[Healeys] electrc fan vs. belt drive

[Bob Spidell]

<part 2> 




An analogy is an air conditioning compressor. With A/C off, at idle, the compressor is not pushing refrigerant through the system. Turn the A/C on, and the compressor has to push the refrigerant through the system, the load on the engine increases and the idle speed will drop (modern cars compensate with the idle control valve/solenoid, so usually you don't get the drop). Similarly, when the load on an alter/generator increases, the unit is required to push more electrons through the resistance of the load. If you were turning the alternator by hand, you would feel more and more resistance--more effort required, i.e. more HP--to turn the alternator, at the same speed, as the load increases. Since the power output of an engine is more-or-less fixed, there will be less HP at the driven wheels when you have more electrical accessories running. 

Actually, electrically-driven accessories are LESS efficient than engine-driven accessories. Let's say the alternator is 90% efficient (it probably isn't), and a typical electric motor is 90% efficient (again, probably optimistic). Since the two are in series--and belt friction/slippage and the inefficiencies of the fan or impeller are a wash with mechanically-driven accessories, basically--the efficiency of the two is .9 times .9, or roughly 80%. Hence, the electrically-driven accessories would be roughly 20% less efficient, all else being equal. 

Again, the value of the electrics is the ability to cut or reduce the power to the motors when not needed. Also, you can crank up the fan motor at idle--when the engine is turning slowly and cars are more inclined to overheat--to keep the engine temps down. 


Bob 





----- Original Message ----- 
From: "Bob Johnson" <bjsbj8 at gmail.com> 
To: "Oudesluys" <coudesluijs at chello.nl> 
Cc: "Bob Spidell" <bspidell at comcast.net>, "healeys" <healeys at autox.team.net> 
Sent: Thursday, February 5, 2009 8:24:29 AM GMT -08:00 US/Canada Pacific 
Subject: Re: [Healeys] electrc fan vs. belt drive 


There is an implication here, I believe, that the greater the power requirement, the harder it is to spin the generator/ alternator. Isn't the relationship between the engine and the gen/alt a fixed relationship between the pulleys? And following that, the the regulator controls the amount of electrical output as required to run whatever you are trying to run on your vehicle? I don't know, but it just seems logical to me that the motor turns, the gen/alt turns and the horsepower to do this work would be essentially constant regardless of electrical power requirements. If you take the mechanical water pump out of the system you would save hp requirement for the engine, IOW the engine would spin more easily. Then the electrical water pump, while requiring hp to run the pump motor would be "free" because more electricity would be released by the regulating system. It was always being produced, just not being used. Is this true or false? 

Bob Johnson 
BJ8