Thanks, Bob - always appreciate the opportunity to learn.

 

  It appears from the diagram  that this is a pulse integration type Tach (I
know you know that-just me thinking out loud).  When a pulse is present at
the base of Q1 it conducts - otherwise it's off.  That the on and off
conduction of Q1 corresponding to the ignition pulse rate causes the voltage
dropped across Q1 and therefore C1 to vary according - in part - to the
resistor value selected designated by 4:6:or 8:.  Apparently the 4: 6: and
8: are values referring to the number of cylinder the tach must be initially
set up for.

 

Looking at the diagram, it does not appear to me that this tach diagram is
designed to use  a spark pulse from the ignition wire itself (as some Tachs
are).  The reason I believe this is that if that were the case then the tach
would always get one spark per 720 deg regardless of 4,6 or 8 cylinders.
The selection resistor would indicate to me that this Tach expects the
"traditional" pulse set up.      

On a typical 4 stroke engine using one ignition coil each spark plug is
fired once per 720 deg (Two revolutions).  So if you have one coil providing
all the sparks and the signal is taken from the ignition coil rather than a
single spark plug wire, then the number of ignition pulses per crank
rotation (360) is the number of cylinders divided by 2.  So for the 4
cylinder 2 pulse per rev, for the six cylinder 3 pulses per rev and for the
8 4 pulses per rev. This would seem to correspond to the 3 resistors 4:6:8:
shown on the diagram.   Frequently the tach signal is taken from the
negative lead of the single spark coil.   

That is probably the simplest case and was the case for most auto engines
for decades.    But now we have some engines with one coil for every 2
cylinders instead of a distributor, the coil fires two spark plugs at the
same time, but one is a waste spark.  A waste spark system would then fire
once per crank revolution (360 deg)but sends out two sparks (one wasted) .
Then some distributor-less 4 stroke engines use one ignition coil for every
spark plug, this ignition system fires each coil once for every 2 crank
revolutions. So things have gotten a bit more complex relative to feeding
the Tach.  So between the 3 basic types of Tachs and the numerous different
ignition systems we have to day - it can get to be a mess.  Especially when
documentation may be lacking

 

 

In this case, it would appear resistor 8:  (the highest resistance) is for
the 8 cylinder as it would produce more sparks per unit time than a 4
cylinder for example and would therefore generate a higher voltage for which
needs to be reduced so that the meter reads the same rpm regardless of
number of pulses.

 

So I would presume that the 3 resistor values would then be selected along
with the value of C1 to give a constant rpm (voltage level) regardless of
the number of cylinders and pulses.  This voltage in turn is reflected by
the rpm meter's needle position.  I would appear that the 200 ohm pot is for
making fine adjustments to this voltage driving the meter.

 

At least that is the way it appears to me for an initial quick look.

But, now I have forgotten what the original problem with the Tach was {:>)

 

Ed

 

 

 

Ed Anderson

 

Rv-6A N494BW Rotary Powered

 

Matthews, NC

 

eanderson@carolina.rr.com

 

http://www.andersonee.com

 

http://www.dmack.net/mazda/index.html

 

http://www.flyrotary.com/

 

http://members.cox.net/rogersda/rotary/configs.htm#N494BW

 

http://www.rotaryaviation.com/Rotorhead%20Truth.htm

 

 

-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On
Behalf Of Bob White
Sent: Friday, May 08, 2009 10:52 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: John Downings Tach

 

Hi John,

 

Presumably I calibrated your tach to read one rpm per pulse.  The

attached schematic is similar to your unit.

 

I changed some of the resistor values.  I don't seem to have the actual

values I used in my notes. Your tach also had a diode between the input

and the junction of R2 and R3.  I then connected it to a pulse

generator and adjusted it for 1 pulse per rpm with the 200 ohm pot.

With 100 pulses per second input, the reading should be 6000 rpm.  I

won't rule out the possibility that my calibration was in error. :) One

possible cause of the failure is I may have over done the input

reduction causing Q1 to fail.  There is also a conditioning circuit

between the coil trigger signal and the tach.  It provides isolation

and supplies a constant short pulse width signal.  

John, most of this is for Ed's benefit as I know you aren't an

electronics guy. ;)

 

Bob W.

