Test: Alternator vs. no alternator = 10% MPG gain @ 70 km/h

[Crashy]

The alternator is extremely inefficient. At 6000rpm engine speed the alternator coils are producing 55v. But we only require 14v so 41v is simply thrown away through heat in the regulator. At high rpm the alternator is 30% efficient. At idle it is 50%.

The rectifier is also rediculously inefficient. We produce voltage then pass it through 2 diodes of 0.7v drop. At idle, a 1.4v drop out of 15.4v is a 9.1% loss. At 100A (low rpm, high rotor current) our alternator is a 1.4 x 100 = 140w heater. We add this heat to bearing and fan friction then wonder why they burn out. At high rpm it's worse because the voltage regulator becomes a 41 x 100 = 4.1kw heater. This heat makes the copper coils less efficient so even more heat is created. Now you can understand why undersizing the alternator is a really silly idea. Luckily we spend very little time at 6000rpm so these rediculous losses are short lived. If we cruise at 2700rpm, our voltage is about 30v so regulator losses at 100A are around 1.5kw (2hp). Now add friction from the belt, fan and bearings. Now add rotor current. Now add rectifier losses. Are we starting to understand why we save 10% fuel with a alternator removal?

How do you kill an alternator? Have a flat battery that gets jump started, then run the headlights, demister and fan on high while doing high rpm.

To improve alternator efficiency, remove the rectifier and regulator and mount them externally. But now you need to keep them cool so add a fan. Let's reduce those losses. Use schottky diodes which have 0.2v drop which will save 50w at 100A. Much less heat = no fan. As for the regulator, get rid of it and instead use a buck converter. 100A at 14v is 1.4kw. At 55v, only 25A is required to get the same 1.4kw. That's right, by using a buck converter you can do 75% less work. I've ignored losses in both cases. Now we are able to undersize the alternator but might still burn it out at idle with high current (because 100A, no surplus volts to convert to amps). You can use an mppt controller of high current rating (eg Fangpusun 70A) which is rated to 150v. But I don't know if the mppt will struggle with changing rpm. At cruise it will be very effective. Or you can buy a 60A 35-60v buck converter but it's not going to work at lower rpm (that might be a good thing) because voltage is below 35v.

Want more efficient? Get rid of the lead battery and replace it with lithium iron phosphate which has a charge efficiency of 97%. Half the weight and 5x longer life. Go one better, double the capacity and charge it from the grid, no alternator

[Crashy]

Originally Posted by bandit86

Battery inefficiency will be offset by electronics inefficiency at lower voltage

False. Every electronic device has a voltage regulator. At any voltage above the minimum, the regulator burns off the excess voltage as heat. So the higher the voltage, the less efficient

[oil pan 4]

Newer cars have alternators that use pulse width modulation to control field power.
Only the coils only make about 15 volts from around 3000 to 12,000 rpm.
The only time you see anything like 40 to 55 volts is on permanent magnet setups like magnetos used on small engines.
The voltage regulators in old school generators and maybe some old fashioned external excited alternators may have put off a ton of heat but anything made after about the 1980s isn't like that.

My automotive alternator welder I made develops about 80 volts DC at no load with 12 volts going to the field while spinning the alternator at around 7,000rpm. So they can make a lot more then 55 volts.

[Stubby79]

Crashy, part of your first and second post on this page contradict each other. If the alt was putting out full power all the time, as your second post suggests, then a smaller alt would put less load on the engine every time, which would use less gas, and therefore be more efficient over all, from the perspective of fuel burned.

They don't work that way anyway(at least not any more); both your posts suggest that alternators are working all-out, all the time. And, besides, alternator efficiency is only one part of the equation of over-all fuel efficiency.

[oil pan 4]

My alternator welder runs an alternator for all its worth when I am actually welding. Trust me under normal conditions a regular car alternator is not being ran anywhere near as hard as my alt welder.

[Crashy]

No, I'm not suggesting alternator is putting out full power all the time. Loads are loads regardless of the alternator size. You don't pull less amps just because the alternator is smaller. The field is adjusted so that all loads are attempted to be met by the alternator. If you have 100A load on a 100A alternator, no problem. But keep all that load and swap for a 60A and 2 things will happen: the alternator will overheat and you will drain the battery. I did not feel it necessary to explain that a 50A load would be ok on a 60A alternator, that should be obvious. A 60A alternator try to do 100A is going to overheat, which means lower efficiency, which means more strain on the engine, not less. Though it might be putting out 65A (and rapidly declining) the strain on the engine would be similar to that of a 100A alternator that is actually putting out 100A

[Crashy]

Originally Posted by Stubby79

And, besides, alternator efficiency is only one part of the equation of over-all fuel efficiency.

Yeah, the biggest part. 10%. Bam!
Usually, when you strike gold, you keep digging in the same area. You don't go off to another random location. I'm actually shocked at the lack of exploration in the electrical department since this alt delete was discovered. There is more gold here

[RedDevil]

For the record, an excellent write-up about alternator efficiency and construction can be found here: http://www.delcoremy.com/documents/h...ite-paper.aspx

Typically, alternators have a voltage regulator that drives the secondary coils by PWM modulation; see page 13 and 14 in that link.
Instead of burning off excess voltage though a resistor, PWM switches the power on and off to match the current required with minimal loss.

The real reason why alternator deletes are efficient is that the whole chain from gasoline through engine efficiency, belt efficiency and alternator efficiency means that just 21% of the potential energy gets converted to electricity - at best. (page 3 and on)

But the alternator is not the worst part, averaging 55% efficiency on its own; nor would a smaller alternator help out much. The alternator should be dimensioned to be able to provide the maximal current the car can draw, but will be most efficient at half that current or less when operating in the 2000-3000 rpm range, where it would be most of the time.
A small alternator that can barely provide the 'normal' current draw would be on its toes and in fact be less efficient than a somewhat bigger one.
(a graph on alternator efficiency per rpm and load can be found on page 23).

[Stubby79]

Originally Posted by Crashy

Yeah, the biggest part. 10%. Bam!

In a Geo Metro with it's tiny 1.0L engine, in top gear at it's most efficient cruising speed of 70km/hr.

Of course freeing up a couple of horsepower is going to have a huge impact on it's fuel efficiency, when the engine is only generating something like 10hp to keep the car rolling.

"Bam" nothing.

[redpoint5]

Originally Posted by Crashy

5. Supercapacitors are a good addition
False. They have high self discharge and adding losses to a system cannot possibly improve efficiency

Good stuff Crashy, but this statement is false. Supercaps have low self-discharge, but what little they do bleed is significant given the relatively low energy capacity. The highest self-discharge is near the voltage limits of the supercap, and minuscule as the voltage drops below max.

6. Alternators don't waste much horsepower when unplugged
False. Bearing, belt and fan friction are still present and are significant. Self excitement of the rotor is still occurring unless you disconnect the main and rotor wires.

Plenty of people have installed an alt-kill switch with great results. Sure, it's not as good as removing it completely, but the improvement in FE is still significant.

Originally Posted by RedDevil

The real reason why alternator deletes are efficient is that the whole chain from gasoline through engine efficiency, belt efficiency and alternator efficiency means that just 21% of the potential energy gets converted to electricity - at best.

The ICE is only 25% efficient itself, so the actual efficiency of converting gasoline energy into electrical must be much less than 21%. I'm assuming the 21% is the efficiency of converting the mechanical energy of the motor into electricity?