What is best to get the spark plug signal?

[Marco_Santos]

What is best sensor to get the spark plug signal?

Hi everyone. My name is Marco Santos, i from Portugal and i need some advices.

I'll need to get the signal from the spark plug wire to build a tachometer. The max rpm is 21000 rpm but the system is to work with 2 cycle engine and 4 cycle engine.

All the help is welcome.

Best regards.
Marco Santos

 

[jimmythefool]

I would imagine that getting a magnetic/inductive pick-up from a scrap yard would be a good basis to start from

Regards
Jim

 

[3v0]

I do not know about best but most of the ones I have seen look like this.
I do not have any experience using it. I seem to remember that there is about 8 turns of wire around the coil or spark plug wire in this sort of setup.

R1 is a pullup resistor that keeps the uC input high. When the base of T1 picks up a signal T1 conducts and grounds out the uC input pin.

EDIT: The schematic was collect some time back. I do not know where it came from but it is simple enough that the author should not mind. If you are he and you do it can be removed.

 

[crutschow]

but the system is to work with 2 cycle engine and 4 cycle engine.

Since 2 cycle cylinders fire every revolution and 4 cycle fire every other revolution, you will need a switch to select which type of engine the tachometer is connected to.

 

[ParkingLotLust]

A hall effect sensor around a spark plug wire would work, but most new cars use a crank position sensor to tell how fast it is going around (which you can derive RPM from)

 

[Marco_Santos]

Hi again.

Thank you for your time, the first idea i had was the hall sensor. But a friend told me to do it with a magnetic/inductive pick-up. i Just want to make a system portable with the 2 engines. I Think its easy to do the change form 2 cycle to 4 cycle on software or with a switch.

i really need some advices what should be the best way.....

Best regards
Marco Santos

 

[ParkingLotLust]

In older vehicles, the tachometer is driven by the RMS Voltage waves from the low tension (LT) side of the ignition coil, while on others (and nearly all diesel engines, which have no ignition system) engine speed is determined by the frequency from the alternator tachometer output. This is a special circuit inside the alternator to convert from rectified sine wave to square wave, and the electrical potential difference is directly proportional to engine speed. Tachometers driven by a rotating cable from a drive unit fitted to the engine (usually on the camshaft) also exist- usually on simple diesel-engined machinery with basic or no electrical systems. On modern engine management systems found on modern vehicles, the tachometer is driven directly from the engine management ECU.

^^ Wikipedia.

This may also help: **broken link removed** You could just add something on the crank and pick it up with the sensor

 

[jpanhalt]

While most 4-cycle engines fire every other revolution, as pointed out above, they also can work fine firing every revolution. It is common in smaller, high-rpm, 4-cycle engines to fire every cycle as it simplifies the coupling to the ignition.

Thus, you will need to be sure what the firing program is for your 4-cycle engine to get proper calibration.

John

 

[ParkingLotLust]

^^ That's called a "wasted-spark" ignition system.

 

[3v0]

Schematic from my frist post.

 

[odps]

can you post the rest of the schematic?

Schematic from my frist post.

can you post the rest of the schematic?

I need to build a tack for chain saw application...

any info appreciated....

 

[xrotor]

Tach

Hi,
I have been looking into this type of pick up to derive a tach reading ala Tiny Tach or ENM the size limitations for use on a chainsaw are considerable

 

[crutschow]

While most 4-cycle engines fire every other revolution, as pointed out above, they also can work fine firing every revolution. It is common in smaller, high-rpm, 4-cycle engines to fire every cycle as it simplifies the coupling to the ignition.

Many motorcycle and automobile engines use this scheme. It uses one coil for each pair of cylinders that fire 360 degrees apart. This eliminates the need for a distributor.

 

[MikeMl]

Many motorcycle and automobile engines use this scheme. It uses one coil for each pair of cylinders that fire 360 degrees apart. This eliminates the need for a distributor.

I have also worked on engines which use a coil per cylinder, which implement the "distributor" in software inside the ECU.