So I've been having the discussion with some other guys at work.....

When a car is idling and you put load on the eletrical system (high beams, etc...) the rpm of the engine drops.

The reason we go into it was he was thinking of going for an electric water pump to give the car more power - and I thought the increased drag on the electrical system would reduce that power gain.

What causes a cars rpm to drop when the electrical system is loaded?

Our first thought was that it could be that when the car requires more current it increases the drag on the alternator to get that current....but I thought the drag in an alternator was constant.

My mate supposed that with less electricity availble the ignition system is now making weaker sparks - less torque created at combustion - less rpm.

Anyone know for sure?

At idle the alternator isn't spinning as fast so isn't putting out full capacity, its not far off as they are pretty efficient. Its not like a generator where your headlights get brighter the faster you go but if you load up the alternator at idle it may struggle a wee bit. Thats my guess anyway.

yeah, i aggree with nic_m

Actually, it's pretty simple. The more power you draw from the alternator, the more power the alternator draws from the engine. 745.6 watts equals 1 horse power. The power to drive the electrical system comes directly from the engine. The alternator is likely 85 to 90% efficient plus there will be losses in the belt driving the alternator so true load from the electrical system is something like 1HP per 600 watts.

At low engine speeds the output of the alternator is limited. This limits how much power the alternator can draw from the engine at idle.

How does the alternator draw this power from the engine - does it make it harder for the engine to turn?

the alternator just does the opposite of an electric motor.

For a given power input and motor efficiency you get a certain amount of power out.

For the alternator, if the power requirement of the electrical system increases then is has to sap more power from the crank

EDIT: yes it becomes harder to turn

mildman basically your mate at work had it right, at idle a heavy load of accessories will rob a little spark and drop rpm because the alt output is low,
but at higher revs the alternator output will be enough to drive everything usually, unless u go right overboard with spotlights and stereo amps etc. the load in hp to spin the alt does not increase with the number of accessories or current drawn.

the brushes in the alternator only put light pressure on the spindle shaft, from light spring pressure, it does not get harder with increased electrical load or current drawn from accessories. The voltage regulator drains off excess current that is not being used.

Well, dunno if this might help. At normal Idle im getting aroun 13.8Volts, but when the Lights come on, I only get 12.6V at idle.

Im thinking load on the alternator causing the rpm to drop. hmmmm.

Sorry 1000Coupe but I'd say you would be wrong.

Basic laws of Physics say that energy can neither be created or destroyed it can only be converted. Therefore to generate electrical energy the alternator needs energy input, in this case energy from the combustion of fuel.

So for whatever output is required from the alternator the engine supplies at least that amount bearing in mind that friction in the belts, bearings, etc. needs to be accounted for also.

All that being said and referring back the original question in the post, an electric water pump will increase horsepower output of the engine.
This is because the electric pump is spinning at a contant RPM that would likely have the same power requirements as a conventional water pump spinning at 3000 RPM. That means you would not need to rob the engine of the extra power required to spin the conventional water pump beyond that RPM and therefore translates to more available horsepower at the wheels.

It should also be noted that there is a reductional in frictional losses from the conventional water pump belt and some of the inertia from the water pump is removed from acting directly on the crank.

Hope that makes sense.

are there any auto sparky's?

an electric water pump will save a couple hp from the engine. ppl generally use these for racing as every hp counts to get an edge.

An electric water pump will not make the alternator harder to turn nor will the headlights, stereo, cig lighter etc.

the alternator puts out a high amount of current, what is not used is "bled" off by the regulator. An alternator spins fairly freely,
regardless of what accessories are on. They do not increase their internal resistance and draw more hp, otherwise u would get worse economy driving with lights, stereo, etc on.