me rethink my numbers. 100 kW is probably a number associated with or representative of peak output. You were probably right to start.

[removed comp.dsp from newsgroups]

You can probably figure the headlights to be roughly 200 watts, based on a

20 ampere fuse for the circuit at 12 VDC. This is about 1/4 HP. An economical vehicle requires something like 10 HP on average, so I would expect about a 3% drop in fuel economy, which would be 1 MPG if you get 30 MPG. YMMV

There may be a slight regen effect when going downhill, adding to the compression braking effect, and the maximum charge current is somewhere between 30 and 70 amps (about 1 HP). It might be possible to tweak the charger circuit to engage only when the brake pedal is touched (or when the battery drops below about 80% charge).

I saw an article on-line about adding an additional, larger motor/generator to the motor belt system, with a separate battery bank and regen drive circuit. It could possibly provide a good portion of the 10 HP or so needed for normal driving, and instantly convert your gas guzzler into a higher performance and more fuel efficient hybrid!

Paul

For 100-120W ? I doubt it.

Barely perceptible for headlights I'd say.

A/C's another matter - and that *does* take a load of power from the engine via the compressor.

Graham

It's almost the same as hp.

It's the wrong way up ! Who wants to know the inverse of mileage ?

Btw - a gasoline car engine is ~ 25% efficient - add in some alternator losses and I'm sure it's possible to deduce the effect on mpg.

Graham

Since no one else calculated it, here goes (not including other lights such as marker and tail lamps, which are typically about 5W a piece (non LED) adding to about 35W on a typical car):

Gasoline = ~32,000 kj/l Engine efficiency (avg, not best case) = 15% Headlamp power = 55W each X 2 Wiring losses = 3% Alternator eff = 95% Belt losses driving alternator = 15%

1 kWh = 3600 kj

Energy needed to light headlights for 1 hour = 0.110 kW => 396 kj/h Adding alternator losses gives: 416.84 kj/h Adding wiring losses gives: 429.73 kj/h Adding belt losses gives: 505.57 kj/h Adding engine efficiency losses: 3,370.46 kj/h

Which gives 0.105 l of fuel consumed per hour.

Interesting, that means with the average car, between 300 and 600 l of fuel is consumed in just lighting the head lamps over the car's life!

snipped-for-privacy@gmail.com wrote: (snip)

I would say closer to 200W for the pair. 30% efficiency for the engine is probably about right.

As far as I understand, Europe uses l/100km instead of km/l.

-- glen

(snip)

You could use a similar argument to say that it should consume more fuel with the heater on high than low, as the heat energy has to come from somewhere, but I believe that one isn't true.

I am not yet convinced that headlight use is more than the measurement error in fuel consumption, though.

-- glen

(snip)

Last I heard, above about 40mi/h the AC takes less power than the increased drag of having the windows open.

A secondary effect is the change in engine temperature with the AC heat from the condenser core going into the radiator.

-- glen

Jeff L wrote: (snip)

Energy needed to light headlights for 1 hour = 0.110 kWh => 396 kj Energy is power x time. or

Power needed to light headlights = 0.110 kW => 396 kj/h

Now consider the cost in energy to build a car multiplied by the increased probability of totaling the car in an accident when the lights are off.

-- glen

Fuel consumption figures are usually quoted as miles per gallon and Litres per 100km rather than km per litre

All of my cars (over the last 20+ years) have had their engine power quoted in kW.

Ian

Indeed, the engine is kept at a constant temperature, whether the heat goes out of the main radiator or the small one from the heating system.

Meindert

Been watching Mythbusters?

That would be a separate radiator in my car IIRC.

Meindert

Last night I drove my wife to the LA airport (about 2 hours). We took her new car, which as an mpg readout in the instrument panel. On the way up, with the AC on, we were running about 26-28 mpg. Returning after dark, with the AC off, we wer getting 31-33 mpg. We were going

65-75 mph both ways, depending on traffic, and hills were trivial either way (Route 91 from Corona to LAX and return).

This is quite common. As well as the change from the A/C, the engine runs more efficiently, when the OAT is lower (within certain limits). In the UK, on a run I used to do very often, a cool, slightly damp morning, returned the 'best' economy.

Best Wishes

Have you ever noticed that the engine is distinctly more sprightly under some weather conditions tooo ?

Graham

As far as I remember the ADAC (German automobile club) calculated the additional consumption as 0.1 liter / 100 km. They did this when some german politicians wanted to enforce car drivers to drive with lights switched on always, even during the day, like they do in scandinavia since many years.

So for a single driver this is below the noise floor (you can spare up to

30% fuel consumption just by using a good driving style[1] - here in Germany you can learn this in special driving lessons), but for all drivers in the country it accumulates to astounding values.

My current milage: 5.2 liters / 100 km (95 octane) = 45.2 miles/gallon on a Honda Jazz.

bye Andreas

-- Andreas Hünnebeck | email: snipped-for-privacy@gmx.de

----- privat ---- | www :

Fax/Anrufbeantworter: 0721/151-284301 GPG-Key: PGP-Key: