circuit to cause engine to run on briefly

This is for a waste-vegetable-oil diesel conversion. Before shutting down, you have to switch the engine over to "regular" diesel fuel and run it for about thirty seconds to clear the wvo out of the injectors and such, so they don't get gummed up. My buddy doesn't want to have to sit in the car after parking it if he forgets to switch over to diesel a block or two before parking and shutting off the engine; or even worse, forget altogether and end up mucking up the engine. So he wants a circuit that will cause the engine to switch over to the diesel line automatically and run for an extra half minute or so after he turns the ignition key off.

Here's a block diagram of my initial idea for the part of the circuit that will cause the engine to run on briefly after turning off the key:

keyed 12 volts | +------------------, | | | ___|___ | | | | | relay |---->to engine | |_______| | | | | | c/ | | '----+-----+-----| npn | | | --- \\ e\\ --- / | | \\ | | / | | | | '-----'-------'-----gnd And use cap and resistor values to give the right amount of run-on time. Obviously it works by maintaining some voltage in the cap for a while, until the resistor (and base current) drains it low enough to where it won't hold the transistor on.

As for causing the plumbing to switch from wvo to diesel when he turns the key off, we can accomplish that by using a 12 volt solenoid in the wvo plumbing line, so that when the operator turns the key off, the solenoid drops out and the engine's plumbing reverts to the diesel fuel line.

Any feedback? I think there might even be a simple circuit that would do the job with just a relay and cap.

Reply to
kell
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switched 12 volts | | 12 volts | | | ___|___ | | | | | relay |---->to engine | |_______| | | | | | c/ | | '----+-----+-----| npn | | | --- \\ e\\ --- / | | \\ | | / | | | | '-----'-------'-----gnd

D'oh, this is the correct diagram.

Reply to
kell

Excuse me, it needs a base resistor too. switched 12 volts | | 12 volts | | | ___|___ | | | | | relay |---->to engine | |_______| | | | | | c/ | | '----+-----+--R--| npn | | | --- | e\\ --- R | | | | | | | | | | '-----'-------'-----gnd

Reply to
kell

That will probably work. If there is anything else on the switched line (that is coming into the capacitor), you will need to put a diode in that line, to keep it from discharging the capacitor too (put the anode to the switched voltage line and the cathode to the capacitor). Also, don't forget to put a diode across the relay too (so it won't feed a spike back into the transistor, when it switches off).

Brian

Reply to
Brian

Yes there is (cap in parallel with coil), but for a 30 second delay, you need a big, and therefore costly, cap. The transistor circuit avoids that. I've gotten some relatively long (up to 10 minutes) delays using a darlington in the circuit - keeps the base current small and therefore discharge rate of the cap is slow so you can use a much smaller cap.

Oh - and you don't need a backwards diode across the relay coil as another poster suggested.

Now for the ugly part: what protects the engine if the ciruit fails?

Ed

Reply to
ehsjr

switched 12 volts | | 12 volts | | | ___|___ | | | | | relay |---->to engine | |_______| | | | | | c/ | | '----+-----+--R--| npn | | | --- | e\\ --- R | | | | | | | | | | '-----'-------'-----gnd

Well, I guess the engine would die. The answer might be to have a redundant circuit wired to the ignition switch. Then, if the circuit fails, at least the car won't die on the highway. The run-on function would fail. Thanks Kell

Reply to
kell

I'd put in an isolation diode, so that when the switched 12V goes to ground, the other switched loads on the line don't immediately discharge the cap:

And don't forget to protect the whole thing with a hash choke and transzorb - automotive (and truck, etc) power is notoriously dirty.

Good Luck! Rich

Reply to
Rich Grise

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