Delaying an Earth signal getting to a controller.

Thread Starter

Robert Gardner

Joined Aug 21, 2018
5
Hi All,

Weird question but here goes.

We are working with an industrial Diesel Engine with Auto Start Controller, our problem is, due to the setup of the engine in it's current form, we have to use Oil Pressure sensing to disconnect the Starter Motor from cranking the engine once it starts.

This is done by the controller receiving a disconnection of the "Earth" signal from the Oil Pressure Switch when it changes from closed to open as the oil pressure rises to open the switch.

Our problem is that the oil pressure rises too soon and cuts out the starter before it turns the engine past compression and starts.

We are hoping there is a way, electronically, to delay the disconnection of the "Earth" Signal reaching the controller, even if by only a few seconds, but then remain, so as to enable low oil pressure shutdown if it eventuates.

Any suggestions based on the above setup?

Thanks
 

dendad

Joined Feb 20, 2016
3,528
How about a relay that is wired across the starter motor. As the starter is energized, so is the relay, and contacts of that relay open the sensor circuit.
When the starter is not powered, neither is the relay, to the sensor circuit operates as normal.
Oh, I just realize, that would not allow the starter to turn off.
Can you just change the oil pressure sensor for one that operates at a higher pressure?
 

Thread Starter

Robert Gardner

Joined Aug 21, 2018
5
Have you got a data sheet on that pressure sensor?
No I don't, the original switch supplied is a .4 - .8 bar switching, I have fitted a .8 - 1.2 with no change, I dare not go too high with the switching as it has to provide shutdown for low oil pressure.

If I manually hold the earth the engine cranks for as long as it takes to start, then I break the contact and all is good. Just need to get this to happen automatically as this will be an unattended site and needs to start/stop by remote signal, we are using a start / stop timer for the remote start signal.

Thanks for your input, it is very much appreciated.

R!G
 

Thread Starter

Robert Gardner

Joined Aug 21, 2018
5
How about a relay that is wired across the starter motor. As the starter is energized, so is the relay, and contacts of that relay open the sensor circuit.
When the starter is not powered, neither is the relay, to the sensor circuit operates as normal.
Oh, I just realize, that would not allow the starter to turn off.
Can you just change the oil pressure sensor for one that operates at a higher pressure?
Hi and thanks for your input,

The original switch supplied is a .4 - .8 bar switching, I have fitted a .8 - 1.2 with no change, I dare not go too high with the switching as it has to provide shutdown for low oil pressure.

R!G
 

dendad

Joined Feb 20, 2016
3,528
Here is a very rough simple circuit. I have not tried it, but it may work.
StartDelay.jpg
The transistor will switch on the relay that disconnects the oil sensor.
Capacitor C (big electrolytic) charging through R and the BE junction of the transistor turns the transistor on. Once the capacitor has charged, the transistor turns off again.
The values of R and C control the time delay.
I'm sure a better circuit using a 555 timer would be better but this is simple, if it works ;)
 

Thread Starter

Robert Gardner

Joined Aug 21, 2018
5
Here is a very rough simple circuit. I have not tried it, but it may work.
View attachment 158487
The transistor will switch on the relay that disconnects the oil sensor.
Capacitor C (big electrolytic) charging through R and the BE junction of the transistor turns the transistor on. Once the capacitor has charged, the transistor turns off again.
The values of R and C control the time delay.
I'm sure a better circuit using a 555 timer would be better but this is simple, if it works ;)
Thanks for taking time out to have a look at this for me, I think we might be able to make it work, I will give it a go and let you know if it works.

Cheers

R!G
 

WBahn

Joined Mar 31, 2012
25,760
I understand the problem that you are trying to solve, but I don't think your proposed solution will actually solve it (or, I should say, it will open you up to other problems).

Think about why the oil pressure switch is being used to disengage the starter in the first place -- because it is being assumed that the presence of oil pressure means that the engine has successfully started and it is therefore safe to disengage the starter.

But the fact that you have a problem means that the presence of oil pressure does NOT, in fact, mean that the engine has successfully started, so delaying the disengage signal based on the oil pressure switch opening will still mean that you are basing the decision to disengage the starter on an indication that does NOT tell you whether or not the engine actually started. Since, apparently, the pressure rises before the engine even turns over completely, you might as well simply put a timer on the start relay and have it crank the engine for a set amount of time whether there is oil pressure or not, because that is really all your proposed solution is going to accomplish.

A better way might be to use an engine speed sensor and disengage the starter once the engine speed rises above a certain RPM.
 

Thread Starter

Robert Gardner

Joined Aug 21, 2018
5
I understand the problem that you are trying to solve, but I don't think your proposed solution will actually solve it (or, I should say, it will open you up to other problems).

Think about why the oil pressure switch is being used to disengage the starter in the first place -- because it is being assumed that the presence of oil pressure means that the engine has successfully started and it is therefore safe to disengage the starter.

But the fact that you have a problem means that the presence of oil pressure does NOT, in fact, mean that the engine has successfully started, so delaying the disengage signal based on the oil pressure switch opening will still mean that you are basing the decision to disengage the starter on an indication that does NOT tell you whether or not the engine actually started. Since, apparently, the pressure rises before the engine even turns over completely, you might as well simply put a timer on the start relay and have it crank the engine for a set amount of time whether there is oil pressure or not, because that is really all your proposed solution is going to accomplish.

A better way might be to use an engine speed sensor and disengage the starter once the engine speed rises above a certain RPM.
Thank you WBahn, your input is very much appreciated, yes you are correct with everything you said, and the Mag Pickup is certainly the answer, and other than the fact the customer won't spend any more money for us to supply and fit one, it would have one on it as we speak, the issue seems to be that the controller is a little sensitive and picks up the breaking of the earth signal within a split second and maybe before the switch has completely opened, I have since found a similar controller that I can use which has a built in adjustable timer for this very reason, I can adjust it from 1 to 9 secs, and is a little cheaper than the one we were going to use.

Thank you WBahn and the others for trying to help me with this, it is very much appreciated, I hope to be in a position to repay this forum in the near future.

Cheers

RIG
 
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