Can Someone Explain - Electrical Drain and Fuel Usage
Discussion
Following on from a conversation I was having with my Dad earlier. Talking about the bad fuel economy his Focus is getting and I mentioned the current climate wouldn't be helping as you'd be driving everywhere with the heater on full, heated screen on, headlights etc. He mentioned this shouldn't make any difference. Now the way I see it, if you have everything electrical on the engine would be under greater load hence have to work harder and use more fuel. The alternator would also be under greater strain but then again surely it can only spin at the same rate as the drive belt? Reason I say this is that even under electrical load the engine idle speed never seems to change, the only exception being air con with a small engine
So can someone with more technical knowledge explain exactly why this would cause a drop in fuel economy?
So can someone with more technical knowledge explain exactly why this would cause a drop in fuel economy?
Well you are partly correct in what you are saying about there being an extra electrical load due to lights/heated screen etc. but I doubt you could even measure the effects on the fuel usage as they are that small.
The only difference is the alternator will be charging for longer not faster.
The largest contribution to decreased economy in cold weather is simply because the car has to bung a load of extra fuel in the engine to keep it running smoothly when the air temperature is as cold as it is.
Its the reason cars can feel a bit more lively in cold weather because its using more fuel and making a bigger bang.
I'm not a mechanic but I am fairly confident in that explanation, happy to be corrected.
The only difference is the alternator will be charging for longer not faster.
The largest contribution to decreased economy in cold weather is simply because the car has to bung a load of extra fuel in the engine to keep it running smoothly when the air temperature is as cold as it is.
Its the reason cars can feel a bit more lively in cold weather because its using more fuel and making a bigger bang.
I'm not a mechanic but I am fairly confident in that explanation, happy to be corrected.
One question, Where does the energy come from to power the electrics?
Common sense tells you that the more energy you need to power the extra electrical equipment, the harder the engine has to work, hence the higher fuel consumption.
As an example, if you run a small petrol powered generator, notice how the engine note changes when you switch on whatever you have plugged in.
Common sense tells you that the more energy you need to power the extra electrical equipment, the harder the engine has to work, hence the higher fuel consumption.
As an example, if you run a small petrol powered generator, notice how the engine note changes when you switch on whatever you have plugged in.
Edited by f1nn on Saturday 11th December 20:22
f1nn said:
One question, Where does the energy come from to power the electrics?
Common sense tells you that the more energy you need to power the extra electrical equipment, the harder the engine has to work, hence the higher fuel consumption.
As an example, if you run a small petrol powered generator, notice how the engine note changes when you switch on whatever you have plugged in.
The alternator is already running at greater efficiency than the car needs and can generally cope with most extra demand without any effective harder work from the engineCommon sense tells you that the more energy you need to power the extra electrical equipment, the harder the engine has to work, hence the higher fuel consumption.
As an example, if you run a small petrol powered generator, notice how the engine note changes when you switch on whatever you have plugged in.
Edited by f1nn on Saturday 11th December 20:22
It's a fair bit more complicated than this but that's as close as we need to go in to it IMO
You can usually hear the alternator "dig in" if the engine is idling and you slam on the main beams and rear screen but mostly the system is pretty well designed for power generation needs
Car headlamps, 2 x 60 watt bulbs
Cars sidelamps, 4 x 5 watt bulbs
Total electrical load from lights is therefore 140 watts.
Car engine typically putting out, say, 25 bhp in normal driving.
1 bhp = 0.75 kilowatts = 750 watts
So that's 25 x 750 = 18,750 watts of power coming from your engine.
As you can see, 140 watts of lights are tiny in comparison to other factors for your car engine. Same goes for other electrical equipment on the car.
Cars sidelamps, 4 x 5 watt bulbs
Total electrical load from lights is therefore 140 watts.
Car engine typically putting out, say, 25 bhp in normal driving.
1 bhp = 0.75 kilowatts = 750 watts
So that's 25 x 750 = 18,750 watts of power coming from your engine.
As you can see, 140 watts of lights are tiny in comparison to other factors for your car engine. Same goes for other electrical equipment on the car.
perdu said:
f1nn said:
One question, Where does the energy come from to power the electrics?
Common sense tells you that the more energy you need to power the extra electrical equipment, the harder the engine has to work, hence the higher fuel consumption.
As an example, if you run a small petrol powered generator, notice how the engine note changes when you switch on whatever you have plugged in.
