Even with an engine using an oil pressure gauge, I've seen the cranking of the engine under adverse conditions (it didn't want to start easily) bring the oil pressure up within the "safe" zone.

Your starter motor is only going to be able to spin the crank at some given maximum RPM. Detecting the RPM and checking for rotation that is well in excess of that should work. The Bendix of the starter should release from a slight overspeed I'd think.

Dean

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Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

 

I'd rather not dismantle the alternator, is there no easy way of interfacing with injectors or something? I'm trying to make a push button engine start switch, but I need a way of detecting engine start/ engine revs to cut the starter solenoid.

 

You shouldn't have to dismantle the alternator. Slap a scope across the alternator output and you'll probably see some pretty severe AC you could refine and use for the sense input. The injector will work on a newer diesel if it's electronically controlled I suppose. All the diesels I've been associated with use mechanical injectors. The only electronics is for the starting and charging system.

Is there a speed sensor on the engine. For instance, the VW diesels have a sensor off the crankshaft that can be used to power the "upshift" dash indicator or for engine service.

Dean

__________________
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

 

how about using the voltage drop on the high side of the starter cable? The current should drop - and so the voltage - to almost zero if the engine is running and the alternator supplies charging current to the battery.

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Proper Planning Prevents Piss Poor Performance

 

You can sense engine revs with the AC ripple on the alternator output (http://www.caesar-datensysteme.net/D...D100-011US.pdf)

However, the signal is very small and it may not be present at cranking speeds. Alternators have a rotor winding, which is fed through the slip rings. That controls the alternator power and is used to regulate the voltage. The current for the rotor comes from the alternator itself, so it won't ever generate anything unless some other rotor current is provided. That comes from the ignition warning light.

When the alternator starts to work, it generates enough voltage to provide its own rotor current and the ignition light goes out. Until that happens, you won't be getting any conduction of the main diodes that feed the battery, so there won't be any ripple. That was why I suggested looking at the alternator voltage before the diodes, where there will be at least a volt of AC even at cranking speeds.

Watch the ignition warning light as you start the engine. On some cars, that doesn't turn off until the engine speed has gone a little over tickover, but I am fairly sure that it won't go out at cranking speeds.

If the ignition warning light does go out as the engine starts, you could use that but be careful. If the ignition warning light bulb fails, there is no rotor current so the alternator never generates. I would also suggest a timeout on how long you run the starter motor.

Alternators are not difficult to take apart enough to get to the diodes. The diodes are huge and on large heatsinks, and the wires are about 1mm in diameter so there is nothing little or delicate.

 

I agree with Dean. The load on the starter motor will reduce dramactically if the engine has started and is running on its own.

Just sense the starter current to determine this condition.

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L.Chung

 

Sense starter current? Won't that be rather complicated? Also sensing alternator voltage doesn't seem like a good idea - what if the alternator has failed? The starter would continue cranking even when the engine has started.

Sensing RPM on injector wire seems the best way, but how would I do this?

xx

 

Hi Mike,

I have built a computerized controller for my diesel engine that does exactly what you are looking for.

I use a hall effect sensor and a magnet on the flywheel to determine RPM.

When cranking, I wait until the RPM exceeds a certain speed to determine if the engine has started. Using oil pressure won't work, because the engine starts long before the oil pressure comes up. Also, the alternator does not start putting out power until the engine is near full speed, so that is not useful in determining when the engine has started.

Here is a video showing the start sequence:
Generator starting

The start sequence is as follows:

1. Move speed control to start position
2. Engage Glow plug for fixed amount of time
3. Start cranking
4. when rpm exceeds 300rpm, disengage starter
5. move speed control to run

I'm also using a PID controller to tune the engine speed to exactly 1800 rpm. If you watch the video closely, you can see the PID overshoot the target RPM, then back off to the correct speed. (The PID controller was not fully tuned when the video was made.)

Unless you are a glutton for punishment, or enjoy building and programming your own controller, I suggest you stick with one of the commercially available controllers. That being said, if you are interested, you can see full schematics and description of the controller hereAutomatic Generator Controller

 

Alternatively, investigate the Lister Startomatic

Electrokinetica - Maintenance

Rick...

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Rick...

Science and sound engineering will always prevail in the end
"for nature cannot be fooled" [Feynman]

 

"I use a hall effect sensor and a magnet on the flywheel to determine RPM."

In the design and manufacture of rotating machinery, it is often critical to have the moving parts dynamically balanced, in order to reduce vibration and wear .... over the lifetime of the mechanism. Even a few grams can make a difference at higher rpm levels ... either causing a knock, or premature failure due to wear.

It may be prudent to determine the weight of the magnet which you attached to the flywheel, and affix a weight of identical magnitude at a point that is diametrically opposite, and symmetrical about the main rotating axis.