|
|
|
|
|||||
|
||||||
From: Arild Jensen (no email)
Date: Fri Aug 01 2003 - 15:04:17 EDT
Philip Odom asks:
What is the easiest way to accurately measure diesel engine rpm? Strobe?
I have tachs that always read differently and I would like to know which is
correct.
REPLY
If you only want to a spot check of the calibration, the strobe effect of a
fluorescent light is as good as any.
If you want a continuously reading meter which is accurate; that is another
matter.
Ever since someone realized that you can pick off pulses from alternators, the
automotive and marine industry has been using this short cut.
Unfortunately this is anything but accurate. As the belt wears, it slips deeper
into the Vee groove on the pulley and the ratio changes.
If there is any slippage, again the speed appears to change.
And finally in the case of twin engines, it is almost impossible to provide
accurate synchronization.
The only way to accurately measure engine RPM is with a dedicated speed
transducer feeding the meter.
Analog meters ( especially low cost ) use resistors and capacitors to integrate
the pulses into an equivalent DC voltage to drive the needle.
Components like resistors and capacitors change their value with temperature and
as they age. This lead to calibration errors.
Digital tachometers are more accurate if they use a dedicated transducer. If
they convert the analog voltage from integrating the alternator pulses, you
still have a question of ultimate accuracy. While they look pretty and give the
appearance of more accuracy, this can be deceiving.
As for the easiest way to accurately measure RPM, that is a function of how
easy it is to install the requisite transducer.
Tachometer senders can use a reflective tape on the crankshaft pulley.
This is often the easiest as long as you don't have a lot of dirt or oil flung
around the pulley.
That approach doesn't work on a truck where dust and dirt rapidly coat
everything.
Some senders require the addition of a small magnet to the flywheel or
crankshaft pulley.
The better quality tachometers use a pickup coil mounted next to the flywheel
and it counts the number of teeth passing by.
This requires a more sophisticated meter than can be programmed to divide by the
number of teeth on the ring gear.
For better resolution at low speeds you need more pulses per revolution. Four
reflective marks is better than one.
Many teeth are better than one magnet.
The reason why alternators are so popular as tach senders is they have six
poles per revolution and the alternator spins much faster than the engine due to
the pulley ratio. Mind you, the ratio changes as belt wears and if you change
to an after market alternator the ratio is dependent on what pulley size you
use.
--- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.423 / Virus Database: 238 - Release Date: 11/25/2002 _______________________________________________ http://lists.samurai.com/mailman/listinfo/trawler-world-list To Unsubscribe send email to Include the word "Unsubscribe" (and nothing else) in the subject or body of the message.
|