How to Detect our Car's Engine is Running or Not?

[Mechatrommer]

i want to build a circuit that will disconnect the engine starter circuitry (before the relay, after the car's key) coz i hate when i accidentally turn my car's key while the engine is running, the starter will goes nasty! and my car doesnt have the protective circuit like other cars do. but in order to do this, i need to detect whether the engine is ON or OFF.

going into the engine sensor will be messy i think, i'm planning to start from the alternator, coz thats the source of electricity during the engine ON, but i have no clue how my car's alternator outputting the electric, is it AC or DC? or every cars are the same architect of alternator? if i cant do with alternator whats the other source to detect?

so my clearer questions will be:
1) how's the car's alternator works?
2) whats the best way to detect the engine is running or not?

any idea... i thank you.

[RayJones]

Tachometer signal.

But you must make sure you can actually complete cranking as you'll get this signal as soon as you start turning over the engine....

Tachometer is used for the safety interlocks on LPG vehicles to allow the gas valves to operate.

[flolic]

2) whats the best way to detect the engine is running or not?

Oil pressure sensor/light  

[Mechatrommer]

ok thank you... but... where i can find those tachometer and oil pressure sensor/light in my car?

[RayJones]

ok thank you... but... where i can find those tachometer and oil pressure sensor/light in my car?

A good starting point is under the bonnet. It's your car though so you have to work out the rest!

[RayJones]

A quick and dirty way to get a tacho signal is to wrap a few turns (Not many needed) about a spark lead. the HV pulse will easily induct enough signal for you to sense, and more importantly manage (spikes etc).

If you have Diesel - all bets are off with this technique

[MightyTwin]

The simplest way to detect if an engine is running would be to check the battery voltage. If it's ~13.8V, then the alternator is spinning, and the engine is running.

And vice versa, if the battery voltage is at ~12.6-12.8 ish volts, then the engine is turned off.

The charging voltage can vary slightly between alternator manufacturers. Alternator wear could, potentially, also make the charging voltage drift a bit. The safest bet would be to measure the voltage across the battery terminals when the engine is idling, and use that measurement in your project.

Nice and simple wire hookup, at least.

Using the oil pressure sensor could potentially give a "false positive" feedback, because they're generally set to latch at 0.8 bars, which isn't a lot. Cranking could easily set it off.

Using the charging voltage wouldn't give such a false positive, as the starter motor draws the battery voltage down when it's cranking.

-MightyTwin.

[Mechatrommer]

@rayjones: that seems practical and non contact type circuit. i did it recently on my camera flash equipment though.
@mightytwin: seems ok too, but i might be calibrating quite frequently as battery wear out (new to old), so does the alternator. but i think detection of a sudden low voltage indicating the starter is cranking, wait until voltage goes high again, means no cranking, but when the engine is already on, no more hi-lo-hi signal and there is no indication that the engine is succesfully started.

[RayJones]

Yeah, if you want a reliable indication of if the engine is running I would not be looking at battery volts.

Hot weather, cold weather, dead alternator, flat battery, etc the list goes on.

How's this for an idea out of left field.

If your car uses a CAN bus, perhaps there is a message to tell you the current RPM.
Latch onto that message and implenet your starter lockout.
 That's probably how they'd do it on a the modern variants you speak of....

If you have an OBD2 connection, you are halfway there as tools like a ScanGuage http://www.scangauge.com/ can easily show you RPM amongst multitudes of other things.

[Mechatrommer]

CAN bus? i know nothing about it. is it connected to the car's computer? no no no! if i accidentally tossed the pc,then i will be in deep trouble!
my car is Toyota Wish 1.8CC 2003 model, i dont know what it has in the computer. and i cannot rely on the computer, other than having no knowledge on it, i also want to do the same with my other local brand elcheapo car.

[kek]

How about simply energizing a relay 20 seconds after you turn the car on and wire the normally closed contact in series with the starter relay? Have your new relay drop out when you turn the car off.

Cheers,
Ken

[Mechatrommer]

How about simply energizing a relay 20 seconds after you turn the car on and wire the normally closed contact in series with the starter relay? Have your new relay drop out when you turn the car off.
Cheers,
Ken

that is exactly what i want to do, the problem is how to correctly energyze the relay according to engine ON?
and i will not put the new relay in series with the starter relay, coz that will require a new "big" relay, i will simply put/tap the new relay to the energizer of the old relay.

something like this (sorry for sloppy drawing) i prefer it Al-Express-Tium hand drawing  
edited: sorry i missed the label... the bottom relay is the new "Engine ON - Cut Off" relay... normally closed.
edited: and sorry, i just noticed the starter motor is not actually directly connected to 12V, it must have gone to a voltage booster to KV level, well just assume it will work that way.
the most important part is the new relay and the control system, thats where i'm stuck.