The alternator is already running at greater efficiency than the car needs and can generally cope with most extra demand without any effective harder work from the engineCommon sense tells you that the more energy you need to power the extra electrical equipment, the harder the engine has to work, hence the higher fuel consumption.
As an example, if you run a small petrol powered generator, notice how the engine note changes when you switch on whatever you have plugged in.
Edited by f1nn on Saturday 11th December 20:22
It's a fair bit more complicated than this but that's as close as we need to go in to it IMO
You can usually hear the alternator "dig in" if the engine is idling and you slam on the main beams and rear screen but mostly the system is pretty well designed for power generation needs
Are you talking about the engine note changing due to the extra load it has just been placed under?
In its simplest terms, An engine has to produce the electrical power that you are asking from it.
If you run this theoretical engine with no electrical accessories on, it has to be more fuel efficient than the same engine running while every button has been pressed, surely?
If I am wrong, then someone should tell the government as they will no doubt want to tax this free energy.
Well the heater shouldn't make a significantly difference because it's presumably not electric? If he really runs the heated screen constantly it will make a significant difference - they draw a lot of power (my Corrado sometimes used to stall if you turned on the rear window demister when it was idling).
I would have thought that the biggest difference would be that it takes longer for the engine to get properly up to temperature so it's running with the choke out for longer.
I would have thought that the biggest difference would be that it takes longer for the engine to get properly up to temperature so it's running with the choke out for longer.
Edited by kambites on Saturday 11th December 20:51
Negative Creep said:
Following on from a conversation I was having with my Dad earlier. Talking about the bad fuel economy his Focus is getting and I mentioned the current climate wouldn't be helping as you'd be driving everywhere with the heater on full, heated screen on, headlights etc. He mentioned this shouldn't make any difference. Now the way I see it, if you have everything electrical on the engine would be under greater load hence have to work harder and use more fuel. The alternator would also be under greater strain but then again surely it can only spin at the same rate as the drive belt? Reason I say this is that even under electrical load the engine idle speed never seems to change, the only exception being air con with a small engine
So can someone with more technical knowledge explain exactly why this would cause a drop in fuel economy?
Your alternator works exactly the same way as the ones in the power station*. It spins at one speed and outputs one voltage but as the load varies the amps vary. To get more power out you must add a larger load to the alternator. So can someone with more technical knowledge explain exactly why this would cause a drop in fuel economy?
W=VA (and V is fixed) and all that.
The reason for the engine revs dropping slightly is due to a greater load(and due to the greater load it is now harder for the alternator to spin) being placed on the engine and the engine controls reacting to open the throttle,inject more diesel to keep the engine rpm at whatever revs.The reason for the loss of mpg is that you are asking for more energy from the engine plain and simple.
- ok maybe not.the National Grid ones are designed to work at 3000rpm(or 50hz clever eh) and not across a range of revs like the one in your car but its close enough.
One horsepower is 756 watts. 756 watts / 12volts = 63 amps.
63 amps is a lot. And if your drawing that much its using up a whole 1bhp.
Edit: that assumes a 100% efficient alternator. Dont know how effiecient a normal alternator is, but i dont see it being that far off.
63 amps is a lot. And if your drawing that much its using up a whole 1bhp.
Edit: that assumes a 100% efficient alternator. Dont know how effiecient a normal alternator is, but i dont see it being that far off.
Edited by sparkybean on Saturday 11th December 21:48
f1nn said:
perdu said:
f1nn said:
One question, Where does the energy come from to power the electrics?
Common sense tells you that the more energy you need to power the extra electrical equipment, the harder the engine has to work, hence the higher fuel consumption.
As an example, if you run a small petrol powered generator, notice how the engine note changes when you switch on whatever you have plugged in.
The alternator is already running at greater efficiency than the car needs and can generally cope with most extra demand without any effective harder work from the engineCommon sense tells you that the more energy you need to power the extra electrical equipment, the harder the engine has to work, hence the higher fuel consumption.
As an example, if you run a small petrol powered generator, notice how the engine note changes when you switch on whatever you have plugged in.
Edited by f1nn on Saturday 11th December 20:22
It's a fair bit more complicated than this but that's as close as we need to go in to it IMO
You can usually hear the alternator "dig in" if the engine is idling and you slam on the main beams and rear screen but mostly the system is pretty well designed for power generation needs
Are you talking about the engine note changing due to the extra load it has just been placed under?
In its simplest terms, An engine has to produce the electrical power that you are asking from it.
