RPM activated switch to save motor from nitrous blow up

[MalcolmV8]

Hey guys,

Got an interesting one. A buddy of mine has a sport bike that he runs nitrous on. The problem is if you hit the engine limiter while spraying it does very bad things like blow up the motor and cost a ton of money and down time. So I was thinking of maybe building some sort of RPM activated switch to interupt the nitrous circuit right before the engine limiter is hit.

It's a 4 cylinder 4 stroke motor so it will have 2 tach pulses per crank shaft revolution. He says the limiter activates at 14500 rpm so we'll make the RPM activated switch happen at 14000rpm which is 28000 tach pulses per minute which is 466.66 pulses a second.
So is it possible to build such a circuit? Something that would be activated at 466hz or higher? I have no clue how to go about something like that. I assume some sort of frequency counter circuit?

Thanks for any pointers on a circuit to build such a thing, if possible.

Thanks
Malcolm

 

[audioguru]

Hi Malcolm,
The datasheet for the LM2907-LM2917 shows exactly what you want called a "speed switch". The IC is available in an 8-pins and in a 14-pins packages which have different circuits inside. You must select which IC has an input that matches the tachometer's sensor or points. The LM2917 has a zener diode voltage regulator inside.

 

[MalcolmV8]

Where do I find these data sheets? does it tell you how to wire things up? I'm quite the newbie with electronics.

Thanks
Malcolm

 

[audioguru]

Where do I find these data sheets? does it tell you how to wire things up?

I get datasheets from www.datasheetarchive.com but Google can also link to the manufacturer's intro page which has a link to download the datasheet and application notes.
They have schematics and formulas for different conditions.

 

[stevez]

A few comments/thoughts:

A. The difference between 14,000 and 14,500 isn't much. You'll want to make sure the time base that your limiter is using is not significantly different than anything you use to measure RPM with. My guess is that you don't quite know what your margin ought to be - but if the error puts the nitrous cutoff above the danger point you've lost the protection you wanted.

B. What does the limiter do to limit RPM? You might be able to piggy-back on that - possibly as a backup to your other shutdown device. If it interrupted ignition pulses you might look for those interruptions to trigger a solenoid to "off" or "close".

C. This is probably not a good way to learn electroics - given that some outcomes are expensive and possibly dangerous. With that said, many of us learned electronics because we had some special project or interests in mind.

Good luck.

 

[Leftyretro]

Your application is pretty straight forward and there are many commercial products available you might consider using/modifying before building from scratch. Even a add on tachometer with an adjustable kill set-point could be used, but instead of killing the ignition system have it kill the Nox injection system. The hardest problem will be in interfacing with the Nox system. Does it already have an electrical cutoff solenoid/valve? If not you will have to find one compatible with the gas and it's operating pressure and flow capacity.

Good luck and keep us posted.

Lefty

 

[MalcolmV8]

A few comments/thoughts:

A. The difference between 14,000 and 14,500 isn't much. You'll want to make sure the time base that your limiter is using is not significantly different than anything you use to measure RPM with. My guess is that you don't quite know what your margin ought to be - but if the error puts the nitrous cutoff above the danger point you've lost the protection you wanted.

Yes my cut off of 14000k is a guess at this point. We would probably set it somewhere around 8 or 10k first and experiment making sure it works and see what delays there are. We would then slowly move up to the cut off that works best for him.

B. What does the limiter do to limit RPM? You might be able to piggy-back on that - possibly as a backup to your other shutdown device. If it interrupted ignition pulses you might look for those interruptions to trigger a solenoid to "off" or "close".

The factory limiter is built into the engine computer and it turns of fuel injectors for what seems to be every other cycle. This creates a very lean condition and hence the motor destroying itself. There is no simple way to tap into that and at that point its to late, we need to shut off the nitrous before this condition occurs.

C. This is probably not a good way to learn electroics - given that some outcomes are expensive and possibly dangerous. With that said, many of us learned electronics because we had some special project or interests in mind.

Well this is not my absolute introduction to electronics, I've played around with it over the years, I just lack the skills to design something from scratch. Once a circuit is build I can usually figure out what's going on or with a some pointers at least understand the circuit.

 

[audioguru]

The max speed governor on my car's engine works only when I am passing a slower vehicle on a 2-lane road and a big truck is coming towards me.
They say it is for tire safety. Maybe I should slow down.

