JasonC SBB

That would implement a simple table-driven 'P' type of control (i.e. not 'PID'), and it will work, BUT, the sampling/decision loop has to be faster than a certain rate else the system will oscillate. I know 500 Hz is fast enough, and IIRC 20 Hz is too slow.

Matt Cramer

I'll have to check what we found when we scoped the stock Miata ECU, but it appears to have been sort of a modified P type. It had a "Use X duty cycle at Y volts" algorithm, but it wasn't linear like a P control.

cordycord

iTrader: (2)

So MS3 controls the alternator...can you do anything funky with it? For example, can you turn the alternator off during WOT?

Joe Perez

iTrader: (8)

At present, the software control for the alternator is in its infancy, piggy-backing on an existing application. Anything is possible, it's just a question of code development.

Shutting off the alternator at WOT could be achieved using existing functionality by switching the field drive with a relay that is activated by the MS under certain conditions. I honestly have no idea how to calculate what the potential benefits of this would be.

Actually, that's not true. I know exactly how to calculate this.

Under a typical load, we'll say that the alternator is putting out 30 amps. (running the engine, a few lights, etc.) At 13.8 volts, that's 414 watts. A typical alternator is about 55% efficient, so it's taking in 753 watts of mechanical energy. 1 hp = 745 watts, so the alternator is consuming 1.01 horsepower.

Hmm.

A typical alternator is around 50-60% efficient

cordycord

iTrader: (2)

Wouldn't it be nice to ask more of the alternator during braking? Poor man's KERS.

JasonC SBB

Or you could do in s/w what my circuit does:

It's a hysteretic controller that simply makes a new decision every 1.5 ms or so.
That is, every 1.5 ms it will decide whether or not to turn the field on or off. The decision is simply: is the voltage is higher or lower than the setpoint or not? The voltage needs to be filtered with about a 100-300 us time constant.

Trust me it works well. If you doubt me, realize that the field voltage to output current characteristic of an alternator is an integrator. A hysteretic controller will innately produce the "PWM" signal you need, and is inherently stable if the sampling period is fast enough.

I can tell you it works better than the alternator control in my 540i. I blip the gas at idle in the 540, the lights flare briefly. Not in the miata.

Edit
P.S. My circuit has about 50 mV of hysteresis, IIRC.
P.P.S. The resulting "PWM" on the field signal, is closer to "pulse density modulation", if you wanna get fancy.

JasonC SBB

There is potential danger with low voltage at full power. The fuel pump will be receiving reduced voltage and its pressure may droop.

If you want to see useful things to do with alternators and MPG, check out what Honda did with the early-mid 90s Civic HX. It was discussed at length at one of the popular MPG geek sites.

Other fancy things you should do:

- use a separate pin for sensing batt volts - in my NB the pin that is connected to the injector power line is not accurate. There's a different pin that is (I forgot which one, I think it's called "room fuse")

- keep alternator off below 500 rpm or so

- pinch off the setpoint below 800 RPM (e.g. 14.2V at 800 RPM, sliding to 12.6V at 600 RPM), to help with anti-stall.

Full_Tilt_Boogie

iTrader: (1)

Whats this about the HX alternator?

I havent seen or heard anything about that, and it would have to be OBD-I to be early mid 90s, and I dont know of anything in the ECU I tried googling but found nothing.

Joe Perez

iTrader: (8)

Thread split off from the "Megasquirt is for the Fishes" discussion.

Braineack

iTrader: (62)

I'm still using Jason's circuit in my builds, but with the addition of a PNP to trigger an NPN to send out the field output. So I'm able to set an output in the MS to activate the alternator only after 300RPM like the OEM ECU, so it would be just as easy to cut it back off above a certain TPS or any other trigger.


I do know alternator control is on the docate for the MS3 code, just unsure when Ken will get to it and hoe he'll implemint it.

NiklasFalk

iTrader: (1)

Be prepared to switch to a different table for the non-alt case, for the different voltage giving a quite different situation for e.g fuel pressure.
14.1V is quite different than 11.1V or whatever the fuel pump and ECU will see sans-alt.

Braineack

iTrader: (62)

That's why the MS (and almost all other ECUS) has voltage compensation correction curves...

Specifically for dwell, deadtime, and the idle valve.

Ben

iTrader: (33)

We have done this for pro level Solo2 cars. A ~130 hp typical 4-banger picks up about 2 whp when shutting off the alternator.

y8s

iTrader: (8)

Random real world experience with alternator control:

The MS3 compensates for voltage, but not for load. With the constant-voltage control of the alternator, there is not really any more need for voltage compensation except for (maybe) at startup where the alternator is only working against the battery.

Sadly, the current MS3 software gets all crazy when you have a steady voltage and varying load alternator: you turn on the headlights, the idle dips and gets sad. turn them off and it swells.

cordycord

iTrader: (2)

The Braille boys were talking about the ALMS cars (Corvettes, etceteras) shutting down the alternator during WOT for the additional horsepower, and then turning it back on during off-throttle or braking to recharge the battery. It's just another way to free up some horsepower.

I wonder when they're engaging the alternator if they're simply turning it back "on", or if it's being ramped up over a short period to avoid damage to the alternator or the belt system. Jeez, I wonder if this was one of the issues with the F1 alternators.

Reverant

iTrader: (10)

Modern alternator control you say?

Zaphod