OT: Petrol consumption

[Mad Prof]

Anybody know if more petrol is consumed when the car headlights are on? If
so how significant is it.
I assume there must be more fuel needed since the batter is only used on
start-up - the generator being the main source of electrical power and it
is driven by the engine.


M.P

 

[Jerry Avins]

Anybody know if more petrol is consumed when the car headlights are on? If
so how significant is it.
I assume there must be more fuel needed since the batter is only used on
start-up - the generator being the main source of electrical power and it
is driven by the engine.

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

 

[John Popelish]

Anybody know if more petrol is consumed when the car headlights are on? If
so how significant is it.
I assume there must be more fuel needed since the batter is only used on
start-up - the generator being the main source of electrical power and it
is driven by the engine.

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.

 

[[email protected]]

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.

 

[[email protected]]

Mad Prof wrote:

[...]

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?

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

 

[Paul E. Schoen]

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.

[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

 

[Eeyore]

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.

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

 

[Eeyore]

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.

It's almost the same as hp.

And forget about km/l. That's just too foreign.

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

 

[Jeff L]

Anybody know if more petrol is consumed when the car headlights are on? If
so how significant is it.
I assume there must be more fuel needed since the batter is only used on
start-up - the generator being the main source of electrical power and it
is driven by the engine.


M.P

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!

 

[glen herrmannsfeldt]

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?

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

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.

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

-- glen

 

[glen herrmannsfeldt]

Jerry Avins wrote:

(snip)

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?

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

 

[glen herrmannsfeldt]

Eeyore wrote:

(snip)

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

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

 

[glen herrmannsfeldt]

Jeff L wrote:
(snip)

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

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

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!

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

 

[Ian]

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.

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

 

[Meindert Sprang]

Jerry Avins wrote:

(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.

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

 

[Meindert Sprang]

Eeyore wrote:

(snip)


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

Been watching Mythbusters?

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

That would be a separate radiator in my car IIRC.

Meindert

 

[Richard Henry]

Eeyore wrote:

(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

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).

 

[Roger Hamlett]

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

 

[Eeyore]

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.

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

Graham

 

[Andreas Huennebeck]

Anybody know if more petrol is consumed when the car headlights are on? If
so how significant is it.

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
--
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