[TR] Alternator Output vs RPMs

[Dave1massey at cs.com]

In a message dated 9/15/2008 1:58:07 PM Central Daylight Time, 
75TR6 at tr6.danielsonfamily.org writes: 
> I'm having a discussion with a TR6 friend who has a Delco 55 amp alternator
> on his car like I do. He says that every time his Perma-Cool electric fan
> comes on (he doesn't have a mechanical fan) he sees a 400 RPM drop (stock
> mechanical tach) in engine speed. His reasoning is that when an accessory
> calls for power the alternator has to work harder and, in turn, the engine
> works harder. 
> 
> I thought that an alternator's output was fairly constant and had no impact
> on engine RPMs and vice versa. I just went out and let my car settle into a
> steady idle with the volt gauge reading a steady 14.x with just the electric
> fuel pump running. Raising the RPMs the had no impact on the gauge reading.
> I turned on the headlights, electric fan, heater fan, radio and electric
> fuel pump and saw the volt gauge only drop to 13.x but there was no impact
> on RPMs. Raising the RPMs with that full load had no impact on the volt
> gauge. This is what I'd expect to see happen, but I'm no expert. Does it
> make sense that his RPMs would drop when his fan comes on?
> 
> 
Your friend is right.  The alternator (or a generator in general) converts 
mechanical energy to electrical energy.  If you draw more electrical energy it 
will need more mechanical energy.  That means torque.  Hence the speed will 
drop.  Haven't you ever ridden one of those bicycles connected to a generator 
connected to some lightbulbs?  No lightbulbs easy pedaling.  One lightbulb, more 
difficult pedaling.  Three or for and I can't keep up.

The voltage will drop also showing that the windings have a finite resistance 
and current flow will cause a voltage to develop along their length.

So, why doesn't your car's RPM drop?  As I recall you have a modern 
electronic fuel injection system.  I don't know all the details but it is common with 
these FI systems to modulate an idle control valve to maintain the idle speed 
at a desired level.  Other methods of regulating idle speed are also used.  
These include adjusting the spark advance.  This is all very effective and can 
mask the additional load applied to the motor by the charging system.

Most of the TR's built during the 70's and 80's also have a spark retard 
module on the distributor.  This has the effect of regulating the idle speed.  The 
way this works is the spark is retarded by the vacuum generated by the idling 
engine.  As the timing retards the engine speed drops causing the vacuum to 
drop reducing the retard.  An engine so equipped will reduce the effect of 
additional mechanical load due to the alternator loading.  But it is common for 
the plumbing to be removed leading to this phenomenon.

Dave