[kek]

Find a source of power in your car that is on when the ignition switch is in the "on" position like the radio or cigarette lighter. Have your new relay energize a period of time after this power source is on (long enough that the engine should be running).

Of course this wouldn't prevent you from trying to start your car more than once during this delay!

[Mechatrommer]

Find a source of power in your car that is on when the ignition switch is in the "on" position like the radio or cigarette lighter. Have your new relay energize a period of time after this power source is on (long enough that the engine should be running).
Of course this wouldn't prevent you from trying to start your car more than once during this delay!

i've tried, but havent found any briliant solution, the closest i can get is to monitor at two place, when the key is in "ON" position, and when the fuel light on the deskboard is off, thats a certain that the engine is running, but two input is not good enuf i think, and have to mess around with the inside of the deskboard. i need better than that.

if you talking about radio or cigarette lighter, that will be ON even we just turn the key from OFF to ON position, what if i want to listen to the radio for an hour, or be it just a little bit more than the delay u talking about, and later want to directly ignite the engine, the control already cutted off then i need to return to OFF position and quickly ignite the engine, workable if i dont do such thing (listen to radio etc), but what if my wife do while i'm not around... she will go panic!

[kek]

If you don't start the engine before the time delay relay energizes then simply turn to "off" then switch to "start your car", your new realy will start timing over again. Remember you would be using the normally closed contacts of the new relay.

Get a "time delay relay (on delay)" they make them with adjustable dials even. They are quite common in industrial controls.

[RayJones]

CAN bus? i know nothing about it. is it connected to the car's computer? no no no! if i accidentally tossed the pc,then i will be in deep trouble!
my car is Toyota Wish 1.8CC 2003 model, i dont know what it has in the computer. and i cannot rely on the computer, other than having no knowledge on it, i also want to do the same with my other local brand elcheapo car.

LOL, if you are not prepared to learn how not to try and restart you car when it is already running, nor really understand how cars normally function (albeit 2003 is rather early), I give up.

I've probably only ever hit the starter once or twice in my 25+ years of driving cars when the engine as already running. It really is not that hard to avoid. 

[MightyTwin]

Yeah, if you want a reliable indication of if the engine is running I would not be looking at battery volts.

It wouldn't be unreliable if you set the voltage low enough, say 13 volts. The battery voltage would never drop below 13 volts when the engine is running (if it does, well then you've got bigger problems), and would never go above 13 volts when the engine is killed. 

A simple zener circuit would do fine.

Why overcomplicate the solution to a simple problem? 

-MightyTwin.

[GeoffS]

A none electronic answer but what's wrong with just looking at the tachometer?
It's a sure fire way to know if your engine is running 

[Simon]

just use the alternator voltage and a time delay circuit to ensure it has time to start the car

[RayJones]

Why overcomplicate the solution to a simple problem? 

-MightyTwin.

Thank you, yes what really is the problem?
It is not that hard to learn to avoid starting a car that is already running.

This is where the rules of commonsense surely must re-enter this generation?

[RayJones]

A none electronic answer but what's wrong with just looking at the tachometer?
It's a sure fire way to know if your engine is running 

Indeed

[NiHaoMike]

Assuming the oil pressure sensor grounds the signal line to turn on the indicator, it can be done with a two pole relay and a diode. One set of contacts connects the start signal to the existing starter relay and the other one grounds the negative side of the coil, while the positive side connects to the start signal. A diode grounds the negative side through the oil pressure sensor to allow the relay to switch. Initially, the oil pressure sensor will provide the ground to allow the relay to switch, while one set of contacts latches it. But once the start signal is off, the relay will not switch until the oil pressure sensor grounds the circuit again.

I'm not sure why such a circuit would be needed since the neutral interlock would already prevent anything from happening if the key is bumped to start while driving.

[tecman]

There is on all car alternators an output that feeds the idiot light is available.  If you monitor this pin (smaller wire on the alternator) for voltage > 12 volts you can see if the engine is running.  When the engine is not running this output will be low and can sink some current.

Paul

[TheDirty]

I thought this discussion was over when MightyTwin chimed in.  I'm not certain how more reliable and easy you want.  Power over 13 volts means car is running.

[joelby]

How often do you try to start the car when it's already running? The way I get around the problem is by having a car with an incredibly noisy exhaust. You should be able to have a noisier sports exhaust fitted fairly quickly. As an added bonus, your car will impress more people and set off car alarms.