Alternator Driven Tachometer

[Byeckerslike]

Hi All,

 

I am mystified as to how an alternator driven tachometer is calibrated, for the alternator's RPM is purely dependant on the relationship of the crank and alternator pulley.

 

I assume that there must be a way of calibrating the tacho, but I can see nothing obvious to adjust.

 

 

Many thanks in anticipation!

 

 

Rob

[Chris Lingwood]

On my vdo tacho there is a little hole in the back with a tiny little plastic adjustment screw. Try shining a torch in all the holes see if there's something in there.

 

Also the ratio of crank to alternator is normally 2:1 I believe. Yours might just be fixed to that ratio, but its unlikely I would have thought.

[MoominPapa]

On my vdo tacho there is a little hole in the back with a tiny little plastic adjustment screw. Try shining a torch in all the holes see if there's something in there.

 

Also the ratio of crank to alternator is normally 2:1 I believe. Yours might just be fixed to that ratio, but its unlikely I would have thought.

The VDO tach I'm planning to use has a combination of switches to set a division ratio, and a trimmer for fine adjustment, according to the manual I downloaded.

 

Of course to do the twiddling requires _another_ way to measure the engine speed. Those shine-a-laser-on-the-flywheel tachos would be ideal, but I don't have one. A cunning plan I'm thinking of trying, (not done yet, so no idea if it will work) is to record the engine sound on the laptop. On my thumper, the ignition events should be blindingly obvious on a waveform display using something like Audacity, and I can just measure the time between them off the X-axis and do the maths.

 

It's either so cunning, you could put a tail on it and call it a weasel, or a mad scheme that will never work.

 

MP.

[paulcatchpole]

The VDO tach I'm planning to use has a combination of switches to set a division ratio, and a trimmer for fine adjustment, according to the manual I downloaded.

 

Of course to do the twiddling requires _another_ way to measure the engine speed. Those shine-a-laser-on-the-flywheel tachos would be ideal, but I don't have one. A cunning plan I'm thinking of trying, (not done yet, so no idea if it will work) is to record the engine sound on the laptop. On my thumper, the ignition events should be blindingly obvious on a waveform display using something like Audacity, and I can just measure the time between them off the X-axis and do the maths.

 

It's either so cunning, you could put a tail on it and call it a weasel, or a mad scheme that will never work.

 

MP.

 

Sounds like a fine idea to me!

 

There's a website out there somewhere with software to do petrol engine analysis from sound files recorded from an inductive pickup - just a wire wound around the HT lead, recorded with a laptop sound card mic connection.

 

PC

[MoominPapa]

Sounds like a fine idea to me!

 

There's a website out there somewhere with software to do petrol engine analysis from sound files recorded from an inductive pickup - just a wire wound around the HT lead, recorded with a laptop sound card mic connection.

 

PC

 

Hmm, computers are great at doing signal analysis, but I'm not sure I'd like to put the contents of a petrol engine ignition system anywhere near my computer, except through a very carefully designed interface, preferably including opto-isolators. Them chips are not known for liking kilo-volts up 'em.

 

MP.

[paulcatchpole]

Hmm, computers are great at doing signal analysis, but I'm not sure I'd like to put the contents of a petrol engine ignition system anywhere near my computer, except through a very carefully designed interface, preferably including opto-isolators. Them chips are not known for liking kilo-volts up 'em.

 

MP.

 

I'd be happy with the air-gap interface to be honest, despite the high HT potential, it's only going to induce a current in the pickup when it's cutting the flux, as it were - so when the HT spike is rising or falling - and a handful of turns of bell wire isn't going to be a very effective pickup.

 

Ah well, maybe I should try it and see? :-)

 

PC

[alan_fincher]

As has been said before, if you can mount a small magnet on any part of the flywheel or drive-train, then a cheap, (less than 10 quid) cyclocomputer can easily be programmed to measure RPM, (by choice of wheel size).

 

Obviously if you measure RPM of the prop-shaft, then you need to factor it up by the reduction of you gearbox to arrive at engine RPM.

 

This can be used to accurately calibrate an alternator sensed rev-counter.

[paulcatchpole]

Whilst looking for the sound-card-dyno software, I spotted this - a model airoplane RPM counter - uses a photocell to sense light/dark/light from the prop - might be useful for something...

 

http://www3.towerhobbies.com/cgi-bin/wti00...LXPT31&P=ML

 

The two projects I was thinking of were StreetDyno and HomeDyno but one seems to have disappeared and the other become commercial. Oh well!

 

PC