Using the heat when it's cold... mpg drain?

[dwtaylorpdx]

Ok I had to check, to easy...
I just measured the blower draw on my Jeep, same power on all low and med more on the high speed, the heating element it uses for speed control is right by the blower.

It was pulling about 5.5 amps low and medium... Jumped to 7.5 on high...

Dave

[jamesqf]

Originally Posted by greasemonkee

Since current is constant through the entire series circuit, the resistors are in fact "burning up" wattage, they're burning up voltage to be specific.

A resistor is not a transistor.

Sure. I was trying to give a simplistic explanation, without invoking Kirchoff's Law &c. But try a simple experiment: take a 12-volt battery, and connect a piece of copper wire (low resistance) between the terminals. Lots of current flow, heat, sparks, etc - and in particular energy "burned up". Now put a piece of wood (very high resistance) between the terminals. Does the wood start to smoke & catch fire from all the energy it's burning up? No, because current isn't flowing because of the resistance.

ETA: Or consider the transistor: it's a device that's switchable between two states, high and low resistance. When it's in the high resistance state, is it "burning up" the power that's not flowing through it?

[micondie]

Originally Posted by jamesqf

Sure. I was trying to give a simplistic explanation, without invoking Kirchoff's Law &c. But try a simple experiment: take a 12-volt battery, and connect a piece of copper wire (low resistance) between the terminals. Lots of current flow, heat, sparks, etc - and in particular energy "burned up". Now put a piece of wood (very high resistance) between the terminals. Does the wood start to smoke & catch fire from all the energy it's burning up? No, because current isn't flowing because of the resistance.

ETA: Or consider the transistor: it's a device that's switchable between two states, high and low resistance. When it's in the high resistance state, is it "burning up" the power that's not flowing through it?

Try your experiment with a piece of #24 or finer wire but be careful because it it going to get HOT!!!!!!!!
The reason a piece of wood won't get hot is that there is no current flowing through it to generate any.
Why do you think they mount power transistors on a heat sink?
Why is your CPU mounted on a heat sink with a fan?

[PaleMelanesian]

Test your car's blower fan, and report back with real results.

Originally Posted by PaleMelanesian

I've measured my blower, and the power draw is proportional to the speed setting - low speed uses less than high speed. Sitting with the engine off, reading battery voltage, and each higher setting drops the voltage another 0.1. Back to a lower setting and that 0.1 comes back. I'm not sure about the why or how, though.

Volts - Fan setting
12.2 - Off
12.1 - 1
12.0 - 2
11.9 - 3
11.8 - 4

Originally Posted by Daox

Darin tested this a while ago.

Quote:
Watts . Item
30 heater fan low speed (1)
49 heater fan 2
66 heater fan 3
81 heater fan high (4)

From: Tested: power draw of Geo Metro accessories & systems (volts, current/amps, watts)

[jamesqf]

Originally Posted by micondie

The reason a piece of wood won't get hot is that there is no current flowing through it to generate any.

Yes, that's exactly my point. You don't have as much current flowing through a resistor (V = IR, remember?), so less energy flow, so no heat. But don't take my word for it, look at a basic electric/electronics text, or put a temperature probe on one of those resistors that are supposed to be turning energy into heat.

Oh, and another question for thought: how come the wires in your toaster or electric heater - or indeed, the tungsten filament in an incandescent light bulb - are made of metal, which has a low resistance? Just for the heck of it, I dug out a few old incandescent bulbs, and measured their resistance: 40 W = 30 ohms, 60 W = 17 ohms, 75 W = 13 ohms. Not high resistance at all (though of course it increases with temperature), yet lots of heat.

Why do you think they mount power transistors on a heat sink?
Why is your CPU mounted on a heat sink with a fan?

Remember the transistor is a semiconductor. It's generating heat when current is flowing, and that's when it's in its lower resistance start.

[moorecomp]

Originally Posted by jamesqf

Oh, and another question for thought: how come the wires in your toaster or electric heater - or indeed, the tungsten filament in an incandescent light bulb - are made of metal, which has a low resistance? Just for the heck of it, I dug out a few old incandescent bulbs, and measured their resistance: 40 W = 30 ohms, 60 W = 17 ohms, 75 W = 13 ohms. Not high resistance at all (though of course it increases with temperature), yet lots of heat.

Exactly. The wire wound resistors in my car are 1.3, .62 and .33 ohms each. Not mega-ohms or kilo-ohms, just ohms. Very low resistance, generate heat. That is why they are typically located in the duct to aid in cooling. Low setting routes through all three resistors, med low through the .62 and .33 only, med high through the .33 only, and high straight to the blower motor.

[greasemonkee]

And those measured values change when current is flowing through them. The resistance will be less without current flow - so using ohms law will not work by calculating it.

[MetroMPG]

Learned something new relevant to the original question of this thread:

BMW engineers say:

Modern direct injection diesel engines are now so efficient that the energy going into the coolant circuit and, therefore, to the heater is no longer always sufficient to meet the customer’s [heating] requirements. It has therefore become quite normal to fit cars with an additional electric heater providing such extra heat on up to 1,000 W of electrical energy.

To deliver such energy for additional heating, the engine has to develop up to 2,000 W -- since about twice the amount of mechanical energy is required to provide one watt of electrical energy. In all, therefore, such additional heating involves an increase in fuel consumption of up to one liter

source: http://www.greencarcongress.com/2009...-new-ther.html[/100 km].

So... with some diesels, using the heat can impose a significant MPG penalty... because of a dedicated electric heating element.

I have read anecdotal stories about the smart fortwo diesel that was sold in Canada having insufficient cabin heat in really cold (think -20 C and below) conditions.

The Toyota Echo sold here also had a beefy electrically heated element in the defrost circuit so you could start to clear the front glass before the engine warmed up. Not sure if it's still offered on the Yaris or other cars.

I have to say in the depths of winter, I am sometimes very thankful for the ~20% inefficiency of my gasoline engine and all the waste heat it produces!

[gascort]

jamesqf is right about resistance and electrical usage.
I remember thinking the same thing though when I was in high school about resistors - they cause a voltage drop and do warm up, so they must waste energy. Luckily this is not the case, or at least they don't waste more than about 1% of what I thought back then....
The block of wood DOES conduct some electricity. It's inversely proportional to its resistance, so it is essentially none.... just like you will conduct electricity at 12V. Not enough to feel it usually, but some.
Just think about how hot those resistors would have to get if they took a high speed fan and took enough energy away from it to make it run 1/3 as fast.... lots of E would transfer to a fiery resistor really quick. Luckily they just get warm.

[jamesqf]

Originally Posted by gascort

Just think about how hot those resistors would have to get if they took a high speed fan and took enough energy away from it to make it run 1/3 as fast.... lots of E would transfer to a fiery resistor really quick.

And just think what having what would be in effect an electric heating coil in your ductwork, when you're using the fan for A/C :-)