12VDC shutdown switches on diesel engine

[Redelectric92]

All:

I have been working tirelessly to make a circuit work and am certain I am over thinking it. It’s is a 12VDC circuit on a diesel engine that I need to engineer some shutdowns into via shutdown switches. I have a fuel solenoid that is 12VDC that just be energized to run the engine, and must be deenergized to kill the engine via lack of fuel. I have two shutdown switches. SPST switches. Normally open under correct circumstances. One is a coolant temp switch that closes when the engine reaches 205 deg F. This would be considered an over temp and would kill the engine via opening the 12VDC circuit for the fuel solenoid. One switch is an oil pressure switch that would be closed when the unit is off and closed when a low oil pressure situation occurs. Switch is open when all is well with oil pressure. In the event a low oil pressure anomaly occurs, this would kill the engine via opening the 12VDC circuit for the fuel solenoid.I need both shutdown switches to independent have change state (close) and open the 12VDC circuit to the fuel solenoid and kill the engine. The solenoid and both shutdown switches are 12VDC, two wire, and require a ground. I expect to have to use a minimum of one relay in this circuit, but am flexible. I expect that with the oil pressure switch being open when the engine is offline, that it may take a second or two for the oil pressure to build up and the switch open, allowing the fuel solenoid circuit to close and energize. The 12VDC source will be a common automotive battery.

By all means, I’m open to tips and recommendations. Thanks.

 

[retirede]

I order to use the switches you have, you need a relay to de-energize the fuel solenoid when the monitoring switches close on abnormal condition.
Best practice would be to have monitoring switches that open on fault making the system fail safe.
You need a timing relay to bypass the oil pressure switch on startup.

 

[ptonsparky]

It's called a Murphy switch. They've been used on Ag irrigation systems for decades.

 

[hillbilly1]

Is this for an emergency stop? Or just remote control? The oil pressure switch would probably have a time delay in the controller, as it is temporarily bypassed on cranking. The best and easiest way would be to have N/C contacts in series with the solenoid power if it’s an emergency shutdown.

 

[kwired]

Murphy 117 relay that was mentioned:
is normally open held closed by a mechanical latch. When solenoid is energized it unlatches the relay which opens the output contact. Is pretty common for safety controls on 12VDC where using machine frame as one of the circuit conductors. The relay isn't connected to frame but the associated switch gauges or other monitor devices typically switch between an isolated terminal and frame of the device.

one thing I don't like about this method is a damaged conductor to a monitoring switch disables the protective function of it.

Something like an oil pressure switch needs to have a bypass switch pressed during starting until sufficient pressure is attained, which can be the mechanical reset that is shown on left side in the drawing.

 

[drcampbell]

... One is a coolant temp switch that closes when the engine reaches 205 deg F. This would be considered an over temp ..

Typo? 205°F is an entirely-normal running temperature for water-cooled engines, maybe even a smidge low. 250°F is usually considered the threshold of overheating.

In normal operation, oil temperature should exceed 212°F by a good margin to assure water removal.

Don't confuse the temperature printed on the thermostat with the actual operating temperature.

Use caution when looking up boiling points vs. pressure. Only absolute pressure is relevant, but most references use gauge pressure.
(not a big concern in Louisiana, since none of the state lies at a high elevation)

... Best practice would be to have monitoring switches that open on fault making the system fail safe. ...

Yes, absolutely. And on the hot side, not the grounded side, to assure that insulation damage and a ground fault do not defeat it.

... You need a timing relay to bypass the oil pressure switch on startup.

Being old-school, I prefer a manual pushbutton located within sight of the oil-pressure gauge. (unless this is something that must start autonomously) Another option is an electric pre-oiler pump that will establish oil pressure before the engine starts turning -- highly recommended for engines that must make frequent cold starts and last a long time.

There are packaged engine-controllers/alarm-panels that provide all the necessary functions for operation and protection.

Engine Generator Parts Digital Speed Controller-Ot2106A Generator Elec

Part Number: OEM NO. OT2106A Application: For Supply voltage: 20~32VDCSpeed frequency: 500~12000HzThe rotational speed signal:1~30VRMSSpeed-trim: ±5%Speed up time: 5~35sPosition signal output: 0~200mAEnvironment temperature : -40¡ãC~+85¡ãCShock: <2GRelative humidity:<95%

www.buymachineryparts.com

(this is an example, not an endorsement)

 

[hbiss]

I have a fuel solenoid that is 12VDC that just be energized to run the engine, and must be deenergized to kill the engine via lack of fuel. I have two shutdown switches. SPST switches. Normally open under correct circumstances. One is a coolant temp switch that closes when the engine reaches 205 deg F. This would be considered an over temp and would kill the engine via opening the 12VDC circuit for the fuel solenoid. One switch is an oil pressure switch that would be closed when the unit is off and closed when a low oil pressure situation occurs. Switch is open when all is well with oil pressure. In the event a low oil pressure anomaly occurs, this would kill the engine via opening the 12VDC circuit for the fuel solenoid.

I think you have your logic states mixed up.

- I have a fuel solenoid that is 12VDC that must be energized to run the engine.

- [A] coolant temp switch that closes when the engine reaches 205 deg F. This would be considered an over temp and would kill the engine via opening the 12VDC circuit for the fuel solenoid? (Maybe it actually opens on overtemp?)

- An oil pressure switch that would be closed when the unit is off and closed when a low oil pressure situation occurs. Switch is open when all is well with oil pressure. In the event a low oil pressure anomaly occurs, this would kill the engine via opening the 12VDC circuit for the fuel solenoid. (Maybe it actually opens on low oil pressure?)

All of this might be true if there were a relay someplace that reverses the logic so that if a switch actually does close it activates and interrupts power to the fuel solenoid. But I can't see why they would do that. Likely both switches are wired in series with the fuel solenoid so when either opens it kills power to the solenoid and the engine stops.

Perhaps this is why you are having such a difficult time providing emergency off buttons? (I'm assuming that is what you want to do?)


-Hal

 

[drcampbell]

Nobody's mixed up. We're recommending replacing those sensors with normally-closed (and non-grounded) switches that complete the circuit when all is well.

 

[hbiss]

Nobody's mixed up. We're recommending replacing those sensors with normally-closed (and non-grounded) switches that complete the circuit when all is well.

If you read the OP's explanation I believe he is. It doesn't work that way. What you suggest makes sense and is what I would expect to find. I would be surprised if something like it wasn't already there.

-Hal

 

[retirede]

If you read the OP's explanation I believe he is. It doesn't work that way. What you suggest makes sense and is what I would expect to find. I would be surprised if something like it wasn't already there.

-Hal

Either that or it was field-engineered by somebody unfamiliar with basic failsafe controls.

 

[kwired]

If you read the OP's explanation I believe he is. It doesn't work that way. What you suggest makes sense and is what I would expect to find. I would be surprised if something like it wasn't already there.

-Hal

The Murphy Switch items I posted earlier is what is commonly used. They do have the downfalls mentioned - switching the grounded conductor, normally open switches that close in a shutdown condition, they are not as fail safe as other methods would be.

If OP has this kind of setup and wants an emergency off button all is needed is a NO contact and connect it from grounded conductor to the S terminal on the 117 relay. Relay solenoid will be actuated when you close the switch.