On a recent "lost"episode of “Top Gear U.K.”, Jeremy drove a diesel Audi from their studio to Edinburgh, Scotland, and back on one tank of petrol. He conserved mileage by not using various electrical items, the heater being one. I have no clue as to why this would conserve fuel. Hypothesis: a drain on the alternator causes a great amount of magnetic resistance which causes the alternator to turn with difficulty, which makes the engine to work harder.
Drawing more electricity from the alternator does increase the load on the engine slightly, but I would be surprised if it made a huge difference. I’ve been suspicious of Top Gear ever since reading the allegations of how they handled their review of the Leaf.
Yes, watts used in the electrical system have to come from burning fuel.
Alternators and generators do not make free electicity. Ultimately, it is the engine that powers the heater fan.
On the Geo Metro I used to have, you could literally hear the engine’s idle speed change slightly when you switched the headlights on or off.
Another factor, when you drive to set fuel economy records, you burn so little fuel that it becomes difficult to keep the engine at its proper operating temperature and since the heater is like a second radiator for the engine, not using it helps keep the engine hot so it can be more efficient.
A lot of cars that are used to set fuel economy records have the radiators completely or partially blocked off so the engine doesn’t get over-cooled.
When I took driver education back when I was 16, our instructor demonstrated to us how the car’s accessories used power. He balanced the car on a hill so that the engine idle was holding it in place (it was an automatic), then had us turn on lights, the heater fan, etc. The engine would eventually have enough of a load on it from the alternator that the car would begin moving backwards. The rear defogger was the biggest drain. The alternator doesn’t use much of the total engine output, but it does contribute. This should be common sense–the energy to generate electricity has to come from somewhere.
If you could drive with the windows up on a hot day and refrain from using the A/C, that would actually save a significant amount of gas, as an A/C compressor can use 5 or more horsepower to run at peak load, but that’s about it.
I remember my first trip to the Museum of Science when I was a teenager. There was a bicycle-generator used to power a 100 watt bulb. I couldn’t believe how difficult it was to pedal the bike to generate 100 watts.
A diesel engine doesn’t produce as much heat as a gas burner. Just ask any truck driver. During the winter, it’s hard to keep their diesel engines hot enough to produce heat for the interior of the vehicle, which is why many truck drivers cover their radiator grills in the winter. When you run the heater, not only does the blower draw power, the heater core pulls heat from the engine, lowering its operating temperature.
FYI, oblivion, it’s really a rear defroster, not a rear defogger. It doesn’t do a thing to get rid of fog, and if your window fogs up, the air conditioner is more effective at removing interior moisture than the rear defroster.
In a non-plugin vehicle, 100% of the energy used comes from the gasoline. There is no other source.
Your theory about the magnetic resistance is correct. An alternator basically works by passing magnets past windings, called stator windings. The up and down fluctuation of the magnetic field as felt by the windings induces into the stator core an electrical current. That current is drawn out by the items using electricity. If more items needing electricity are in operation, the greater current flow creates more of a magnetic field in the stator core and thus more mechanical resistance in the interacting magnetoc fields between the rotor and the stator windings. The creates more mechanical load on the alternator. Since the alternator rotor is being turned by the engine’s crankshaft, it creats more load on the crankshaft, which needs more gas to overcome the added load.
Most of the car’s accessories draw minimal current, but they can add up. Even the motor and drivetrain necessary to assist electrical power steering has been determined to ultimately place less of a load on the crankshaft than a power steering pump, although that pump is just recirculating fluid against little resistance whenever you’re not steering (like at a stop light).
I remember my first trip to the Museum of Science when I was a teenager. There was a bicycle-generator used to power a 100 watt bulb. I couldn’t believe how difficult it was to pedal the bike to generate 100 watts.
According to the good folks at Wolfram|Alpha, 85 to 100 watts is “human daily average power”, so lighting that bulb involves as much effort as you would be expected to generate in an entire day.
Horses apparently produce about seven times as much power, so they’d have much less trouble keeping the bulb lit. (The hard part there is getting a horse to ride a bicycle.)
Good point. I guess lights take more power than I’m recognizing. I recall trying one of those bicycle generator-powered headlights, and that generator too a lot of effort to turn sufficiently to get a decent amount of light out.
“FYI, oblivion, it’s really a rear defroster, not a rear defogger. It doesn’t do a thing to get rid of fog, and if your window fogs up, the air conditioner is more effective at removing interior moisture than the rear defroster.”
I beg to differ… The heating elements on the rear windows of both of my cars do a fine job of defogging the windows as well as defrosting them in the winter. I used it today, as it’s been raining all day. A matter of semantics I suppose, since it does both pretty well.
Actually, what I warn against most in that regard is against using it in warm weather. Here in Florida, people seem to use it in warm weather to get rid of water vapor, and I once heard a warning not to do that because the heating element, combined with the cold air of the air conditioner, could stress the rear window enough to make it crack in several places. I’ve seen a few Floridians’ cars with rear windows that had small cracks all over them, and I was told the cause was that they used the rear defroster in the summer. It might be BS, I suppose, but it’s the reason I carry a small squeegee in the trunk for cleaning the rear window before I drive on humid mornings.
I agree with Oblivian on the defogger/defroster issue.
Fog is condensation. It forms because warmer moistrure-laden air (high relative humidity) cools at the boundry layer where its heat energy is transferred to the colder glass. Since its boundary layer, now cooled, can no longer hold the same amount of moisture, it deposits it on the glass. The heat elements in the rear window defroster/defogger heat the boundary layer and the glass, eliminating the cooling of moisture-laden boundary air, thus eliminating the deposition of the moisture on the glass, and even allowing that heated air to absorb the mist that was deposited.
I’d just like to point out one MAJOR flaw in the OP; he didn’t run a diesel Audi on petrol, he ran it on diesel. Petrol(gasoline) in a diesel is a very bad thing, though the reverse is even worse.
I’d also like to know how much he overfilled the tank before he set off. Did he put so much in that it came splashing back out?