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  #16  
Old 02-08-2006, 11:32 PM
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Quote:
Originally Posted by Skippy
This frequency is directly proportional to the rotating speed of the alternator which in turn is directly proportional to the crankshaft speed. The only issues with this method of measuring engine rpm are that you have to know the ratio of the crank pulley to the alternator pulley

skippy is smart.

On the little diesel on my boat the ratio was approx one turn of the crank to 3.2 turns on the alt. There is a setting to adjust on the marine tachometer. (its prolly closer to 4 or 5 on the benz.)

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  #17  
Old 02-09-2006, 12:37 PM
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OK, I can see filtering out the AC pulses from the alternator and correlating that to crankshaft RPM. But they would have to know the pulley diameters of the crank and alternator. Maybe they have a database of all model diesels with such info?

Quote:
Originally Posted by Skippy
To expound on what SD Blue said, an automotive alternator is a (typically 3-phase) AC generator that has its output rectified to DC. They use diodes to do this, and each diode is essentially a half-wave rectifier. The output current is direct in that it stays on the same side of zero all the time, but it has sinusoidal changes in voltage that have a measurable frequency. This frequency is directly proportional to the rotating speed of the alternator which in turn is directly proportional to the crankshaft speed. The only issues with this method of measuring engine rpm are that you have to know the ratio of the crank pulley to the alternator pulley (E-test shop probably has a one-size-fits-most "approximate" value that they use for most vehicles), and the fact that electrical noise from other electrical system components can make the alternator output frequency hard to measure.

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