Newbie with some questions

[Hypersprint]

What am I laughing about, that is probably true

[KLA]

Originally Posted by Hypersprint

What am I laughing about, that is probably true

And IF we drink beer, it should be this one :

http://www.spiegel.de/international/...-a-686305.html

[Hypersprint]

Originally Posted by KLA

And IF we drink beer, it should be this one :

http://www.spiegel.de/international/...-a-686305.html

I remember that one

[KLA]

Sorry to dig up this old thread, but I thought a few people might be interrested.

After some more research, I think a series hybrid golf car can be done with a single small engine of ~11hp and a single, run-off-the-mill 14V/120A car alternator. No need for two big 24V truck alternators.

A single car alternator can be used to produce 42, 48, or 56V to charge a 48V battery pack. Such an alternator can produce up to 5.2 kW with an efficiency of ~65% if the internal regulator and rectifiers are removed and replaced with some custom electronics.

These papers show how:
http://itee.uq.edu.au/~aupec/aupec04.../PaperID82.pdf
http://dspace.mit.edu/bitstream/hand...pdf?sequence=1

The first paper above shows the results obtained with such a modified alternator. The second paper is the doctoral dissertation (from MIT, no less) of the guy who invented this method, which describes it in detail and shows also how to produce two different output voltages from a single alternator at the same time. For a 48V golf cart only one voltage needs to be produced, making the whole thing a little simpler.

With some further mods to the electronics, the alternator could also be used (temporarily) as a 3-phase synchronous motor to start the engine. That means no separate 12V battery is needed for auto-starting the range extending generator and no additional starter/generator would be needed. That additional electronic would not be very expensive, as 1/2 of what's needed in power transistors is already there and used for the voltage step-up to 48/56V.

I realize most of what's in the papers makes sense only to electronics engineers, but I want to point to Fig. 16, 19 and 20, and Section 5. "Conclusions" in the first paper that show experimental results obtained from a 14V/120A car alternator.

[JohnnieB]

I do understand the papers.

The basis for them is a theoretical reduction in an automobile's carbon footprint by converting various systems that are hydraulically, pneumatically or mechanically powered by the gasoline engine to systems powered by electric motors, which are in turn powered by a more efficient 42V electrical system rather than the current 12V electrical system, which they consider to be a 14V system.

Essentially, the automobile's main gasoline engine only supplies motive power and electrical power for the electrical motors that do everything else is supplied by a small, highly efficient, gasoline engine and alternator combination, which of course, uses batteries to buffer the variations in load.

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Can something along the ideas expressed by used to extend the run-time or range of an electric golf cart? Yes.

In fact, EZGO already does it. http://www.ezgo.com/Exceed.html

[KLA]

Originally Posted by JohnnieB

I do understand the papers.
....[/URL]

I knew YOU would .

However, what I understand about the Exceed is that it uses a conventional AC generator, re-wound to produce 48/56V.

What I found interresting about the switched rectifier alternator is that it can use a conventional 12V/120A alternator. Those are produced of course in very large quantities and therefore would be much cheaper to use in a limited production vehicle like a golf cart. And that translates into substantial cost savings.

As for the engine, unfortunately no small engine manufacturer produces rotary (Wankel) engines anymore. Such an engine would be ideal for this because of its high volumetric and mass power density.

Although rotary engines have the reputation of having problems with seals and not being very efficient, the seal issue has been solved by Mazda since quite a while.
The low efficiency is also not true for an application like this because those engines can be more efficient (and a lot cheaper) than a piston engine if they are used only at a fixed rpm and load as required for a range extender.

Audi thinks so too:
http://www.caranddriver.com/features...-alive-feature

[JohnnieB]

I'm not sure what type AC generator EZGO uses in the Exceed system, but all alternators are AC generators.

The line frequency of the AC depends on the number of poles and the RPM the rotor is spun at, but when the output is being converted to DC, line frequency isn't critical.

Basically, car alternators produce three phase AC that is rectified to DC and the output voltage is regulated by varying the excitation current.
The net result is an efficiency of about 50% or less.

On the other hand, just by using an optimal excitation current and optimal rotor speed, efficiencies of 65% and above can be obtained, but the output voltage is far higher than what is needed for the 12V electrical system of a car, so an electronic voltage regulator of some sort has to be used.

It looks like the voltage available when the alternator is operating at its peak efficiency is high enough to charge 36V and possibly 48V golf cart systems.

Also, an engine (gasoline, diesel, propane, hydrogen or whatever fuel) that is ran at a constant RPM can be designed and tuned for maximum efficiency at that RPM.

Then it is just a matter of using the right gear/pulley ratio to spin the alternator at its most efficient RPM.

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It is an interesting concept, but I think I leave my sparky, pure sparky.

[KLA]

You are right in that a normal car alternator can easily produce enough voltage to charge a 36V or 48V battery system if run fast enough, and the output voltage controlled by exitation current.
However, in the papers they don't do that. They run a fixed exitation current. To produce an output voltage high enough, they short the produced AC to ground during part of a 100 kHz PWM cycle. This means the inductance of the stator coils are used to build up a current up to the alternator peak current limit and that stores energy in their magnetic field. When the "short" is switched off, that energy discharges to charge the batteries, filtered and smoothed by capacitors.

This is similar to a buck-boost DC-DC converter. It means the high target voltage can be achieved at much lower alternator rpms and the output power can be regulated by the duty cycle of the 100 kHz PWM of the switched rectifier.
Pretty nifty.

Also, regarding the Audi range extender shown in the above link:
It says it can produce up to 15 kW. I did a quick calculation and it seems that the generator efficiency calculated as output in electrical kWh over raw energy of fuel is about 30% total. That is very high for a gasoline powered generator.

The size of that range extender (without fuel tank) is a cylinder of 9.4" diameter and 9.8" long.
That means it will fit into about the same space as a single T105 battery, which stores roughly 1kWh.
If a second T105 is replaced by a fuel tank, the available electrical energy would be 36 kWh, replacing 2 kWh of stored energy in the two T105s it replaces.