Stupidist Plan yet to save gas -- muzzling the alternator

Sorry; that sounds like some "tin-foil-hat" stuff. That seems like using a flame thrower to light a cigarette. While auto start-stop systems are mandated; putting a clutch on the alternator pulley seems a bit overkill and would never pay for itself. The math simply won't pencil out and would add to the overall price of the car. I do admit, strangers things has happened; like Bigfoot....

 

My gen 3 Prius reduces the DC/DC converter output voltage to around 13.6 whenever it's in motion and the battery is above some threshold level of charge. It uses 14.7 or so when the battery is less charged, or when stopped/parked. (It is, apparently, making no effort to improve MPG-while-stopped, given that any percentage improvement of zero would still be zero.)

So the idea's been around at least since model year 2010 for Toyota.

 

For longer, and in non hybrids. The voltage fluctuated in the 2006 HHR. I think Honda was the first to use such 'smart' alternators. It helped a little with efficiency by not having the alternator be at full drag on the engine all the time.

 

It likely just regulates the charge voltage and current to extend the life of the battery instead of overcharging it.

 

And how many things labeled smart actually aren't?

 

this won't save gas, if the alternator's drag is still on the crank pulley. As you stated earlier, drops of gas - but drops do add up over time, assuming no evaporation.

 

Correct. One-way alternator clutches on pure-ICEs have been around a while now, which prevents the mass of the alternator rotor / pulley / belt affecting the crank only when power is applied... and not robbing energy from it when on trailing throttle.

Makes sense and one of only a list of things which combined raise ICE mpg... but more mechanical complexity and more difficult fault diagnosis... but what else is new for pure-ICEs trying to get 40+ mpg combined. Having owned one... never again.

 

Huh? On normal generators (and thus alternators too), the torque on the input shaft is a function of the electric load on the output terminals. Reduce the electric load, and the mechanical torque is also reduced. That is how some gas should be saved.

 

^ This.

We also get into the same subject in occasional threads wondering why the Prius MGs are a drag on the car when regen braking, but not just when spinning with no electrical load.

 

The difference is so slight in energy efficiency its not worth it!

Over at InsightCentral.net we had the same but oppostie conversation for Gen1 Honda Hybrids... There's a specifc wire that you can snip to send a continuous charge to the 12volt battery rather than letting the dumb computers decide when to charge the battery.

It's been debated often as to the value of snipping this wire or not and because hybrids and EVs in general send a weak charge to 12v compared to what a standard alternator can do in an ICE vehicle, its fairly accepted to want to do all you can to keep your 12v.

Whenever I put a new 12v battery in a Prius for someone I tell them that they need to put it on a worthy smart charger this time of year when days are so short because you need to drive your car for 12 hours non-stop to fully recharge with that weak charge signal to avoid the risk of a dead battery. So if you're reading this and you haven't charged your 12v in a long time, this is the month you'll want to do it!

 

The "weak charge" being sent in a Prius is, at least in part, the result of the exact behavior described here (since model year 2010 for sure, I don't know for sure about the earlier gens). Before programming that behavior on purpose, Toyota must have mulled over the "is it worthwhile?" question and arrived at an affirmative answer.

Maybe the jury's still out on how worthwhile it has proven to be in practice, or whether they paid enough attention to the risk of inadequate charging in low-usage situations. But it seems unlikely they'd have gone to the trouble without seeing some numbers big enough to catch their interest.

 

Sorry, are we still talking about a Subaru alternator? I don't see how a reduced load would all of a sudden reduce the mass of that alternator. It's just wires spinning around in a magnetic field. As the Subaru engine accelerates, it needs to drag that alternator mass with it. I'll give you that a higher load may require slightly more turning force. Unless you disengage the alternator and your just spinning the pulley = no additional mass to slow down the engine = more gas saved. Think old AC compressor clutch. Building on this example, wouldn't a car tuning an AC compressor burn more gas than one that has it's compressor disengaged? That's a measurable difference not a theoretical one.

 

I read an article a year ago that most all auto makers have made less than robust decisions regarding how hybrids and EVs distribute 12v power via the DC to DC converter. The amount of power you can generate from an alternator in an ICE car is so much more than what a DC to DC converter does and most all auto makers have underestimated the value of this.

 

I don't remember anyone claiming that it reduced alternator mass. And this effect is not specific to a brand, it is universal.

I must challenge the "slightly" part. Any generator will have bearing and windage friction losses, but it will also have a drag element that, at minimum, reflects the electric load. The electric power of the load (plus associated electric resistance losses inside the generator) should directly translate to added mechanical force (in this case, a torque) dragging on the generator's shaft and pulley.

This isn't "just wires spinning around in a magnetic field", this is the First Law of thermodynamics: conservation of energy. Energy is not created or destroyed, only transformed. When spinning around in a magnetic field, the wires sending out electric energy also experience drag from the magnetic field, a mechanical force resistance that is reflected at the spinning shaft. Disconnect the load, and this magnetic drag vanishes.

I'm not thinking about AC compressors. Instead, I'm think of the big generators in powerhouses such as at Grand Coulee Dam that power our homes and businesses and streetlights. The load-induced torque is both theoretical and measurable, and very strong. And it doesn't go away no matter how much smaller you make the load and generator. It can only be swamped by other drag losses in the most inefficient devices. It is in car makers best interests to not make inefficient alternators.

FWIW, I am a retired electrical engineer. After some classes and labs and looking around, I went in a different direction, but my career was still based around things like this.

 

Thanks; got it - big Hover Dam turbines same as itity bitty car alternators....

 

The alternator is a tiny regenerative brake being applied whenever the engine is spinning. Reduce its electrical output, and that will reduce the braking force it is applying. The savings from it alone are tiny, but the manufacturer is chasing savings from all over the car design to meet regulations.

 

The physics is indeed the same.

The primary difference between them is scale. But even at car alternator scale, though it may be small, it isn't "itty bitty". Even the smallest car alternators can consume a full horsepower in the electric drag portion "from wires spinning in a magnetic field" alone, not counting the bearing and other frictions.

Though that isn't the operating point where Subaru is "muzzling" it.

 

The "Smart Alternator" has been around for a very long time, I closed my mechanical workshop in '98 and they were already showing up outside new vehicle warrant periods, with all the problems they create.

The first idea was to fast charge the battery to replace the over night losses and the starter drain, then throttle the voltage back to 13v to "maintain" the battery at the "fully charged" state. The problem was, no one who knew anything about lead acid batteries were listened to while this "great new idea for fuel saving" was put out there for technical adoption.

A lead acid battery required that high end voltage to continue until the current acceptance drop to a low percentage over consecutive hrs, 5 mins isn't going to do that, but it does sulphate the plates so they can't store as much capacity and then access it when needed ...... the slow cranking in cold weather is the telltale this is what has happened.

The other problem was rapid fan belt deterioration, that "squeal" when first started .... then finally, the flat battery. Who looked at the alternator belt being the cause?

I'll cut this into two posts, just so no one drops into a comma

T1 Terry