Forum

[PeteF]

I like people who think sideways but I don't think this is will work. The chances of you picking up a signal from a magnet which is inside the piston are nil IMO.
You would have more luck with a magnet on the flywheel. If you machined a pocket in the flywheel to set the magnet in it would have a good chance of staying put and it would be easier to get a sensor somewhere near it.
Modern engines use flywheel sensors like this I think - perhaps you could adapt something.

[Bullet Whisperer]

From my own experiences with lightening flywheels, swapping pistons and conrods etc, I would say as long as the balance factor is 'there or thereabouts', that is near enough for the Bullet engines. What has far greater influence on vibration is good alignment of the crank mainshafts for smooth running in the main bearings and long main bearing life. I have seen terrible run out on some cranks I have removed from 'running' engines - the worst was 1.75mm !!! More than a few thou is not ideal ! Regards, Paul.

[jefrs]

The combustion chamber temperature is very high and subject to explosive pressures. When the magnet comes off it will destroy the piston, the head and the valves in the process. When not if.

[jefrs]

Back of envelope - with the engine running at 1200rpm (an 'easy' number) the engine makes 20 revs per second, which is 40 strokes per second. Stroke is 90mm so 90x40 = 3,600mm/s or 3.6 metres per second. At 4800rpm then 14.4 metres per second or 32mph. Colliding with a flying magnet is going to make a mess.

[jfw]

Brilliant responses


I think one or two of you thought that the magnet would be in the combustion chamber, which was not the idea. Gudgeon pin is a great idea. I like to think that bonding technologies have moved on since boiling down horses for glue.


I do question whether you'd pick up any signal through all the metal, you'd certainly require a bit of signal processing, if you'd detect anything at all. As for 1200 RPM that makes for higher induced current in a sensor but don't mean it'll be high enough to be detectable.


Whilst the crank would be possible with a Hall effect sensor that in my head would be more messy. I'll leave it as a thought experiment for the moment. Loads of other stuff to do but it'll probably sit in the back of my head for a while.

[jefrs]

I only used "1200rpm" because it divides easily by 60. Tickover is nominal 1050rpm and max revs approx 5500rpm. The piston speed is average, and because the thing stops twice on each stroke (top and bottom), peak velocity is higher. There's a circular motion involved but for a crude estimate we don't need to work out sine functions. The piston moves pretty fast.



Magnetism is called a "weak force" for good reason and it follows something like an inverse cube rule (actually very complicated). In other words the sensor for the magnet has to be very, very close to the magnet - where it would get crushed by flying piston.



Most cars and the EFI do have a crank sensors, some cars have sensors on the cam shafts. Technically the Bullet does have twin-cams lifting the push rods. The rocker shaft might be a good place for a sensor. But things in the motor move very fast with a lot of force.



So do remember Murphy's Law - anything that can come loose will come loose, and its extension, Sod's Law, says it will do very expensive damage.

[Adrian]

Thinking about it, you surely don't want magnetized rings or gudgeon pin, they'll only attract fine ferrous debris from the bore, when this stuff should be scraped off and picked up by whatever sump magnet and/or oil filtration you have in use.



If you really want to replace the points ignition with something magnetically triggered with no extra moving parts, our hosts sell the crank-mounted PVL CDI ingition kits. These replace the alternator on a classic Bullet, but the PVL versions are ignition only. If you want crank-mounted CDI ignition WITH some lighting coils thrown in, the Electrex World STK 100D will fit the classic Bullet.



http://www.electrexworld.co.uk/acatalog ... d_350.html



A.

[Tim NZ]

Heath-Robinson has nothing on this! I have not laughed so much for a long time...

[Nettshubby]

I recently contacted our hosts regarding crank bellance if I converted to 535cc, or fitted a higher comp piston. Their reply was that a re ballance is not necessary. So it seems there or thereabouts is the rule!

[Nettshubby]

Nice one Tim! Certainly brightened my wife's and my morning coffee!