If you run this theoretical engine with no electrical accessories on, it has to be more fuel efficient than the same engine running while every button has been pressed, surely?
If I am wrong, then someone should tell the government as they will no doubt want to tax this free energy.
yes
When a (usually, TBH) smallish engine is idling and you bang on a lot of demand (mains, heated rear, etcetera) you can hear the engine note change often when the load is imposed but that is usually 'cos the alternator is only idling too and has to take up the load.
Possibly much of the noise can be heard to be coming from the alternator itself too. I'm not sure I'm putting this clearly, sorry.
I still think the engine is usually carrying the demand load easily under power.
Please don't tell Dave anything
"cold weather" increase in fuel consumption comes from 2 primary factors:
1)air density: cold air is more dense, As you car needs to push this aside to pass, your cars aero drag increases when it is cold
2) engine friction & extra fuelling during cold starting: Starting an engine from say 0degC will use approx 30% more fuel during the warm up period due to higher internal engine friction. The "warm up" phase is also elongated with coolant and oil staying colder, especially if the cabin heater is used on full from start up
These 2 factors tend to increase fuel consumption by approx 10%
Also additional (but small factors):
Cold tyres have a higher rolling drag
Slippy wet roads means people tend to stay in lower gears for a feeling of control (meaning more power lost to engine friction)
small extra electrical load (not just lights & wipers, but heated seats/screen and a more discharged battery (cold start takes more power out during cranking etc) that needs re-charging.
All the cars lubricating fluids start colder and hence denser with more friction(gear / diff oil, wheel bearing greese etc)
Regarding the alternator load, yes this is extra load (it's rpm might be the same, but in order to supply the extra current (amperage) the alternator's rotor magnetic field strength is increased (by the built in voltage regulator) which generates a greater stator current. This IS added as extra load to the engine, but all modern engine management systems automatically account for this extra load so the driver should not be able to tell the difference (this is better calibrated on some cars than others!)
There is also actually a small positive offset, as fuel is denser when it is cold, so your car actually goes further per litre/gallon (although this is more than offset by the total losses increasing)
1)air density: cold air is more dense, As you car needs to push this aside to pass, your cars aero drag increases when it is cold
2) engine friction & extra fuelling during cold starting: Starting an engine from say 0degC will use approx 30% more fuel during the warm up period due to higher internal engine friction. The "warm up" phase is also elongated with coolant and oil staying colder, especially if the cabin heater is used on full from start up
These 2 factors tend to increase fuel consumption by approx 10%
Also additional (but small factors):
Cold tyres have a higher rolling drag
Slippy wet roads means people tend to stay in lower gears for a feeling of control (meaning more power lost to engine friction)
small extra electrical load (not just lights & wipers, but heated seats/screen and a more discharged battery (cold start takes more power out during cranking etc) that needs re-charging.
All the cars lubricating fluids start colder and hence denser with more friction(gear / diff oil, wheel bearing greese etc)
Regarding the alternator load, yes this is extra load (it's rpm might be the same, but in order to supply the extra current (amperage) the alternator's rotor magnetic field strength is increased (by the built in voltage regulator) which generates a greater stator current. This IS added as extra load to the engine, but all modern engine management systems automatically account for this extra load so the driver should not be able to tell the difference (this is better calibrated on some cars than others!)
There is also actually a small positive offset, as fuel is denser when it is cold, so your car actually goes further per litre/gallon (although this is more than offset by the total losses increasing)
To answer some of the other questions:
A "claw pole" alternator (which is what is used for a car) has an efficency that ranges between approx 62 and 75% (i.e. not very efficient)
The road load of a typical car at 50mph is approx 7kW, so an "extra" 1 bhp for electrical load is a significant proportion of the mean road load
Typical alternators are now between 110 and 130 amps rated, and immediately post cold start (especially if car has been standing a few days) with the electrical acessories on, they will be at or close to maximum output (heated seats 25A each, heated rear window 35A, heater front window 45A, lights, wipers, heater blower 15A, Restoring batter SOC 15A, Glow plugs, fuel pump, ems 15A.
A "claw pole" alternator (which is what is used for a car) has an efficency that ranges between approx 62 and 75% (i.e. not very efficient)
The road load of a typical car at 50mph is approx 7kW, so an "extra" 1 bhp for electrical load is a significant proportion of the mean road load
Typical alternators are now between 110 and 130 amps rated, and immediately post cold start (especially if car has been standing a few days) with the electrical acessories on, they will be at or close to maximum output (heated seats 25A each, heated rear window 35A, heater front window 45A, lights, wipers, heater blower 15A, Restoring batter SOC 15A, Glow plugs, fuel pump, ems 15A.
