Electronics > Projects, Designs, and Technical Stuff

Capacitive rpm meter

<< < (3/3)

TheRevva:

--- Quote from: Zad on May 11, 2013, 09:40:22 pm ---You could alternatively take the pickup from the low tension side of the coil.
--- End quote ---
The OP mentioned it's a 2-stroke, and thus you'd probably find it's a magneto-style system.  (The LT side of the coil is slightly 'uglier' than a traditional ignition system, but not by much)
Having said that, if the engine is a single cylinder, or if it uses a single coil per cylinder with completely independent magneto inputs, then this is probably a god plan!

TheRevva:

--- Quote from: SeanB on May 11, 2013, 09:49:32 am ---Not DC, a short pulse every revolution, so probably only 2kW of power needed. Remember only 2 fire at any time, and there are only 4 firing pulses per revolution of the crankshaft. The 50kV is needed to fire the gap, but the arc discharge that it creates is only around 400V at peak combustion pressure, so the power dissipated is much lower. There probably will be as much power dissipated in the coil as in the combustion chamber from the spark, with probably 10% being done by the plug wiring and the distributor.

--- End quote ---
Pretty close to the money there Sean.  The magnetos do NOT suck 7000 HP from the engine!  (The engines _ONLY_ develop about 8000-10000 HP in total...  Hang on, did I just say ONLY??? Gulp!)
The only minor correction is that they don't fire 2 magnetos at exactly the same time.  Yes, there ARE 2 plugs per cylinder, but the twin magnetos are phased to give a degree or two of timing between the 2 plugs on any given cylinder firing.
Here's some other 'trivial facts' on these nitromethane gulping monsters...
The supercharger sitting up top needs more power to drive it than a typical Nascar engine produces in total!
The nitormethane fuel that is injected into these beasts is almost compressed into a SOLID rather than the liquid it starts out at.
The engine only rotates a few hundred revolutions during it's run down the quarter mile, but it requires an entire rebuild / inspection after every run.
Several of the clutch plates (and there's MANY of them) end up 'welded together' by the end of each pass.
Assuming nothing major goes wrong, each pass costs several thousand dollars just in 'routine maintenance'.
Given the sheer power produced by these engines, the crankshaft is intentionally 'skewed' to account for the amount that it 'twists' when under full power.

It's totally INSANE (which is probably why I like it so much?)

NiHaoMike:
And if NASA were to design it, they would just put a rocket engine in the back and call it done...

Circlotron:

--- Quote from: TheRevva on May 09, 2013, 03:39:41 am ---(those 8000+ horsepower engines that consume nitromethane as fuel)...
These puppies use a pair of magnetos to set the fuel mixture alight...  But here's the kicker...  Each of these magnetos is generating about 50kV at somewhere near 44 AMPS!!! 

--- End quote ---
Very old thread that caught my eye.
I'd say you are talking about the MSD ProMag 44. They are indeed rated at 44 amps output, but this feeds into a 35:1 step-up ignition coil to get the 50KV and as a consequence the spark plugs get fed "only..." 1.25 amps.

Navigation

[0] Message Index

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod