Alternator question

[wh33t]

A friend of mine and I are having a discussion about alternators and I had some questions for some alternator experts out there.

Does pulling more amperage off of your Alternator create/increase drag on the motor? If so, how does it do this?

We are trying to debate whether or not having a super loud stereo in your car, having fog lights, power windows etc reduces fuel economy.

[Mark]

I'm no expert but I believe the answer to be 'yes'. Basically I think the alternator belt slackens or tightens to a degree depending on the load requirement and thats determined by the battery charge level. I don't think the difference will be massive tho.

Of course it's always running to some extent. You'll have to make sure that your alternator is capable of meeting your power requirements otherwise you'll drain your battery to death :p

[leopard]

In the debate probably your friend was right that additional feature or accessories used in your car wouldn't reduce fuel consumption.

[MrJan]

In Pimp My Ride (don't judge :D ), they often add extra batteries (see the one with ridiculous bass, where they put a 27" (??) Sub in the back seat :cheese:

[Dave B]

We are trying to debate whether or not having a super loud stereo in your car, having fog lights, power windows etc reduces fuel economy.
It will, but the effect will be barely noticable.

Even a modest engine is capable of about 60kW, standard foglights are usually 50W (each bulb), a standard stereo at full volume about the same.

Electric windows obviously aren't going to be in constant use, so it becomes a question of mass: does the motor and wiring weigh more than a manual mechanism? Again the result will be so small as to be irrelevant.

:)

[Kneeslider]

Well there are two main ways of generating an electric current. A Dynamo, and an Alternator.

A dynamo is very simple, it's exactly the same as a tin can electric motor out of a toy car, Scalextric or similar, but in reverse. If you get an electric motor, and attach a voltmeter to the terminals, then spin the motor with your fingers, you should see a voltage (and a current too) register on the scale. With a dynamo, the quantity of voltage is directly proportional to the speed the dynamo is turning, but the problem with this setup is that it's tricky to make it regulate efficiently, so that the voltage and current produced matches the demand placed upon it by the lights, battery charging etc etc.

The dynamo is simple, but has these drawbacks, because the magnets inside are permanent magnets, like a bar magnet.

A Dynamo produces Direct current, and if your battery is 12v, then it needs to spin fast enough to generate 12v before you can do anything useful with it.

An alternator is different, because, while it still relies on moving a magnet around to induce a current in a coil, amongst other differences, the magnet is an electro magnet, which needs a current through it to enable the alternator to produce electricity. If you increase the current in the electro magnet, you get more electricity out of the alternator for a given speed on the shaft.

This sounds great, because it looks like you are getting something for nothing, but the crucial thing here is that while the alternator is spinning at the same speed, the torque (turning effort applied to the shaft to maintain the speed) required will go up too.

The practical upshot of this is that you can spin an alternator at a fairly low speed, and it will meet the demand needs of everything on your car, and still have enough to charge the battery, by having a high current trough the electro magnet in the middle of the alternator (this current is often called the excitation). And you can also run it at a high speed, when the engine is turnign over faster, but the actual output required might turn out to be lower, so the excitation current would be much less.

So unlike Mark's suggestion of slipping the pulleys which drive the alternator from the engine, you can be much cleverer, and do it all electronically, so the mechanical drive is able to run at a constant tension, without slipping and therefore lasting longer!

[wh33t]

Wow guys. Great replies!

Do they make Dynamos for cars?

[Steve Boyd]

Before power diodes & transistors that could withstand the temperatures around the engine became available, all cars had dynamo's (unless you go back so far they had magneto ignition & no other electrical equipment. Kneeslider obviously isn't old enough to remember these or he'd know that dynamo's can be made with electromagnets rather than permanent magnets. They also used to turn at around the same speed as the crankshaft, whereas an alternator runs at about twice crankshaft speed. Dynamo's only produced up to 20A - 25A, modern alternators can deliver three or four times this ammount.
And, yes, switching on more electrical equipment will take power from the engine. An alternator delivers its current at about 14 volts to keep the battery charged. 70A at 14V is 0.98 kW or 1.3 hp. Allowing for the power losses in the drive belt and the fact that the alternator isn't 100% efficient means about 2 hp (1.5kW) from the engine will go to drive the alternator if you've got enough switched on to need 70A of load (lights, heated widows, super loud stereo's are very inefficient - 100W per channel on a 2 channel stereo can be a 400W drain on the car electrical system - for surround sound you'll need to tow a power station). 1.5kW might not sound a lot if your engine can produce 100kW (135 hp), but remember it's only producing 100kW at full throttle and high revs. Trundling along in city traffic, it might only be producing 10kW & an extra 1.5kW will be noticed in the fuel consumption.

[Mark]

If I run the a/c on fan setting 4 will that use more fuel that it being on fan setting 1? Or is the a/c just on or off?

[schmenke]

I've often questioned that myself. Does cranking the a/c setting to max put additional load on the compressor or merely cause the circulation fan to turn faster?


