Engine Speed Controller

[RJR]

I took a 1 liter engine out of a Geo Metro and built it into a gen-set. It is connected to two 160 amp alternators and charges a pack of 14 batteries. In general, it works great but here is my problem. When the batteries are low, it needs more throttle and as the batteries come near being fully charged, it only needs to idle. There is an electric throttle motor which can increase or decrease the RPM so what I need is method to control this automatically. Right now I am constantly adjusting the RMP manually so as not to waste fuel. What I need is an electronic tachometer which can be set to a specific speed. 1200 RPM for example. If load causes the engine to slow down, then it must increase throttle or decrease if the speed exceeds the set RPM. I was thinking of using a magnetic pickup on the flywheel for the speed sensor. The output must control the polarity of the throttle speed control motor which can operate on 3 to 12 volts. Low voltage is better as it makes the unit less sensitive so preference is to not exceed 4 volts. Although I have an understanding of electronics, I do not possess the skill to design such a device. Is there anyone who would like to take on this challenge?

Thank you,

Roy

[email protected]

[Hero999]

Why not just increase the speed when the battery voltage drops below a certain level?

[RJR]

As long as the engine is running, the Voltage measured at the batteries is that of the Alternator output. Typical alternator voltage is 14 volts. I have thought of a circuit that would speed up the engine if voltage out put is less than 13.7 and slow it down at 14 volts. The idea to keep the voltage at 13.8 to 13.9  but I would still need a tach circuit to prevent engine run-away in case of alternator failure. All ideas and suggestions are appreciated.

[Hero999]

Aren't engines limited anyway?

Unfortunately they're normally set at the start or the red line but you might be able to adjust it to a more sane maximum speed.

Alternatively you could use an optical tachometer.

Are you using  a microcontroller?

If so it should be easy to count the pulses.

If not there are other ways, build a counter with digital logic to count the pulses, use an oscillator to reset the counter at a regular intervals. Use a digital comparator or logic gates to monitor the counter, if it exceeds a certain count the engine could be cut-off or its speed reduced. For example if the tachometer gives one pulse per revolution and you reset the counter every second, if the count exceeds 50 which is 3000 rpm cut-off the engine.

You'll need to add a Schmitt trigger to get a square waveform whichever method you choose to count the pulses.



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