SSG Steggy

I have always seen people talk about the pros and cons of underdrive pulleys. Whether or not the "freed" horsepower is worth the cost is, I guess, a personal decision. As such, I have been thinking that why just free some power, as you are still driving pulleys just at a slower rate. Why not free up the crank pulley altogether and drive everything else by electricity? I was thinking I could mount a 3/4 to 1HP 12VDC motor and pulley and bypass the crank pulley.

Any thoughts or ideas as to how much power I would really be freeing? Cost effective? Just a cool idea?

Siber Express

You are basically talking about Perpetual Motion, thinking you would make more power than you are using
http://en.wikipedia.org/wiki/Perpetual_motionSoon enough you would not have enough power to power your Electronics and the electric motor and your car would quit running.

SSG Steggy

Siber Express, even if my current draw on the motor was significantly less than the output of my alternator?

jz78817

none. it'd cost you power. right now your engine is burning some power driving the accessories. Doing it your way has two problems:

1) if you're driving everything with the electric motor (including the alternator) then due to nothing being 100% efficient the motor can't spin the alternator hard enough to power itself. You need to understand that all of the power the alternator can put out comes from the engine. This concept completely violates the Laws of Thermodynamics*.

2) if you decide to let the engine keep spinning the alternator and use the electric motor for everything else, then you don't get any benefit because the motor needs as much power to drive the accessories as the engine did, plus now you've got the losses of converting mechanical energy to electrical (via the alternator) and electrical back to mechanical (via the motor.)

now, there are cases where engines have electric water pumps from the factory (e.g. BMW) but that's more so the pump can be driven at variable speeds. That way the engine isn't wasting power pumping water at a rate it doesn't need.

alternator output is demand-based. a "130 amp" alternator is capable of delivering that if the load on the electrical system demands it. if it doesn't, it's putting out less current than the max.

here's the flaw in your thinking. let's say for the sake of argument, it would take a 1 hp electric motor to run all of the accessories including the alternator. 1 hp is 746 watts. But, that electric motor is only about 70% efficient, so it would need roughly 969 watts electrical power from the alternator. So it would have to spin the alternator hard enough for it to generate that 969 watts. But the alternator is also only about 70% efficient, so the electric motor would have to be putting out 1260 watts mechanical power (1.7 hp) to get that 969 watts from the alternator.

see how quickly that falls apart?

* The Laws of Thermodynamics, in layman's terms:

1) You can't ever do any better than breaking even
2) You can't break even unless you're at Absolute zero
3) You can't reach Absolute zero.

Siber Express

jz said it better than I could, but basically you will always use more than you can put out.
This is why an Electric car still needs to be plugged in, if what you were thinking would work the whole world would be energy independent.

Choppertwo

Well, being new to the 3.7, I can't help you, but being a pevious owner of a 2.4 eclipse, I can tell you that it definately helped, mostly on top end,I would like to see what you guys think, because I've been thinking about this myself seeing the gains I got from the Mits

WindyMustang

Great explanation jz!

There is ONE cheat - and it's not really a cheat...

For drag racing purposes, you can charge a battery to run the accessory drive via an electric motor. For as long as the battery lasts, all the engine power goes to the drive train. It's a small gain, and comes with a weight penalty, but in some rare instances it's worth it.

Most of the time though, physics says no.