(Whatever happened to the automotive technology thread...? :mark: )

[wh33t]

So it sounds like dynamo is the way to go if you want to create some 'free' power. Although I know it won't be entirely free. I realize the juice coming off of it will vary, but it could be used to power low power things, like a small dvd player lighter adapter socket type dealio.

Where do I get a dynamo? Will any auto parts store have one for my car?

[Steve Boyd]

So it sounds like dynamo is the way to go if you want to create some 'free' power. Although I know it won't be entirely free. I realize the juice coming off of it will vary, but it could be used to power low power things, like a small dvd player lighter adapter socket type dealio.

Where do I get a dynamo? Will any auto parts store have one for my car?

There's no such thing as free power, if there was we'd have invented a successful perpetual motion machine by now & no one would give a toss about oil prices.
A dynamo takes power from the crankshaft in exactly the same way as an alternator, and contrary to what Kneeslider says, they don't necessarily have permanent magnets (not that this would make much difference to the power equation). The ones fitted to cars had windings to create the magnetic field, otherwise there would be no way of controlling the output & you'd fry the battery. Alternators are smaller & lighter for the same power output than a dynamo as the field windings are on the rotor and the power windings are static. It has to be the other way round for a dynamo as the power has to be generated in the rotor so that it can be converted to dc by the commutator. Both dynamos and alternators actually generate ac, its the action of the commutator in a dynamo that turns this into dc. This results in a heavier rotor & a limit to the speed it can spin at. This means a dynmo is generally limited to about the same speed as the engine crank, and so doesn't produce any power when the engine is idling. The most powerful automotive dynamo that I can remember only produced 25A maximum, and would never cope with all the electrical equipment on a modern car. The alternators fitted these days typically produce three times the output of the old dynamo's and, as they usually run at twice crakshaft speed, generate a useful output even when the engine is idling. There's no point in replacing an alternator with a dynamo, in fact when alternators first became available most people who'd fitted things like heated windows & spot lights chucked thier dynamo's away & fitted alternators. There's even less point in running both - you already have an electrical generator, why fit another bigger, heavier, less efficient one?
If you really want free power use re-chargeable batteries & charge them with electricity from someone else's socket outlet! (not that I'm advocating doing anything illegal of course).

[Kneeslider]

Yup, what Steve said... But... I do know that dynamos can have electro magnets within their stators!!! But almost all I have come across and rebuilt, for automotive applications have permanent magnets! Lucas ones on Ariel bikes of the 40s and 50s, Miller units on Velocettes (which are best junked immediately and replaced with Lucas) as well as Bosch units on early '70s VW Beetles, all had mechanical voltage regulators, which were nothing but a pain in the A**, so were best replaced with solid state regulator units. It was easiest to change a 6v system into a 12 volt one this way, after changing all the bulbs of course, then there is the black art of turning a posative earth system into a negative earth one... Perhaps this should be best left to another day???

Although there must be a better way of explaining things... I was trying to keep it as simple as possible, and get away from saying something like "Yes, but it depends!"

Basically, an automotive dynamo produces DC, and an alternator (hence it's name) produces AC. Both are machines which turn rotational mechanical energy produced by the engine into electrical energy.

Because you can't produce energy (mechanical or electrical) from thin air, then whenever you switch on any electrical gizmo in your car, you are taking power from the engine, and making it do something else. So you can't get anything from nothing.

From what Mark and Schmenke asked:-

When you run your airconditioner in your car, you are using the electrical energy from the alternator to turn a motor, which turns a heat pump.

Heat pumps come in lots of different types, but the most common sort uses the principle of evaporative cooling. If you spray water on your face, as it dries, it turns from liquid, into vapour (or a gas), in doing so, it takes heat from it's surroundings, cooling them down. The electric motor in an air conditioner turns a pump which pumps it's working fluid round a closed circuit, where it is alternately evaporated, and then condensed. On evaporation it takes heat away from it's surroundings (the inside of your car, or the inside of a fridge) and pumps the fluid, now with the extra heat, to where it can condense, and give up it's heat to the surroundings under the bonnet of the car.

So, on my car at least the heat pump is either being switched on or off by the A/C switch, only the heater fan switch draws a different current according to it's position. The heat pump does the majority of the work, and the heater fan uses much less power. In theory it would be possible to have a multi speed heat pump, but that is probably over complicating things. However I could be wrong!

If you want to find out more, then look up heat pumps in something like howstuffworks they are as near as you can get to getting something for nothing, look up Carnot and Sterling cycles too! I wish that I could install a heat pump to provide heating and hot water to my house. Way more efficient than simply burning gas to heat the place! I have strayed off topic a little now :p

[Mark]

So what you are saying is that having the ac on fan position 4 only uses very little extra fuel compared to pos 1?

[Kneeslider]

Yes! :p

[MrJan]

My old man has a 1938 MG and recently changed the dynamo in favour of a alternator purely because he uses it for touring Europe and when he turned on the lights the dynamo couldn't create enough charge. Now that he has the alternator he's even managed to rig up a cigarette lighter to charge his phone one, although the ammeter (sp?) can't handle the alternator and doesn't show whether you are charging or not :S