[Healeys] electrc fan vs. belt drive

[Bob Spidell]

This got me to thinking (always a dangerous proposition) ... 

Aren't we forgetting about the Law of Conservation of Energy? (http://en.wikipedia.org/wiki/Conservation_of_energy). If I read the law correctly (I'm not a lawyer) if it takes X HP to drive the water/air through/over the engine you'll use X HP from a) the engine, for a mechanical pump or fan or b) the alternator for an electric pump/fan. Assuming little or no slippage from the belt, the engine-driven pump/fan will be operating at near 100% efficiency (minus the fluid 'slippage' at the fan blade or pump impeller--essentially the same for both electrical or engine-driven units). I don't know the actual number, but alternators are probably around 80-90% efficient at converting mechanical energy to electrical (by rectifying the AC output to DC you're losing some of the electrical power, and some goes up as heat and there's always friction). 

IOW, if you convert X HP to watts, you'll increase the load on the alternator by (X HP->Y Watts), and the alternator will produce the additional watts by converting more engine HP to electricity. 

The advantage of electrically-driven accessories--it seems to me--is the ability to reduce or shut off the motor power when not needed. Yes, you can run them off the battery, but usually you'll be running off the alternator. 


Bob 



----- Original Message ----- 
From: "Eric (Rick) Wilkins" <e-wilkins at cox.net> 
To: "Oudesluys" <coudesluijs at chello.nl> 
Cc: "healeys" <healeys at autox.team.net> 
Sent: Wednesday, February 4, 2009 9:48:55 AM GMT -08:00 US/Canada Pacific 
Subject: Re: [Healeys] electrc fan vs. belt drive 

Not that simple math. 

There is the efficiency of each item to be considered. You can run 
the water pump from a battery for maximum HP, but an alternator may/ 
may not use more mechanical energy than your belt driven pump. Also, 
once the pump is removed you are reducing the engine from driving an 
alternator AND a pump, to just a an alternator. 

Wilko 

On Feb 3, 2009, at 2:20 AM, Oudesluys wrote: 

> A water pump and a mechanical fan need a fair amount of power to be 
> driven. They are not perpetu|m mobilae, so that power needs to come 
> from somewhere: ultimately the engine, and it does not make any 
> difference if that is through the alternator/dynamo and battery. So 
> the power needed for the alternator that will feed the battery and 
> powers the water pump will not be available to drive the wheels. 
> Simple mathematics. 
> Kees Oudesluijs