Engine tach

[one_dusty_hoot]

steve,
sorry for the lower case, shoulder operation yesterday.

WARNING! LONG POST

did you make it to bristol? would you say jarrett was a
bit ticked?

first, i would check with cummings to see where they
mount a sender. i would guess they have a place
somewhere. probably a reluctance mount maybe on a
timeing gear, somewhere that has gear teeth, then by
adjusting the air gap to specifications a good pulse
to supply a tachometer can be obtained. they may also
have a tach spec that would mate the pick-up.

if not, then gary's suggestion about using an alternator
with tach output. you will need to know drive and driven
pulley sizes to set up the tach. i believe the alternator
has dip switches that you set according to the ratio of
the two pulleys.

your indicator, the tach, is an analog device, generally
using a diode to rectify the incoming pulses with a
capacitor and resistor to correlate incoming pulses to
an analog reading, or, using induction coils and a permenant
magnet.

a dc generator pick-up is no good after time mainly due
to variation in output voltage caused by the brush point
of contact with the armature which would very easily get
a film covering in time.

so for accuracy you might limit your search to a digital
to analog tach. you could go digital to digital but unless
you had periodic updating rather than instantaenous readings
the tach would be too busy.

calibration..
unless you wanted to keep your pit road speed as close to,
but not exceed 55 mph, or, do some serious drive line
calculations, i'm not sure that +/- 250 rpm makes that
much difference, until you establish a pattern,then what
you do want however is repeatability.

for greatest accuracy an analog indicator sould be set to
full-scale deflection, and, unless the zero set is way
off i would leave it as you would need to open the housing.
you wouldn't want to run the diesel at that speed anyway,
and the only way this calibration can be accomplished is
to have both sender and tach on a bench set-up.

i have a tach that operates much the same way a tuning
fork is used. it has a calibrated dial that is matched
to the length of a calibrated rod. as the rod is extended
the dial has a pointer indicating a lowerer frequency, and,
vice versa, as in f=1/t. when the vibration from the motor
is equal to the rod length the rod resonates.

i would guess that there are others, maybe better, but the
one i have and use is a briggs and stratton, birometer-vibra
tach, model 19200,$20.87.you want find this doing a google
search, or, on the shelf but if you ask the man where you
buy briggs and stratton parts he can order one for you. i
just called to make sure they were still available, and they
are.

i ue mine mainly to set or check the frequecy of the genset.
where frequency equals number of poles times revolutions per
minute divided by 60 seconds, f=2x1800/60. the motor operates
at 1800 rpm turning a generator that has two poles for each
revolution.

i do this with a load on the motor, the two ac's draw about
13 ampe each leg, or, about 1/2 the load of a 6.5 kw genset.
you can plug in the battery chargers for more or even connect
an 8 ft heating strip, fan, whatever. p=i x e, power equals volts
times current, or, 54.17 amps=6500 watts x 120 volts the max
rating for this generator.

first, after the motor has warmed i apply the load, the ac's.
then setting the dial at 1800 rpm and held firmly to the
cylinder head shroud, 4" connector box, or generator, i rotate
the dial lower or higher slowly till the wand resonates.first
setting the wand to 1800 which in fact corresponds to 60 cycles.
once i find the motor speed i can adjust the motor govenor
speed up or down to the desired 1800 rpm/60 cps. you can
expect the motor to speed up slightly when the load is removed.

now to check the engine tach. hold the tach on a solid surface.
set the wand to 1000, 1200, and 1500 rpm by rotating the dial.
on each setting slowly depress the accelerator pedal
watching the wand, slowly as you can pass the spot easily.
when the wand length is in resonance with the motor vibraton
the wand will vibrate wildly. check the engine tach to see if the
two readings correspond, make note of the difference.

there are frquencies, harmonics, and sub-harmonics or a running
motor. these vibrations will be indicated by the vibrating
wand but harmonics with a lesser intensity. some practice with
observation and you can master the speed testing.

whew! all with one hand. sorry to those who don't like long
posts.

bob janes




--- In WanderlodgeForum@yahoogroups.com, "transit360"
wrote:
> 2 part question:
> What's the best way to install a tach on a typical diesel engine?
I'd
> like to add a dash gauge to mine but am unsure or where to get
them.
> Installation I'm not scared of, just need a source and
recommendation
> on brand.
>
> Where's a good place to get a handheld RPM reader? I need to make
> sure of the RPM range on my 8.3 Cummins and would like to buy a
unit.
> I've seen some that use reflective tape and a laser to read RPM.
> Suggestions?
>
> Thanks a bunch!
> Steve
> 2 '98 Bluebird QBRE Partybuses - 1st official paying trip just
make to
> Bristol Raceway and back this weekend!!!