Typical serpentine "belt horsepower"?

[lasitter]

I've always known that turning the belt takes power, without regard to the particular accessory being powered. But how much?

At its most simple, if you could replace everything but the crank pulley with an idler or a tensioner so that no load were being driven, how many horsepower would it take to turn the belt at X,000 RPM (pick a number).

[IsaacCarlson]

I think you are looking at less than 2 hp at the most.

[redpoint5]

Not even 1hp for x,000 RPM would be my guess. My only frame of reference is a 1hp electric car I built in high school that could probably do 50mph if given the right gearing and a long enough straight. I got it up to 43mph around an oval 1/4 mile track, but tire grip was the limiting factor.

1hp is a lot of power.

[lasitter]

I had seen people talk about 5-7 belt horsepower, but I guess it would vary based upon the number of pulleys, turns, etc.

[GRU]

it depends on what the belt is driving. If the belt is just attached to (let's say) alternator, but there is NO load, then the amount of power it's taking is very small

Alternator, with all accessories ON is probably robbing about 2 hp on an average car

A/C, when ON, probably 10 hp on most cars. I know on my little civic the A/C drops my MPG by about 12% at 60 mph steady speed. On a Lincoln MKX I drove, the A/C dropped the MPG by about 9% on 60mph

[drmiller100]

the belt itself is well under 1 horsepower.

one way to measure it is to feel the heat in the belt. The belt "robs" horsepower, and turns it to heat. A serpentine belt is VERY cool to the touch.

If you try to bend the belt into too small of a diamater, it takes more power.

Once upon a time I ran a supercharger I built on a 2 stroke. The engine had an 8 inch diamater pulley, and the supercharger had a 1 inch diameter pulley.

The ultimate speed of the belt is limited by feet per second, not RPM. at 8000 rpm on an 8 inch pulley I was WAAAAAAAAAAAY past the design spec of the belt.

They got HOT very quickly, and blew VERY frequently.

[lasitter]

Originally Posted by drmiller100

one way to measure it is to feel the heat in the belt. The belt "robs" horsepower, and turns it to heat. A serpentine belt is VERY cool to the touch.

Don't forget the heat transfer to the pulleys and the effect of air cooling from a moving belt ...

[redpoint5]

Originally Posted by GRU

it depends on what the belt is driving...

The OP's theoretical load is just the belt and idle pulleys; no accessories.

Originally Posted by drmiller100

one way to measure it is to feel the heat in the belt. The belt "robs" horsepower, and turns it to heat. A serpentine belt is VERY cool to the touch.

Heat is a great way to imagine how much power is lost in turning the belt. To consume 1hp, the belt and pulleys would have to dissipate 746 watts of heat. Imagine a portable electric space heater running at half power. That's 1hp.

[gone-ot]

ANALOGY -- rubber belt on pulley as to rubber tire on asphalt.