 

[MalcolmV8]

Your application is pretty straight forward and there are many commercial products available you might consider using/modifying before building from scratch.

Yes there are commercial products that do this but they are expensive and we like a challenge and I enjoy electronics.

The hardest problem will be in interfacing with the Nox system. Does it already have an electrical cutoff solenoid/valve? If not you will have to find one compatible with the gas and it's operating pressure and flow capacity.

Yes luckily it does. In fact a solenoid is activated when he's spraying and all we need to do is interrupt that circuit with a relay or similar.

 

[MalcolmV8]

The max speed governor on my car's engine works only when I am passing a slower vehicle on a 2-lane road and a big truck is coming towards me.
They say it is for tire safety. Maybe I should slow down.

Or bypass governor and see how tires hold out No not really. Just joking.

I looked up that data sheet and see the basic IC. I have no idea how to incorporate that in a circuit from scratch or how to set the trigger frequency to activate the relay. Are there any sample circuits I can look at? if someone has built something like this before?

Thanks

 

[audioguru]

My son was driving on the highway and a front wheel came off. He was lucky that the car scraped straight to a stop and nobody hit him. There was nowhere to park.
Guess who installed a wheel that day for the very first time? (It wasn't me)

Guess who retrieved the wheel from the very busy highway? I nearly got killed.

 

[MalcolmV8]

Yes freeway side repairs are very dangerous. If I ever break down on the side of the highway its worth the tow bill to me. I refuse to work on the side of a highway and get smacked by a semi.

 

[nickelflippr]

The LM18200 datasheet shows yet another speed switch circuit. Its interesting how good a motor runs right before they blow up from a lean condition, at least on a two stroke.

 

[Nigel Goodwin]

The LM18200 datasheet shows yet another speed switch circuit. Its interesting how good a motor runs right before they blow up from a lean condition, at least on a two stroke.

My first road bike was a 1964 Royal Enfield 250cc, it used a Villiers 4T two stroke twin engine. Now these engines tended to be used for dragster racing, but never road racing - because while they performed quite well for their time, the engine cooling wasn't good enough. Under sustained high revs the pistons tended to expand faster then the barrels, and all of a sudden the engine would rapidly slow down and stop. Leave it ten minutes, and it was fine again - I had this happen to me a number of times

One nasty occurance that did happen, I was changing gear from 3rd to 4th, at a fair number of revs (the impetuousness of youth!) and the gear selector must have broken. So it apparently stayed in 3rd, and went into 4th, at the same time - oops! - big pile of broken bits in the bottom of the gearbox!

Used to love my two strokes!.

 

[nickelflippr]

So it apparently stayed in 3rd, and went into 4th, at the same time - oops! - big pile of broken bits in the bottom of the gearbox!

Ouch!

A friend of mine obtained a racing cart chassis, and put a little two stroke 80cc motor on it. He invited me and another guy to test it out on a deserted piece of blacktop. After he ran it thru the gears he realized that the carburetor main jet was a size or two too small. He suggested short shifting, as it definetely went into overrev when it started to lean out. I grasped the situation after a short test, but the other guy didn't, and melted the piston from the overheating!

 

[MalcolmV8]

Sounds like you guys know all to well what happens when the motor leans out. With nitrous it is bad

So can anyone help me build a circuit? I really lack the skills to design one from scratch but I can certainly build the circuit/board etc.

Thanks guys
Malcolm

 

[Nigel Goodwin]

There have been two links to datasheets that specifically show what you want.

 

[MalcolmV8]

There have been two links to datasheets that specifically show what you want.

I need more help than that, sorry I'm no good at designing circuits. How do I make those circuits trigger at 14000rpm?

 

[Nigel Goodwin]

The chips output a voltage depending on the speed, you compare that with a fixed voltage (which you adjust for what you want) using a comparator.

The LM series chips even have the comparator built-in.

Check

https://www.electro-tech-online.com/custompdfs/2007/09/AN-162.pdf

figure 9 for a speed switch, and figure 14 for connecting to the ignition circuit.

 

[power58]

I have looked over an MSD soft touch rev limiter box and it has an LM2917 F/V converter as it's main IC. They use resistors to set the RPM limet. Nitrous solenoids take time to close and open so a 500 RPM band is not much as pointed out before. you could also buy a shift light and use the lamp output to shut off the N2O, they use LM2917 IC also.