Introduction: Rev Limiter Flame Throwing

About: diy electronics and car related projects

Hey guys and welcome

In today's project we r going to build a rev limiter from scratch

Step 1: The Theory Behind It

In case you do not know rev limiter is the device that cut the fuel or spark to the engine to stop it from revving at certain rpm.

All cars come with built in rev limiter in the car computer that is set to a certain rpm by the manufacturer. And almost all of them cut the fuel not the spark which does make sense.

for more info :

Step 2: About the Project

when it comes to cars modification it is more fun to cut the spark and keep the fuel goes so we can have a good amount of fuel going out of the exhaust which makes a nice flames.

So the goal is to build a rev limiter that cuts the ignition spark , with the ability to control the time delay between cuts which is known as a gain, in that way we will have a nice bang bang from the exhaust with the flames.

and by adding a spark plug at the muffler we can ignite the fuel to make flames.

After all the theory being said let's now move on to work

Step 3: The Ignition Spark

most of the cars that throw flames the cause of the flame is the excessive amount of fuel coming out from the muffler and it burns coz of the high temperature in the exhaust pipes , I'm not sure whether my car will reach that hot temperature which will ignite the fuel , so what I've done is installing a spark plug at the end of the muffler to do the job. I build frequency generator using 555 timer that controls the current via mosfet passing through a coil that I installed.

Step 4: Building the Limiter

As u know the way that car computers get the engine rpm is by the crankshaft sensor , for example the crankshaft sensor generates a pulse every one revolution , so by knowing the number of pulses in a known time the rpm can be calculated.

As u can see in the video I did connect the shaft position sensor ( coz my car doesn't have crank sensor but it ia the same thing) to the oscilloscope The signal is clear as I explained , and the frequency increases by the increase of engine speed.

So now we have our input to the rev limiter. And the goal is to make a specific action which is stop the ignition signal at a specific frequency or rpm.

And to make it work and simple , we will convert the frequency to a voltage in that way we can set the threshold easily by setting the potentiometer to make a voltage divider. And the best way to do this conversion is by using the lm2907/2917 ic , that does the job we need.

The lm2907 design was taken from it is data sheet which explain every thing about it
At the input we have our signal from the crank sensor connected to a dc blocking capacitor to get red of any dc offset and low frequencies , and with a pull down resistor which of course prevents any false triggers or noise ,

The rest of the circuit is followed by the data sheet , and there is equations in the data sheet to calculate the value of F and R

By building the circuit on a breadboard and I will use an arduino just to emulate the crank signal so I can test the circuit first.

As u can see in the video how does the voltage increases by the increasing of the frequency

At the other side of the circuit where is action will be done , we will use transistor

Now in the next step i will make a voltage divider of one resistor and one potentiometer at the output signal.

The reason of that is we need to active the npn transistor at certain adjustable rpm , and we know that the current will flow at base emitter starting at the volt around 0.6 0.7 v

Until this point we converted the freq to voltage and set adjustable threshold that turns on and off the npn transistor.

Step 5: The Action

Now we want to to switching our ignition coils off and for doing that there two ways , first way us by grounding the coil signal which will stop the coil from charging and will generate a spark by this way the spark would occur at anytime regardless of the piston position , and you know guys how bad is it if the spark occured at the incorrect time , it will damage the engine,

The other way for doing it is by supplying the ignition signal with 5v and in this way the ignition coil will keep charging and held the charge so there is no spark going to occur until the ignition signal goes back to normal and the spark will occur normally again .

And to control the time delay or gain , i will use simple monostable 555 timer circuit with potentiometer to control the time , so the signal from the first npn will trig the 555 timer on and the 555 timer will send high 5v signal to the ignition by the delay time

At this way when the engine reaches the rpm that we set , the 555 timer will stop the ignition for sat time , so the rpm goes down , again , and after the time finish the whole cycle will repeat and repeat..

and don't forget the switch on the spark plug that we have mounted at the end of the muffler to ignite the fuel.

and above is the full circuit schematic.

Step 6: Results

you can watch the end of the video to see the result , I kept playing with the rpm threshold and gain (time delay) so I got different results

I hope everything is clear for you guys.

thank you !!