I Eat, Therefore I Am

For those of you who drive, you may have been advised to turn off your radio/air con before you switch off your engine. I have alwis opposed to that simply because I am lazy AND because no one has really gave me a proper rationale or evidence to do so.

Well, thanks to the power of the internet…a short search on google showed a few things…and a simple meta-analysis reveals the following…(I have sieved out the better ones and posted it here for your reading pleasure and conclusion.)

The Question/s: “Do electrical devices running on your car’s electrical system affect petrol consumption OR Do we need to turn off the radio and A/C before switching off the car engine? Do you use more gas, all else being equal, with the lights, radio, plug-in accessories running than without?”

“The question is whether this is a noticeable drain. Your car is producing electricity whether you’re using it or not. Thats what the alternator does. How much stuff would you have to run to see a difference sounds like a question for someone with the time and equipment to run some decent experiments.”

“Some of the energy in the gasoline burned by your engine turns an alternator which, in turn, generates the electricity used by lights and gadgets. Alternator efficiency is only 50-65%, so the amount of gas used by lots of devices may be non-trivial.”

“Do you? Yes. Is it measureable? No. Even the strongest alternator, at say 135 amps, is drawing 135*12.6/745=2.3 hp, when it’s balls-out, which is pretty much never. You could gain that back by putting two more PSI in your tires. It’s miniscule.”

“My theory: it would technically put more resistance on the alternator, which is run by a pulley on the engine’s crankshaft, and through which it would send some feedback to the engine. In theory, just like the a/c compressor. But unlike turning on the a/c, I think the effect of plugging in a laptop would be negligible, and hard to measure, in gas used or in noticeable engine strain.”

“The air conditioner is belt driven, not electric, in 99% of cars. (Certain hybrids and electric cars have electric air conditioning.) Turning on the air conditioner in a normal car activates the compressor (engaged via a clutch) and places additional load on the engine. So yes, it does reduce your available engine power and your fuel economy.”

“For some cars the increased load on the alternator is noticeable. On my old Honda, for example, when you turned on the headlights the idle speed dropped a bit.”

“As someone who drives stick it’s very easy to tell that the A/C puts additional load on the engine; it’s much easier to stall it when the AC is on.”

“This definitely depends on the car. I usually drive manual cars, and with my current 220HP V6, I couldn’t tell you if the AC was running or not, just based on the driveability. When I drove a 92HP Civic, I could definitely tell the AC was dragging the engine.”

“…The rear defroster is going to put significant load on the electrical system (and hence the alternator will resist a bit more in order to meet that load). They didn’t directly answer what the effect on mileage would be, though, just some estimates. I understand that energy isn’t free, but my question centers around whether the alternator has to work harder when there are more things on, or if it just kind of maintains a steady state, delivering a certain amount of current to the battery whenever the motor is running at a certain speed, regardless of whether electrical devices are running. Turns out that the resistance increases when you turn devices on, so theoretically there is increased use of fuel.”

“Also looks like no one can agree on whether it is trivial or not; I’ll bet that can vary depending on the car you’re driving.”