 

On Fri, 8 May 2009 21:09:56 -0400

"John" <downing.j@sbcglobal.net> wrote:

 

> Jeff, are you saying that with the one lead that the engine is actually > revving twice the reading.  It is no wonder the plane goes over the chocks


> with a reading of 3,000 rpm before the tach quit working.  It actually > starts jumping up and down before it makes the leap and I can get it shut > down, that is why I tie it down good before I start it.  JohnD

> ----- Original Message ----- > From: "Jeff Whaley" <jwhaley@datacast.com>

> To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>

> Sent: Friday, May 08, 2009 12:48 PM

> Subject: [FlyRotary] Re: John Downings Tach

> > > Hi Bob, the unit has 2 pick-ups, so with a 4/4 you need both coil pick-ups


> to get 2 pulses per rev, same as with the 2/2; if you had a 1 cylinder/2 > stroke it would give 1 pulse per rev; therefore the number of cylinders > matter. With the V6, 2 pick-ups also produce 2 pulses per rev, but since > they are occurring a slightly slower rates the frequency to voltage > conversion was not accurate; I changed the R/C filter on the input and > adjusted the internal reference pot to increase the rpm readout ... drove > around for a few weeks with it in my Sonoma to verify calibration with > Sonoma Tach. The rpm max reading is 9999.

> Jeff

> > -----Original Message-----

> From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On > Behalf Of Bob White

> Sent: Friday, May 08, 2009 12:18 PM

> To: Rotary motors in aircraft

> Subject: [FlyRotary] Re: John Downings Tach

> > Hi Jeff,

> > Thanks for the tip.  There is something I don't understand about the

> specs for this unit.  It's good for 4 cylinder 4 stroke or 2 cylinder 2

> stroke.  If I use the spark plug wire for the pickup, a 4 stroke

> (piston) engine will see one spark per two rev.  Why does it matter how

> many pistons there are?  If it's a wasted spark system, then one spark

> per rev. just like a two stroke piston engine.  There must be something

> basic I don't understand.  What sort of mod did it need for the V6?

> > BTW, what is the maximum rpm reading for that unit?

> > Thanks,

> Bob W.

> > On Fri, 8 May 2009 07:48:47 -0700

> Jeff Whaley <jwhaley@datacast.com> wrote:

> > > I like the belt and suspenders!!

> > I am using a UMA digital tachometer 2 1/4" from Aircraft Spruce that was


> > originally designed for 4 cylinder engines using an electrical impulse > > pickup from spark plug leads. I modified it to mate with the Ford V6; I > > have modified it again to connect to the EC2 driving the R1 PRI
injector. > > The rotary firing system is similar to a 4 cylinder so if this tach is > > still available (Part # 10-24885) it would work off spark plug impulse > > couplers.

> > Jeff

> >

> > -----Original Message-----

> > From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On > > Behalf Of Bob White

> > Sent: Friday, May 08, 2009 12:14 AM

> > To: Rotary motors in aircraft

> > Subject: [FlyRotary] John Downings Tach

> >

> > John is using a standard tach that is normally connected to a > > distributor/coil ignition system.  The straight 12V pulse used for the

> > LS1 coils wouldn't activate it.  I built an interface for him that > > involved taking that pulse as an input and generating a shorter constant


> > pulse width output.  I modified the tach to trigger on the lower voltage


> > signal, and adjusted the scale factor to read correctly with one > > pulse/rev.  As John reported, it worked for about 0.8 hours, then quit. > > He is sending everything back to me so that I can find out where the > > problem is.  I would like to find a tach solution that John can use > > without modifying anything.  The parameters are he has a 2 1/16 inch > > mounting hole, the tach needs to read correctly with 1 pulse/rev (would
be > > the same as a two cylinder piston engine with coil and distributor), > > should trigger directly on a 12V pulse.

> >

> > I've found tachs on the web that mention working with distributorless or


> > electronic systems, but none of them were the 2 1/16 inch type.  I found
a > > converter, the MSD 8913, but on the Summit Racing catalog it
specifically > > says it won't work with the LS-1 system.  It work by sensing the current


> > pulses to the 12V power lead on the coil.  I don't know why it wouldn't > > work with an LS-1 coil, but it doesn't seem like a good investment when > > the LS-1 is specifically mentioned as not working.

> >

> > Can anyone recommend a tach solution that John can use with his LS-1
Coil > > rotary engine combination?

> >

> > On another topic, John has the only system I've heard of that has the > > capability to run on either EFI or a carburettor.  John posted a picture


> > of his Tailwind a few days ago.  It's a very good picture but
considering > > the redundancy of the EFI/carb system, I felt that the picture was > > lacking.  I submit this edited version as being more indicative of
John's > > desire for uncompromised safety.

> >

> > Bob W.

> >

> > --

> > N93BD - Rotary Powered BD-4 - http://www.bob-white.com

> > 3.8 Hours Total Time and holding

> > Cables for your rotary installation - http://roblinstores.com/

> >

> > --

> > Homepage:  http://www.flyrotary.com/

> > Archive and UnSub: > > http://mail.lancaironline.net:81/lists/flyrotary/List.html

> >