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OT: Petrol consumption
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Jeff L
electronics forum beginner

Joined: 04 Sep 2005
Posts: 47

PostPosted: Fri Jul 21, 2006 9:02 am    Post subject: Re: Petrol consumption Reply with quote

"Jeff L" <levy_jeff@hotmail.com> wrote in message

"glen herrmannsfeldt" <gah@ugcs.caltech.edu> wrote in message
Jeff L wrote:

Since no one else calculated it, here goes (not including other lights
as marker and tail lamps, which are typically about 5W a piece (non
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

Yes, oooooopppppps, gotta stop posting at 4:00 AM! The numbers still work

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
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

I wouldn't worry about that small amount of fuel, and besides most cars
mandatory DRL's (daytime running lights, which are run at reduced power).

Warming up an engine uses way more fuel. For example warming up from
sitting all night at say 10 deg C, my old 2.8L Camaro with multi port
injection would take 2 - 3 L in the first 5 km (yes, I actually managed to
measure this (ALDL data, and confirmed with an *emergency* fuel pump and
tank)), then start sipping fuel. eg I would commonly get about 100km per
8 -
9 L when driving down the highway at 140 km/h, with a few higher speed
bursts when dropped down to 4th gear. Mixed city driving was generally 10
12 L per 100 km, and driving granny mode (100 km/h, using the highest gear
as soon as possible, and *normal* acceleration) down the highway could get
down to the 7 l per 100 km mark.

One other thing I noticed is that the idle fuel consumption and driving at
50 km an hour in steady state conditions burned fuel at about the same rate
(based on time)!

Very excessive highway (not likely steady state condtions) speed burned
about 1.28 l per minute, which gave an economy of 30L per 100 km.


Generally, with the above calc's it looks like a car will burn 0.95 L per
hour, per 1 kw load, assuming that the energy is not drawn during

There are a ton of other things that use power in a car also, such as
estimates of energy usage):
- heater blower ~ 300W on full
- window defrost ~ 300W for ~20 min
- Dash lights ~30W
- exterior lights ~35 W
- Stereo ~10 to 50W
- Cooling fan 200W to 400W intermittent
- Fuel Pump ~50 - 75W average
- Ign system 30 to 50W
- emission pump if equipped - 200 - 300 W
- MAF sensor 75 to 350W
- Injectors and controller 50 to 150W
- power accessories - large draw that is intermittent, so little energy

Mechanical loads
- water pump - 500 to 4000 W, generally several kW on the highway!
- power steering pump - 100 to 3000W
- belts - 50 to 400W
- emission pump 300 to 1000W
- idler pulley - 5 to 50W
- air conditioning compressor - 750 to 7500 W dependant on size and design
- Valve train losses - this is large - A company that builds racing valve
springs built a machine to spin a typical SBC chev V8 up to 8000 RPM. The
motor had no crank, just valve train parts, and they needed an electric
motor with something like 40 HP!
- Oil Pump - ???

Other losses:
- Brake drag
- Tire friction (can be substantial)
- Wheel bearing friction
- Transmission losses and differential losses, generally 10 to 15%,
generally higher at lower power levels due to the energy needed to
friction, also can be higher if it's an automatic
- Oil viscosity drag

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electronics forum Guru

Joined: 22 Jun 2006
Posts: 642

PostPosted: Fri Jul 21, 2006 11:10 am    Post subject: Re: OT: Petrol consumption Reply with quote

glen herrmannsfeldt wrote:

Richard Henry wrote:


Some old military aircraft engines injected water to make the engines
run better.

There was an article, I believe in Popular Science some years
ago on running car engines on hydrogen. They put water through
the carburetor.

That doesn't sound terribly likely actually !

(That was in the days before fuel injection was

With all the talk about a hydrogen economy and fuel cells I haven't
heard anything lately about hydrogen in internal combustion engines.

They haven't forgotten but it's a truly daft idea.

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