Max_Torque said:
To answer some of the other questions:
A "claw pole" alternator (which is what is used for a car) has an efficency that ranges between approx 62 and 75% (i.e. not very efficient)
The road load of a typical car at 50mph is approx 7kW, so an "extra" 1 bhp for electrical load is a significant proportion of the mean road load
Typical alternators are now between 110 and 130 amps rated, and immediately post cold start (especially if car has been standing a few days) with the electrical acessories on, they will be at or close to maximum output (heated seats 25A each, heated rear window 35A, heater front window 45A, lights, wipers, heater blower 15A, Restoring batter SOC 15A, Glow plugs, fuel pump, ems 15A.
Erm 7kw is approx 533 amps !!!!!A "claw pole" alternator (which is what is used for a car) has an efficency that ranges between approx 62 and 75% (i.e. not very efficient)
The road load of a typical car at 50mph is approx 7kW, so an "extra" 1 bhp for electrical load is a significant proportion of the mean road load
Typical alternators are now between 110 and 130 amps rated, and immediately post cold start (especially if car has been standing a few days) with the electrical acessories on, they will be at or close to maximum output (heated seats 25A each, heated rear window 35A, heater front window 45A, lights, wipers, heater blower 15A, Restoring batter SOC 15A, Glow plugs, fuel pump, ems 15A.
More like 2kw on a good day at 13.8 volts. (Figures rounded up to make it easier)
2000/745=3bhp (ish)
And to answer one of the original questions about idle speed. All modern engine management systems will maintain the correct idle speed automatically as part of there normal operation.
I stand corrected on any of the above if my calcs are wrong
Air Con is the one that saps power, but not as much as a Supercharger, some of the big Merc ones take 100 bhp to drive it, good job they give a lot more back !
I think basically the electrical kit does use quite a bit of juice, but in comparison to making a 2 tonne vehicle move at speed and accelerate in a suitably brisk manner it is very little and most cars are equipped with an engine that is more than up to the job of moving it and can spare a bit to drive the alternator.
As for using more fuel, mine feels so damn perky in this weather so it gets plenty of extra accelrator.
I think basically the electrical kit does use quite a bit of juice, but in comparison to making a 2 tonne vehicle move at speed and accelerate in a suitably brisk manner it is very little and most cars are equipped with an engine that is more than up to the job of moving it and can spare a bit to drive the alternator.
As for using more fuel, mine feels so damn perky in this weather so it gets plenty of extra accelrator.
sonarbell said:
Max_Torque said:
To answer some of the other questions:
A "claw pole" alternator (which is what is used for a car) has an efficency that ranges between approx 62 and 75% (i.e. not very efficient)
The road load of a typical car at 50mph is approx 7kW, so an "extra" 1 bhp for electrical load is a significant proportion of the mean road load
Typical alternators are now between 110 and 130 amps rated, and immediately post cold start (especially if car has been standing a few days) with the electrical acessories on, they will be at or close to maximum output (heated seats 25A each, heated rear window 35A, heater front window 45A, lights, wipers, heater blower 15A, Restoring batter SOC 15A, Glow plugs, fuel pump, ems 15A.
Erm 7kw is approx 533 amps !!!!!A "claw pole" alternator (which is what is used for a car) has an efficency that ranges between approx 62 and 75% (i.e. not very efficient)
The road load of a typical car at 50mph is approx 7kW, so an "extra" 1 bhp for electrical load is a significant proportion of the mean road load
Typical alternators are now between 110 and 130 amps rated, and immediately post cold start (especially if car has been standing a few days) with the electrical acessories on, they will be at or close to maximum output (heated seats 25A each, heated rear window 35A, heater front window 45A, lights, wipers, heater blower 15A, Restoring batter SOC 15A, Glow plugs, fuel pump, ems 15A.
More like 2kw on a good day at 13.8 volts. (Figures rounded up to make it easier)
2000/745=3bhp (ish)
And to answer one of the original questions about idle speed. All modern engine management systems will maintain the correct idle speed automatically as part of there normal operation.
I stand corrected on any of the above if my calcs are wrong
To awnser the OP's question, you are correct that the extra electrical equiptment will require more power from the engine, and so lower fuel consumption.
However, there are far greater influences (which have already been meantioned in this thread) that will make an impact on consumption.
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