Disconnecting alternator from car....

That was the maximum the alernator could ever draw with worst case assumptions piled on for good measure. Usually in the real world an alternator will be drawing much much less - just enough to run the fuel pump, engine control system and accessories once the battery reaches full charge. Probably most of the time it is pulling less than 0.3 hp (unless you are constantly running a 1000 watt stereo).

Again, that 0.4 gallons per hour is using worst case assumptions with the alternator at maximum draw. For normal running it is a fraction of this (less than 0.05 gallons per hour).

Likely less than 5% if you are driving 30 mph. And if you are really worried about that draw, then don't run the radio, heater, A/C, use the power windows or seats, leave off all lights all the time, and push start the car. Get it down to where your only draw is the fuel pump and the engine control system.....then you probably can get the alternator draw to under 2%. If you really want to know, you should put a ammeter on the main alternator power feed. I bet once the battery is charged it won't exceed 15 amps most of the time. 15 amps at 14 volts is 210 watts (about 0.3 hp).

Ed

If you disconnect the alternator, be sure you have a real good battery and don't drive too far. cuhulin

With most current vehicles disconnecting the alternator will cause far more problems then limited range. The ECM will not like the loss of power from the alternator. Even with a good deep cell battery you will probably only make it about 50 miles before the system shuts down.

I know that on the older cars running a generator they would pull the belt off on the drag strip and gain about 5HP. On those vehicles the belt was separate from the water pump drive on most vehicles.

If you want to try it all you have to do is unplug the alternator. Be prepared to watch the dash light up like a Christmas tree. Don't turn on ANY accessories like A/C, radio, power windows, whatever else.

With it disconnected you might gain about 2 HP. Of course after you drive till the battery discharges plan on buying a new battery. Oh and if you connect up the alternator to charge the battery back up plan on buying a new one of them as well. They are not designed for long term high output any longer. Most will fry if they have to put out much over half the rated max for any length of time.

Much better to keep on letting those 3 or 4 or 5 Horses keep on pulling that alternator. No Doubt about it! cuhulin

I agree.. A 100 amp alternator putting full current would be about 1300-1400 watts, just about 2 hp. But in real life you dont pull that much current all the time, most likely.

In rough terms, 2 hp compared with 200 hp (when fully putting out), would be 1%. And 1% would probably resolve to 0.2-0.3 mpg.

At least the boy is using his thought processes. Thought experiments worked well for Einstein, Hawking, Penrose, et alia.. But on cars, it is better to engineer it and test the result

Do you really need to test the fact that at 60 mph, a small car is drawing

10-20 hp, and not 200 hp? In rough terms, 2 hp is then 10-20%, not 1% -- which, btw, I already pointed out.

And, altho the electrical draw on average may be modest, the residual frictional load is proly on the order of a better part of a hp. You could have an engineer test that particular detail.....

There is a lot of wind resistance to overpower at 60mph

Frictional load is nowhere close to a hp. Have you ever spun an alternator by hand? Its almost friction-less. The only effort needed is to rotate the mass and that is only when accelerating it to a higher speed. You do know that alternators by their very nature are automatic... they automatically respond to electrical load presented... no electrical load presented, no magnetic drag... an electrical load presented, a corresponding magnetic drag.

You know you can achieve the same thing WITHOUT disconnecting the alternator. Actually you can achieve more... the equivalent of a belt less disconnected alternator, and still produce electricity! All you have to do is weigh all the rotating mass (pulley that drives the belt, belt that drives the alternator rotor, the alternator rotor itself) and add a small fraction of a percent for the bearing friction, brush friction, and windage (air that the rotor pushes around) and then remove "meat" from the flywheel! Not enough "meat" on the flywheel to equal the rotating mass? Remove "meat" from the pistons, piston pins, connecting rods, and crankshaft! Be careful though... removing "meat" can be dangerous to the health of an engine! Think "balance". Also check out the availability (and weight savings) of titanium connecting rods for your engine.

meh. You want light? Just start with a carbon car:

and . . . it's Italian.

Yeah, the Italians make some good stuff.

Emma comes first,,, cuhulin

In message , Existential Angst writes

The biggest draw on your fuel will be moving air around your car.

from a recent post on rcm:

I see where Bosch are touting their new 70% efficient 'smart' alternator that allows digital control so that field current can be turned off during acceleration and cranked up during braking, boosting engine efficiency by as much as 3%. Smart Indeed!

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If they're claiming 3% just by tweaking an alternator, imagine disconnecting it.

Apropos of other comments/realities, not saying disconnecting the alternator is currently the best idea, but mebbe it would be a good idea for mfr's to make such a provision.