Alternator To Crankshaft Ratio

[SydneyRoverP6B]

According to the workshop manual, both the 11AC and 18ACR alternator as fitted to the V8 engine have a nominal output of 45 Amps @ 6000 alternator rpm.

So what is the ratio of alternator rpm to engine rpm? In other words, how many times does the alternator spin for one revolution of the crankshaft?

Ron.

 

[westOz74P6B]

simple matter of dividing alternator pulley diameter (78mm) into crankshaft pulley diameter (154mm) which gives you 1.97 so ratio is 1.97:1, basically not far off 2:1 for rpm,
Regards,
Scott

 

[SydneyRoverP6B]

Great, thanks Scott.. Just a matter of division,...how good is that!

The output current won't follow a linear relationship, so I image if it were to be observed on a CRO, it would look something like a log curve.

Ron.

 

[darth sidious]

Great, thanks Scott.. Just a matter of division,...how good is that!

The output current won't follow a linear relationship, so I image if it were to be observed on a CRO, it would look something like a log curve.

Ron.

I think it's something like a steep [1 - exp (-x)] curve.

 

[darth sidious]

simple matter of dividing alternator pulley diameter (78mm) into crankshaft pulley diameter (154mm) which gives you 1.97 so ratio is 1.97:1, basically not far off 2:1 for rpm,
Regards,
Scott

Ah, that explains why they specify testing the alternator output voltages with hardly any load, and then repeat with a hefty load, both with the engine running at 3000rpm.

 

[SydneyRoverP6B]

darth sidious wrote,...

I think it's something like a steep [1 - exp (-x)] curve.

That would climb very quickly indeed. You would need to use a multiplication factor of say 1000 for the independent variable x (where x is alternator rpm) otherwise the corresponding current values would be too inaccurate.

So approx 3000 engine rpm will see close to maximum current delivery. After that figure, output gains would be very minimal indeed.

Ron.

 

[testrider]

What's the maximum RPM for the alternator then? Perhaps it migh be worth gearing up the alternator to spin faster at a lower engine RPM to avoid too much battery drain in traffic?

 

[DaveHerns]

I think the problem is that the ACR range don't produce power at tickover speeds whereas later ones do . I think alternators from Sierras and Capris fit and produce more power at all engine speeds

 

[darth sidious]

darth sidious wrote,...

I think it's something like a steep [1 - exp (-x)] curve.

That would climb very quickly indeed. You would need to use a multiplication factor of say 1000 for the independent variable x (where x is alternator rpm) otherwise the corresponding current values would be too inaccurate.

So approx 3000 engine rpm will see close to maximum current delivery. After that figure, output gains would be very minimal indeed.

Ron.

I'm not too sure of the range of x, I just think it's sort-of that shape of curve! Don't hold me to it though!